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Fabrication and FMR studies of ferromagnetic iron-gallium arsenide waveguide structures and application to microwave bandstop filters

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UNIVERSITY OF CALIFORNIA
IRVINE
Fabrication and FMR studies of Ferromagnetic
Fe-GaAs Waveguide Structures and Application to
Microwave Bandstop Filters
DISSERTATION
Submitted in partial satisfaction o f the requirements
For the degree of
DOCTOR OF PHILOSOPHY
IN ELECTRICAL AND COMPUTER ENGINEERING
by
Wei Wu
Dissertation Committee:
Professor Chen S. Tsai, Chair
Professor Chin C. Lee. Co-Chair
Professor G. P. Li
2002
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UMI Number: 3039234
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© 2002 Wei Wu
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T h is dissertation o f W e i W u
is ap p rov ed and ;s a c c e p ta b le ir quality
and form for publication o n m ic r o film :
' V m m u i c c f\.-(.'haiv
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C o m m ; - te e Chair
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2002
ii
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D E D IC A T IO N
This dissertation is dedicated to
My parents
Xiangyun Wu and Ailian Chen
My wife Liangyan Wang & My Daughter Jessica
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TABLE OF CONTENTS
LIST OF FIGURES................................................................................................................... vi
LIST OF TABLES..................................................................................................................... xi
ACKNOWLEDGEMENTS..................................................................................................... xii
CURRICULUM VITAE......................................................................................................... xiii
ABSTRACT.............................................................................................................................xvi
CHAPTER 1 Introduction........................................................................................................ 1
I.
I Research background.........................................................................................1
1.2
Purpose o f dissertation and summary o f results..............................................3
1.3
Outline o f dissertation....................................................................................... 7
References....................................................................................................................... 8
CHAPTER 2 Fe/Ag/Fe multilayer growth and characterization........................................ 9
2.1
Introduction to M B E .........................................................................................9
2.2
Ultrathin Single/multilayer ferromagnetic Fe/Ag film grow th .................... 13
2.3
Characterization o f Fe/Ag/Fe ferromagnetic thin films................................ 19
References..................................................................................................................... 27
CHAPTER 3 Microstructural studies o f ferromagnetic Fe films........................................28
3.1
Principles of Magnetic Kerr effect (MOKE)...............................................28
3.2
MOKE measurement setup and results..........................................................30
3.3
X-ray diffraction and X-Ray Read-camera measurements.......................... 38
References..................................................................................................................... 47
CHAPTER 4 FMR studies o f Fe-GaAs waveguide structures............................................ 49
4.1
Introduction to ESR measurement...................................................................49
4.2
Principles o f ESR measurement....................................................................... 51
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4.3
Experimental results o f ferromagnetic Fe/Ag/Fe samples........................ 54
References....................................................................................................................67
CHAPTER 5 Wideband Microwave Band-stop filter fabrication and measurement
69
5.1
Fe films-GaAs microwave band-stop filter fabrication...............................69
5.2
Microwave Transmission measurement......................................................... 72
5.3
Principles o f Band-stop filter measurement.................................................. 80
5.4
Microwave Measurement................................................................................ 83
5.5
Conclusions and Discussions.......................................................................... 92
References.................................................................................................................... 94
CHAPTER 6 Antiferomagnetic coupling in Fe/Cr/Fe/GaAs............................................. 95
6.1
Introduction...................................................................................................... 95
6.2
Magnetic coupling effect................................................................................ 101
6.3
Growth o f Fe/Cr/Fe multilayers on GaAs (100) substrate
6.4
MOKE measurements..................................................................................... 104
6.5
AFM studies o f the magnetic samples...........................................................108
6.6
FMR microwave measurements o f Au/Fe/Cr/Fe-GaAs..............................112
References....................................................................................................................128
CHAPTER 7: Conclusion......................................................................................................130
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LIST OF FIGURES
Figure 1.1
Schematic diagram o f microwave bandstop Filter........................................ 4
Figure 2.1
MBE system used for this dissertation.......................................................... 10
Figure 2.2
AFM image o f GaAs layer grown on the GaAssubstrate............................. 13
Figure 2.3
AFM image o f Fe thin film grown on GaAs substrate................................. 14
Figure 2.4
Single cell model o f GaAs lattice....................................................................15
Figure 2.5
The 4x4 cell o f the GaAs lattice model.......................................................... 16
Figure 2.6
Schematic diagram o f single layer Ag/Fe structure...................................... 17
Figure 2.7
Schematic diagram o f multilayer Fe/Ag/Fe structure...................................18
Figure 2.8
Standard AES spectrum o f Iron......................................................................20
Figure 2.9
Standard AES spectrum o f silver................................................................... 20
Figure 2.10
AES of GaAs substrate cleaned by the standard process and AES
o f as-deposited Fe/Ag/Fe/GaAs structures by MBE................................... 21
Figure 2.11
Cross-section view SEM image o f sample A ................................................ 23
Figure 2.12
Magnified SEM image o f the distortion region in sample A ...................... 23
Figure 2.13
SEM image o f GaAs substrate after cleaning process (for Sample B)
Figure 2.14
SEM image o f sample B...................................................................................25
Figure 2.15
SEM image o f sample C ...................................................................................25
Figure 3.1
Schematic diagram o f Kerr and Farady effect.............................................. 29
Figure 3.2
Schematic diagram o f MOKE measurement setup.......................................30
Figure 3.3
Diagram depicting longitudinal MOKE effect.............................................. 31
Figure 3.4
Schematic diagram o f a magnetic hysteresis loop........................................32
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24
Page
Figure 3.5
Longitudinal MOKE spectra for Fe magnetic films deposited on GaAs
substrate at (a) room temperature, (b) 80 °C (hard axis), (c) 80 °C
(easy axis), (d) 120 °C (hard axis), (e) 120 °C (easy axis).......................... 33
Figure 3.6
Longitudinal MOKE spectra o f ultrathin Fe magnetic films deposited
on GaAs substrate at 150 °C...........................................................................36
Figure 3.7
Longitudinal MOKE spectrum o f ultrathin Fe film deposited on GaAs
substrate at 170 °C...........................................................................................37
Figure 3.8
The layout of a XRD 0 - 2 0 scan................................................................41
Figure 3.9
X-ray diffraction spectrum o f Ag/Fe structure grown on Si substrate
Figure 3.10
XRD spectra o f (a) A gfe/G aA s single layer structure and
(b) Ag/Fe/Ag/Fe/GaAs multilayer sample....................................................44
Figure 3.11
X-Ray read-camera diffraction patterns o f (a) GaAs (100) substrate
(b) Ag/Fe/Ag/Fe films deposited on GaAs by MBE at 120 °C (c)
Ag''Fe/Ag/Fe films deposited on GaAs produced by thermal
evaporation at 120 °C (d) Ag/Fe/Ag/Fe films deposited on GaAs
by MBE at 170°C...........................~ .............................................................. 45
Figure 4.1
Schematic diagram o f Electron spin resonance setup................................... 50
Figure 4.2
Schematic diagram o f electron spin induced magnetic moment.................52
Figure 4.3
FMR spectra of single layer Fe films.............................................................. 55
Figure 4.4
FMR spectrum of multilayer Fe film grown at room temperature............. 57
Figure 4.5
FMR spectrum o f multilayer Fe film grown at 100 °C................................. 59
Figure 4.6
FMR spectrum o f multilayer Fe film grown at 120 °C.................................60
Figure 4.7
FMR spectrum of multilayer Fe film grown at 170 °C.................................61
Figure 4.8
FMR response of single layer Fe film by thermal evaporation................... 62
Figure 4.9
Linewidth 5fres o f ferromagnetic resonance signal observed in the
multilayer samples...........................................................................................65
Figure 4.10
The values o f the FMR linewidth AH(J) o f multiplayer samples
as a function o f microwave frequency......................................................... 66
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Page
Figure 5.1
Sketch o f Fe-GaAs microwave bandstop filter............................................ 71
Figure 5.2
Diagram o f experimental setup for microwave measurement....................73
Figure 5.3
Schematic diagram o f a straight microstrip line.......................................... 74
Figure 5.4
Transmission Characteristics o f straight microstrip lines...........................75
Figure 5.5
Basic equivalent circuit o f a straight microstrip transmission line............ 76
Figure 5.6
Equivalent circuit o f multi-finger low-pass filter transmission line..........78
Figure 5.7
Transmission Characteristics o f low-pass filter type microstrip lines...... 79
Figure 5.8
Schematic diagram o f flip-chip microwave bandstop filter....................... 80
Figure 5.9
Diagram o f magnetic easy and hard axes o f Fe film................................... 81
Figure 5.10
Transmission characteristics o f a single layer Fe based flip-chip
bandstop filter................................................................................................. 83
Figure 5.11
Calculated and measured peak absorption versus bias magnetic field
with the magnetic field applied along the easy axis o f Fe film.................84
Figure 5.12
Transmission characteristics o f the bandstop filter with the magnetic
field applied along the hard axis o f Fe film.................................................85
Figure 5.13
Transmission characteristics o f a bandstop filter using the Fe/Ag/Fe
multilayer structure.........................................................................................86
Figure 5.14
Calculated and measured peak absorption versus the bias magnetic
field applied along the easy axis o f the Fe film.......................................... 87
Figure 5.15
Transmission characteristics o f the bandstop filter
with the magnetic field applied along the hard axis o f Fe film.................87
Figure 5.16
Comparison o f calculated and measured peak absorption frequency
versus bias magnetic field applied along the hard axis o f the Fe film..... 88
Figure 5.17
Tuning o f peak absorption carrier frequency o f the bandstop filter
while the magnetic field is applied along the easy axis o f the Fe film.... 89
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Figure 5.18
Comparison o f calculated and measured peak absorption frequency
versus bias magnetic field in easy axis o f Fe film...................................... 90
Figure 5.19
Relationship between FMR linewidth (measured at 9.6 GHz)
and maximum microwave signal absorption dip intensity........................ 91
Figure 6.1
(a) The magnetization in domains usually cancel in bulk materials (b)
magnetizations in domains are aligned along external magnetic field......97
Figure 6.2
Domain structures under (a) no external magnetic field (b) weak applied
magnetic field and (c) strong applied magnetic field..................................97
Figure 6.3
Internal magnetic field H i m e m a i (along z direction) induced by a
magnetization distribution o f Mz
100
Figure 6.4
Spin configurations o f ferromagnetic coupling between Fe layers induced
by Cr space layer............................................................................................ 101
Figure 6.5
Spin configurations of anti ferromagnetic coupling effect between Fe
layers induced by an additional layer o f Cr atom.......................................102
Figure 6.6
The spin configuration in the Fe and Cr layers (a) perfect interface (b)
rough interface case. Cr layer's coupling is frustrated (c) In interface
region. Cr/Fe coupling is frustrated (d) The frustration in Fe layers...... 103
Figure 6.7
MOKE spectrum for sample with 120 A Fe layers and 15 A Cr layer grow
at room temperature....................................................................................... 109
Figure 6.8
MOKE spectrum for sample with 120 A Fe layers and 21 A Cr layer grow
at room temperature...................................................................................... 109
Figure 6.9
MOKE signal of sample was grown at 150 °C with
a Cr layer thickness o f 31 A .........................................................................110
Figure 6.10
MOKE spectrum for Au/Fe/Cr/Fe samples with 120 A Fe layers and 15 A
Cr layer grown at a substrate temperature o f 150 °C................................. I l l
Figure 6.11
MOKE spectrum for Au/Fe/Cr/Fe samples with 120 A Fe layers and 26 A
Cr layer grown at a substrate temperature o f 150 °C................................. I l l
Figure 6.12
Room temperature growth o f Cr layer on Fe film (AFM)........................ 114
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Figure 6.13
Three-dimentional AFM image o f Cr layer
deposited on Fe film surface at room temperature................................. 114
Figure 6.14
AFM image o f Fe/Cr/Fe films grown on GaAs substrate at 150°C...... 115
Figure 6.15
Magnetic force microscopy (MFM) image o f Fe/Cr/Fe grown on
GaAs substrate at 150 °C without any external magnetic field
annealing....................................................................................................... 116
Figure 6.16
MFM image of Fe/Cr/Fe thin films grown on GaAs substrate at
150 °C. The sample has been annealed at 2000 Oe................................... 117
Figure 6.17
ESR spectrum o f Au/Cr/Fe/Cr (15 A)/Fe films grown at 150 °C.......... 118
Figure 6.18
FMR absorption spectrum at 9.6GHz with the external magnetic field
varied along magnetic hard axis...................................................................118
Figure 6-19
Cross-section schematic of Au/Cr/Fe/Cr/Fe structure............................... 119
Figure 6.20
FMR spectrum for Au/Fe/Cr (26 A)/Fe films grown at 150 °C................121
Figure 6.21
Cross-section schematic o f Au/Fe/Cr (26 A)/Fe structure on GaAs
Figure 6.22
FMR absorption spectrum at 9.6GHz for Au/Fe/Cr (31 A)/Fe films grown
at 150 °C.......................................................................................................... 122
Figure 6.23
Cross-section schematic of Au/Fe/Cr (21 A)/Fe structure on GaAs........ 123
Figure 6.24
FMR absorption spectra o f Au/Fe/Cr (21 A)/Fe films grown at (a) 25 °C
(b) 100 °C (c) 150 °C (d) 320 °C................................ ~ ........................... 124
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121
LIST OF TABLES
Page
Table 2.1
Basic lattice constant and crystal structure o f Fe. Cr and Ag film ........... 15
Table 2.2
Comparison o f substrate cleaning conditions and microstructure
Analysis by SEM and ESR for three Fe/Ag/Fe samples deposited by M BE...................22
Table 6.1
Electronic configurations o f Cr2+, CrJ+ and Fe............................................98
Table 6.2
Cry stal lattice constant and crystal structure o f Fe and C r...................... 105
Table 6-3
Comparison o f cleaning condition and ESR linewidth
o f four Fe/Cr (2.1nm)/Fe samples deposited by MBE at various temperature...............124
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ACKNOWLEDGEMENTS
I would like to express the deepest appreciation to my committee chair and
faculty advisor. Professor Chen S. Tsai, whose dedication to science and high standard of
research work are always an inspiration for every student. I have benefited greatly from
his guidance, encouragement and invaluable advice through all the time o f my Ph.D.
research. Without his support and persistent help this dissertation would not have been
possible.
I will forever be grateful to my co-advisor. Professor Chin C. Lee. who has been a
constant source o f support and understanding. I feel privileged to have worked under his
guidance. I wish to thank Professor G. P. Li for serving on my dissertation committee: his
knowledge in semiconductor devices and his dedication to both research and teaching
make him a very popular model for students. Special thanks to Professor H. Hopster o f
Physics Department for his continuous guidance and useful discussions in my four-year
research work, for his support in using the MBE system in his Laboratory. I am grateful
to other colleagues Dr. Jun Su. Dr. Nhan Do. Dr. William So and Jae Yoo. for their
supports during my work towards this dissertation.
Financial support was provided by the University o f California. Irvine, SMART
program and U.S. Army Force Office.
I owe a special thanks to my parents and brother, who have been always source of
moral support. Finally, I owe the deepest thanks to my wife Liangyan for her love and
understanding.
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CURRICULUM VITAE
Wei Wu
1989-93
B. S. in Physics. Nanjing University. China
1993-96
M. S. in Semiconductor Physics. Nanjing University, China
1996-98
Research Assistant in Electronics Engineering, Chinese University
o f Hong Kong. Hong Kong
1998-2002
Ph. D. in Dept, o f Electrical and Computer Engineering.
University o f California. Irvine
FIELD OF STUDY
Magnetic microwave thin film growth by MBE and microwave band-stop filter devices.
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PUBLICATIONS
(1). W. Wu. C. C. Lee. C.S.Tsai. J. Su. W.So, H.J.Yoo. R. Chuang. and H.J. Hopster.
"Epitaxially grown Fe/Ag ultrathin films on GaAs and their application to wideband
microwave devices”. Journal of Crystal Growth. Vol. 225.534-539 (2001)
(2). W. Wu. C. C. Lee. C.S.Tsai. J. Su. W.So. H.J.Yoo. R. Chuang.
"Fabrication o f ferromagnetic/semiconductor waveguide structures and application to
bandstop filters”. Journal of Vacuum Science and Technology, Vol. A 19 (4) 17581762. July/Aug. 2001
(3). C. C. Lee. W. Wu. C.S.Tsai
"Ferromagnetic resonance and Microstructural studies o f ultrathin Fe/Ag films-GaAs
waveguide structures”. Journal of Applied Physics, May 2002
(4). C. S. Tsai. C. C. Lee. J. Su. W. So. W. Zuo. W. Wu. H. J. Yoo. G. Giergiel. H.
Hopster. and D. L. Mills. "Wideband Electronically-Tunable Microwave Band-Stop
Filters Using Ultrathin Iron-Gallium Arsenide Waveguide Layer Structure”. Presented at
1999 MRS Spring Meeting. San Francisco. April. 1999
(5). Jun Su. Nhan Do. W. Wu and Chen. S. Tsai. " AO guided-mode to substrate-mode
conversion in proton exchanged LiNb03 waveguide”. Presented in 7th international
conference on Modem Acoustic and Ultrasonics. Nanjing, China, Oct. 11-14. 1998
(6). C. S. Tsai. W. Wu. C. C. Lee. J. Su. W. So, H. J. Yoo, R. Chuang
"Ferromagnetic resonance study o f ultrathin magnetic films and application to
microwave bandstop Devices " Presented at Nanometer materials and technology 2000
conference. Sendai. Japan. August 25-28.2000
xiv
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(7) W. W u , C. C. Lee , C. S. T s a i. J. Su, H. J. Yoo, R. Chuang
•'Magnetic properties and Ferromagnetic resonance study o f ultrathin Iron films grown
on GaAs (100)” Presented at 12th American Crystal growth Conference. Colorado,
August 13-15. 2000
(8) W. Wu. C. C. Lee . C. S. T sa i. J. Su. W. So, H. J. Yoo. R. Chuang
“Fabrication o f Ferromagnetic/Semiconductor Waveguide Structures And application to
High Frequency Notch Filter “ Presented at 47th American Vacuum Society International
Symposium. Boston. Oct. 1-5. 2000
(9). Nhan Do, J. Su. W. Wu. A.M. Matteo and C. S. Tsai.
“High efficiency acoustooptic guided to leaky-mode conversion in PE lithium Niobate
waveguides and application” Presented at IEEE International Ultrasonics Symposium.
Lake Tahoe, Nevada. October 17-21. 1999
(10). W. Wu. C. S. Tsai. C. C. Lee. H. Hopster and D. L. Mills
“ Ferromagnetic metal/ semiconductor structures for wideband integrated microwave
notch filter Devices” Presented at IEEE 56th Devices Research Conference, Denver.
Colorado. June 19-21. 2000
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ABSTRACT OF THE DISSERTATION
Fabrication and FMR studies of Ferromagnetic Fe-GaAs Waveguide
Structures and Application to Microwave Bandstop Filters
By
Wei Wu
Doctor o f Philosophy in Engineering
University o f California. Irvine. 2002
Professor Chen S. Tsai. Chair and Professor Chin C. Lee. Co-Chair
Epitaxial growth o f magnetic ultrathin films on semiconductor substrate has been
attempted for the integration o f the magnetic/semiconductor material hybrid devices. Of
all the Fe/III-VI semiconductor system, growth o f iron film on GaAs (100) has received
the most attention due to their smallest lattice mismatch o f only 1.4%.
In this dissertation, we report on the growth o f high-quality single crystal Fe/Ag
multilayer structures on GaAs (100) substrate. The X-ray diffraction (XRD), Magnetooptic Kerr effect (MOKE) and ferromagnetic resonance (FMR) measurements were
performed. High quality Ag/Fe multilayer crystalline structures have been grown, as
confirmed by read-camera XRD results. We studied the coupling between the
electromagnetic signal and the spin excitations in the ultrathin Fe films. In the microwave
region this coupling arises when the film magnetization vector M is driven by the
magnetic field component o f the radiation field. For single crystal Fe films, the
corresponding resonance occurs near 10 GHz when no magnetic field is applied. We have
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observed FMR linewidth broadening on Ag/Fe/Ag/Fe/GaAs samples due to intrinsic
damping effect in a frequency range o f 10 to 35 GHz. This is a very useful frequency
regime for many microwave devices.
Tunable
microwave
bandstop
filters
were
successfully
fabricated
using
ferromagnetic Fe/Ag/Fe-GaAs and ferromagnetic/anti ferromagnetic Cr/Fe/Cr/Fe-GaAs
layer structures. The resonant absorption frequency can be tuned electronically by
varying the magnitude of external bias magnetic field. This Fe film-based microwave
devices possess an important advantage over their Yttrium-Iron-Gamet (YIG) -based
counterparts in that for a given operating carrier frequency, it requires a much smaller
bias magnetic field than that of YIG devices. This is so because the saturation
magnetization o f the Fe films is more than one order o f magnitude larger than that of
YIG. Accordingly, a significantly higher device operating carrier frequency with large
electronic tunability could be achieved more readily using the Fe film structures,
compared to the previous report on ferromagnetic YIG-based devices.
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CHAPTER 1 INTRODUCTION
1.1 Research background
1.1.1 Basic concept o f magnetization
Many materials can be considered as magnetic in the sense that there may be a
magnetic moment associated with each individual atom. This moment may arise either
from the orbital motion o f the electron or may be associated with the electron spin. In our
study, we are concerned with ferromagnetic materials where the spin moments are
aligned parallel to each other so that a net magnetic moment is presented in the specimen,
even in the absence o f an external magnetic field. Due to the quantum mechanical
exchange interaction between electron spins on adjacent atoms, it tends to align the spins
parallel or antiparailel to each other. The exact mechanism o f the interaction will not be
provided here but it is equivalent to that of an internal magnetic field acting on the
individual atomic moment. It is found that in most ferromagnetic materials, applying
small magnetic field, normally at the order o f only a few Oe, can change magnetization.
1.1.2 Magnetocrystalline anisotropy
Suppose there are two atoms, A and B. each carrying a magnetic moment. Instead
o f a random array o f A and B, AA. AB and BB pairs may be aligned in a particular
direction with respect to the applied field so as to reduce the magneto-elastic energy [I].
This introduces an anisotropy in the magnetostatic field associated with the atomic
moments, which is compared to the isotropic with a random arrangement o f atomic
moments.
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Magnetocrystalline anisotropy arises from the coupling between the electron spin
and the orbital motion o f the electron [2]. The directional properties o f the electron orbit
are highly oriented with respect to the crystal lattice; thus the effect o f an applied
magnetic field on the spin moments depends on the spin orientation with respect to the
crystal lattice. In most ferromagnetic materials there are preferred directions of
magnetization, usually along one or other o f the major crystalline axis. A preferred
direction o f magnetization may be induced in a specimen by applying a magnetic field.
1.1.3. Magnetic materials
Magnetic materials are composed o f a large number o f domains separated by domain
walls, where domain wail is defined as a boundary between two regions with
magnetizations orient differently. The domain configurations considered are those formed
in single crystals, with a uniform crystalline orientation throughout the specimen [3].
Most o f the materials are observed to be polycrystalline composed o f large crystalline
grains with various orientations, the domain structure within a single grain follows the
pattern o f domain structures observed in large single crystals. Many crystals have an
ordered magnetic structure, which means that in the absence o f an external magnetic
field, the mean magnetic moment is non-zero. In the simplest type o f magnetically
ordered crystals, i.e. ferromagnets such as Fe, Ni and Co. the mean magnetic moments o f
all the atoms have the same orientation provided that the temperature o f the ferromagnet
does not exceed a critical value, i.e. the Curie temperature Tc. For this reason
ferromagnets have spontaneous magnetic moments even in the absence o f an external
magnetic field.
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In anti ferromagnets (include the transition metals Mn, Ni and Co), the mean
atomic magnetic moments compensate each other within each unit cell (in zero external
magnetic field). In other words, an antiferromagnet consists o f a set o f sublattices (called
magnetic sublattices), each of which has a non-zero mean magnetic moment. This type o f
magnetic order occurs if the temperature o f the antiferromagnet is less than a critical
temperature, known as the Neel temperature Tn. The magnetic order in ferromagnets and
antiferromagnets is the result o f correlation between the directions o f the electron spins
on individual atoms. This correlation is in turn due to the fact that the space symmetry o f
the wave function depends on the magnitude o f the resultant spin o f the system o f
electrons [4].
1.2 Purpose of dissertation and summary of results
The goal of this dissertation is to grow the single crystalline ferromagnetic Fe films,
study their microstructural and magnetic properties, and also utilize them to fabricate a
special spin electronic devices, namely, electronically tunable wideband bandstop (notch)
filter. Such bandstop filters possess potential applications in RF signal processing and
communication systems.
For the Fe-GaAs system, the lattice mismatch is very small, i.e.. 1.4%. making it
possible to grow epitaxial Fe films on GaAs [5]. It has been reported that, for Fe films
deposited directly on GaAs substrates at high temperature, interdiffusion o f As species
into the iron layer could occur [6]~[10]. The resultant Fe:As compound forms a
magnetically dead layer at the interface, which can severely degrade the magnetization o f
the sample, and leads to a disruptive effect at the semiconductor/metal interface. As a
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result, the measured spin-dependent electron transmission at the interface is very weak
due to the magnetic dead layer.
To eliminate this unfavorable intermixing effect,
multilayer Fe/Ag structures were grown.
Studies showed that the in-plane mismatch
between fee Ag and bcc Fe is only 0.8%, allowing the growth of single crystal Fe films
on Ag.
Microstrips— ^
RF Input
Fe/Ag-GaAs
Structure
RF Output
Figure 1.1 Schematic diagram o f microwave band-stop filter device
One o f the most useful properties o f ferromagnetic materials is that they can couple
to a radiation microwave signal. This coupling occurs when the magnetization vector M
is driven by the H component o f the radiation. In the magnetic thin film structures, the
magnetization lies in the plane of the film. If plane-polarized microwave signal
propagates to the plane o f the film, it causes to exhibit gyroscopic rotation. This system is
driven into resonance in accordance with the following equations [11][12]:
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fres = y [ ( H o + H a n )(H o +
fres
= y [(H o
Han + 4tiMs)] 112.
- H a „ )(H o +0.5 H a„ + 47tMs)],/2.
H 0 // Easy axis
( 1- 1)
H 0 // Hard axis
( 1-2 )
where y is the gyromagnetic ratio. H0 is the external magnetic field. Han is the anisotropy
field, and 47tMs is the saturation magnetization o f the ferromagnetic film. The coupling
causes strong absorption of the input microwave power at the FMR frequency / rcs. This is
a very useful frequency regime for many microwave devices.
The device structure fabricated in this study is shown in Fig. 1.1. a schematic
drawing illustrating a microwave stripline device concept. Striplines are an effective way
to carry microwave signals in planar circuits. They are guided wave devices into which, a
microwave signal is injected from a coaxial cable. The thick Ag'Cr metal layers
deposited on top o f Fe/Ag/Fe samples act as an electrode and the structure sustains an
electric field between the conducting ground plane and the metal layers located above the
insulating layer o f GaAs. The microwave radiation field is thus plane-polarized with its
electric vector E vertical and the corresponding magnetic vector H horizontal, and it
propagates down the device structure, and again picked up by another coaxial cable.
If the magnetization vector M o f Fe film is parallel to the direction o f microwave
propagation, the ferromagnetic resonance condition can be satisfied and the radiation
field can couple to the magnetization. In the Fe/Ag conducting microstrip region, the
radiation field can transfer energy into the ferromagnetic metal. Maximum coupling
occurs at the ferromagnetic resonance (FMR) frequency given in above equations (1-1.12). therefore, a display o f power transmitted versus frequency will show a sharp decrease
in transmission at f ns [12]. This device is called a bandstop or notch filter. The location
5
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o f the “notch” could be moved by applying an external magnetic field (parallel to the
magnetization direction M) on the device. If an external magnetic field is applied
perpendicular to the direction o f M and has sufficient strength to reorient the direction of
M perpendicular to the direction o f signal propagation, the radiation field will no longer
couple to M. since now M is parallel to the magnetic vector o f microwave signal. This
will effectively turn the filter off [13]. In this study, the epitaxial Fe films grown on GaAs
have shown the narrowest linewidths ever observed for a ferromagnetic Fe (23 Oe at
9.6GHz).
We studied the coupling between the electromagnetic signal and the spin
excitations in the ultrathin Fe films. Due to the elimination o f magnetic dead layer in our
samples, strong coupling between microwave signal and spin wave occurs in magnetic Fe
films. For single-crystal Fe film, where the saturation magnetization 4jt M s is very high
(around 22.000 Oe). the corresponding resonance occurs near 10 GHz with no magnetic
field applied. This is a very useful frequency regime for many microwave devices. Also,
the resonant frequency can be tuned electronically by varying the magnitude o f an
external magnetic field. It is possible to achieve higher operation carrier frequency for the
resulting microwave bandstop filter under a relatively low applied magnetic field.
Experimental results on FMR Iinewidth broadening due to intrinsic damping have been
reported for the first time in a wide frequency range from 10 to 35 GHz.
6
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1.3 Outline of dissertation
•
Molecule beam epitaxy (MBE) technique
•
Epitaxial growth o f single crystalline ultrathin Fe/Ag single and multilayers on GaAs
substrate
•
Characterization o f Fe/Ag/Fe-GaAs waveguide structures by X-ray diffraction
(XRD), X-ray read camera technique
•
Magnetooptic Kerr effect (MOKE) and Electron spin resonance (ESR) studies o f
ultrathin magnetic film quality and properties
•
Fabrication o f Fe/Ag/Fe-GaAs based wideband bandstop filters
•
Development o f a flip-chip technique for study o f wideband tunable band-stop filter
•
Growth o f Au/Fe/Cr/Fe multilayer structures on GaAs substrate for studying the
ferromagnetic/ antiferromagnetic coupling effect and sample characterization by
MOKE and ESR
7
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REFERENCES
1. N. M. Atherton, Electron spin resonance, theory and application, (1973)
2. Alex Hubert and Rudolf Schafer. Magnetic domains Chap. 1. (Springer 1998)
3. K. H. J. Buschow. Handbook o f magnetic materials Vol. 12 (North-Holland 1999)
4. J. A. C. Bland and B. Heinrich. Ultrathin Magnetic structures I (Springer 1994)
5. K. Park. L. Salamanca-Riba. B.T. Jonker, J. Appl. Phys. 79 (1996) 5195
6. M. Gester. C. Daboo and J.A.C. Bland. J. Mag. Magn. Mater.; 165, (1997) 242
7. G. Gladyszewski, C. Jaouen, J. C. Girard and P. Guerin. Thin Solid Film 319
(1998)44
8. B. K. Kuanr and Alka V. Kuanr. J. Mag. Magn. Mater.; 165 (1997) 275
9. A. Filipe. A. Schuhl. P. Galtier. Appl. Phys. Lett. 70. (1) (1997) 129
10. F. P. Zhang. P. S. Xu and E. D. Lu. et.cl. J. Appl. Phys. 86(3) (1999) 1621
11. M. Zolfl. M. Brockmann. M. Kohler. S. Kreuzer, T. Schweinbock, S. Miethaner
and FBensch. G. Bavreuther, J. Mag. Magn. Mater. 175 (1997) 16
12. B. Heinrich and J.A.C. Blands. Editors. Ultrathin Magnetic Structures vol. II.
Springer-Verlag. Berlin (1994)
13. N. Cramer. D. Lucic. R. E. Camley. Z. Celinski. J. Appl. Phys. 87 (2000) 6911
8
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CHAPTER 2 Fe/Ag/Fe MULTILAYER GROWTH AND
CHARACTERIZATION
Properties o f magnetic ultrathin films and multilayers have received considerable
attention both experimentally and theoretically [l][2]. Some unique characteristics o f
ultrathin films include enhanced magnetic moment, existence o f surface anisotropy and
perpendicular magnetization [3], Epitaxial growth o f magnetic layers on semiconductor
substrates has been widely attempted for the integration o f magnetic/semiconductor
hybrid devices [4]-[7]. For single crystal Fe films, where the saturation magnetization
Ms is very high (around 22.000 Oe). the corresponding resonance occurs near 10 GHz at
the absence o f external magnetic field. This is a very useful frequency regime for many
microwave devices [8]-[ 10]. In this chapter, we report the growth o f single crystalline
ferromagnetic Fe films by MBE technique, their microstructural properties, and their
crystalline structures. The deposited samples were characterized by Low energy electron
diffraction (LEED). Auger electron spectrum (AES) and Atomic force microscope
(AFM) techniques.
2.1 Introduction to MBE
2.1.1 Induction to MBE and MBE system setup
Ultrahigh vacuum (UFTV) is a prerequisite for a complete control o f the
environment in the growth and study o f a material. At the extreme low-pressure level o f
10‘10 torr (comparable to the Earth's ionosphere), a gas molecule travels 300 miles on
average before colliding with another gas molecule. It can be sure that at such pressure
level the material under study is not affected by contamination or oxidation.
9
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Figure 2.1 MBE system used for this dissertation
Molecular beam epitaxy (MBE) technique is a UHV system developed to grow highpurity epitaxial layers o f semiconductors and metallic materials. MBE can produce highquality films with very abrupt interfaces and good control o f thickness, doping and
composition. Because of the capability o f high degree o f control with MBE. it is a
valuable tool in the development o f electronic and optoelectronic devices. In MBE. the
elements o f sources in the form o f 'molecular beams' are deposited onto a heated
crystalline substrate to form thin epitaxial layers. The 'molecular beams* are typically
from thermally evaporated elemental sources. To obtain high-purity layers, it is critical
that the material sources be extremely pure and that the entire process be done in an ultrahigh vacuum environment in the level o f 10' 10 torr. Another important feature is that
10
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growth rates are typically on the order o f a few A/min and allowing for nearly atomically
abrupt transitions from one layer to another.
2.1.2 Principles o f MBE growth
As shown in Fig. 2.1. our MBE system consists o f two vacuum chambers: a
growth chamber (the larger one) and a buffer load chamber (the smaller one). The load
chamber is used to bring samples into and out o f the vacuum environment while
maintaining the vacuum integrity o f the growth chamber. Also it is used for preparation
and storage o f samples.
A clean surface is an important prerequisite for epitaxial growth, since
contaminants from the atmosphere or other sources can easily contaminate a GaAs wafer
and. therefore cause defects and degrade the optical and electrical characteristics o f the
epitaxial layer produced. AES was an essential tool for determining wafer cleanliness.
GaAs wafers are cleaved into pieces o f 10 by 10mm in size and then bonded on a
molybdenum boat. This process will result in some dust on the wafer surface that will
produce defects in sample growth. However, with proper care the amount o f dust can be
reduced by acetone cleaning and nitrogen gas blowing. The mounted substrates are then
brought into the load chamber and heated for several hours to outgas the substrates before
moving into the main chamber. The substrates are outgassed again at 550-600° C in the
main chamber at a vacuum level around 10‘9 torr.
The substrates are loaded into the growth chamber and moved to face the sources.
The surface crystallographic structure o f the sample can be determined by bombarding
the surface with low energy electrons (approx. 10-200 eV) and diffracted electrons were
observed as spots on a phosphorescent screen. The relative position o f the spots on the
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screen shows the surface crystallographic structure, and the intensity o f the spots as a
Junction of incident electron energy reveals information about surface reconstructions.
The substrate annealing temperature is ramped up, until a diffraction pattern appears in
LEED, showing that the oxide layer has been removed from the surface. The temperature
o f the substrate is monitored using a thermocouple in direct contact with the molybdenum
holder.
Growing epitaxial layers requires an approximate knowledge o f the flax and thus
calibration o f the growth rate is essential. The growth rate is measured using a crystal
monitor gauge, which, for a given system and material, the monitor reading is
proportional to the flux at the sample surface and hence the growth rate. The resolution o f
the gauge is capable o f measuring the rates to a precision o f 0.1 A per minute. The
biggest difficulty, however, is that while measuring the atom flux, the gauge gets coated,
causing a rapid loss o f its sensitivity.
We have used two UHV systems to grow samples for device fabrication. The big
UHV system has sample characterization facilities available: AES for chemical surface
analysis and LEED for structural characterization. While the small system offers the
same preparation and deposition facilities as in the big MBE chamber, it does not have
any in-situ characterization technique available. Thus, it can only be used for routine
growth o f samples following established recipes.
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2.2 Ultrathin single/multilayer ferromagnetic Fe/Ag film growth
Epitaxy refers to the ordered growth o f one crystal upon another crystal. There are
three main modes o f epitaxial growth: (a) monolayer, (b) nucleated, and (c) nucleation
followed by monolayer.
Figure 2.2 AFM image o f GaAs layer grown on GaAs substrate
Monolayer growth occurs when the deposited atoms are more strongly bound to
the substrate than they are to each other. The atoms aggregate to form monolayer islands
o f deposit, which enlarge, and eventually a complete monolayer coverage has taken
place. The process is repeated for subsequent layer growth. As displayed in Fig. 2.2, the
Atomic force microscopy (AFM) image (\um by 1um in size) of GaAs thin film grown
on GaAs substrate, clear layer-by-layer epitaxial growth is shown. The step height
between neighboring monolayers is measured and exactly equal to the crystal lattice
constant o f GaAs. In case o f nucleated growth, the initial deposited atoms aggregate as
small three-dimensional (3D) islands, which increase in size as further deposition
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continues until they touch and intergrow to form a continuous film. This mode is favored
where the forces o f attraction between the deposited atoms are greater than that between
them and the substrate. In Fig. 2.3. AFM image o f Fe film deposited on GaAs substrate is
shown. Three-dimensional islands are distributed everywhere and stacked together from
top to bottom. In the final growth mode, growth starts with the formation o f a single or
few monolayers on the substrate followed by subsequent nucleation o f 3D islands on top
o f these monolayers.
Figure 2.3 AFM image o f Fe thin film grown on GaAs substrate
Epitaxial growth requires that the atomic spacing and the lattice constant o f the
material differ by no more than a few percent, and that they have the same crystal
structure. The lattice matching imposes a serious constraint on the range o f compositions
that can be grown on a given bulk substrate. The epitaxial behavior o f one metal at the
surface o f another metal has been discussed in recent years, since such information is
essential in order to create new materials that having electronic properties as well as
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chemical properties not available in single metals [11]~[15]. Ultrathin Fe films on single
crystalline GaAs are typical o f a man-made materials system. It is well known that for
bulk Fe the phase transition from bcc to fee occurs at 1189 K, i.e., fcc-Fe can only be
present at temperature above 1189 K. The close match in lattice constant between bcc-Fe
and GaAs makes it possible to grow thin bcc-Fe films on GaAs using MBE techniques
even at room temperature.
Fe (Iron)
Ag (Silver)
Cr (chromium)
Lattice Constant
2.87 A
Crystal Structure
BCC
Lattice Constant
4 .09 A
Crystal Structure
FCC
Lattice Constant
2.88 A
Crystal Structure
BCC
Table 2.1. Basic lattice constant and crystal structure o f Fe. Cr and Ag
Figure 2.4 Single cell model o f GaAs lattice. The blue balls
correspond to Ga ions and the green balls correspond to As ions.
15
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Table 2.1 gives the detail crystal parameters o f the materials involved in our
sample growth. For GaAs crystal, its lattice consists o f two FCC sublattices displaced by
[1/4,1/4.1/4], GaAs has equal numbers o f gallium and arsenic ions distributed on a
diamond lattice so that each has four o f the opposite kind as the nearest neighbors, as
shown in Fig. 2.4. The blue sites correspond to gallium (Ga) ions and the green sites
correspond to arsenic (As) ions. The 4x4 cell o f the GaAs lattice is shown in Fig. 2.5.
Figure 2.5. The 4x4 cell o f the GaAs lattice model
Prior to deposition o f the Fe films by MBE. GaAs (100) substrates were cleaned
in UHV by heating at 550-580 °C and mild sputtering (Ne ion. 500 V and 1000 V for a
few minutes). Surface structures were examined in situ by low-energy electron diffraction
(LEED) and Auger electron spectroscopy (AES). The experimental results confirmed that
the abovementioned cleaning steps provided a well-ordered surface free o f carbon and
oxygen species. It is generally believed that the initial Fe growth is crucial. The first few
atomic layers o f Fe determine the crystalline properties o f the whole film. It is clear that
16
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some compound formation takes place at the Fe/GaAs interface [ 16]—[ 19]. Even though
some groups have now grown Fe films epitaxially on semiconductor substrates
(especially MgO). very little is known on an atomic scale about the initial growth phase
[20]~[24].
Ag (5.0 nm)
GaAs (100) substrate (350 nm)
Figure 2.6 Schematic o f single layer Fe/Ag structures
The Fe films were grown at various substrate temperatures with a deposition rate
o f a few atomic layers per minute. A quartz crystal oscillator was used to monitor the
evaporation rate. The pressure during deposition is in the 10 ‘10 torr range. These
conditions lead to high-quality single-crystal Fe films. Two kinds o f sample structures,
single layer o f Ag (5nm)/Fe (45nm) and multilayer structure o f Ag (5.0 nm)/Fe (45
nm)/Ag (10 nm)/Fe (1.0 nm) were grown on the GaAs substrates. For Fe films deposited
directly on GaAs substrates (Fig. 2.6) at high temperature, interdiffusion o f As species
into the upper iron layer could occur. The resultant Fe^As compound forms a
magnetically dead layer at the interface, which can severely degrade the magnetization o f
the sample, and leads to a disruptive effect at the semiconductor/metal interface. If Ag
film was grown firstly on GaAs, polycrystalline Ag films would be obtained because o f
large lattice mismatch and different crystal structures. Studies showed that the in-plane
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mismatch between fee Ag and bcc Fe is only 0.8%, allowing the growth o f single crystal
Fe films on Ag [25]. Therefore, multilayer structure Ag (5.0 nm)/Fe (45 nm)/Ag (10
nm)/Fe (1.0 nm) was grown on GaAs substrate, as shown in Fig. 2.7. For the multilayer
structure, the 1.0 nm Fe seed layer was grown first. After cooling down to room
temperature, the lOnm Ag buffer layer was deposited. This Ag buffer layer was grown to
isolate the 1.Onm magnetic dead layer from the 45nm main Fe layer, while still allowing
epitaxial growth of this Fe layer on top o f it because the in-plane mismatch between fee
Ag and bcc Fe is only 0.8%. Adding a buffer layer o f Ag seems to suppress the spin
pinning and may also lead to narrower linewidth.
Ag (5.0 nm)
Ag(10nm)
Fe(l.Onm)
V
*‘ v -
Figure 2.7. Schematic o f multilayer Fe/Ag structures
It was also reported that strong magnetic moments could be induced in non­
magnetic Ag metals deposited on Fe (100) by direct exchange interaction. The magnetic
moment is enhanced at the Fe/Ag interface. This will improve the coupling between
microwave signal and spin wave, as described in the following chapter. Then the films
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were characterized ex-situ by magnetic measurements using the magnetooptic Kerr
effect. We obtained very square hysteresis loops with a low coercive field. In addition,
some o f the films were characterized by ESR equipment at 9.6 GHz. Narrow linewidth
(around 30 Gauss) are usually obtained provided the growth conditions are adequate.
For comparison, samples were also grown by conventional thermal evaporation
technique. The vacuum level is around lO'6 torr during evaporation. The source material
was held in a tungsten boat, which was heated by electric current to melt and vaporize the
source material. The substrate was not baked, nor was it cleaned in situ by sputtering.
2.3 Characterization of Fe/Ag/Fe ferromagnetic thin films
Auger Electron Spectroscopy studies the release o f electrons with characteristic
energies from elements in the surface o f the material. This technique is derived from the
Auger effect. AES identifies the chemical elements in the surface o f the material. The
energy o f an Auger line is independent o f the initial excitation process (photon- or
electron-induced) and can be found in Auger-tables. In electron-induced AES an electron
beam o f typically 3 to 10 keV energy is used to create the primary core hole.
Figure 2.8. 2.9 shows the standard electron-induced AES spectra of iron and
silver, respectively [26]. The x-axis represents the electron energy (in eV unit), which
gives the characteristic energy level corresponding to specified elements. For Fe. a 3keV
electron-beam was used, there are three characteristic peaks, namely. 598 eV. 651 eV and
703 eV. For silver, also three characteristic peaks are seen to position at 266. 302 and 356
eV with a 5keV electron beam detection source used in the measurement.
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iro n , Fe
Figure 2.8 Standard AES spectrum o f Iron material
Silver. Ac c = 4 ;
C*«SkeV
Figure 2.9 Standard AES spectrum o f silver
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(a) G aA s Subtrate
I
(b) Fe/A g-G aA s
Figure 2.10. Auger electron spectroscopy o f GaAs substrate cleaned by the standard
process and AES o f as-deposited Fe/Ag/Fe/GaAs structures by MBE
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Figure 2.10 presents the AES o f (a) GaAs substrate after cleaning process and (b)
the Fe/Ag films deposited on GaAs substrate by MBE technique. Compared with
standard AES data, no peaks corresponding to carbon or oxygen-related species were
found in the spectrum for the cleaned GaAs substrate, which proved that our substrate
cleaning process has given a technically cleaning substrate in chemical means. In
physical means, scanning electron microscopy has been used to examine the GaAs
substrate.
Sample A
Sample B
Sample C
Annealing Temperature (UC)
650
580
580
Annealing Time (minutes)
10
10
10
Sputtering Voltage (kV)
1.0
0.5. 1.0
0.5. 1.0
Sputtering Time (min.)
10
5. 5
j. j
Sputtering Temperature (UC)
180
170
170
Sputtering Vacuum (torr)
6E-4
6E-4
3E-4
FMR linewidth (Oe)
86.310
47. 66
26.42
Crystal structure (SEM)
Seriously tilt
Tilt
Good
Table 2.2. Comparison of substrate cleaning conditions and microstructure analysis by
SEM and ESR for three Fe/Ag/Fe samples deposited by MBE
Electron Microscopes were developed due to the limitations o f light microscopes,
which are limited by the physics o f light to 500x or lOOOx magnification and a resolution
o f 0.2 micrometers. Scanning electron microscopy is the best known and most widely
used o f the surface analytical techniques, which, accompanied by X-ray analysis, is
considered a relatively rapid and non-destructive approach to surface analysis. SEM
functions exactly as their optical counterparts except that they use a focused beam of
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electrons instead o f light to image the specimen and gain information o f sample crystal
structure and composition.
Figure 2.11. Cross-section view SEM image o f sample A
Figure 2.12. Magnified SEM image o f the distortion region in sample A
In operation, a stream o f electrons is formed by the electronic source and
accelerated toward the specimen using a positive electrical potential. This stream is
confined and focused using metal apertures and magnetic lenses into a thin, focused,
monochromatic beam. Interactions occur inside the irradiated sample, affecting the
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electron beam. The primary electrons enter a surface with energy o f 0.5 - 30 keV. and
generate many low energy secondary electrons. The intensity o f these secondary
electrons is largely governed by the surface topography o f the sample. An image o f the
sample surface can thus be constructed by measuring secondary electron intensity as a
function of the position of the scanning primary electron beam. High spatial resolution is
possible because the primary electron beam can be focused to a very small spot (< 10
nm). High sensitivity to topographic features on the outermost surface (< 5 nm) is
achieved when using a primary electron beam with energy o f < 1 keV. Hence, some
qualitative elemental information can be obtained. This examination can give us the
topography and crystallographic information
Figure 2.13. SEM photo of GaAs substrate after cleaning process (for Sample B)
High-resolution SEM cross-section view images are presented in Fig. 2.11-Fig.
2.15. For sample A. under growth condition (described in detail in table 2.2). serious tilt
region was observed around 10 micrometers deep below the Ag/Fe films-GaAs interface
in Fig. 2.11 and the magnified image o f tilt region was shown in Fig. 2.12.
24
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Figure 2.14. SEM picture o f sample B, this sample is covered by additional Ag layer to
protect it from oxidation, crystal tilt region is seen around lum below the Fe/GaAs
interface
Figure 2.15. SEM picture o f sample ARO-C
After careful examination o f the growth condition o f this sample, we speculated
that it is due to the high annealing temperature at 650 °C and relatively long time of
sputtering at 1000 eV energy level, which hurt the surface crystal lattice and result in
roughness on the surface region, and therefore produce defects and dislocations in
deposited magnetic films. FMR experimental result shows a very wide line width o f 310
Oe, as described in table 2.2, and the deposited film is polycrystalline. In sample B. we
25
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reduced the substrate annealing temperature to 580 °C and sputtered the sample in a
lower energy of 500eV. the tilt phenomenon is relieved compared with that in sample A.
and the FMR line width is reduced to be 66 Oe. Further reduction o f the sputtering time
and sputtering vacuum level (Ne ion density) gave us the cleaned substrate without
crystal damage and thus a good single crystalline magnetic film, and the FMR linewidth
is reduced to as small as 26 Oe. No distortion was seen in the cross-section SEM images
in Fig. 2.15. In conclusion, the best cleaning process has been established for our
substrate and it is ready for the sample growth and the subsequent devices fabrication.
Reference
1. A. Flipe and A. Schuhl, J. Appl. Phys. 81(8) (1997) 4359
2. F. P. Zhang, P. S. Xu and E. D. Lu. J. Appl. Phys. 86(3) (1999) 1621
3. M. Brockman. M. Zolfl. S. Miethaner and G. Bayreuther. J. Mag. Magn. Mater. 198199(1999)384
4. S. Datta and B. Das. Apply. Phys. Lett. 56 (1990) 665
5. Magnetism in ultrathin films, ed. D. Pescia. Appl. Phys. A (1998) 49
6 . B. Lepine. S. Ababou and G. Jezequel. J. o f Appl. Phys. 83(6) (1998) 3077
7. M. Zolfl. M. Brockman, F. Bensch and G. Bayreuther. J. Mag. Magn. Mater. 175
(1997) 16
8 . C.S. Tsai. Proc. IEEE Vol. 84 (1996) 853
9. Jun Su. Chen S. Tsai, Chin C. Lee, J. Appl. Phys. 87 (2000) 5968
10. N. Cramer, D. Lucic, R. E. Camley. Z. Celinski. J. Appl. Phys. 87 (2000) 6911
11. B. K. Kuanr and Alka V. Kuanr, J. Mag. Magn. Mater. 165 (1997) 275
26
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
12. M. Neubauer, N. Reinecke and W. Felsch, Nucl. Instru. And Meth. In Phys. Rew. B
139(1998)332
13. Y. B. Xu. D. J. Freelad and J.A.C. Bland, J. Appl. Phys. 85(8) (1999) 5369
14. E. Schloman. R. Tutison. H. J. Van hook and T. Vatimos. J. Appl. Phys. 63 (1988)
3140.
15. K. Bierleutgeb, H. Krenn and H. Seyringer Phys. Slat. Sol. B 220 (2000) 41
16. H. Sitter and W. Faschinger Thin Solid Films 225 (1993). p. 250
17. R. Meckenstock, O. von Geisau. J.A. W olf and J. Pelzl J. Appl. Phys. 77 12(1995).
6424
18. C. Daboo et al.J. Appl. Phys. 75 (1994). p. 5586.
19. B. Heinrich and J.F. Cochran Adv. Phys. 42 5 (1993). p. 523.
20. G. A. Prinz and J.J. Krebs. Appl. Phys. Lett. 39 (1981). p. 397.
21. S.B. Quadri. M. Goldenberg. G.A. Prinz and J.M. Ferrari. J. Vac. Sci. Technol. B 3
(1985). p. 718.
22. D. J. Freeland. Y. B. Xu and J. A. C. Bland. Thin Solid Films 343-344 (1999) 210
23. K. Park. L. Salamanca-Riba. B.T. Jonker. J. Appl. Phys. 79 (1996) 5195
24. B. K. Kuanr. Alka V. Kuanr. J. Mag. Magn. Mater. 165 (1997) 275
25. G. Gladyszewski. K. Temst and Y. Bruynseraede. Thin Solid Films 275 (1996) 180
26. Lawrence E. Davis. N. C. Macdonald and R. Z. Weber. Handbook o f Auger Electron
Spectroscopy. Perkin-Elmer (1976)
27
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CHAPTER 3 MICROSTRUCTURAL STUDIES OF
FERROMAGNETIC Fe FILMS
Epitaxial Fe thin films grown on GaAs by molecular beam epitaxy are being
studied for the purposes of fundamental magnetic properties research and device
development [l]~[7]. The research interests concern the relationship between the
structural and magnetic properties o f thin-films. and the study o f the mechanisms o f film
epitaxial growth and interface quality as well as the structural characteristics o f the
sample grown [8 ][9].
3.1 Principles of Magnetooptic kerr effect (MOKE)
Magneto-optic methods make use o f the effect o f the domain magnetization on
the polarization of plane-polarized light observed by reflection from the surface o f the
specimen (the Kerr effect), or by transmission through the specimen (the Faraday effect),
as shown in Fig. 3.1. The Kerr and Faraday effects occur because the magnetization in
the material produces a change to the dielectric tensor o f that material [I0 ][ll]. The
magnetooptic Kerr effect (MOKE) is commonly used to characterize magnetic materials
due to the sensitivity of this effect to the magnetization strength and orientation in these
materials, and is a powerful probe for studying surface and ultrathin film magnetism with
submonolaver sensitivity.
The physical principle o f MOKE is that when plane polarized light is incident on
a magnetized material, the plane o f polarization o f the reflected light is rotated with
respect to the plane o f polarization o f the incident light.
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M a g n e tic m ateria ls
P la n e o f p o la ris a tio n
is ro ta te d -fa ra d a v ro ta tio n
P la n e o f p o la ris a tio n
is ro ta te d -K e rr ro ta tio n
Transmitted light
Reflected liaht
Figure 3.1 Schematic diagram o f Kerr and Farady effect
Any plane-polarized light can be decomposed into two circularly polarized lights.
The theoretical explanation of the Kerr effect is that the index o f refraction o f the righthanded circularly polarized light and the index o f refraction o f the left-handed circularly
polarized become different when a material is magnetized. The detected change in the
Kerr intensity (which is proportional to the Kerr rotation and to the sample's
magnetization) versus the applied magnetic field gives the hysteresis loop. Generally,
reflection o f plane polarized light from a magnetized surface is elliptically polarized.
However, reflection could also be plane polarized if the plane o f polarization is either
parallel or perpendicular to the plane o f incidence. The magnitude o f the rotational angle
o f the Kerr effect for a ferromagnetic material is generally between I O'4 to 10‘J degrees.
The angle o f rotation is more pronounced when the incident angle is increased.
29
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3.2 MOKE measurement setup and results
3.2.1 MOKE setup
UHV
S a m p le S u p e rc o n d u c tin g
UHV w in d o w
P o la riz e r
H e -N e
L aser
P h o to d io d e ,
Figure 3.2 Schematic diagram o f MOKE measurement setup
The MOKE effect is shown very simply in Fig. 3.2. The experimental setup normally
involves passing laser light through a polarizing filter and then reflecting the light o ff the
sample [12]. The light then passes through another cross-polarizing filter. Slight changes
in the plane o f polarization will thus cause variations in the detected light intensity after
the second filter. MOKE is frequently used to measure the hysteresis loops o f thin
magnetic films, by studying the light intensity as a function o f applied magnetic field.
As mentioned before, if the plane o f polarization o f the incident light (defined as the
plane containing the electric vector) is either parallel or perpendicular to the plane o f
incidence then the reflected light is also plane polarized. However, this symmetry is
destroyed by the presence o f a magnetic field. If the reflecting surface is magnetized, the
30
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reflected light will be elliptically polarized even if the incident light is polarized parallel
or perpendicular to the plane o f incidence. The degree o f ellipticity imparted to the
reflected beam is small and the effect can be regarded as a rotation o f the plane o f
polarization o f the light on reflection. Among the ferromagnetic metals, ferrites, and
paramagnetic metals, the first material type possesses the greatest effect.
Plane of m ad e nee
Surface normal
►
M
Figure 3.3 Diagram depicting longitudinal MOKE effect
Longitudinal MOKE effect is shown in Fig. 3.3 [13]. In the longitudinal case the
magnetization vector is in the plane o f the surface and parallel to the plane o f incidence.
This geometry provides a signal proportional to the component o f magnetization that is
parallel to the film plane and the plane o f incidence o f the light. The effect is that
radiation incident in the linearly polarized state is, on reflection, converted to elliptically
polarized light. The major axis o f the ellipse is often rotated slightly with respect to the
principal plane and this is referred to as the Kerr rotation. A polarization rotation is
detected using crossed Polarizer.
31
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3.2.2 MOKE experimental results
Magnetization (M)
Saturation
Magnetization
Coetcivity
Applied
ruM
R etd iu
(H|
Figure 3.4 Schematic diagram of a magnetic hysteresis loop showing the remanence
Mr/Ms, coercivity Hc, and saturation magnetization Ms o f a ferromagnetic material. In
MOKE measurement. MOKE signal intensity I is proportional to magnetization M.
Ferromagnets will tend to stay magnetized to some extent after being magnetized
by an external magnetic field. The tendency to remember their magnetic history is called
hysteresis. The parameters associated with the hysteresis loop (Fig. 3.4) are summarized.
The fraction o f the saturation magnetization retained when the magnetic field is removed
is called the remanence of the material Mr [13]. The coercivity Hc refers to the reverse
field required to bring the magnetization to zero. The magnetic susceptibility on the
hysteresis loop is defined as I/H and the differential susceptibility is defined to be dl/dH,
where I is the intensity o f MOKE signal, which is proportional to magnetization M.
32
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Mr/Ms
(a). RT
30 Oe
Figure 3.5 Longitudinal MOKE spectrum for ultrathin Fe magnetic film deposited
on GaAs substrate at (a) room temperature.
Longitudinal MOKE measurements were carried out at room temperature with the
samples grown. A linearly polarized He-Ne laser (0.6328 um) beam was reflected from
the sample surface with an incident angle 45 0 from the normal. The reflected light
propagated through an analyzing polarizer and was detected with a photodiode. This
geometry provides a signal proportional to the component o f magnetization that is
parallel to the film plane and the plane o f incidence o f the light. Since the change in the
polarization state o f the reflected light is proportional to the magnetization in the
material, it is possible to examine the magnetic properties of the Fe films.
33
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^ Mr/Ms
i
(b). 80°C (Hard)
(c). 80°C (Easy)
Mr/Ms=92%
"X%
k M r /M s
Mr/Ms=97%
—
20 Oe
20 Oe
Figure 3.5 Longitudinal MOKE spectra for ultrathin Fe magnetic films deposited
on GaAs substrate at (b) 80 °C (hard axis), (c) 80 °C (easy axis).
MOKE measurements were carried out for the Fe/Ag films grown at various
temperatures with a magnetic field applied along the Fe film plane. Neither smoothing
nor background subtraction was performed. We measured the coercive field Hc and the
relative remanence magnetization M/Ms. where Mr is the remanence magnetization and
Ms is the saturation magnetization. For the sample grown at RT. the hysteresis cycles
obtained (see Fig. 3.5(a)) indicates that the film is ferromagnetic, having nearly isotropic
distribution o f the coercive field Hc= 20 Oe, and relative remanence o f Mf/Ms=76%
within the sample plane. There’s no significant difference between the measurements
along magnetic easy axis [100] and hard axis [110] o f the Fe films. The above
longitudinal MOKE data suggests that polycrystalline Fe films, which originally had no
preferred magnetization, were formed at room temperature. Fig. 3.5 (b)(c) shows the
34
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longitudinal MOKE spectra measured along hard and easy axes o f the Fe film grown at
80 °C. With the growth temperature increased to 80 °C, relative remanence Mr/M s was
increased to be 92% (hard axis) and 97% (easy axis), respectively, there appeared an
anisotropy o f the remanence magnetization along the sample plane. Shown in Fig. 3.5
(d)(e) are the longitudinal MOKE spectra for the sample grown at 120 °C: Mr/Ms (hard
axis)=78%. Hc (hard)=7 Oe. M/M* (easy)=94%, and Hc (easy)=l 1 Oe.
4 Mr/Ms
4 Mr/Ms
(e). 120°C (Easy)
(d). !20°C (hard)
1
20 Oe
1 1 1 *v•
_Mr/Ms=94%
20 Oe
-► H
H
Figure 3.5 Longitudinal MOKE spectra for ultrathin Fe magnetic films deposited on
GaAs substrate at (d) 120 °C (hard axis), (e) 120 °C (easy axis)
35
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Mr/Ms
‘Mr/Ms
(a)
Mr/Ms=98%
(b)
Mr/Ms=90%
«?
•
t
•
•20 Oe
' 10 Oe
>H
-►H
*
f
/
i i i
Magnetic field (Oe)
Figure 3.6 Longitudinal MOKE spectra o f ultrathin Fe magnetic films deposited on GaAs
substrate at 150 °C. The magnetic field is applied along the easy axis (a) and the hard axis
(b) o f Fe film.
Fig. 3.6 displays the longitudinal MOKE spectra o f the sample grown at 150 °C.
We observed the in-plane magnetic anisotropy behavior with the easy and hard axes
parallel to [100] and [110] directions, respectively. The anisotropy did not change much
after further increase in the growth temperature. The clear rectangular loop (Fig. 3.6(a))
obtained along [ 100 ] indicates that this direction corresponds to a magnetic easy axis.
The coercive field is almost the same as that o f sample in Fig. 3.5(e), except that the
magnetization is much stronger, indicating that better magnetic texture has been formed.
In Fig. 3.6 (b), it shows the presence o f a magnetic hard-axis case. Apparently, when an
external magnetic field is applied, the magnetic domain walls in the Fe films are aligned
along the direction o f the magnetic field. This alignment becomes much easier as the film
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approaches single crystal quality. Therefore, strong MOKE signal intensity (proportional
to the component o f magnetization) was observed.
i
kMr/Ms
Mr/Ms=78%
(0- 170°C (Easy)
•
•
10 Oe
%
Figure 3.7 Longitudinal MOKE spectrum o f ultrathin Fe film deposited on GaAs
substrate at 170 C. The magnetic field is applied along the easy axis o f the Fe film.
At the growth temperature o f 170 °C (Fig. 3.7). the narrowest Hc (4.0 Oe) and the
strongest MOKE signal intensity (proportional to the component o f magnetization) were
obtained, indicating transition to single crystalline structure with a magnetic texture. The
coercive field Hc was seen to decrease with increasing growth temperature in a range
from RT to 170 °C. The measured values are 20 Oe at RT and 4 Oe at 170 °C.
respectively. Apparently, the displacement o f the domain walls becomes much easier
when the film structures change from polycrystalline to single crystalline at higher
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growth temperature. Therefore, the applied magnetic field required for reaching
saturation magnetization is reduced and the coercive field Hc becomes smaller.
3.3 X-ray diffraction and X-Ray Read-camera measurements
One valuable technique to characterize the sample crystalline structures is X- ray
diffraction. X-ray diffractometry (XRD) is used to determine the phase content in many
minerals and materials. It is used as an adjunct to chemical analysis to identity the
crystalline phases and measure the lattice parameters o f artificially produced structures
such as the epitaxially grown materials in modem electronic materials. The wavelength
o f X-rays is o f the same order o f magnitude as the distances between atoms or ions in a
molecule or crystal (A). A crystal diffracts an X-ray beam passing through it to produce
beams at specific angles, depending on the X-ray wavelength, the crystal orientation, and
the structure o f the crystal. X-rays are predominantly diffracted by electron density and
analysis o f the diffraction angles produces an electron density map o f the crystal.
X-ray diffractometer consists o f an X-ray generator, a goniometer and sample
holder, and an X-ray detector such as photographic film or a movable proportional
counter. X-ray tubes generate X-rays by bombarding a metal target with high-energy (10
- 100 keV) electrons that knock out core electrons. An electron in an outer shell fills the
hole in the inner shell and emits an X-ray photon [14]. Two common targets are Mo and
Cu. which have strong K (alpha) X-ray emission at 0.71073 and 1.5418 A. respectively.
These sources produce a continuous spectrum o f X-rays and require a crystal
monochromator to select a single wavelength. In the following paragraph, we will give a
mathematical expression o f XRD principle.
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First let’s define a three dimensional lattice simple cubic unit cell vector:
r = M,a, + u2a2 +
where (M1.H2.M3) are integers. Next, define G to be the allowed momentum vectors in
Fourier space. Then the discrete three-dimensional Fourier decomposition for the electron
density is given by:
«(r) = I « / ; '
(3-1)
The basis o f G . {b\.bi.b'>) must satisfy the following condition:
a, - b : =2KSu
( 3- 2 )
_ _
2k
2k
2k
Lettmg (a,.a-,.a3) = (— x . — y. — ?) in the cartesian coordinates, therefore:
b
b
b
2k
2k
2k
Gh k l = — kx + — ky + — l.=
a
a
a
(3-3)
where (h. k. I) are integers. And the length o f G is:
IG f =(— )2(hr+k2+l2)
1 '
a
(3-4)
Let F be the amplitude o f the diffracted wave, k be the wave number o f the incident plane
wave, k ' be the wave number o f the reflected wave. Then:
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To have maximum amplitude, the exponent must be zero. Since it must be true for all r \
G = k'-k
(3-6)
Therefore, we can solve for the diffraction angle using \k\ = \k" =
2k
k
!g1‘ = G •G = (k'-k) ■(k'-k)
(3-7)
:Ci: = i k \ '- 2 k - k '
(3-8)
;Gp = 4 ( — ): (l- c o s 2 0 )
(3-9)
X
where 20 is the angle between k and k \ Solving for A. we get:
k = ^ s in #
'G\
(3-10)
Now. using 3-10, we solve the lattice constant a:
a = - ^ — v' / r + * : + /:
2 sin#
(3-11)
Therefore, all we have to do is find the proper (h.k.P) that would give diffraction peaks.
40
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q = 'kf ~ k\ = 2k, sin# = 4;r/lsin# = 2m l d
Figure 3.8 The layout o f a 9 -2 0 scan. As the sample surface rotates by angle
9 . the angle between the incident and reflected rays increase by 2 0 .
From Figure 3.8. it can be seen that when a measurement o f the angle between the
incident and reflected X-rays is made, the angle obtained is actually twice the Bragg
a n g le # . To perform the measurement, the sample is rotated by angle 9 with respect to
the incident ray k , and the detector is rotated by angle 2 9 with respect to k ,. The
sample and the detector are to be rotated synchronously by stepper motors, while k is
kept unchanged.
We are now ready to analyze the data using Equation 3-11. The angles given in
the data are actually times 2 . which is the angle by which the detector is rotated.
41
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10000
9000
80007000—
6000-
Fe (100)
3
•2-
5000-
g
4000-
c
3000-
Fe(110)
■
2000 -
30
ZLA
40
50
60
70
80
90
Degree
Figure 3.9 X-Ray diffraction spectrum o f Ag/Fe structure grown on Si substrate
In this study, the XRD measurements were performed using a conventional
diffractionmeter with Cu-Ka radiation. The Fe samples were examined in a 0-20 scan.
As indicated in Fig. 3.9, we first measured the Ag/Fe films grown on a Si (100) substrate.
We see that peaks at around 44° and 6 4 0 appears, which come from the diffractions o f Fe
(100) and (110) crystal structures. Compared with all other diffraction peaks resulted
from the Si substrate, these two Fe crystal diffraction peaks are very weak, indicating that
the deposited Fe film on Si substrate is essentially polvcrystalline in texture with weak
diffraction ability. That's the reason we choose GaAs substrate instead o f Si. GaAs will
give a better crystal lattice match with Fe film than that o f Si. and thus facilitates
epitaxial growth o f a single crystal Fe film structure as required for devices fabrication.
42
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Fig. 3.10 displays the XRD spectra o f Ag/Fe/GaAs single (Fig. 3.10a) and
Ag/Fe/Ag/Fe multilayer structures (Fig. 3.10b) at a growth temperature o f 150 °C. The Fe
diffraction peaks at 44.5° and 64.7° in both o f the XRD spectra correspond to interplanar
distances o f 0.203 nm and 0.144 nm. which are identified as the a-Fe (110), (200).
respectively. The peak at 66.5° is identified as the diffraction o f GaAs (400) phase. No
other crystal orientation was observed. From the XRD spectra, it is evident that the films
are composed o f bcc crystal phase. The full width at half maximum (FWFIM) o f the Fe
(200) diffraction peak is measured to be less than 0.3 degree, which indicates an excellent
crystalline structure. A major difference between the diffraction pattern o f the single
layer and the multilayer structures lies in the diffraction intensity corresponding to Fe
( 110 ). where the intensity of the single layer sample at ( 110 ) reflection is much higher
than that of the multilayer sample. These data suggest that the multilayer structures are
preferentially orientated to the (200 ) direction, while the single layer structures have two
equivalent options: (110) and (200) direction. Therefore, the multilayer sample can
simply be considered as having single-phase crystalline structure, while the single layer
sample is composed o f two major crystalline phases. The XRD data presented here do
support the view that the multilayer design exhibits substantially enhanced structural
coherence over single layer design, which leads to the observed FMR features. This
difference will influence our microwave device performance.
43
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(a)Single layer s a m p l e
3
CD
(0
C
d)
30
35
40
45
50
55
60
65
70
75
80
85
90
95 100
; ( b) M u l t i l a y e r s a m p i e
3
CD
(0
C
CD
30
35
40
45
50
55
60
65
70
75
80
85
90
95 100
20 D e g r e e s
Figure 3.10 X-Ray diffraction spectra o f (a) Ag/Fe/GaAs single
layer structure and (b) Ag/Fe/Ag/Fe/GaAs multilayer sample
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Figure 3.11 (a) X-Ray read-camera diffraction patterns o f GaAs (100) substrate
A more direct means of studying the sample crystal quality is read-camera X-ray
diffraction [15]. Fig. 3.11 shows the photos o f diffraction patterns recorded by the readcamera. The compositions of the films are identified by measuring the angular positions
o f the rings and then compared with the standard data.
Figure 3.11 (b) X-Ray read-camera diffraction patterns o f Ag/Fe/Ag/Fe
films deposited on GaAs substrate by MBE at 120 °C.
45
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Figure 3.11(c) X-Ray read-camera diffraction patterns o f Ag/Fe/Ag/Fe films deposited
on GaAs substrate produced by thermal evaporation at 120 °C
Figure 3.11(d) X-Ray read-camera diffraction patterns o f Ag/Fe/Ag/Fe films
deposited on GaAs substrate by MBE at 170 C
Fig. 3.11(a) displays the diffraction pattern o f a GaAs substrate without Fe film
while Fig. 3.11(b) shows that o f a Ag/Fe/Ag/Fe multilayer sample grown by MBE on
GaAs at 120 °C. These two patterns are practically identical, indicating that Fe film
crystalline structure is well matched to the GaAs substrate. To ensure that the diffraction
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pattern o f multilayer sample does come from the Fe film rather than from the GaAs
substrate alone, another sample was produced by thermally evaporating the same
thickness o f Fe on GaAs. The diffraction result is exhibited in Fig. 3.11(c). As can be
seen, ring pattern appears, indicating that the thermally evaporated Fe film is poly­
crystalline.
Higher growth temperature results in better epitaxial growth, and in turn, leads to
a better crystal quality. Clear diffraction spots are observed for sample grown at the 170
°C. as shown in Fig. 3.11(d). These diffraction spots originate from the diffraction of
GaAs substrate and Fe (200) phase. This fact shows that single crystal bcc Fe films with
orientation (100) has been stabilized on GaAs (100) substrate, proving that the well
ordered crystalline metal/semiconductor system has been established. Compared with the
diffraction spots from GaAs substrate alone as shown in Fig. 3.11(a). the multilayer
sample grown at 170 °C produces much enhanced diffraction spots. We believe that this
diffraction pattern mainly comes from the Fe film. The XRD diffraction patterns are
consistent with our FMR analysis to be reported in the next chapter.
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References
1. F.J.A. den Breeder, W. Hoving and P.I.H. Bloemen J.
p. 562.
2 . S.S.P. Parkin
Phys. Rev.
L e tt.
3 . C. Chappert and P. Bruno
M a g n . M a g n . M a te r .
93 (1991),
6 7 (1991), p. 3598.
J. Appl. Phys.
64 (1988), p. 5736.
4. M. Stampanoni. A. Vaterlaus. M. Aeschlimann and F. Meier Phvs.
(1987), p. 2483.
R e v .L e tt.
59
5. L. He and W. D. Doyle. J. Appl. Phys. 79. (1996) 6489
6 . B. Heinrich. A.S. Arrott. J. F. Cochran. K. B. Urquhart. K. Myrtle.Z. Celinski and Q.
M. Zhong. Mat. Res. Soc. Svmp. Proc. Vol. 151. 177 (1989)
7. J. F. Cochran, J. Rudd, W. B. Muir. B. Heinrich and Z. Celinski. Phys. Rev. B 42.
(1990) 508
8 . B. Heinrich. J. F. Cochran and R. Hasegawa. J. Appl. Phys. 57. 3690 (1985)
9. C. S. Tsai. J. Su and C. C. Lee. IEEE Trans. Magnetics. 35 (2000) 3178
10. Ultrathin magnetic structures, vol. I ed. By B. Heinrich and J.A. C. Bland (1994)
11. A. Azevedo, C. Chesman and F. M. De Aguiar. Phy. Rev. Lett. 25 (1996) 4837
12. Interaction o f electromagnetic radiation with magnetic media. Ron Atkinson (1998)
13. Hans Steidl. Magnetic Kerr effects. Experiment and Theory (1996) and Megan
Shumaker and Selman Hershfield. Micromagnetic Modeling (1999)
14. J. Ph Ansermet, Spin Dynamics and Spectroscopy (1998)
15. Q. Z. Liu, S.S. Lau, N.R. Perkins and T.F. Kuech. Appl. Phys. Lett.. 69 (1996) 1722
48
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CHAPTER 4 FMR STUDIES OF Fe-GaAs
WAVEGUIDE STRUCTURES
A spectroscopy is a determination o f the energy levels o f molecules, atoms and
nuclei. These energy levels are created by interaction between matter and radiation.
Various regions of the spectrum may be exploited for analytical purposes and the
interactions that occur in the absorption o f specific radiation
frequencies are
characteristic of the matter involved. A principal means of characterizing our samples is
Electron spin resonance (ESR) or called as ferromagnetic resonance (FMR) in our case.
Through such a measurement, one may extract direct information on a key material
parameter, the FMR linewidth [ 1]—[3 ]. This controls the maximum coupling between
microwaves and spins that may be achieved, and the linewidth as well as line shape
provide information on the quality o f the MBE deposited films [4]. In addition, from such
data, one also obtains quantitative information on the strength and character o f the
anisotropy present.
4.1 Introduction to ESR measurement
The absorption o f energy that gives the ESR response occurs because there is a
change in the magnetic moment o f the atoms or molecules, and what is in fact detecting
is a change in the complex magnetic susceptibility arising from this change [5].
Therefore, the fundamental to any ESR spectrometer is its ability to detect small changes
in the complex magnetic susceptibility o f the sample. The susceptibility o f the sample is
defined as the magnetic moment divided by the magnetic field. In the presence o f a radio
frequency field, the susceptibility becomes complex. The complex susceptibility x
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contains two components: an imaginary part x’\ a change in which gives rise to pure
absorption, and a real part x \ which gives a pure dispersion [6 ], If the sample is in a
resonant cavity, then the change in complex magnetic susceptibility at resonance will
cause a change in the complex reflection or transmission coefficient o f the cavity.
I
M icrowave
frequency m eter
Pow er supply
~
*
isolator
Detector
attenuator
r
C om puter system
I
M icrowave cavity
Figure 4.1 Schematic diagram o f Electron spin resonance setup
The ESR spectrometer consists basically o f a microwave source, a sample cell or
cavity, and a detector [8 ]. The sample cell or cavity is placed between the poles o f the
magnet. Fig. 4.1 is a schematic diagram o f the microwave spectrometer instrument: A
Bruker 200D-SRC with an X-band (9-10 GHz) bridge. The measurement temperature is
varied from 4K to 350K. Basically, microwaves irradiate the sample and we monitor the
absorption o f the microwaves. As most microwave samples have a fairly narrow band
performance, normally it is easier to sweep the magnetic field and keep the microwave
frequency constant. In our measurement, the frequency is fixed at 9.6GHz. We apply a dc
magnetic field, which causes the spins to line up. If the microwave magnetic field is
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perpendicular to the dc field, the spin precesses at a certain frequency, which depends
upon the dc field. At the resonant condition, if that frequency matches the frequency o f
the microwave signals, the sample absorbs the microwave signal, resulting in a
ferromagnetic resonance. The output from the crystal detector is passed to an
oscilloscope to monitor and record the response. The resonance field and its width give a
great deal o f information about the magnetic properties o f the sample. The area under the
absorption response is proportional to the total number o f unpaired spins in the sample
mThe absorption of microwave power has a maximum at f iei and is usually
symmetrical on either side of f res. For nearly all samples it approximates to a Gaussian or
Lorentzian distribution, depending on which type o f interaction is the main source o f
broadening. The measured ESR gives a direct and accurate description o f the effect o f the
crystalline environment on the energy levels. Normally useful results can only be
obtained with single crystal structures [8 ]. Most spectra are anisotropic, and in powdered
or poly-crystalline samples the spectra are smoothed out and resonance may not be
detected.
4.2 Principles of ESR measurement
We begin with a ferromagnet at T =0. All the atomic magnetic moments have the
same directions which corresponds to minimum energy o f the ferromagnet. Any change
o f direction will not remain localized in the original atom, but owing to the presence o f
the exchange interaction, it will propagate through the crystal in the form o f wave
motion. Such waves are called spin waves. Spin waves can propagate at zero and finite
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temperatures both in ferromagnets and in antiferromagnets, and they are characterized by
a dependence o f the frequency on the wave vector. They may also be regarded as
oscillations in the magnetic moment density, propagating through a magnetically ordered
crystal [9], Spin wave is the fundamental dynamic excitation o f an ordered magnetic
system [13]. Measurement of the frequencies provides unique information on magnetic
order, properties and interactions in a material. It was shown that if the magnetic moment
o f any given atom in the ferromagnet is pushed from the equilibrium direction and then
left alone, a spin wave would propagate through the crystal. It is clear that the spin-wave
energy must be equal to the excitation energy o f the crystal required to cause the change
in the orientation o f the atomic spin.
/
v
Figure 4.2 Schematic diagram o f electron spin induced magnetic moment
Since no resonance is infinitely narrow, there must be some damping effect. In
ferromagnet. this is described via the Landau-Lifschitz-Gilbert equation [10][11].
Damping is due to the interaction o f spin waves with each other and also with lattice
vibrations and conduction electrons. Spin waves are always damped although the
damping is very small at low temperatures ( T « Tc).
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Schematic diagram o f electron spin induced magnetic moment in magnetic
materials is shown in Fig. 4.2. It can be described by the following equations:
(4.1)
(4.2)
where L is angular momentum. M is the magnetization. Ms is saturation magnetization
and H is the effective magnetic field. The Gilbert term a is related to the spin-orbit
coupling [11]. y represents the gyromagnetic ratio, g is electron g-factor or Lande factor.
Pb is Bohr magnetron (0.927 x 10'2j Am2) and h represents Planck’s constant (6.626 x
10'34 Js). What makes FMR a valuable tool is that H includes not only the applied
external field but also other internal fields such as magnetic anisotropy and stress-induced
anisotropy.
Each orbiting electron possesses a magnetic moment equal to 1 Bohr magnetron.
In isolated ions, the net magnetic moment is equal to the sum o f the orbital and the spin
contributions. Each filled orbital gives a net zero contribution to the magnetic moment
since the two electrons spin oppositely. Net magnetic moments are generated only in
atoms or ions with incomplete electronic shells. Filled shells contribute zero magnetic
moment, since they are the sum of filled orbital. The most important subshells likely to
be incompletely filled are the 3d (first transition row) and the 4 f (rare earth elements).
The second and third transition rows (4d and 5d electrons) also produce magnetic
moments but the elements, and hence the minerals, are rare [ 12 ].
53
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4J Experimental results of ferromagnetic Fe/Ag/Fe samples
We performed ESR measurement o f the magnetic thin films grown at the
microwave frequency o f 9.6 GHz, with emphasis on peak-to-peak linewidth o f the
resonant signal. The phenomenon o f ESR is described. Under a uniform static magnetic
field H o f the order o f 1 kG. and place the sample in a microwave cavity, a resonance is
observed at a frequency given by [14]
hco = gMll ■\H + (:V, - N?_)A/]- [H +(.\\ - \ r2 ) V/]f 5
(4.3)
where the applied magnetic field is along the z axis. ,Vt, ,Vv, V. are demagnetization
factors. H represents biased external magnetic field and M is magnetization, g is the
electron g-factor and hb is Bohr magnetron.
The effective field can be derived from the following expression for magnetic energy:
(4.4)
£ = -
By replacing the classical magnetic moment MP by:
gMaJ = MV
(4.5)
where J is angular momentum. J* Jy J: represent the angular momentum along three
axis, and V is volume.
54
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It is used to predict the configurations o f magnetic structures, in particular o f
small particles that a single-domain behavior prevails. In thin film structures, the
magnetization lies parallel to the plane o f the film and parallel to an easy axis.
3000
-
2400
-
1800
-
1200
-
2 9 oe T=170 °C
!
’ 38 Oe T= 120 °C
-90 oe T=80 °C
(
1
3
j/
600
•J1
C
u
N
y
/ ’
52 oe T=250 °C
J 1 350 oe T=RT
;
0
d :
-600
t ^
-1200
-
-1800
-
-2400
-
^
*
I
-3000
1 ^ 1 ----- •— L
^ _ J -- .-- 1-- ---------- 1— ± —1---^
---...
1
1
i. i
.
0 200 400 600 8001000120014001600180020002200
Magnetic field (G)
Figure 4.3 FMR spectra o f single Fe film. The applied magnetic field was along
the hard axis. No resonance absorption was observed for the case with magnetic
field applied along the easy axis.
Compared with that o f bulk Fe materials, which has FMR linewidth of 45 Oe
[15][16]. ultrathin Fe/Ag films have demonstrated significantly narrower linewidth as
indicated in the following measurement. It is generally believed that spin-orbit interaction
in the ferromagnetic film plays a dominant role in the damping mechanism, as interpreted
using Landau-Lifshitz equation with an intrinsic Gilbert type o f damping. In real (non­
perfect) crystal structures. spin-Iattice and other imperfections-spin interaction will
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contribute to broadening o f the linewidth. Therefore, AHPP consists o f a homogeneous
part, which is the intrinsic linewidth occurring in perfect sample (proportional to
frequency), and inhomogeneous part, which is due to sample imperfections, depending on
sample preparation conditions. It can be seen that the resonance magnetic field. FMR
signal intensity, and the linewidth AHPP are all sensitive to the Fe film growth
temperature.
Fig. 4.3 shows the dependence o f the measured ferromagnetic resonance
linewidth AHPP on the growth temperature for the Ag/Fe single layer structure samples
grown. FMR measurement was performed at 9.6 GHz with the magnetic field applied
parallel to the easy axis of Fe film plane. It is clear seen that the resonance field. FMR
intensity, and linewidth AHPP are all sensitive to the growth temperature. As the growth
temperature increased from room temperature, the FMR line-width becomes narrower,
the signal intensity becomes much stronger, and the resonant peaks shifts towards lower
magnetic field. This indicates a trend o f clear ferromagnetic feature and better film
quality. The narrowest measured AHPP o f the single layer films is about 29 Oe, obtained
at the growth temperature o f 170 °C. In general. FMR signal is proportional to the
magnetization o f ferromagnetic film. The narrowest linewidth AHPP corresponds to the
iron film of the best crystal quality, which is single crystal film, as confirmed by in situ
LEED measurement. The variation o f the sample magnetization should be related to the
modification o f film crystal structure by changing the growth temperature. The spectra
also show weak and broad satellite FMR peaks, which appear at a magnetic field around
300 Oe. It is suggested that if the Fe film is grown directly on the GaAs substrate at
relative high temperature (> 100 °C). "spin pinning” may occur at the Fe/GaAs interface.
56
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which will in turn degrade its magnetic response [17]~[20], as discussed in Chapter 2.
Therefore, in our subsequent sample preparation, multiplayer structures at various growth
substrate temperatures were pursued.
258 oe
s
0
300 600 900120015001800
Magnetic Held (G)
Figure 4.4 FMR spectrum o f multilayer Fe films grown at room temperature. The
external magnetic field is applied along the Fe [110] magnetic hard axis.
Fig. 4.4 shows the FMR spectra o f multilayer structures grown at room
temperature. The resonance condition is established by sweeping the external dc
magnetic field H. which is parallel to the hard axis o f the Fe film. At an appropriate value
o f magnetic field H, the natural frequency o f gyroscopic motion will match the
microwave frequency and resonant absorption o f energy from the microwave radiation
field occurs. The maximum absorption occurs at the FMR field Hres In real (non-perfect)
crystal structures, spin-lattice and other imperfections-spin interaction contribute to the
linewidth broadening.
57
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The measured FMR signal (field derivative o f the absorption cx” / dH. where x”
is the out-of-phase microwave susceptibility) is proportional to the total magnetic
moment of the ferromagnetic film [2 1 ]:
I - Ms [(Hres+ 47rMefr) / (2Hres + 47rMdT)]
(4.6)
where effective magnetization 47tMctr = 4rc Ms - 2KU < Ms. Ms is the saturation
magnetization. Hres is resonant magnetic field, and Ku is the strength o f perpendicular
anisotropy. The resonance field Hres corresponds to the zero crossing o f cx’7 cH and the
FMR linewidth is given by the field interval between the extrema o f c%"/ cH.
For the sample grown at RT. a weak FMR absorption peak appeared with a broad
linewidth around 258 Oe. Narrowing o f the FMR linewidth and enhancement o f signal
intensity occurred as the growth temperature was increased, which indicates a trend of
clear ferromagnetic features and better crystal quality. In the mean time, it was seen that
the main resonant peaks (positioned at a magnetic field above 1000 gauss) shifted
towards a lower magnetic field. Also, a satellite resonance peak appeared at around 300
Oe (as indicated in Fig. 4.5). Due to the multilayer structures, that is. a 1.5 nm Fe seed
layer and a 10 nm Ag buffer layer were grown before the deposition o f main Fe layer, the
potential magnetic dead layer were removed from the main Fe layer. Increase o f magnetic
moment at the Fe/Ag interface will also contribute to greater ferromagnetic signal.
Therefore, the phenomenon o f spin pinning virtually disappeared in the main Fe layer.
The satellite resonance peak became stronger and sharper as the growth temperature
increased, but shifted to a higher magnetic field region, which was opposite to that o f the
main peak.
58
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T= 100 C
31 Oe
s
iS
SI oe
B
4*
s
0
300 600 900 120015001800
Magnetic Field (G)
Figure 4.5 FMR spectrum at 9.6 GHz o f multilayer Fe film grown at 100 °C. The
external magnetic field is applied along the [110] magnetic hard axis o f Fe film.
The 9.6 GHz measurement shows that the 45nm Fe layer behaves like the bulk Fe.
i.e. no FMR is observed with the magnetic field applied along the easy axis [100] o f Fe.
However, when the field is applied along the [110] hard axis, two resonance peaks are
observed. We believe that these two FMR peaks are caused by the in-plane anisotropy.
The resonant peak at higher field (positioned at magnetic field above 1000 Oe) shifted
towards a lower magnetic field as the growth temperature was increased, indicating an
increase o f effective saturation magnetization. The resonance peaks at lower field were
observed at a magnetic field around 300 Oe. According to the theory given in Ref. [22].
the diagram o f spin wave frequency versus external magnetic field exhibits a cusp-like
feature for the fields around 1000 Oe.
59
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T= 120 C
25 Oe
3
28 Oe
s/l
B
s>
s
0 300 600 900120015001800
Magnetic Field (G)
Figure 4.6 FMR spectrum at 9.6 GHz o f multilayer Fe film grown at 120 °C. The
external magnetic field is applied along the [110] magnetic hard axis o f Fe film.
The resonance at higher field corresponds to the saturated sample where the
saturation magnetization is parallel to the external magnetic fields. The FMR at lower
field corresponds to the non-collinear state where the saturation magnetization is dragged
behind the magnetic field. Fig. 4.6 displays the FMR responses of Ag/Fe/Ag/Fe/GaAs
sample grown at 120 °C. Drastic narrowing o f the linewidth for the samples grown at
120°C suggested a trend o f greater ferromagnetic features and better crystal quality. The
measured FMR linewidth is as narrow as 23 Oe for the sample grown at 170 0 C. as
shown in Fig. 4.7. which is very close to that o f the perfect crystal structure with AHPP
=21 Oe. corresponding to the best crystal structure quality o f deposited Fe films. This
value is comparable to the FMR linewidth reported in [10] where ultrathin Ag/Cu/Fe
films were grown at room temperature. The narrowest linewidth corresponds to the best
60
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crystalline quality o f deposited films, which has been confirmed to be single crystal in
LEED measurement.
From Fig. 4.6, it is clear that, as the resonant peak shifted to lower Hr, FMR
intensity as well as the sample magnetization increased, in good agreement with the
equation (4.6). The effective magnetization reached its largest value at the growth
temperature of 170 °C (Fig. 4.7). As the growth temperature was further increased to 250
°C. the main resonance peak was seen to shift to a higher Hr, the signal intensity was
decreased, and the linewidth AHPP was broadened. The above observations clearly suggest
a degraded film quality. Based on MOKE and XRD data we concluded that high-quality
ferromagnetic single crystal Fe films were grown at a growth temperature o f 170 °C.
T= 170 C
26 Oe
23 Oe
3
>>
'3!
e
0
300 600 900 120015001800
Magnetic Field (G)
Figure 4.7 FMR spectrum o f multilayer Fe film grown at 170 °C with the external
magnetic field applied along the [110] axis of Fe film.
61
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6
cd
a)
>
260 Oe
450 Oe
ra
_>
d
0)
T3
i.
i
C
o
Q.
w
O
(A
-O
(0
<r
i.l
5
if
0
250 500 750100012501500175(2000
M a g n e tic Field (O e )
Figure 4.8 FMR response o f a single layer Fe film grown by thermal evaporation.
Comparing to Fig.4.7. the signal in Fig. 4.8 was magnified by 15 times.
For comparison. FMR measurements were also performed on the 5nm thick single
layer Fe film grown on GaAs directly by MBE and 45nm thick Fe film thermallyevaporated on GaAs. For the 5nm Fe sample grown by MBE. FMR measurement shows a
resonance peak at a magnetic field around 1100 Oe. with linewidth around 42 Oe. The
origin o f this high field resonance in this single Fe film is the same as that in the
multilayer samples, except that the lower resonance peak disappears, because in this case
only one Fe layer is involved in FMR measurement, and no non-collinear state exists. For
the sample produced by thermal evaporation, the FMR signal is very weak compared to
the MBE grown samples. No FMR peaks were detected in the same experimental
condition as that o f in Fig 4.7, unless we magnified the signal by 15 times. As shown in
Fig. 4.8, a weak and very noisy signal is displayed. Its resonant field has been shifted
towards a higher magnetic field o f 1400 Oe. Simulation result indicates that this thermal
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evaporation sample has null anisotropy field, i.e.. Han=0 Oe. In other word, the sample is
magnetically isotropic.
We now present a method to obtain quantitative information on the behavior o f
linewidth AHPP as a function o f microwave frequency from the frequency-dependent
FMR measurements. FMR provides maximum coupling between microwaves and spins
that may be achieved. The linewidth as well as the line shape provide information on the
quality o f deposited films. In addition, from such data, one also obtains quantitative
information on the strength and characteristics o f the anisotropy. In previous studies,
FMR linewidth has been commonly expressed in the following form [22]:
AHPp(co ) = AH(0) + 1 .1 6 (to/yKG/yMs)
(4.7)
where AH(0) is the frequency-independent linewidth, y is the gyromagnetic ratio, for Fe
film, y = 2.8 MHz/Oe. w is the microwave signal frequency and G is the Gilbert damping
parameter as described in Laudau-Lifshitz equation. We see that AHpp(w) consists o f a
homogeneous part, which is the intrinsic linewidth occurring in a perfect sample
(proportional to the applied frequency), and an inhomogeneous part (assumed to be
independent o f the applied frequency), which is due to sample imperfections.
As we reported previously [23]. high quality samples (FMR linewidth 23 Oe) o f
Fe/Ag layer structures on GaAs substrate were used to fabricate the integrated bandstop
filters. When ferromagnetic resonance condition is satisfied, maximum coupling o f
microwave signal occurs at the following ferromagnetic resonance fre q u e n c y ^ o f the Fe
film.
fre s
= 7 [(H0 + Han)(H0 + H^ + 4jtMs)] ‘ '
(4.8)
63
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where y is the gyromagnetic ratio. Ho is the external magnetic field. Han is the anisotropy
field, and 47tM s is the saturation magnetization o f the ferromagnetic film. For Fe film,
Han=650 Oe and 4ttMs =22.000 Oe. As a result, a display o f transmitted microwave
power versus frequency would show a sharp decrease at f res and forms a "notch".
Tuning o f the peak absorption FMR frequency was accomplished as the magnetic
field was applied and varied along the easy or the hard axis o f the Fe film. For a magnetic
field applied along the easy axis, the peak absorption frequency has been tuned in a range
o f 10.6 to 36 GHz with the magnetic field varied from 0 to 4650 Oe. Detail experimental
results will be reported in the next chapter. If we differentiate the absorption intensity (I)
with the microwave frequency, we get the relationship between cl/cf and the microwave
frequency f. The resonant frequency f res corresponds to the zero crossing o f dl/cf.
Linewidth 6fres is given by the frequency interval between the extrema o f cl/df
Data taken at several microwave frequencies allows one to extract the linewidth
and understand the mechanisms, which contribute to explore spin pinning, the magnitude
and nature o f anisotropies present in samples o f interest, and related matters. The analysis
o f such data is complicated by the fact that the magnetic films o f interest to the present
research are in fact conducting. Thus, a proper description o f the ferromagnetic resonance
spectrum o f conducting ferromagnetic films must be utilized to analyze the data, for a
wide range o f thickness.
64
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(0
res
<0
>
(0
>
0)
Q
c
o
a
w
o
in
a
m
o
o
m
a:
CO
CM
5
U_
8
10 12
14 16
18 20 22 24 26 28
30
Micr ow ave Signal Frequ enc y ( G H z )
(a)
Figure 4.9 Linewidth 8 frcs o f ferromagnetic resonance signal observed in the
multilayer samples. Changing the magnetic field varies the center resonance
frequency fres.
Fig. 4.9 presents the linewidth Sfres of the ferromagnetic resonance signal under
various external magnetic fields in a frequency range from 10 to 30 GFlz. Very good
signal-to-noise ratio has been observed at all frequencies, which allows determination of
the linewidth precisely.
65
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400
O
-G
350
300
T3
250
—
2
U
i-
200
150
-
too -
Mi cr ow ave Frequency (GHz )
(b)
Figure 4.10 The values o f the FMR linewidth J H(j) o f
multiplayer samples as a function of microwave frequency.
Using equation in [24]:
AH(/) =Sfres x [(y2 12'./) x (2Happ + 2Han + 4km ,)]*1
(4.9)
We obtain the FMR linewidth AH(/) o f the multilayer Fe samples as a function o f
microwave frequency, as shown in Fig. 4.10. The frequency dependence o f AHPP (/) in
Fig. 4.10 shows a typical linear dependence with a near-zero offset linewidth AH(0).
which indicates a near-perfect crystalline quality of the deposited multilayer magnetic
Fe/Ag film. This is the first time that the experimental results on the Ag/Fe-GaAs layer
structures have been carried out with sufficient resolution to measure the linewidth
broadening due to intrinsic damping, where the FMR linewidth is resolved to vary
between 0.5 and 1.1 GHz in a frequency range o f 30 GHz. compared to previous report
66
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on the Fe film linewidth varying from 1.5 to 3.5 GHz in a frequency range o f 15GHz
[25]. From the slope o f the curve, the Gilbert damping parameter G is extracted to be
1.45xl0 8 sec'1.
In conclusion, high-quality single-crystal Fe/Ag films have been grown on GaAs
( 100) substrate under preparation conditions employed in this study. Experimental results
on the grown samples have been obtained with sufficient resolution to measure the FMR
linewidth broadening due to intrinsic damping. The frequency-dependent FMR linewidth
AH(/) shows a typical linear relationship with the microwave frequency. The frequencyindependent linewidth AH(0) is extracted to be zero, indicating a near-perfect crystalline
quality o f the magnetic Fe/Ag films grown.
References
1. M. Brockman. M. Zolfl. S. Miethaner. G. Bavreuther. J. Mag. Magn. Mater. 198
(1999)384
2. R. J. Hicken. C. Daboo and J. A. C. Bland. J. Appl. Phys. 78(11) (1995) 6670
3. Magnetism in ultrathin films, ed. D. Pescia. Appl. Phys. A (1998) 49
4. A. Azevedo. C. Chesman and F.M. Aguiar. Phys. Rev. Lett. 76 (25) (1996) 4837
5. B. Heinrich, A.S. Arrott. Z. Celinski, MRS Vol 151. (1989) 177
6 . Ultrathin Magnetic Structures II. eds by J.A.C.BIand and B. Heinrich (1994)
7. Z. Celinski. B. Heinrich, and J. F. Cochran. J. Apply. Phys. 73 (1993) 5966
8 . M. Gester. C. Daboo. J.A.C. Bland. J. Mag. Magn. Mater. 165. (1997) 242
9. S. Datta and B. Das. Apply. Phys. Lett. 56 (1990) 665
10. Z. Celinski and B. Heinrich. J. Appl. Phys. 70, (1991) 5935
11. M. Zolfl, M. Brockmann. M. Kohler. S. Kreuzer. T. Schweinbock. S. Miethaner and
67
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
F. Bensch, G. Bayreuther. J. Mag. Magn. Mater. 175 (1997) 16
12. B. Lepine, S. Ababou, G. Jezequel. J. o f Appl. Phys. 83(6) (1998) 3077
13. R. E. Camley. D. L. Mills. J. Appl. Phys. 82 (1997) 3058
14. Charles Kittel. Introduction to Solid State Physics. John Wiley (1996) Chap. 16:
N. M. Atherton. Electron Spin Resonance. Ellis Horwood (1973)
15. M.W. Prins. D. L. Abraham and H. Van Kempen. J. Mag. Magn. Mater. 121. (1993)
152
16. W. Platow. A. N. Anisimov. G. L. Dunifer, et.cl. Phys. Rev. B 58 (9) (1998) 5611
17. B. Dieny. V.S. Speriosu. S.S.P. Parkin. B.A. Gurney, D.R. Withoit and
D. MaunPhys. Rev. 5 43 (1991). p. 1297.
18. B. K. Kuanr. Alka V. Kuanr. J. Mag. Magn. Mater. 165 (1997) 275
19. A. Flipe. A. Schuhl and P. Galtier. Appl. Phys. Lett. 70(1) (1997) 129
20. M. Johnson. Science 260 (1993) 320. Phys. Rev. Lett. 70 (1993) 2142
21. J. F. Cochran. J. Rudd. W. B. Muir. B. Heinrich and Z. Celinski. Phys. Rev. B 42.
(1990)508
22. B. Heinrich. J. F. Cochran and R. Hasegawa. J. Appl. Phys. 57. 3690 (1985)
23. C. S. Tsai, J. Su and C. C. Lee. IEEE Trans. Magnetics. 35 (2000) 3178
24. Ultrathin magnetic structures. Vol. I. ed. By B. Heinrich and J.A. C. Bland, (1994)
25. B. Heinrich, S. T. Purcell. J. R. Dutcher. K. B. Urquhart. J. F. Cochran and A.S.
Arrott. Phys. Rev. B 38, (1988) 12879
68
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CHAPTER 5 WIDEBAND MICROWAVE BAND-STOP
FILTER FABRICATION AND MEASUREMENT
5.1 Fe films-GaAs Microwave band-stop filter Fabrication
5.1.1
Advantages of Fe-Based Microwave Devices
Microwave devices fabricated using ferromagnetic materials possess a unique
capability o f tuning microwave carrier frequencies electronically, as a result o f strong
coupling between an electromagnetic wave and the magnetostatic waves (MSW or spin
waves) [1][2][3]. The ferromagnetic resonance (FMR) frequency
is given by the
following equations [4][5]:
fres
= Y [( H o
+ H a n X H o + Han + 4tiMs)] i,:.
fres = y [(H0 - HanXHo +0.5 Han + 4ttMs)J i,:.
Hq // Easy axis
(5-1)
H0// Hard axis
(5-2)
where y is the gyromagnetic ratio. H0 is the external magnetic field. Han is the anisotropy
field, and 47tM s is the saturation magnetization o f the ferromagnetic film. The coupling
causes strong absorption of the input microwave power at the FMR frequency / rcs. For
the YIG film. Han=l00 Oe, 47tMs=1750 G. Thus. /res=14.0 GHz at Ho=4.150 Oe [5]. Note
that for the Fe film the saturation magnetization is as large as 22.000 G and Han=650 Oe.
and therefore the corresponding / res is as high as 32.0 GHz. This example clearly shows
that for a given magnetic field the Fe film-based microwave devices can operate at a
much higher carrier frequency than the YIG-based counterparts [6 ]. For example, as to be
detailed in the following section, bandstop filters using the Fe-GaAs structure have
69
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demonstrated a frequency tuning range as wide as 22 GHz (from 10.7 to 32.5 GHz) with
the corresponding magnetic field tuning from 0 to 4.500 Oe. Noticed that this tuning
range is much smaller (by a factor o f two) than that required in the YIG-based filters for a
comparable frequency tuning range [6 ].
5.1.2 Device Fabrication
After the growth o f single crystal Fe/Ag/Fe layer on GaAs substrate using MBE.
the magnetic films were coated with a very thin layer of silver for protection against
oxidation. Then samples were cleaved into pieces for material characterization and FMR
measurements. Chemical processing was utilized to fabricate the integrated-tvpe
microwave bandstop filters. To facilitate device fabrication, a relatively thick Ag films
(around 1pm) were deposited on Fe/Ag/'Fe-GaAs samples by E-beam evaporation.
Photolithography is used to pattern the wafer surface for defining the exact
dimensions o f devices and circuits using an ultra violet source. This involves spinning a
thin layer o f a light sensitive polymer, known as photoresist, on the wafer surface. Ultra­
violet light is shone onto this layer through a mask containing the pattern to be
transferred. Areas exposed to UV light can then be removed using a developer solution.
The remaining pattern on the wafer can now be used for etching the wafer or depositing a
metallic layer onto it. The thickness o f the resist layer is critical. It is usually set by the
duration and the speed o f the spinner for a given photoresist. In our case, photoresist was
used and spun at 4.500 rpm for 35 seconds to produce a thickness o f approximately 1mm.
Best results were achieved for wafer dimensions o f roughly 1cm x 1cm. Etching process
is an integral part o f fabricating devices and microstrip lines on GaAs. certain metal
70
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shape can be defined by etching after deposition. We have successfully fabricated
microstrips on semi-insulating GaAs substrates and characterized their performance.
Even though GaAs is not a very good insulator, its semi-insulating property permits the
construction o f microstrip lines with relatively low loss in certain frequency range.
Microstrii
RF Input
Fe/Ag-G&As
Structure
RF Output
Figure 5.1 Sketch o f Fe-GaAs microwave bandstop filter. The Fe Film has been etched in
the shape of Microstrip line on GaAs substrate.
Fig. 5.1 shows the waveguide structure and the device configuration o f the
integrated-type bandstop filter. The Fe/Ag/Fe films with a typical thickness o f
40nm/5nm/lnm were first grown on a 350 pm thick GaAs (100) substrate using MBE. A
silver layer was then deposited on top o f the Fe film in order to prevent its oxidation, and
deposition o f Cr/Ag layer was done by electron beam evaporation to serve as an electrode
for subsequent excitation and propagation o f microwaves. Subsequently, chemical
etching, ion milling, and reactive ion etching techniques have been explored to develop
the relevant etching processes. These etching processes led to form a microstrip
71
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transmission line with 50-ohm characteristic impedance. The dielectric GaAs will be used
as passivation layer of the devices and as dielectric for transmission line structure.
The thickness and the relative dielectric constant o f the GaAs substrates that we
have used in experiments are 350 pm and 13. respectively. The device consists o f three
parts, in two o f which 1.26 mm wide (equivalent to 50 Q)- striplines are prepared on a
Teflon-Aluminum board. In the central region, a slot is engraved for the magnetic
samples. The microstrip lines are linked using the bonding method. 50 GHz coaxial
connectors were mounted and connected onto the device for measurement. The
transmission characteristics of the device are evaluated by measuring the microwave
powers in the input and output ports.
5.2 Microwave Transmission measurement
As stated before. ESR technique has been used to measure the FMR of Fe/Ag
samples along hard axis o f Fe film at a relatively low frequency o f 9.6GHz. But the
resonance frequency is out o f the range if measured in the easy axis o f Fe film using this
technique. With the goal o f pushing the resonance frequency o f the films up. we setup a
new FMR system operating at higher frequency up to 50 GHz. This would also bring the
frequency at which the films are tested by FMR into the frequency range o f the
microwave devices.
26GHz/ 50GHz-microwave measurement has been setup in our
laboratory. Key device parameters that can be measured include reflection coefficient,
transmission coefficient, scattering parameters in frequency domain, and impulse
reflection response in time domain with rise time less than 18ps.
72
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Fig. 5.2 shows the setup for the ultrahigh frequency FMR measurement, it
consisted o f high frequency signal generator, direction coupler and power meter. Data
collection is controlled by computer program. Strong attention was paid to design and
fabricate the microstrip transmission line, in order to satisfy the matching condition at
very high microwave frequency.
CZUSgo
□□na a
0 0 0 0
HP-VEE
HP-83630B Signal
Generator
*
HP-438A
Power Meter
(10 MHz- 26.5 GHz)
D jrectiona]
(10 MHz- 26.5 GHz)
HP-83650B Signal
Generator
CouPler
(10 MHz- 50 GHz)
under
Test
v
HP-E4419A
Power Meter
(10 MHz-50 GHz)
Figure 5.2 Diagram o f experimental setup for microwave measurement
To send microwave electromagnetic signals into the sample o f planar structure.
microstrip-Iine is commonly used. The microwave signal is confined in spaces between
the strip conductor and the ground plane and propagates along the microstrip-line. In Fig.
5.1. the Fe-GaAs bandstop filter structure, the microstrip-Iine was fabricated on Fe/AgGaAs sample to launch microwave signals. The line length is 8 mm and the line was
designed to have characteristic impedance o f 50 ohms to match the impedance o f coaxial
73
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connectors, coaxial cables, and the sweep generator. This filter-type microstrip structure
has demonstrated lower loss over large bandwidth. This study extends the frequency
range o f the resulting filter from 26 GHz as reported in [5] to 36 GHz by this improved
microstrip packaging and better crystal quality (due to the removal o f FexGai.x magnetic
dead layer) [7]~[11]. The microwave signal is confined in the spaces between the strip
conductor and the ground, and propagates along the line.
Microwave
f Output
Microwave
Au Ground
Plane
— GaAs Substrate
Figure 5.3 Schematic diagram o f a straight microstrip line
A straight microstrip is fabricated on GaAs substrate with a width o f 256 um.
which corresponds to 50 Q-impedance as calculated by the software, as shown in Fig.
5.3.
74
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Straight Transm ission line a
7
-5CO
2,
in
in
Straight Transmission line b
O
-
10-
c
o
r
»
in
c
~ -15
Microstrip Loss Measurements
-20
5
10
15
20
25
30
Microwave frequency (GHz)
Figure 5.4 Transmission Characteristics o f straight microstripline
Fig. 5.4 exhibits the measured insertion loss o f two straight microstrip
transmission lines. The circuit being measured includes the microstrip line and two
coaxial connectors. We can see that the insertion loss for line a is within 5dB for
frequency up to 25 GHz. At frequencies above 25 GHz. oscillatory response is observed.
This oscillatory response is caused by multiple reflections and dispersion o f signals in the
line. Even though the microstrip line and the coaxial connector both have 50-ohm
impedance, impedance match alone does not guarantee match o f electrical field
distributions. The reflection is in turn due to the mismatch o f electrical field distribution
in the microstrip line and that in the coaxial connector. At higher microwave frequency,
the field distribution mismatch becomes more severe, causing higher reflection and thus
75
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the oscillatory response. Further experimental studies are needed to achieve flat response
o f transmission line for frequencies up to 50GHz.
For a basic Transmission Line, as shown in Fig. 5.5:
♦
R
i(x+x1, t)
+
L
v(x,t)
c
v(x*x1,t)
Figure 5.5 Basic equivalent circuit o f a straight microstrip transmission line
cX
dt
di(x.t)
^dV (x.i)
— — = - O l ( x .t ) - C — ---cX
dt
0 -4 )
Suppose it's a lossless transmission line: R =G =0,
We got:
dV{x J)
di(x.t)
di(x.t) _ ^dV (x.t)
° - 6)
Z = °)X ^
P
The character impedance is defined as:
^
\C
76
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^
^
^
x K1+ 12 h/J ° 5 + 0040 - t ) 21
(5-7)
The wavelength o f microwave signal in the microstrip line is [12]:
a
=
(5-8)
A
l + 0.63x(*r - l ) ( » y - ,2S
The maximum frequency that the microstrip could support with no-loss assumption is:
2l x l°6
/» * = ------ , .T7 -
(w+2d)jer+
7
(> 9 )
1
where d is Fe film thickness (45 nm). w is the width o f microstrip (256 um ) and er of
GaAs is 13. If we define the thickness of waveguide structure as h. in our case it is the
thickness o f GaAs substrate thickness 350 um.
The above formulas are valid at frequencies where the quasi-TEM assumption can
be made, otherwise etf and Z will be frequency-dependent. The microstrip becomes
dispersive with sef (J) increases with frequency. From equation 5-9, we calculate the
value o f f 0 to be around 30 GHz. this is actually the cutoff frequency that the waveguide
structure can support before the dispersive effect appears. To push our device operating
frequency further, we must redesign the microstrip pattern, therefore, low-pass filter
microstrip line is considered.
77
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Following is the equivalent circuit o f Low-pass filter:
Lx1
Zs
Lx1
1x1
exl
exl
ZL
cx1
Vs
Figure 5.6 Equivalent circuit o f multi-finger low-pass filter transmission line
yi
.40
BO'
CO
DO
yi
cosh(—)
2
KOsinh(^)
yl
ZO sinh(^) '1
O'
c o sh (-)
Y
1
yi
cosh(^)
KOsinh(y)
ZO sinh(^)
(5.10)
co sh (^)
.40 = DO = cosh yi + (----- )sin h ^
2 K0
For the same N series networks:
'.4
B'
'.40
BO'
C
D
CO
DO
For lossless transmission line: y = j(3
cosh n = .40 = cos Bl
(OCXI
— sin/31
(5.11)
2 K0
Since the periodical capacitors provide shunting effects, the circuit will be low-pass
filtering, if
/3l = colxyiL*C « 1. cosh /7 = 1~
(5. 12)
-> i^r
1L
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L and C are inductance and capacitance o f the microstrip per unit length,
respectively. The region o f imaginary propagation constant, which is called the passband
of the filter, is deduced from:
jcosh/7j < l
(5.13)
Combine Eq. 5-12 and 5-13, the cutoff fre q u e n c y is calculated to be [13]:
/
= co / I
k
= ( ------- -------- )05 x -
Cx\*l*L
k
The transmission line sections, is short compared with the signal wavelength, act
as series inductor. Combined with the shunt capacitors the microstrip line will produce a
low-pass filter. Furthermore, since the cutoff frequency is dependent on the values of
microstrip width and length as well as its character impedance, with proper design of
these parameters, we will be able to push the bandwidth to much higher frequency than
the straight microstrip line.
-10
Low -pass filter-type M icrostnp line a
■o
Low-pass filter-type M icrostnp line O
o -20
•30
-40
Transmission Charactenstics of Low-pass
filter-type Microstrip lines
-50
10
15
20
25
30
Microwave Frequency (GHz)
Fig. 5.7 Transmission Characteristics o f Low-pass filter type microstrip lines
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Fig. 5.7 shows the transmission Characteristics o f a low-pass filter-type
Microstrip transmission line. We see that compared with the curve in Fig. 5.4 (straight
transmission line), very flat transmission response was obtained. In the following
microwave measurement, we will utilize this transmission line to accommodate the
samples and make the bandstop filters.
5.3 Principles o f Band-stop Device Measurement
For the flip-chip type filter (Fig. 5.8). a 50-ohm microstrip low-pass filter
transmission line was first formed in a separate 350 pm thick GaAs substrate. The
measured transmission characteristics o f the microstrip line for a quasi TEM mode extend
to a frequency as high as 35 GHz. The diagram o f magnetic easy [100] and hard [110]
axes o f Fe film is illustrated in Fig. 5.9. Microwave signals are introduced along the
magnetic easy or hard axes o f Fe film and the transmitted signals are measured in the end
o f microstrip line.
C onnector
GaAs
Losv-Pass
Filter
A lum inum
H older
C onnector
Figure 5.8 Schematic diagram o f flip-chip microwave bandstop filter
80
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R F out
* <100> - Easv Axis
RF out
* <UQ> - Hard Axis
H o*
RF in
^
Fe-GaAs Film
RF in
Figure 5.9 Diagram o f magnetic easy and hard axes o f Fe film
A Fe-GaAs sample was flipped and laid upon the microstrip low-pass filter
transmission line, and a magnetic circuit with large tuning range o f magnetic field was
used to provide the bias on the Fe film. Only the samples with best magnetic quality
(confirmed by MOKE and ESR measurements) demonstrated good filter characteristics.
It was found that although the flip-chip-type filters provided a lower level o f signal
absorption than that in the integrated-type devices, it was a quick and effective way to
select the qualified Fe/Ag samples before proceeding the complicated evaporation and
chemical etching processing required in fabrication o f the integrated-type devices. We
have consistently found that the same sample that provides good filtering characteristics
in the flip-chip measurement also does so in the integrated-type device.
Magnetic moment M is aligned along applied magnetic field H. and H will
provide restoring force such that disturbance o f M (provided by a varying microwave
field) will cause it to process about H in gyroscopic motion. At an appropriate value o f H.
the restoring force will cause the natural frequency o f gyroscopic motion to match the
81
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microwave frequency and resonant absorption o f energy from the radiation field will
occur. The role of magnetic anisotropy is to change the value o f field at which resonance
occurs.
In operation, an incident microwave signal propagating along the microstrip line
is coupled into the Fe-film to excite FMR in the Fe film, which in turn results in
absorption of the propagating microwave signal. Thus, the operating carrier frequency at
which the absorption o f the microwave power peaks is readily tuned by varying the
magnetic field. Such frequency tuning o f the peak absorption has been analyzed using a
theoretical model similar to that reported in Ref. [14]. It is shown that maximum coupling
and thus the peak attenuation o f the microwave power occur at the FMR frequency /res.
which is determined by the equation (5-l)(5-2). The microwave signal came into contact
with the spins in Fe films through the skin effect. If the conducting films have a thickness
comparable to or larger than the microwave skin depth, their influence is quite minor.
Thus, it would assume that the optimum Fe film thickness would be in the order o f Fe
film skin depths. If only the conductivity o f Fe is employed to estimate the skin depth. Fe
film thickness should be in the micrometer range. However, when the ultrahigh frequency
microwave signal excites spin waves in the Fe film, the skin depth shrinks rapidly at
resonance [17]. Therefore, the optimum Fe film thickness is much smaller than one
micrometer. In our case, the Fe films are grown to be 40nm. Such ultrathin layer makes
the ohmic dissipation introduced at the interface between GaAs and Fe film increase
markedly. If. however, we consider thin films o f Fe coated with a thick Ag layer, then the
ohmic dissipation remains minor. In our samples, there is such a thick Ag or Cr cap layer.
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5.4 Microwave Measurement
-
2
-
as
3
-3-
5 -4e
e
w
St
s
-7-
6
8
10
12
14
16
18
20
22
Frequency (GHz)
Figure 5.10 Transmission characteristics o f a single layer Fe based
flip-chip bandstop filter
We begin with a single layer Fe films on GaAs in which the easy magnetization
axis and constant external magnetic field Ho lie along one o f the principle axes o f the
ellipsoid, for example, the z axis. The above formula (5-1) determines the frequency o f
the uniform ferromagnetic resonance mode when external magnetic field lies along the
easy magnetization axis. Fig. 5.10 shows the insertion loss versus microwave frequency
with the easy-axis o f single Fe layer aligned to the microstrip-line. A series o f attenuation
dips were observed. The dip frequency was controlled by the bias magnetic field.
Tunable wideband FMR absorption was observed in a frequency range from 9.6 to 21
GHz. The maximum dip intensity was around 1.5 dB.
83
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E xternal M agnetic Field (O e)
Figure 5.11 Calculated and measured peak absorption versus bias magnetic field while
the magnetic field is applied and varied along the easy axis of the single layer Fe film.
Fig. 5.11 show the FMR frequency as a function o f the external magnetic field
applied and varied along the easy axis o f single Fe film sample. In the hard axis case, the
FMR absorption was observed in the frequency range o f 8.5 to 11.5 GHz. However, the
maximal absorption was just 1 dB.
High quality samples (as confirmed by ESR o f having the narrowest linewidth)
were seen to show much better filter characteristics. Fig. 5.12 shows transmission
characteristics o f the bandstop filter fabricated using a single layer Fe film with measured
ESR linewidth o f 35 Oe. with the external magnetic field is applied and varied along the
magnetic hard axis. The resonance frequency at first decreases with increasing magnetic
field H. It vanished for H around 550 Oe and then increased with increasing magnetic
field. Therefore, for a constant frequency o f the microwave, the ferromagnetic resonance
occurs twice as H varies, one in a weak and then again in a strong external magnetic
field. This has been observed in ESR measurement at 9.6 GHz. Fig. 5.12 shows that a
84
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tuning range of 4.2 to 15.3 GHz has been measured as the bias magnetic field varied from
0 to 1900 Oe along the magnetic hard axis [110].
co
T3
(A
(0
O
C
o
Q.
t_
O
(A
XI
<
3
4
5
6
7
8
9 10 1 1 12 13 14 1 5 16
F r e q u e n c y ( G H z)
Figure 5.12 Transmission characteristics o f the bandstop filter
with magnetic field applied along the hard axis o f single layer Fe film.
As described in the section on sample growth. Chapter 2. for a single layer Fe
film grown on the GaAs substrate, undesirable interfacial mixing may occur. If the
GaAs/Fe hybrid structure is to be utilized in high frequency microwave devices, the spin
pinning occurs right at the interface where one wishes the microwaves to drive the Fe
spins. In order to alleviate the interfacial mixing effect, multilayer samples were
subsequently grown with a 50-100 Angstrom buffer layer between the Fe film and the
GaAs substrate. This layer structure should eliminate the spin pinning and thus enhance
the ferromagnetic coupling, as confirmed by the FMR measurements.
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6
9
12
15
18
21
24
27
30
Frequency (GHz)
Figure 5.13 Transmission characteristics o f a bandstop filter using the Fe/Ag multilayer
structure. The magnetic field is applied along the easy axis o f Fe film.
Fig. 5.13 shows the transmission characteristic o f the devices with the magnetic
field aligned along the easy axis vs. the carrier frequency o f the incident microwave.
Compared with the single layer sample, sharper and narrower dips were measured with a
maximum dip level o f 5.0 dB. FMR absorption was observed in the frequency range from
10.7 to 27 GHz with the corresponding magnetic field tuned from 0 to 2900 Oe. It is
worth noting that single and multilayer exhibit different anisotropic fields. For the
multilayer. Han=650 Oe, and for the single layer, Han-550 Oe. As a result, at zero
magnetic field, the measured resonance frequencies are 10.7 and 9.6 GHz. respectively.
Figure 5.14 shows the dependence o f calculated and measured peak absorption frequency
on the bias magnetic field. Note that the magnetic field is applied and varied along the
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easy axis o f the Fe film. The experimental results are in good agreement with the
theoretical predication.
[
30 j-
Transmission Characteristics of
the Filter with Fe/GaAs Ripped
A
Easy
Experimental
Theoretical
er
.■I1--- 1----- !— ,— >
-
3
-400
0
400 800 120016002000240028003200360040004400
Bias Magnetic Reid (Oe)
Figure 5.14 Calculated and measured peak absorption versus the bias
magnetic field applied along the easy axis of the Fe film.
-3---------- 1---------- 1---------- ----------5
10
15
20
25
Frequency (GHz)
Figure 5.15 Transmission characteristics o f the bandstop filter
with the magnetic field applied along the hard axis o f Fe film.
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27
N
24
_------- : T heoretical
X
x
o
ac 21
S
>*»■
>» 18
U
c
o
3
15
(b)
/ „ = 7(<Ha -H J (H 0 + 0.5H„ ♦
Experim ental
-
/
'/
'wX
y
H a rd -A x is
0
x/
12
X
3
‘XX*
9
6
i
1
i
\i
/
/
X X
X
—
C a se
^
1___ 1
600
/
/l___ 1___ ____ I
1200 1800
i
•
2400 3000
Bias Magnetic Field (Oe)
Figure 5.16 Comparison o f calculated and measured peak absorption frequency versus
bias magnetic field applied along the hard axis o f the Fe film.
Fig. 5.15 shows the measured insertion loss versus microwave frequency with the
magnetic field Ho applied along hard axis o f the Fe film. The FMR absorption was
observed in the frequency range of 8 to 22 GHz with the maximal absorption around 2.0
dB. Fig. 5.16 shows that the experimental data is in good agreement with the theoretical
predication. We see a dramatic dip in frequency for the case where the magnetic field H0
is applied along the hard axis. Its origin is as follows. For very low applied fields, the
magnetization is aligned along the easy axis. As the magnetic field is increased, it rotates
to align with the hard axis. As it rotates, the spin wave frequency softens, to reach zero at
Ho = Han, where alignment along the hard axis is achieved. Further increases in magnetic
field stiffen the spin wave, as described above in Eq. (5-2). Note that the calculated plot
in Fig. 5.16 does not take into account the misalignment between magnetization and bias
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magnetic field, when H0 < Han- Further theoretical study needed to simulate this part of
experimental result.
1.0
0.5
o.o
3
-0.5 -
C/5
5/ )
•1.5
-2.0 i-
-3.0 r
-3.5
Microwave Frequency (GHz)
Figure 5 .17 Tuning o f peak absorption carrier frequency o f the bandstop filter
while the magnetic field is applied along the easy axis o f the Fe film.
After the fabrication the best low-pass filter-type microstrip transmission line
(insertion loss is less than 1.5db for frequency up to 32 GHz), a Fe/Ag multiplayer
sample with measured ESR linewidth as narrow as 23 Oe was used to produce an
integrated band-stop filter device. Fig. 5.17 shows the corresponding frequency tuning
obtained when the magnetic field was applied and varied along the easy axis o f the Fefilm. Clearly, a tuning range as large as 10.7 to 36 GHz has been accomplished. This
tuning range represents a nearly three-fold increase over that reported in Ref. [16]. Note
the highest magnetic field required was 4650 Oe. The absorption dips ranged from 1.5 to
4.0 dB, depending on the operation carrier frequency. For the 0.6 cm long microstrip-line
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suggests a maximum attenuation o f 7 dB/cm. In this study, a much higher filtering
frequency (36 GHz) was achieved with a larger frequency tuning range (25.4 GHz). In
comparison. Ref. [17] reports the highest filtering frequency o f 18 GHz with a frequency
tuning range o f only 6 GHz.
35 r
=
Magnetic easy-Axis of Fe
>ws 25
LL
li.
0
1000 2000
3000 4000
5000
External Magnetic Field (Oe)
Figure 5.18 Comparison o f calculated and measured peak absorption frequency
versus bias magnetic field in easy axis o f Fe film.
Fig. 5.18 shows a comparison between the measured data and the theoretical
prediction on the peak absorption frequency. We see in Fig. 5.18 that the experimental
results are in excellent agreement with the theoretical prediction for the case in which the
magnetic field is applied along the easy axis. However, it is noticed that according to the
measurement of transmission characteristics o f Ag/Fe-GaAs filter, insertion loss still
exists due to the electrical field mismatch in the microstrip line itself, especially at higher
microwave frequency (above 25 GHz).
90
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10
20
30
40
50
60
70
80
FMR linewidth (Oe)
Figure 5.19 Relationship between FMR linewidth (measured at 9.6 GHz)
and maximum microwave signal absorption dip intensity.
The problem arises from the interaction between the microstrip and the
conductive magnetic film. Therefore, we need to take into account the effect o f the
effective permeability of the ferromagnetic films on the impedance matching for
designing the low-loss bandstop filter. With proper design, it's possible to push the cutoff
frequency to a higher value.
It is clear that the ferromagnetic resonance linewidth is a key parameter that will
affect device performance. Also, the performance o f semiconductor/ magnetic film
hybrid structures based on excitation o f spin waves in the magnetic film will be degraded,
if there is strong "spin pinning” at the interface between the two layer [ 18]—[21 ]. Fig.
5.19 shows the relationship between the FMR linewidth (at 9.6 GHz) and the resonance
absorption dip intensity o f the filters. In order to achieve reasonable device signal
response, namely. FMR absorption dip intensity larger than 1.0 dB. the Fe film FMR
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linewidth measured at 9.6 GHz must be narrower than 30 Oe. FMR data taken at several
frequencies may be used to determine the values o f frequency-dependence linewidth and
understand the intrinsic mechanisms, which will in turn help understanding on spin
pinning, the magnitude and nature o f anisotropies present in the Fe samples.
The maximum microwave signal attenuation can be increased dramatically by
making the GaAs dielectric waveguide thickness much thinner, according to the
theoretical calculation reported in [22]. Therefore, it is proposed that if one wishes to
realize very strong coupling to the spins in the Fe film, iron/dielectric multilayer
structures should be used, in which the microwaves are confined into very thin (several
micrometer) dielectric films.
5.5 Conclusions and Discussions
In conclusion, tunable wideband microwave band-stop filters have been
constructed and tested using ultrathin Fe-GaAs waveguide layer structures. Transmission
characteristics o f the filters are evaluated using the flip-chip and integration techniques.
A frequency tuning range as large as 10.7 to 32.5 GHz has been accomplished in the
integrated-type filters. The experimental results are in excellent agreement with the
theoretical prediction for the case in which the external magnetic field is applied along
the easy and hard axes of the Fe film. Owing to the very high saturation magnetization of
the Fe film, the tuning range for the carrier frequency o f such devices is greatly increased
using only moderate magnetic field.
The mismatch o f the electromagnetic field distribution becomes more severe at
ultrahigh microwave frequency, which leads to high reflection and thus the oscillatory
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response. Further theoretical and experimental studies are needed to achieve flat
transmission response from the GaAs-based microstrip line up to 50GHz. At the higher
frequency range, the microstrip-Iine supports not only the fundamental mode o f
microwave electromagnetic signal but also higher order modes. Thus, the theoretical
study will focus on analysis o f modes o f the electromagnetic wave and the calculation of
electrical field distributions using numerical techniques. It is suggested that in order to
enhance the interaction of the electromagnetic wave with the thin iron film underneath
the strip electrode, the thickness o f dielectric GaAs needs to be reduced so that more
electric and magnetic fields of the wave incur in the iron film [17]. Stronger coupling and
thus deeper absorption dips will be achieved with suitable modification o f the sample
geometry. To realize very strong coupling to the spins in the Fe films, multilayer
structures may be used in which the microwaves are confined to very thin dielectric
films. A magnetic multilayer fabricated by ferromagnetic films, which alternated, with
dielectric films with both films having a thickness in the micron range may lead to very
strong attenuation o f the microwaves.
References
1. Jun Su. Chen S. Tsai. Chin C. Lee. J. Appl. Phys. 87 (2000) 5968
2. J A.C. Bland and B. Heinrich. Ultrathin Magnetic Structures II. Springer (1994)
3. C.S. Tsai, Proceedings o f the IEEE, vol.84. (no.6 ). IEEE (1996) 853
93
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
4. C. S. Tsai and J. Su, Appl. Phys. Lett., Vol. 74 (1999) 2079
5. C. S. Tsai, J. Su and C. C. Lee, IEEE Trans. On Mag., Vol. 35 (1999) 3178
6 . C. S. Tsai, J. Mag. Magn. Mater., Vol. 209 (2000) 10
7. C. C. Lee. W. Wu and C. S. Tsai, J. Appl. Phys. May (2002)
8 . W. Wu, C. C. Lee and C. S. Tsai, J. o f Cry. Growth 225 (2001) 534
9. B. Dieny, V.S. Speriosu. S.S.P. Parkin, B.A. Gurney. D.R. Withoit and D. Mauri/Vtys.
Rev. B 43 (1991), p. 1297.
10. B. K. Kuanr. Alka V. Kuanr. J. Mag. Magn. Mater. 165 (1997) 275
11. A. Flipe. A. Schuhl and P. Galtier. Appl. Phys. Lett. 70(1) (1997) 129
12. G. L. Matthaei. L. Young and E. M. Jones. Microwave Filters. Impedence-matching
Networks and coupling structures, Artech House. Norwood. 1980
13. Simon Ramo, John R. Whinnery and Theodore van Duzer. Fields and Awaves in
Communication electronics, Wiley. 1994
14. B. K. Kuanr. Alka V. Kuanr. J. Mag. Magn. Mater. 165 (1997) 275
15. D. J. Freeland. Y. B. Xu. M. Tselepi. J. A. C. Bland. Thin Solid Films 343-344 (1999)
210
16. E. Schlomann. R. Tutison. J. Weissman. T. Vatimos. J. Appl. Phys. 63 (1988) 3140
17. N. Cramer. D. Lucic. R. E. Camley. Z. Celinski. J. Appl. Phys. 87 (2000) 6911
18. Hitoshi Ohta, Seisaku Imagawa and Eiji Kita. J. Phys. Soc.. Japan 62 (1993) 4467
19. S. Datta, B. Das. Apply. Phys. Lett. 56 (1990) 665
20. M. Gester, C. Daboo. J.A.C. Bland. J. Mag. Magn. Mater. 165 (1997) 242
21. M. Zolfl. M. Brockmann. M. Kohler, S. Kreuzer, T. Schweinbock, S. Miethaner and
F. Bensch, G. Bayreuther, J. Mag. Magn. Mater. 175 (1997) 16
22. R. E. Camley. D. L. Mills, J. Appl. Phys. 82 (1997) 3058
94
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CHAPTER 6 ANTIFERROMAGNETIC EXCHANGE
COUPLING IN Fe/Cr/Fe/GaAs STRUCTURES
The discovery o f exchange couplings in magnetic multilayers has stimulated
considerable interest in this material system for both the fundamental research and the
potential applications. The study thus far has focused on the following points: (1) the
improvement o f the exchange coupling strength in magnetic multilayers, ( 2 ) the transport
properties in order to understand the physical origin o f the GMR [1]~[5]. The interest in
the study o f multilayers arises from their peculiar magnetic behavior: magnetic coupling
between the layers and oscillations in the coupling itself as a function o f the thickness o f
the nonmagnetic spacer layer [3].
In this chapter, the influence o f micro-magnetism on the magnetic coupling was
studied. Moreover, microstructural properties o f Fe/Cr multilayers have been studied to
determine the magnetic coupling o f Fe thin film layers. These studies allow development
of a model that determines the magnetic coupling effect and. therefore, the potential
technological applications in microwave bandstop filtering.
6.1 Introduction
Magnetic materials could be divided into three groups: (1) Paramagnetic:
Transition metals ions are present but the magnetic moments are randomly distributed.
Although an external field will produce some alignment o f the magnetic dipoles, the
alignment will disappear when the external field is removed. (2) Ferromagnetic: Adjacent
magnetic moments are aligned. When an external field is applied the magnetic dipoles
interact and the field remains locked in. The magnetism is due to unbalanced electron
95
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spins in the inner orbits o f the elements concerned. In unmagnetized metal magnetic
domains are randomly oriented. After a magnetic field is applied the domains align with
each other and the material remains a strong magnet after the external field is removed.
(3) Anti ferromagnetic: Alternate atoms have oppositely directed magnetic moments. The
magnetic susceptibility is low but increases with the temperature up to the Neel
temperature. Above this temperature the susceptibility falls and the material becomes
paramagnetic. Examples include Cr metal, and compounds like MnO. MnS. and FeO.
Iron, nickel, cobalt and some o f the rare earths exhibit a unique magnetic behavior
called
ferromagnetism.
Ferromagnetic
materials
exhibit
a
long
range-ordering
phenomenon at the atomic level, which causes the unpaired electron spins to line up
parallel with each other in a region called domain. The long-range order in ferromagnetic
materials arises from a quantum mechanical interaction at the atomic level [6 ]. This
interaction could lock the magnetic moments o f neighboring atoms into a parallel order
over a large number o f atoms, in spite o f the thermal agitation, which tends to randomize
any atomic-level order [7]. Sizes o f domains range from a 0.1 mm to a few mm. Within
the domain, the magnetic field is intense, but in a bulk sample the material will usually be
unmagnetized because the many domains will be randomly oriented with respect to one
another.
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t J k lA i
J n
n
t v x
Figure 6.1a
T
Figure 6.1b
Figure 6.1a The magnetization in domains usually cancel in bulk materials,
leaving the materials unmagnetized
Figure 6.1b If an external magnetic field is applied, magnetizations in the domains
will be aligned along the direction o f external magnetic field and the material is
magnetized.
For ferromagnetic materials, a small externally applied magnetic field can cause
the magnetic domains to line up with each other and the material is magnetized.
Figure 6.2a
Figure 6.2b
Figure 6.2c
Figure 6.2 Domain structures under (a) no external magnetic field (b) weak
applied magnetic field and (c) strong applied magnetic field.
When an external magnetic field is applied, the domains will align in the direction
o f this field and grow at the expense o f their neighbors. If all the spins are aligned in a
piece o f Fe, the magnetization would be about 22.000 Oe. A magnetization of about
10.000 Oe can be produced in annealed iron with an external field o f about 5 Oe. a
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multiplication o f the external field by a factor o f 2000. The magnetic constant p 0 = 4rc x
10' 7 T m/A is called the permeability o f space. The permeabilities o f most materials are
very close to p 0 since most materials will be classified as paramagnetic. But in
ferromagnetic materials the permeability may be very large and it is convenient to
characterize the materials by a relative permeability. The effect o f external magnetic
fields is to cause the domain boundaries to shift to those domains parallel to the applied
field. The induced magnetization may be aligned parallel or in some other direction to the
external field. Induced magnetization not parallel to the external field results from
magnetic anisotropy, and may oppose the external field. For a given ferromagnetic
material the long-range order abruptly disappears at the Curie temperature for the
material. The Curie temperature o f iron is about 1043 K.
V
Cr
Fe
Cr"
C r'
Co"
Mn4'
Mn3'
Fe:‘ __
Numbers of 3d
electrons
3
4
6
Magnetic
Moment (nB)
3
4
4
Element
Ions name
Table 6.1 Electronic configuration o f metal
In table 6.1. the electron configurations o f Cr and Fe ions are presented. The 3d
electrons have large spin and relatively low orbital contributions to magnetic moments.
Since the 4s electrons are outside the 3d electrons, the latter are partially shielded and the
orbital contribution will not be entirely negated. Thus for the 3d electrons, the spin
contribution is largely responsible for the magnetic moment and is proportional to the
number o f unpaired d electrons [8 ]. In 4 f electron containing elements, the electrons are
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well shielded by outer electrons, both orbital and spin effects contribute to the total
magnetic moment.
The magnetic field energy can be separated into two parts, the external field and
internal magnetic field. The second part of magnetic field is related to the magnetic field
generated by the magnetic materials body itself. The internal magnetic field Hm is defined
as the field generated by the divergence o f the magnetization M:
From Maxwell's equation:
divB = div({tQH + M) = 0
We have: divHm = -div( M / ju0)
(6 - 1)
The field can be calculated like a field in electrostatics from the electrical charges. The
energy connected to the internal field is expressed as [9]:
=
i
=
( 6 -2 )
ait
space
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II
•^itiiernal
Figure 6.3 Internal magnetic field
distribution o f Mz.
H mIc m a i
(along z direction) induced by a magnetization
As an example, we study the internal magnetic field induced by a one­
dimensional magnetization distribution. Suppose the magnetization direction depends
only on z-axis (Fig. 6.3). In this one-dimensional case, the differential equation 6-1 is
easy to be integrated, producing the internal field H,„ and its energy density Ej:
m(r) = M {r ) / A/,
\ f s is the surface magnetization density.
/ ^ 0)m,(r)f
Ej
=(M':/2M„)mUs)
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The energy density has the form o f uniaxial anisotropy energy; the anisotropy energy
coefficient is generally defined as:
Kd -
!2 jjq
6.2 Magnetic coupling effect
The use o f MBE to produce smoother films, has finally led to the observation o f
oscillatory interlayer coupling in (111) Co/Cu [10]. These studies emphasize that subtle
structural modifications of magnetic multilayers can lead to dramatically altered
properties. This oscillation was caused by an oscillation in the sign o f the interlayer
exchange coupling between the ferromagnetic layers. Oscillatory coupling was shown in
magnetic multilayer systems in which the nonferromagnetic layer comprises one o f the
3d, 4d, or 5d transition metals. The oscillation period was found to be just a few atomic
layers, typically about 10 A. but varying up to -4 0 A [11-15].
Figure 6.4 Spin configurations o f ferromagnetic coupling effect between Fe layers
induced by Cr space layer.
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Figure 6.5 Spin configurations o f anti ferromagnetic coupling effect between Fe
layers induced by an additional layer o f Cr atom.
It had previously been found that the magnetic moments o f the Fe layers in a
(lOO)-oriented Fe/Cr/Fe sandwich are aligned antiparallel or parallel to one another in
zero fields with a proper Cr layer thickness [12]. Therefore, the coupling was shown to
oscillate between anti ferromagnetic and ferromagnetic coupling. In this chapter, we
studied the multilayer structures for which the interlayer coupling is antiferromagnetic. In
a local moment model, the Fe-Fe interactions favor ferromagnetic alignment o f spins,
while the C rC r and C rF e interactions favor antiferromagnetic alignment. As shown in
the following schematics Fig. 6.4 and 6.5. the magnetic moments are all aligned in the
same direction in Fe layers due to the ferromagnetic exchange coupling between Fe-Fe.
while in Cr layers, the neighboring magnetic moments are aligned to be anti-parallel with
each other because o f the antiferromagnetic coupling effect within Cr-Cr. At the Fe/Cr
interface, the neighboring Cr / Fe magnetic moments will also be aligned to be anti-
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parallel due to the antiferromagnetic coupling between Fe-Cr. Therefore, if odd number
o f Cr layers are grown, the induced magnetic moment o f the second Fe layer grown on
top o f Cr will be aligned in the same direction as in the first Fe layer, this is the case
shown in Fig. 6.4. [16]. For the even number o f Cr layers grown, the induced magnetic
moments in the second Fe layer are aligned to be in the reverse direction o f the first Fe
layer, as shown in Fig. 6.5.
Figure 6.6 The spin configuration in the Fe and Cr layers (a) perfect interface (b) rough
interface case, only Cr layer's coupling is frustrated (c) In interface region. Cr/Fe
coupling is frustrated (d) finally, the roughness led to the frustration in Fe layers.
The spin configuration in the Fe and Cr layers is affected by interface roughness
[17]. For perfect Fe/Cr interfaces, the spin configurations are shown in Fig. 6 .6 a. in
which all pairs of spins have their preferred alignment. Magnetization is produced in Fe
layer, but for Cr. the net magnetization is zero due to Cr-Cr antiferromagnetic coupling
between neighboring Cr atoms. In real sample growth, due to lattice mismatch
dislocations exist and result in roughness at the interface, as shown in Fig. 6 .6 b~d.
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Consequently, it is impossible to obtain the preferred alignment for all pairs o f spins.
Some pairs will not be in their minimum energy configuration, that is. the coupling will
be frustrated'. In Fig. 6 .6 b, the Fe-Fe and Fe-Cr interactions are satisfied, but the C rC r
interactions are frustrated at the interface. The frustration o f the Fe-Cr interaction at the
interface is shown in Fig. 6 .6 c and in Fig. 6 .6 d. Note that the frustration occurs in the Fe
layer. The degree o f the frustration depends on the interface quality (step size and
distribution) and interdiffusion o f different atoms. In-situ techniques like LEED or AFM
will enable us to study the atomical level o f surface quality and find the best growth
condition.
The magnetization described above reveals the presence o f an exchange coupling
between two ferromagnetic layers separated again by a non-magnetic layer [ 18]—[20].
Here, a thick ferromagnetic layer Fe easily orients itself in the applied magnetic field.
The Cr layer is chosen such that the layer Fel is always antiferromagnetically coupled to
Fe2. The Cr layer is the target o f this study. So in our sample growth, the growth
condition for the Fe films is always fixed, while Cr layer thickness and growth
temperatures are varied to study the different coupling effects. In Fig. 6.5, the case where
Fel and Fe2 are coupled antiferromagnetically is shown. It was clearly a model o f the
indirect coupling among localized spins via conduction electrons [21 ].
6.3 Growth of Fe/Cr/Fe multilayers on GaAs (100) substrate
The magnetic properties o f a thin film are very sensitive to its atomic scale
neighborhood and the morphology o f interfaces is found to have a large influence on
magnetic coupling effect. Thus, the capability to grow multilayers with extremely flat
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interfaces is needed. For this purpose, a good starting point is to find the excellent lattice
constant matching o f neighbor metal films to obtain both physically and chemically
abrupt interfaces. When depositing a metal layer on another metal, the difference between
the crystal parameters does not allow forming a perfect stacking; structural defects may
appear, even though the lattice constant mismatch between Cr and Fe is only 1% and both
o f them are body centered cubic (bcc) crystal structures. The difference can reach 2% for
Fe and Cr deposited on a GaAs substrate. Using the technique like electrons diffraction or
microscopy, it is found that the deposit o f the first layers occurs with a tetragonal
deformation and remains coherent with the substrate (pseudomorphous deposit) [22]. For
thicker layers, the deposition recovers its bulk structure, i.e. bcc for Fe. by forming
crystal twinning boundaries. For the Fe/Ag model interface, it has been shown that Fe
grows in a layer-by-layer mode after several initial layers growth. The in-plane crystal
lattice o f the Ag layer matches the lattice o f the Fe atoms. For the case o f the Fe layer
grown on GaAs. a good epitaxy quality is obtained.
Fe (Iron)
Lattice Constant;
i!
Crystal Structure;
Cr (chromium)
Lattice Constant;
Crystal Structure:
0.2867 nm
: BCC (body center cubic) j
1
0.2905 nm
BCC
Table 6-2 Crystal lattice constant and structure o f Fe and Cr
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j
i
The subject o f the magnetism o f Cr layer in the vicinity o f Fe layers has been studied
over the last few years [23][24], It was pointed out that the interlayer exchange coupling
is highly sensitive to the magnetic properties o f the Cr layers, and various phase
transitions can be observed when varying the Cr thickness or the temperature in Fe/Cr/Fe
structures [25]. This chapter is concerned with the temperature and thickness dependence
of the interlayer exchange coupling in the Fe/Cr/Fe sandwiches grown epitaxially by
MBE.
The free energy in the coupling magnetic thin films is expressed as:
( 6 .3 )
£ = £ J+ £ .z + £ cx
where E: is the Zeeman energy for a magnetic body in an external field:
E. = -A , f:V? • Had V .
r
.1/ is magnetization. V is volume of the magnetic material and Ha is the applied magnetic
field.
Ea is the anisotropy energy: £ , - 2 A',
Where A'/ is the cubic anisotropy constant, for Fe film, the value o f 2A >.l/ = 550Oe.
which is called anisotropy field.
The interlayer exchange coupling energy £ « is expressed in the following form:
(6.4)
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In which Mi and M2 represent the magnetization vectors o f the two Fe layers and A12 and
B 12 are the bilinear and biquadratic coupling constants, respectively.
The expression for the saturation field is as follows [26]:
ff„
j K*
(6.5)
M ■a
in which Ki, M, and d are the cubic anisotropy constant, magnetization, and thickness of
the Fe layers, respectively. The positive and negative signs represent the hard- and easvaxis directions, respectively. Note that these expressions apply only when the coupling
field is anti ferromagnetic in nature, i.e.. the second term (including the
sign) in Eq.6.5
is greater than zero. If the coupling field is ferromagnetic, we simply have
= ±2
K
. where the negative case refers to the coercive field o f a square easy-axis
hysteresis loop [27].
In fact Cr is a particularly interesting choice of spacer material because bulk Cr is
known to exhibit incommensurate spin density wave antiferromagnetism. Epitaxial
Fe/Cr/Fe sandwiches were grown on the GaAs (100) substrates by MBE capped with Au
layer. It is well known that the chemistry o f the Fe/GaAs interface is complicated by
interdiffusion, which occurs at around a 20 A thick region o f the Fe/GaAs interface. The
thick Fe layer will provide a more flat substrate that will not interdiffuse with the
additional deposited Fe film. The surface o f the GaAs substrate has an oxidized surface
layer that is removed by annealing at a temperature o f 550 °C for 10-30 min. Sputtering
and annealing at 500 °C are then employed to remove any carbon present and to smooth
the surface. An Fe layer o f thickness 120 A is grown at a substrate temperature o f 150 °C
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before the Cr layer o f approximately 5-60 A thick is grown at room temperature to 170
°C at a rate o f about 1 A /min. We also grew the Cr layers at 300 °C. this higher
temperature growth resulted in a smaller tetragonal distortion o f the Cr layer, but induced
more interfacial layer interdiffusion between Fe and Cr. A second Fe layer is grown at
120-150 °C with a same thickness o f 120 A. The completed structure is capped with a
Au layer about 50 A thick to prevent oxidation o f the underlying Fe layers. The
thicknesses o f the various films are obtained by monitoring the deposition rates with
quartz crystal oscillators, which are calibrated by alpha-step profilometer measurements.
6.4 Magneto-optic Kerr effect (MOKE) measurement
The Magneto-optic Kerr effect (MOKE) corresponds to a change in the intensity or
polarization state of light reflected from a magnetic material. The amount o f change in
the polarization state or intensity is proportional to the magnetization in the material.
Ferromagnets will tend to stay magnetized to some extent after being magnetized by an
external magnetic field. The tendency to remember their magnetic history is called
hysteresis. The fraction of the saturation magnetization retained when the external field is
removed is called the remanence o f the material. Hysteresis loops were recorded with the
field applied parallel to Fe film easy [100] and hard [110] axes. The MOKE saturation
field has a nonlinear dependence on the coupling field and is sensitive to both FM and
AFM coupling. The saturation field was taken to be the field at which the Kerr intensity
reached 96% o f its maximum value. The in-plane easv-axis MOKE loop was found to be
square. MOKE signal was detected in all the deposited samples. The longitudinal
amplitudes (proportional to saturation magnetization) o f the MOKE loops were found to
vary with the film deposition conditions.
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Mr/Ms
Mr/Ms
Mr/Ms=95%
Mr/Ms=96%
1J
Sr
- V .v .
20 Oe
40 Oe
t
i
-► h
>■ -
t
► H
92153
Figure 6.7
Figure 6.8
Figure 6.7 MOKE spectrum for sample with 15 A Cr layer grown at room
temperature.
Figure 6.8 MOKE spectrum for sample with 21 A Cr layer grown at room
temperature.
Figure 6.7 shows the MOKE spectrum for the samples with 120 A Fe layers and
15 A Cr layer grown at room temperature.
The measured MOKE intensity is much
weaker than that o f the Fe/Ag/Fe multilayer samples presented in the previous chapters.
The coercive magnetic field Hc is around 40 Oe. This value is much higher than that o f
the Fe/Ag/Fe samples. We believe this difference is caused by magnetic coupling effect
between the two Fe layers separated by 15 A Cr and in turn causes the changes in
magnetization and magnetic response. For a thicker Cr layer thickness o f 21 A. the
MOKE signal intensity is still weak, as shown in Fig. 6 .8 . In order to improve the
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magnetic response o f MOKE signal, normally the Fe layer will be grown at a thickness
more than 100 A.
Mr/Ms
10 Oe
1
Oe&_
Figure 6.9 MOKE signal o f the sample grown at
150 °C with a Cr layer thickness o f 31 A.
Figure 6.9 shows a much stronger MOKE signal with rectangle shapes. The
coercive field He is seen to decrease to 7 Oe. This sample was grown at 150 °C with a Cr
layer thickness o f 3 1 A. We speculate that under such Cr layer thickness, there would be
no antiferromagnetic coupling effect between these two Fe films. The overall magnetic
moments will be the sum o f the two Fe layers and thus the magnetization is increased.
The clear-cut rectangle loops obtained along the (100) direction indicates that this
direction corresponds to a magnetic easy axis. In this measurement, in order to obtain a
better signal to noise ratio, we performed the MOKE measurement with the samples
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placed in high vacuum, all the data was collected in a vacuum level around 10° torr. The
magnetic remanence ratio (M/A/,) obtained from the hysteresis cycle is almost 95%,
which also indicates that good quality single crystalline Fe films have been grown.
Mr/Ms
i i Mr/Ms
I
Mr/Ms=78%
Mr/Ms=80%
400*
t—
>H
Magnetic Field (Oe)
Magnetic Field (Oe)
Figure 6.10
Figure 6 .11
Figure 6.10 MOKE spectrum for Au/Fe/Cr/Fe samples with 120 A Fe layers and 15 A Cr
layer grown at a substrate temperature o f 150 °C.
Figure 6 . 1 1 MOKE spectrum for Au/Fe/Cr/Fe samples with 120 A Fe layers and 26 A Cr
layer grown at a substrate temperature o f 150 °C.
Figure 6.10 shows the MOKE spectrum for Au/Fe/Cr/Fe samples with 120 A Fe
layers and a 15 A Cr layer grown at a substrate temperature o f 150 °C. From the figure,
the magnetic remanence ratio (M/Ms) obtained from the hysteresis cycle is much smaller
(less than 80%) than that o f the Fe/Ag/Fe samples presented in Chapter 3. Due to the
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limitation in the amplitude o f the magnetic field available, we were unable to apply high
enough magnetic field to get the saturation magnetization, as indicated in the above
figure. It has been reported that this external magnetic field could be around 1000 Oe
level to force the antiferromagnetic coupling films to be in saturation magnetization. In
Figure 6.1 1. the Cr layer is 26 A thick with all the other growth parameters remained the
same as the sample in Fig. 6.10. The measurement was performed at vacuum tube, so
very clean signal is obtained. The MOKE curve looks very similar to that in Fig. 6.10.
and it suggests that a much stronger magnetic field would be needed to force the sample
into saturation. We believe that for the above two samples, the two Fe films coupled
anti ferro magnetical ly.
6.5 AFM studies of the magnetic samples
The atomic force microscope (AFM) probes the surface of a sample with a sharp
tip. with feedback mechanisms that enable the piezoelectric scanners to maintain the tip
at a constant force to obtain height information above the sample surface. Tips are
typically made from Si3N.» or Si. As the tip scans the surface o f the sample, moving up
and down with the contour o f the surface, the laser beam is deflected off the attached
cantilever into a dual element photodiode. The photo detector measures the difference in
light intensities between the upper and lower photo detectors, and then converts to
voltage. Feedback from the photodiode difference signal, through software control from
the computer, enables the tip to maintain either a constant force above the sample, the
piezoelectric transducer monitors real time height deviation. AFM can be used to study
insulators and semiconductors as well as electrical conductors.
112
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The Atomic force microscope (AFM) and in-plane magneto-optical Kerr-effect
(MOKE) measurements are combined in order to investigate how the bilinear and
biquadratic coupling strengths depend upon the value o f the Cr thickness, and how the
growth condition influences the Fe/Cr interface roughness, as well as epitaxy
relationship. A systematical study o f the thin film growth as a function o f growth
conditions will help us find the best growth parameters to obtain flat interface and
epitaxial relationship. In a growth regime, atoms randomly deposited on a cold substrate
tend to form disordered metastable states, while mobility tends to lead the system towards
smooth states of lower energy by forming islands. A great variety o f morphologies appear
as a result o f the competition between these two processes, which could be controlled by
the growth temperature and the atom flux o f source materials. The atoms are trapped on
the terrace where they are deposited. This yields a stable growth on vicinal surfaces.
As reported in the previous chapter, at a high growth temperature o f 170 °C, for
the Fe films grown on GaAs. interdiffusion occurs at the Fe/GaAs interface and formed a
semiconductor-metal compound material for about 2 nm thick. Additional layer o f Fe
film will recover its crystal structure and facilitates epitaxial growth to obtain single
crystal films, which has been confirmed by LEED and XRD data. When depositing on
the Fe layer, ultrathin Cr layers will dissolve into the bottom Fe bulk structure. In fact,
within a certain temperature range, the dissolution is kineticallv blocked in the superficial
region where intermetallic compounds are formed. They are referred to as surface alloys.
113
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Figure 6 .12. Room temperature growth o f Cr layer on Fe film
100 nm
Figure 6.13 Three-dimensional AFM image o f Cr layer
deposited on Fe film surface at room temperature
The structure and magnetism o f ultrathin layers are closely related. For a range of
less than 4 monolayers. Cr grows on the Fe surface with interdiffusion and the layer is
not magnetic. The Cr islands appear via an ordered dislocation network. Above four
114
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monolayers, the Cr relaxes to take on its bulk lattice parameter, defects are filled up with
Fe. and a perpendicular magnetic anisotropy appears. This lasts until a thickness o f six
monolayers is reached, above which the anisotropy rotates into the surface plane and
epitaxial growth of layer-by-layer model begins. If the Cr layer is grown under higher
temperature (above 300 °C). thicker alloy structures will form, which remains
pseudomorphous even for a 200 A thick layer.
100 nm
Figure 6.14. AFM image o f Fe/Cr/Fe thin films grown on GaAs substrate at
150 °C without any external magnetic field annealing
However, even at the optimum temperature for layer-by-layer growth, there is
some interchange o f the deposited Cr atoms and the Fe bottom layer atoms at the
interface leading to an interfacial alloy. This can be seen in the AFM images o f Cr layer
deposited on the Fe film shown in Fig. 6.12. Figure 6.12 is AFM image o f 15 A Cr layer
sample grown at room temperature, while Fig. 6.13 is the 3-D AFM image o f the same Cr
layer sample. Island of about 10 A height is evident, as are many little bumps and the
islands. The islands contain Fe as well as Cr atoms and there are Cr atoms in the Fe layer.
115
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Because the GaAs (10 0) surface is very flat, the roughness o f the Fe/Cr layer determines
the thickness fluctuations. From the AFM images the rms roughness, <r, and mean island
separation, R, could be calculated. From the inequivalence o f R for Fe and Cr and from
modeling, the roughness o f the Fe bottom layer was not correlated with the roughness at
the top of the Cr layer. On the basis o f this assumption, the rms thickness fluctuation of
the Cr spacer layer is calculated as <r,={(rFe+<rcr)12- Typical value o f R is around 200 A
and a, value is about 5-10A.
100 nm
Figure 6.15 Magnetic force microscopy (MFM) image o f Fe/Cr/Fe thin films
grown on GaAs substrate at 150 °C without any external magnetic field annealing
The stronger magnetization area looks brighter in the image.
Figure 6-14 shows the AFM image o f Fe/Cr/Fe sample with 31 A Cr film grown
at a temperature o f 150 °C. For a Fe film deposited on Cr, a perfect epitaxy occurs
because the lattice parameter difference is only 1%. The Fe structure is then bcc type for
the (100) surface. From the images, we see that the higher growth temperature at 150 0 C
leads to a more uniform Fe film metal layer even it seems not a perfect epitaxy relation
116
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with the bottom Cr. In Fig. 6.15, magnetic force microscopy o f the same topography
region in Fig. 6.14 are presented, these two images are recorded simultaneously in
measurements. The brighter region indicates stronger magnetization. Even without any
external magnetic field annealing, we see that the Fe magnetic domains aligned along the
magnetic easy axis and form a net magnetizations. Figure 6.16 shows the MFM image o f
the same samples annealed at a magnetic field o f 2000 Oe. much stronger and more
uniform magnetization are shown, most o f the magnetic domains in the Fe layers are
align along magnetic easy axis.
Figure 6.16 MFM image o f Fe/Cr/Fe thin films grown on GaAs substrate at 150
°C. the sample has been annealed at an external magnetic field o f 2000 Oe.
117
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6.6 FMR microwave measurements of Au/Fe/Cr/Fe-GaAs multilayer
-J21 Oe
31 Oe
3
O
)
c
:/ / /
■
o ■' / ' J
:
^
:
j
c
x
C/5
LU
22 Oe
-
r
1
I
0
'
'
1
1
■
'
I
1
'
1
'
I
1
I
'
I
'
I
' ‘T “
'
I
100 200 300 400 500 600 700 800 900 1000 1100 1200
M agnetic Field (Oe)
Figure 6.17 ESR spectrum (9.6 GHz) for multilayer Au/Cr/Fe/Cr (15 A)/Fe films grown
at 150 °C. The external magnetic field is applied along the Fe [110] magnetic hard axis,
no ESR found along easy [100] axis.
3
(Q
C
o
!
o(A
H ,..= 2 4 5
<Q
<
X
sLi.
:
O e :'
i
\
Hi„=424 Oe
\
H'„ = 2 7
0e
I—.— i— .— i— i— i— .— |— .— \— >— |— .— i— i— |— i— i— i— |— i— ,— i— |
100
200
300
400
500
600
700
800
900 1000 1100 1200
M agnetic Field (Oe)
Figure 6.18 FMR absorption spectrum at 9.6GHz with the external magnetic field varied
along magnetic hard axis.
118
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The ESR spectrum for Au (5nm)/Cr (5nm)/Fe (12nm)/Cr (1.5nm)/Fe (12nm)
films grown at 150 °C is displayed in Fig. 6.17. The 5 nm Cr layer that grown on top of
Fe layer is used to eliminate the potential interdiffusion between Au and Fe atoms when
Au layer is deposited directly on Fe layer, which occurs because o f the strong mobility of
Au atoms. The external magnetic field is applied along the Fe [110] magnetic hard axis,
no ESR signal was found along easy [100] axis. Four resonant peaks were found with a
narrowest linewidth 21 Oe. As explained in the previous chapter, it indicated a high
quality crystalline Fe films have been grown. Compared with the ESR signal o f the
Fe/Ag films, all o f the resonant peaks occur at a magnetic field below 500 Oe.
Figure 6-19. Cross-section schematic o f Au/Cr/Fe/Cr/Fe structure
We believe that in this measurement, an additional internal magnetic H,„ has been
produced due to the antiferromagnetic coupling effect occurs in the sample. Since two
layers o f Fe films were involved in this particular FMR measurement instead o f only one
in the previous FMR measurement, the relevant analysis on FMR response would be
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more complicated. The magnetizations in the Fe films will not be kept as a constant value
as before, its amplitude and direction are varied by the external magnetic field. So in this
case, there are two factors that influence the resonant situation, one is the magnetic field
(internal plus external) and the other is the variation o f magnetizations. But the overall
effect is that the magnetic field needed to satisfy the resonant condition will be reduced.
As indicated in Fig. 6.18, the FMR absorption spectrum was measured at a microwave
frequency o f 9.6GHz with the external magnetic field varied along magnetic hard-axis of
the Fe film. The absorption peaks were found at 27 Oe. 245 Oe and 424 Oe. respectively.
Figure 6.19 shows the detailed dimensions of the sample cross-section configuration.
Finally, the filtering characteristics o f the Fe/Cr/Fe/Cr films were measured in our lab
using the flip-chip arrangement described in Chapter 5. Peak absorptions o f microwave
signals were found with a maximum dip intensity o f 1.0 dB. FMR absorption was
observed in the frequency range from 10 to 32 GHz with the corresponding magnetic
field tuned from 0 to 3500 Oe. Although the level o f FMR absorption is still weak at this
point, this is the first time a tunable filtering o f microwave signals is observed using the
Fe/Cr/Fe/Cr multilayer structure. It is to be noted that the sample was grown in a
relatively simple small MBE system in which no in-situ analysis instrument was
incorporated.
120
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3
(0
Hr»=1173 0 6
oH =117 Oe
c
o
a.
w
•• H =65 Oe
’ 6H =32 Oe
0
100 200 300 400 500 600 700 800 9001000110012001300140015001600
M agnetic Field (Oe)
Figure 6.20: FMR absorption spectrum for Au/Fe/Cr (26 A)/Fe films grown at
150 °C. The external magnetic field varied along hard axis o f Fe film.
Figure 6.21 Cross-section schematic o f Au/Fe/Cr (26 A )/Fe structure on GaAs
121
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3
Q
C
o
ak»
HfM=385 Oe
'■ 6H =72 Oe
o
(0
n
<
O'
2
LL
0
500
1000
1500
2000
2500
Magnetic Field (Oe)
Figure 6.22 ESR absorption spectrum at 9.6GHz for Au/Fe/Cr (31 A )/Fe films grown at
150 °C with the external magnetic field varied along the Fe [110] magnetic hard axis
Figure 6.20 present the ESR measurements o f another sample with Au/Fe/Cr(26
A)/Fe thin films grown on GaAs substrate at 150 °C. Detailed sample structure is
displayed in Fig. 6.21. Flip-chip measurement shows the sample is microwave active
with resonant frequency tuned from 10 GHz to 35 GHz. while the external magnetic field
was varied from 150 Oe to 5500 Oe.
Figure 6.22 shows the ESR measurements o f Au/Fe/Cr (31 A)/Fe thin films
sample grown on GaAs substrate at 150 °C. ESR shows the absorption peaks at 67 Oe
and 1165 Oe with the linewidth 40 Oe and 54 Oe. respectively. The crystal quality seems
not as good as the previous two samples because linewidth is wider, according to our
experience, to get a reasonable microwave response, normally the FMR linewidth
measured at 9.6 GHz should be less than 35 Oe. Flip-chip microwave measurement
shows the sample was relatively microwave inactive because only very weak absorption
122
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o f the microwave signal was seen. Compared with the sample obtaining in Fig. 6-19 and
6 - 2 1 . the only difference is the Cr layer thickness is increased to 31 A . We speculate that
under this space layer thickness, magnetic coupling will be drastically reduced and
internal magnetic field built in the Cr/Fe interface is rather weak.
Figure 6.23 Cross-section schematic o f Au/Fe/Cr (21 A)/Fe structure on GaAs.
the Cr layer was grown at various temperature from 25 - 320 °C
We also studied the dependence o f magnetic property and microwave response o f
the Fe/Cr/Fe samples on the growth temperature o f the Cr layer. Figure 6.23 show's the
cross-section schematic of Au/Fe/Cr (21 A)/Fe structure on GaAs. the Cr layer was
grown at various temperature from 25 - 320 °C.
123
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Sample A
Sample B
Sample C
sample D
550
550
550
550
10
10
30
10
0.5. 1.0
0.5. 1.0
0.5. 1.0
0.5. 1.0
Sputtering Time (min.)
3.3
3.5
3 .4
3.3
Sputtering
500
510
500
540
5E-4
5E-4
4E-4
5E-4
25
100
150
320
57. 96
49.81
41.65
75.102
Annealing
Temperature (°C)
Annealing Time
(minutes)
Sputtering Voltage
(kV)
Temperature (°C)
Sputtering Vacuum
(torr)
Cr Growth
Temperature (°C)
FMR linewidth (Oe)
Table 6-3. Comparison of cleaning condition and ESR linewidth
o f four Fe/Cr (21 A )/Fe samples deposited by MBE at various temperature
a
n
0
200
400
600
800
1000
1200
1400
Magnetic Field (Oe)
Figure 6.24 (a) FMR spectrum o f Au/Fe/Cr (21 A )/Fe films grown at 25 °C
124
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in
c
»
c
IT
H ,„=680 O e
aH =81 O e
2
IL.
o
200
400
600
BOO
1000
1200
1400
1600
Magnetic field (Oe)
Figure 6.24 (b) FMR spectrum o f Au/Fe/Cr (21 A )/Fe films grown at 100 °C
c
o
Q.
w
o
nin
ra
£T
2
u.
0
200
400
600
800
1000
1200
1400
1600
Magnetic Field (Oe)
Figure 6.24 (c) FMR spectrum o f Au/Fe/Cr (21 A )/Fe films grown at 150 °C
Table 6-3 shows the detailed growth condition and sample cleaning process.
Figure 6.24(a)~(d) display the FMR absorption spectra measured at 9.6 GHz with the
magnetic field applied and varied along magnetic hard axis o f Fe films. In flip-chip
125
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measurement, we see that except for the sample grown at 150 °C. very weak response
was observed in all remaining samples. Considering the spin configuration model in
Fe/Cr/Fe system, as shown in Fig. 6-4. we speculate that the magnetic coupling in these
samples will be either ferromagnetic or no coupling, therefore, the two Fe layers could be
regarded as just a simple Fe layers with a thickness o f 24 nm, all the FMR absorption
signals measured at 9.6 GHz are very similar to the single Fe samples grown on GaAs
substrate, except that the microwave absorption response becomes weaker. Because in
this study the thickness of Fe films are much thinner (12 nm), thus, the corresponding
absorption ability is certainly weaker than that o f thick Fe samples. According to our
experience, the single Fe films structure grown by the small MBE system has never
provided significant filtering response even for the samples prepared under different
growth conditions.
0
200
400
600
800
1000
1200
1400
1600
Magnetic Field (Oe)
Figure 6.24(d) FMR spectrum for Au/Fe/Cr (21 A )/Fe films grown at 320 °C.
Even without microwave response in our flip-chip measurement, the information
coming from Fig. 6.24 still help us find the best growth condition for Cr layer. Although
126
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some reports show that Cr layer grown on room temperature will have the advantage o f
no interdiffusion effect [17][28], the FMR linewidth shown in Fig. 6.24(a) is pretty wide
(57 Oe). At a growth temperature o f 100 °C. the linewidth was reduced and resonant
peaks shift to lower magnetic field, which is similar to what we observed in Fe/Ag/Fe
samples. The narrowest linewidth we got so far is 41 Oe positioned at a magnetic field o f
990 Oe for sample grown at 150 °C. Some studies indicated that at a growth temperature
o f 300 °C. the Cr film grew in a layer-by-layer mode [16][29][30]. this would lead to a
better crystal structure and flat interface. The magnetic quality o f Cr layer was degraded,
as shown in Fig. 6.24 (d). the linewidth is expanded to be 75 Oe with a lower resonant
peak become very weak.
127
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References
1. A. Vega, A. Rubio and L. C. Baibas. J. Appl. Phys.69 (1991) 4544
2. Y. Kobayashi. H. Sato. Y. Aoki, R. Loloee and W. P. Pratt. Jr. J. Magn. Magn. Mater.
238.(2002) 84
3. S. M. Rezende. C. Chesman, M.A. Lucena, A. Azevedo, F.M. Aguiar and S.S.P.
Parkin. J. Appl. Phys. 84 (1998). p. 958.
4. S. Colis. A . Dinia. P. Panissod, G. Schmerber and C. Menv. J. M agn. Magn. Mater.
226(2001) 1725
5. M. Guth, S. Colis. G. Schmerber and A. Dinia, Thin Solid Films, 380 (2000) 211
6 . Magnetism in ultrathin films, ed. D . Pescia. Appl. Phys. A (1998) 49
7. K. H. J. Buschow. handbook o f magnetic materials Vol. 12 (1999)
8 . N. M. Atherton. Electron Spin Resonance. Ellis Horvvood (1973)
9. J. Rauluszkiewicz and H. K. Lachovvicz. Physics o f Magnetic Materials (1984)
10. M. A. Mangan. G. Spanos. R. D. McMichael. P. J. Chen and W. F. Egelhoff.
Metallurgical and Materials Transactions A, 32 (2001) 577
11. H. R. Khan J. Magn. Magn. Mater. 165. (1997) 297
12. H. A. M. Van den Berg, W. Clemens. G. Gieres. G. Rupp, M. Vieth. J. Wecker and S.
Zoll. J. Magn. Magn. Mater. 165 (1997) 524
13. E. M. Ho and A. K. Petford-Long, J. Magn. Magn. Mater. 156 (1996) 65
14. E. D . Whitton. D . B. Jardine. R. E. Somekh. J. E. Evetts. M. J. Hall and J. A. Leake
Thin Solid Films. Volume 275. (1996) 195
15. T. Zimmermann, J. Zweck and H. Hoffmann. J. Magn. Magn. Mater. 148. (1995) 239
16. D . T. Pierce. J. Unguris and M. D . Stiles. J. Magn. Magn. Mater. 200 (1999) 290
17. R. J. Hicken, C. Daboo. M. Gester and J.A.C. Bland. J. Appl. Phys. 78 (1995) 6670
18. H. Hopster, J. Appl. Phys. 87 (2000) 5475
19. K. V. Rao, V. Korenivski. D . M. Kelly. I. K. Schuller. K. K. Larsen and J. Bottiger J.
Magn. Magn. Mater. 140 (1995) 549
128
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
20. B. K. Kuanr and A. V. Kuanr, J. Magn. Magn. Mater. 165, (1997) 275
21. J.F. Cochran and J.R. Dutcher. J. Magn. Magn. Mater. 73 (1988), p. 299
22. E.E. Fullerton, M.J. Conover, J.E. Mattson, C.H. Sowers and S.D. Bader. Phys. Rev.
B 48 (1993) 15755
23. A. B. Drovosekov. D. I. Kholin. N. M. Kreines. O. V. Zhotikova and S. O.
Demokritov. J. Magn. Magn. Mater. 226 (2001) 1779
24. K. Temst. E. Kunnen. V. V. Moshchalkov. H. Maletta. H. Fritzsche and Y.
Bruynseraede. Physica B 276 (2000) 684
25. V. V. Ustinov. M. M. Kirillova. I. D. Lobov, L. N. Romashev. V. M. Maevskii. M. A.
Milyaev and O. N. Kiseleva. J. Magn. Magn. Mater. 198 (1999) 24
26. J.F. Cochran, J. Rudd. W.B. Muir. B. Heinrich and Z. Celinski. Phvs. Rev. B 42
(1990)508
27. A. Azevedo, C. Chesman and F. M. de Aguiar. Phys. Rev. Lett. 76 (1996) 4837
28. J. Dekoster, J. Meersschaut and G. Langouche. J. Magn. Magn. Mater. 198 (1999)303
29. Y. U. Idzerda. L. H. Tjeng and J. Gutierrez. J. Appl. Phys. 73 (1993) 6204
30. J. J. Krebs, P. Lubitz and G. A. Prinz. Phys. Rev. Lett. 69 (1992) 1125
129
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CHAPTER 7 CONCLUSION
In this dissertation, the growth and characterization o f single and multi-layer
ultrathin Fe layers on GaAs substrates using molecular beam epitaxy (MBE) are
presented. Good quality single-crystal Fe films were obtained under appropriate growth
conditions. A Cr layer o f varying thickness was also used as a spacer between two Fe
layers to affect the degree o f exchange coupling.
The FMR linewidth of the Fe film has been shown to be an important parameter
for the construction of tunable microwave bandstop filters. Tunable wideband bandstop
filtering has been demonstrated in both the flip-chip and the integrated configurations.
Transmission characteristics o f the resulting filters are evaluated with frequency tuning
range as wide as 10.6 to 33 GHz. Due to the large saturation magnetization o f the Fe film,
the electronic tunability o f both the operation carrier frequency and the bandwidth o f the
resulting microwave are significantly greater than the YIG counterparts. Such wideband
integrated microwave devices possess many potential applications in RF signal
processing and communication systems.
130
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