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Accepted Manuscript
Ferroelectric, Magnetoelectric and Photoelectric Properties of BiFeO3
/LaNiO3 Heterostructure
Fei Fan , Mengmeng Duan , Bingcheng Luo , Changle Chen
PII:
DOI:
Reference:
S0577-9073(18)30696-8
https://doi.org/10.1016/j.cjph.2018.08.002
CJPH 598
To appear in:
Chinese Journal of Physics
Received date:
Revised date:
Accepted date:
11 June 2018
29 July 2018
10 August 2018
Please cite this article as: Fei Fan , Mengmeng Duan , Bingcheng Luo , Changle Chen , Ferroelectric, Magnetoelectric and Photoelectric Properties of BiFeO3 /LaNiO3 Heterostructure, Chinese Journal
of Physics (2018), doi: https://doi.org/10.1016/j.cjph.2018.08.002
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ACCEPTED MANUSCRIPT
Highlights
BiFeO3/LaNiO3 heterostructure was grown on quartz substrate by RF sputtering.
The dense and uniform surface morphology was observed from AFM.
BFO layer shows good ferroelectric and weak ferromagnetic character at RT.
Photoelectric effect was observed in BFO/LNO heterostructure.
The photoconductivity increases with the enhanced power of light.
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Ferroelectric, magnetoelectric and photoelectric properties of
BiFeO3 /LaNiO3 heterostructure
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Fei Fan†, Mengmeng Duan, Bingcheng Luo, Changle Chen
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Shaanxi Key Laboratory of Condensed Matter Structures and Properties, School of Science,
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Abstract:
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Northwestern Polytechnical University, Xi’an 710129, China
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BiFeO3/LaNiO3 (BFO/LNO) heterostructure was fabricated on quartz substrate via RF
sputtering method. The microstructure and surface morphology of the BFO/LNO
heterostructure was demonstrated. BFO layer shows good ferroelectric and weak
ferromagnetic characters at room temperature. The dielectric constants of the
heterostructure under an applied magnetic field 1.2T and zero field are both
†
Author to whom correspondence should be addressed; email: fanfei@nwpu.edu.cn
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decreased with increasing frequency at room temperature and the dielectric
constant under the applied magnetic field is larger, which is attributed to the
coupling between the electric and magnetic dipoles, and further demonstrated in
the framework of the Ginzburg-Landau theory for second phase transition.
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Additionally, the photoconductivity of the heterostructure under blue-laser
illumination was observed, and the photoconductivity increase with the enhanced
power of the blue-laser.
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Keywords: multiferroic; dielectric constant; hysteresis loop; photoconductivity.
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1. Introduction
Multiferroic materials with potential coupling between ferroelectric and magnetic
order parameters are the center of attention because of potential application in
multifunctional devices [1-3]. BiFeO3 (BFO) is just one such a material of particular
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interest, and it is known to possess a high ferroelectric ordering (transition
temperature TC ~1103 K) and an antiferromagnetic ordering (transition temperature
TN ~ 643 K) at room temperature [4]. The main interest of BFO resides in the possible
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use of its magnetoelectric coupling properties that could allow to write a magnetic
information electrically and vice verse. The potential application of BFO in memory
devices, integrated circuits, spin electronic devices and other integrated ferroelectric
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devices have lead to an explosion of interest in its growth and properties. However,
in bulk BFO, the magnetoelectric effect is weak due to the presence of the cycloidal
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order. Consequently, several approaches have been considered to unwind the
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cycloid and hence modify the magnetoelectric effect. It was found that the cycloid
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can be destroyed by applying a magnetic field of 20 T in bulk BFO [5] but also by
chemical substitution at Fe site [6-9]. In addition, epitaxial strains in films can
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destabilize the cycloidal order and drive a transition towards a homogenous weakly
ferromagnetic order. Thus, thin films of BFO offer advantage over bulk counterpart
because of lattice strain induced tailoring of multiferroic properties at room
temperature [4, 10]. On the other hand, LaNiO3 (LNO) was often chosen as the
electrode or buffer layer in ferroelectric films due to its simple composition, easy
availability and good conductivity [11-13]. Furthermore, it is also proved that
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saturated hysteresis loop can be observed in BFO films on Pt bottom electrode by
introducing a LNO intermediate layer or BFO films on LNO electrode [14-16].
Therefore, in this paper, using LNO as the bottom electrode and buffer layer, we
deposited BFO/LNO heterostructure on quartz substrate by RF sputtering and
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investigated its multiferroic properties, which has been reported few. Besides, due
to its small band gap and unique characteristics of polarization, BFO has overcome
the constraints of the traditional solid-state solar cell band gap, so that BFO thin film
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can perform the photovoltaic effect. The photoelectric property of our BFO/LNO
heterostructure was also reported, which could widen its application of BFO thin film
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in the field of novel magnetic-optical-electric device.
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2. Experiments
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For this study, a sputtering target of Bi1.1FeO3 was prepared by sol-gel method [9]
and the BFO/LNO thin film heterostructure was fabricated on a quartz substrate by
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RF sputtering. The sputtering was performed at power density of 1 W cm-2 with a
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highly purified gas atmosphere (O2 20%+Ar 80%) at a working pressure of 2 Pa.
Before deposition, the pressure of sputtering chamber was evacuated below 1×10-4
Pa. The LNO layer as bottom electrode of thickness 150 nm was firstly deposited on
the quartz substrate at temperature of 500 0C. Then, the BFO layer of thickness 250
nm was deposited on the LNO layer at temperature of 600 0C. After deposition, the
BFO/LNO thin film heterostructure was cooled down to room temperature at a rate
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of 8 0C min-1 under 200 Pa oxygen pressure. The thicknesses of the LNO and BFO thin
films were estimated by SpecEI-2000-VIS ellipsometer, respectively. The crystal
phase structure of the BFO/LNO thin film heterostructure was characterized by X-ray
Diffraction (XRD) using θ-2θ scans with the Cu Kα radiation (D/max2200PC, Rigaku),
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the wavelength is 0.15432nm. The surface morphology was showed by Atomic Force
Microscopy (AFM) working in the intermittent contact mode with an Olympus
AC240TS cantilever (Asylum Research, MFP-3D). The valence state of Fe iron was
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checked by X-ray Photoelectron Spectroscopy (XPS) (PHI-5400). Polarization-electric
field (P-E) and magnetization-magnetic field (M-H) hysteresis loops were measured
using a Radiant precise workstation (Radiant Technologies) and superconducting
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quantum interference device (SQUID) magnetometer, respectively. The dielectric
constant was carried out using an Impedance Analyzer (HP4294A) and the leakage
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current behavior was performed using a Keithley 2182A nanovoltmeter and a 6487
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picoammeter with a delay time of 5 s. A 150 mW blue-laser coupled into a slit was
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used as the light source during the photoelectric property experiment.
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3. Results and discussion
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Fig. 1 XRD pattern of the BFO/LNO heterostructure.
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Fig. 1 shows the X-ray diffraction pattern of the BFO/LNO heterostructure. No
second phase or other oriented grains were detected besides the diffraction peaks of
BFO and LNO, indicating the BFO layer is a single phase. Moreover, the BFO thin film
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does not exhibit preferred orientation, but also consists of randomly oriented
perovskite structure, which is attributed to the large film thickness [15]. The pseudo-
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cubic lattice parameters of LNO and BFO layers calculated from the positions of the
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diffractive peak by Bragg expression are 0.3867 nm and 0.3953 nm, respectively.
Fig. 2 (color online) AFM graphs of the LNO layer (a) and
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BFO/LNO heterostructure (b).
The typical surface morphology of LNO and BFO/LNO heterostructure over 1μm×
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1μm scan size are shown in Fig. 2 (a) and (b), respectively. The images illustrate our
sample is uniform, compact and dense. The corresponding root-mean-square (rms)
roughnesses estimated by Igor software for LNO and BFO/LNO heterostructure are
about 1.79 nm and 4.95 nm, respectively, indicating a well growth of the
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heterostructure. The polycrystalline structure and uniformity of the morphology of
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our sample are believed to significantly influence the multiferroic properties.
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Fig. 3 (color online) (a) P-E hysteresis loop measured at room temperature and 1 kHz.
(b) M-H hysteresis loop measured at room temperature. Inset: the XPS of Fe 2p
orbital (left) and modified view of M-H hysteresis curve (right).
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Fig. 3 (a) shows the polarization behavior of the BFO thin film, which was measured
at 1 kHz and 300 K. The trend of saturation can be observed from the loop and the
shape of the hysteresis loop indicates the BFO film exhibits good ferroelectric
behavior at room temperature. The field dependence of magnetization of the BFO
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film is measured at room temperature, as shown in Fig. 3 (b), which exhibits a
hysteresis loop indicating a weak ferromagnetic (see inset of Fig. 3 (b) (right))
character at room temperature. It is widely accepted that oxygen vacancies are
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easily formed during growth and cause a portion of the Fe 3+ ions to become Fe2+,
thus a cumulative of mixed Fe2+/ Fe3+ valence formation will contribute to the
ferromagnetic character of the BFO film [17, 18]. According to the XPS spectra of Fe
2p orbit of the BFO film (see inset of Fig. 3 (b) (left)), the weak peak (indicated by the
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arrow) located at 708.9 eV corresponds to Fe2+ [19] and the other peaks correspond
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to Fe3+, implying that the BFO film is a Fe3+ and Fe2+ mixed-valence system.
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Therefore, the weak ferromagnetic character of our sample originates from a
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cumulative of mixed Fe2+/ Fe3+ valence formation.
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Fig. 4 (color online) (a) The dielectric constants as a function of frequency with zero
field and 1.2 T magnetic field measured at room temperature. Inset shows the
frequency dependence of MC. (b) The dielectric constants as a function of
temperature with zero field and 1.2 T magnetic field. Inset shows the temperature
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dependence of MC.
Fig. 4(a) plots the dielectric constant as a function of frequency under applied
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magnetic fields of 1.2 T and zero field at room temperature. The dielectric constants
are both reduced with increasing frequency, which could be due to the inability of
the electric dipoles to be in pace with the frequency of applied electric field at high
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frequency. Moreover, the dielectric constant increases with the applied magnetic
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field, which is similar to that reported, e.g., in Sc-doped BFO films [20]. The variation
is characterized by the magnetocapacitance (MC) effect and defined as: MC=[ε(H) -
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ε(0)]/ε(0)=Δε/ε(0), where ε(H) and ε(0) are the dielectric constant at applied
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magnetic field and zero field, respectively. It is seen that the value of MC decreases
with increasing frequency (see inset of Fig. 4(a)) and is about 0.61% for the magnetic
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field change of 1.2 T at an applied frequency of 10 kHz. Additionally, the
temperature dependence of dielectric constant under applied magnetic fields of 0 T
and 1.2 T was measured at 10 kHz, as shown in Fig. 4 (b). The dielectric constant
increases with increasing temperature and shows an anomaly around 635 K. The
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positive value of MC reaches the maximum 1.39% near TN (see inset of Fig. 4 (b)) and
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rapidly decreases to zero.
Fig. 5 (color online) (a) The J-E curves of the BFO/LNO heterostructure measured at
room temperature under zero field and 1.2 T magnetic field. Inset shows the MR
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value. (b) Variation of MC with M2 at room temperature. Inset shows M-H curve
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measured at room temperature.
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For the origin of MC effect, current reports attribute it to the magnetoelectric
coupling effect [7, 20-22] or magnetoresistance (MR) effect combined with Maxwell-
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Wagner effect [23]. In order to understand MC effect in our BFO/LNO
heterostructure well, we carried out the following experiment. J-E plots under
applied magnetic fields of 0 T and 1.2 T for our sample were measured at room
temperature, as shown in Fig. 5 (a). Here we define MR as: MR=[R(H) -R(0)]/R(0),
where R(H) and R(0) are the resistances at applied magnetic field and zero field,
respectively. It is found that MR of our sample (see inset of Fig. 5 (a)) is less than
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0.1%, so MR effect can be ruled out. Therefore, such apparent MC in the present
case could be primarily attributed to the coupling effect between the electric and
magnetic dipoles. Additionally, the variation of dielectric constant could be explained
in the framework of the Ginzburg-Landau theory for the second-order phase
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transition [24]. It is presented that the difference of the dielectric constant Δε will be
proportional to the square of the magnetic-order parameter, i.e., Δε~ γM2, where M
is the magnetization and γ denotes the magnetoelectric interaction. Fig. 5 (b) shows
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the Δε/ε(0) vs M2 plot. It should be noted that the data approximately falls to a
single line. This further verifies that the observed phenomenon is basically due to the
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magnetoelectric coupling.
Fig. 6 (color online) The J-E curves of the BFO/LNO heterostructure taken in dark, 50
mW/cm2, 100 mW/cm2 and 150 mW/cm2 blue-laser illumination.
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BFO, as a typical photo-ferroelectric material [25-28], the resulting current of photoexcited carriers will be driven by the intrinsic polarization, causing a change in
conductivity. The current can be described by the following equation [25]:
(1)
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J=(σd+σph)ξ
where σd and σph represent the dark and light components of the conductivity,
respectively. Fig. 6 shows the leakage current density versus electric field (J-E)
characteristic of the BFO/LNO heterostructure taken in dark, 50 mW/cm2, 100
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mW/cm2 and 150 mW/cm2 blue-laser illumination (λ=450 nm), respectively. Under
the applied electric field of 40 kV/cm, the results show that the dark conductivity is
4.68×10-6 Ω-1 cm-1, and illuminated conductivities are 5.15×10-6 Ω-1 cm-1, 5.88×10-6 Ω-1
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cm-1 and 6.19×10-6 Ω-1 cm-1 corresponding to the illuminations of 50 mW/cm2, 100
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mW/cm2 and 150 mW/cm2, respectively. This data suggests that photoconductivity
increases with the enhanced power of blue laser, which is consistent with the model
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present in Eq. (1) and with our previous experimental results in
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BiFeO3/La0.7Sr0.3MnO3 heterostructure [29], indicating the obviously photoelectric
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properties in BFO.
4. Conclusions
The BFO/LNO heterostructure was grown on quartz substrate by RF sputtering. X-ray
diffraction analysis revealed that the randomly oriented perovskite BFO film was
obtained with no impurity phases. The dense and uniform surface morphology was
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observed from AFM. At room temperature, the BFO film shows a good ferroelectric
character and a weak ferromagnetic character as result of a cumulative of mixed
Fe2+/ Fe3+ valence formation. Additionally, the frequency and temperature
dependence of dielectric constant were investigated with and without magnetic
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field. The anomaly around 635 K in ε(T) is observed, which is due to the influence of
vanishing magnetic order on the electric order. Moreover, the dielectric constant
changes under applied magnetic field with frequency and temperature, which is
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attributed to the coupling between the electric and magnetic dipoles. This is further
demonstrated in the framework of the Ginzburg-Landau theory for the second-order
phase transition, indicating that the BFO film shows multiferroic property. In
addition, we report the photoconductivity in BFO/LNO heterostructure under dark
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field and illumination from 50 mW/cm2, 100 mW/cm2 and 150 mW/cm2 blue-laser,
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and the photoconductivity increase with the power of light enhanced, indicating the
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potential photoelectric effect in BFO/LNO heterostructure. We hope our observation
will motivate more scientific research in multiferroic and photoelectric properties of
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BFO thin films and widen the application of BFO-based novel magnetic-optical-
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electric device.
Acknowledgments
This work was supported by Scientific Research Foundation for New Faculty of
Northwester Polytechnical University (No. G2018KY0305).
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