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Optical and magnetic properties of KBiFe2O5 thin films fabricated by chemical solution deposition
Materials Letters 161 (2015) 423–426
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Optical and magnetic properties of KBiFe2O5 thin films fabricated by chemical solution deposition Xuezhen Zhai a, Hongmei Deng b, Wenliang Zhou a, Pingxiong Yang a,n, Junhao Chu a, Zhi Zheng c a Key Laboratory of Polar Materials and Devices, Ministry of Education, Department of electronics, East China Normal University, 500 Dongchuan Road, Shanghai 200241, China b Instrumental Analysis and Research Center, Institute of Materials, Shanghai University, 99 Shangda Road, Shanghai 200444, China c Key Laboratory for Micro-Nano Energy Storage and Conversion Materials of Henan Province, Institute of Surface Micro and Nano Materials, Xuchang University, Henan 461000, China
art ic l e i nf o
a b s t r a c t
Article history: Received 7 July 2015 Received in revised form 2 September 2015 Accepted 4 September 2015 Available online 5 September 2015
KBiFe2O5 thin films grown on quartz substrates by a chemical solution deposition method have been investigated by means of optical and magnetic measurements. The effects of synthesized temperature and the molar ratios of K/Bi on the phase formation of KBiFe2O5 thin film were assessed. Microstructural characterizations using X-ray diffraction and scanning electron microscopy indicate that the sample annealed at 750 °C shows better crystallinity. Optical transmittance measurement and magnetization curves show optical and magnetic tunability of KBiFe2O5 thin film with increasing growth temperature, respectively, where the possible physical mechanisms were provided and discussed. These results are helpful in the deeper understanding of relation between crystal structure and physics in perovskite-like oxides and show the potential role, such materials can play, in solar-energy devices and multiferroic applications. & 2015 Elsevier B.V. All rights reserved.
Keywords: Multiferroics KBiFe2O5 Magnetic materials Sol–gel preparation Thin films
1. Introduction In recent years, multiferroics are regarded as an intelligent material owing to its simultaneous effects of ferroelectricity and ferromagnetism [1–2]. A lot of efforts on multiferroic materials concentrate on ABO3 perovskite structures, which is composed of a three-dimensional framework of corner-sharing BO6 octahedra [3– 5]. But, it is a major challenge for obtaining multiferroics that two order parameters are both either strong at any temperature or nonzero at room temperature and above. To address the above issues, some studies on brownmillerite structure A2B2O5 provide an alternative approach to prepare multiferroic materials due to its characteristic oxygen-deficient perovskite structure [6]. A2B2O5 is composed of perovskite-like three-dimensional framework of corner-sharing BO6 octahedra alternating with slabs containing rows of corner-sharing BO4 tetrahedra, formed by the deficiency of oxygen during the formation of the structure [7–8]. As indicated in the literature [9], the coexistence of electric and magnetic properties could be achieved in KBiFe2O5 crystal structure which contains tetrahedral Fe3 þ in a n
Corresponding author. E-mail address: [email protected] (P. Yang).
http://dx.doi.org/10.1016/j.matlet.2015.09.018 0167-577X/& 2015 Elsevier B.V. All rights reserved.
[Fe2O3] block that alternates with a [(K, Bi)O2] block. In addition, this new multiferroic KBiFe2O5 features a narrow band gap (Eg) without losing the useful ferroelectricity, which is attributed to its smaller coordination number and the inverted t2g/eg orbitals of tetrahedra with lack of inversion symmetry. Compared with the nanopowders, thin films occupy more important position in the field of actual device preparations. However, experiments on KBiFe2O5 thin films have not been conducted yet. In this paper, KBiFe2O5 thin films were deposited on the quartz substrates by a chemical solution deposition method. The microstructure, surface morphologies, optical and magnetic properties were analyzed. The effects of growth temperature and the ration of K/Bi on the phase formation of KBiFe2O5 thin film as well as the physical properties were further discussed. We hope that our work will inspire more experimental explorations on KBiFe2O5 as a candidate material for use in multiferroic and solar-energy applications.
2. Experimental section The multiferroic KBiFe2O5 films were deposited on quartz substrates by a sol–gel process. Analytically pure C2H3KO2, Bi (NO3)3 5H2Oand Fe (NO3)3 9H2O were used as starting materials.
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The starting materials were dissolved in CH3COOH and C2H6O2 mixture and the concentration of the final solution was adjusted to 0.25 mol/L. The precursor solution was spin-coated on quartz substrate at 6000 rpm for 30 s. The thermal treatments of thin films were carried out in a rapid thermal processor. The spincoating and thermal treatments were repeated several times until the thicknesses of thin films are 200 nm. The crystalline structure of the KBiFe2O5 films was investigated by X-ray diffraction (XRD, Bruker D8 Advance) with Cu-Kα radiation source. The surface morphologies were characterized by scanning electron microscopy (SEM, Philips XL30FEG). The magnetic properties of the samples were investigated by physical property measurement system (PPMS-9, Quantum Design). The optical properties were characterized by transmittance spectra (PerkinElmer UV/VIS Lambda 2 S). All measurements were performed at room temperature.
3. Results and discussion Room-temperature XRD patterns of the as-prepared samples at different annealing temperatures are shown in Fig. 1(a). These XRD data show the films to be consistent with a polycrystalline like-perovskite structure, which is similar to Zhang's results [9], at least within the detection limits of the instrument. In Fig. 1(a), a slight impurity peaks marked by special symbol have been observed in KBiFe2O5 film annealed at 650 °C, which corresponds to K2FeO4. The absence of impurity phases for samples at 750 and 850 °C indicates that appropriately increased annealing temperature can improve the crystallinity of KBiFe2O5. Moreover, it is notable that the sample exhibited a phase transition behavior with the increase of the temperature from 750 °C to 850 °C, which may be attributed to the strong interface effects [10]. Fig. 1 (a)–(c) show SEM images of the thin films with smooth surfaces. The films annealed at 650 and 750 °C are well crystallized since favorable grains can be identified clearly. However, the surface morphology for the sample at 850 °C is ambiguous possibly because the surface is too smooth to be observed visible grain boundary. To confirm the effects of chemical composition of the KBiFe2O5 on its crystal-structure and surface morphology, the XRD patterns
and SEM images of the samples with different molar ratios of K/Bi were shown in Fig. 2. From the XRD results, the thin films were well crystallized but a weak impurity peak at 2θ ¼25° exists in the composition with K/Bi ¼ 0.95 and K/Bi ¼1.00, then disappears gradually with increasing K content, which might originates from the stronger volatility of element K than Bi. The EDX analysis results of the KBiFe2O5 thin films are summarized in inset table of Fig. 2a. It can be seen that chemical composition of sample K/ Bi¼1.05 is most close to the standard molar ratio 1:1:2 for Bi:K:Fe (in at%), which agrees with the above XRD analysis. These results show that annealing temperature 750 °C and molar ratio K/Bi ¼1:05 are the optimum condition for forming the pure KBiFe2O5 thin film in our experiment. SEM surface morphologies of three different compositions appear to be the same, indicating that the effect of K/Bi ratio is inferior to that growth temperature on the crystal growth. Fig. 3(a) shows the optical transmittance spectra of the KBiFe2O5 thin films in the wavelength range from 200 to 1500 nm. The spectra also indicates that the absorption edge is around 500 nm and the absorption edges of KBiFe2O5 films show a blue shift with the increasing growth temperature. The corresponding optical band-gaps can be estimated from the tangent lines in the plot of the Tauc’s law (αhv)2 versus hv for the direct band-gap material, where α is the absorption coefficient and hν is photon energy, as presented in Fig. 3(b) [11]. The slope of the linear part suggests the band gap of KBiFe2O5 is estimated to be 2.43, 2.54, and 2.57 eV for growth temperature 650, 750 and 850 °C, respectively. In the fundamental absorption edges region, the absorption is due to the band-to-band transition from the top of valence band to the bottom of conduction band directly. The valence band and conduction band of KBiFe2O5 are composed of O 2 p and Fe 3d (egt2g) states, respectively [9]. Compared with BiFeO3[12], the KBiFe2O5 film presents a sharply lower band gap, which is attributed to a distorted crystal field at the tetrahedral environment leading to the further splitting of the t2g and eg and their orbital inversions [9]. Finally, the measured magnetization versus magnetic field (M– H) curves of the samples at room temperature, are shown in Fig. 4. In crystal-structure of KBiFe2O5, the antiparallel array of nearest neighbor Fe3 þ spins give rise to net magnetic moments. Compared with the KBiFe2O5 powders [9], thin films exhibit larger
Fig. 1. (a) XRD patterns of the KBiFe2O5 thin films with annealing temperature of 650, 750 and 850 °C, respectively; and (b)–(d) SEM micrographs corresponding to the samples.
X. Zhai et al. / Materials Letters 161 (2015) 423–426
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Fig. 2. (a) XRD patterns of the KBiFe2O5 thin films with different molar ratios of K/Bi; (b)-(d) SEM surface morphologies corresponding to the films, respectively. The inset table of (a) illustrates the composition ratios of the KBiFe2O5 thin films with different molar ratios of K/Bi at annealing temperature of 750 °C.
Fig. 3. (a) Optical transmittance spectra of the as-grown KBiFe2O5 thin films. (b) Plots of (αhν)2 versus hν for KBiFe2O5 thin films with annealing temperature of 650–850 °C.
magnetization and clearer hysteresis loops at room temperature. It may be attributed to the strong interface strain-induced more structure distortion [13]. All films exhibit weak ferromagnetic behavior which is further improved by modifying the growth temperature. Only the M–H curves corresponding to 750 °C show the saturation tendency, and the saturation magnetization (Ms), 1.1 emu/cm3, has been also observed. The observed remanent magnetization (Mr) and coercive filed (Hc) versus growth temperature (T) relation shown in Fig. 4(d) can be explained considering the modification of the short-range G-type antiferromagnetic order [14], namely, Fe3 þ may have three different spin state configuration; high spin (HS, S ¼5/2, t2g3eg2), intermediate spin (IS, S¼ 3/2, t2g4eg1) and low spin (LS, S ¼1/2, t2g5eg°). The comparable crystal field splitting energy and spin–spin exchange splitting energy leads to spin state transition. The thin film annealed at 750 °C shows a better hysteresis loop due to Fe3 þ HS state in KBiFe2O5, which further confirms that 750 °C is the most suitable temperature to synthesize high-performance KBiFe2O5 films.
4. Conclusions In summary, KBiFe2O5 thin films have been grown on quartz substrates by a chemical solution deposition method. The effects of growth temperature and the molar ratios of K/Bi on the phase formation of KBiFe2O5 thin film have also been studied. Optical transmittance measurement and magnetization curves show optical and magnetic tunability of KBiFe2O5 thin film with increasing annealing temperature, respectively. These results are helpful in the deeper understanding of relation between crystal structure and physics in perovskite-like oxides and show such materials have promising applications in solar-energy devices and multiferroics.
Acknowledgments This work was supported by the National Natural Science Foundation of China (61474045), the State Key Basic Research Program of China (2013CB922300).
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X. Zhai et al. / Materials Letters 161 (2015) 423–426
Fig. 4. (a)–(c) The magnetization hysteresis (M–H) loops of the KBiFe2O5 films. (d) Mr and Hc as functions of thin film annealing temperature.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
W. Eerenstein, N.D. Mathur, J.F. Scott, Nature 442 (2009) 759. S.W. Cheong, M. Mostovoy, Nat. Mater. 6 (2007) 13. L.J. Zhai, H.Y. Wang, Eur. Phys. J. B 87 (2014) 250. M. Staruch, L. Kuna, A. McDannald, M. Jain, J. Magn. Magn. Mater. 377 (2015) 117–120. L. Tan, L. Yang, X. Gu, W. Jin, L. Zhang, N. Xu, J. Membr. Sci. 230 (2004) 21–27. S. Shin, M. Yonemura, H. Ikawa, Mater. Res. Bull. 13 (1978) 1017. J.B. Goodenough, J.E. Ruiz-Diaz, Y.S. Zhen, Solid State Ion. 44 (1990) 21. S. Tanasescu, N.D. Totir, D.I. Marchidan, Solid State Ion. 134 (2000) 265–270. G.H. Zhang, H. Wu, G.B. Li, Q.Z. Huang, C.Y. Yang, F.Q. Huang, F.H. Liao, J.H. Lin,
Sci. Rep. 3 (2013) 1265. [10] W.L. Zhou, H.M. Deng, L. Yu, P.X. Yang, J.H. Chu, J. Appl. Phys. 117 (2015) 194102. [11] J. Tauc, R. Grigorovici, A. Vancu, Phys. Status Solidi 15 (1966) 627–637. [12] W.L. Zhou, H.M. Deng, H.Y. Cao, J. He, J. Liu, P.X. Yang, J.H. Chu, Mater. Lett. 44 (2015) 94. [13] R.J. Zeches, M.D. Rossell, J.X. Zhang, A.J. Hatt, Q. He, C.H. Yang, A. Kumar, C. H. Wang, A. Melville, C. Adamo, N.A. Spaldin, L.W. Martin, R. Ramesh, Science 326 (2009) 977–980. [14] M. Haruta, H. Kurata, K. Matsumoto, S. Inoue, Y. Shimakawa, S. Isoda, J. Appl. Phys. 110 (2011) 033708.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Optical and magnetic properties of KBiFe2O5 thin films fabricated by chemical solution deposition Xuezhen Zhai a, Hongmei Deng b, Wenliang Zhou a, Pingxiong Yang a,n, Junhao Chu a, Zhi Zheng c a Key Laboratory of Polar Materials and Devices, Ministry of Education, Department of electronics, East China Normal University, 500 Dongchuan Road, Shanghai 200241, China b Instrumental Analysis and Research Center, Institute of Materials, Shanghai University, 99 Shangda Road, Shanghai 200444, China c Key Laboratory for Micro-Nano Energy Storage and Conversion Materials of Henan Province, Institute of Surface Micro and Nano Materials, Xuchang University, Henan 461000, China
art ic l e i nf o
a b s t r a c t
Article history: Received 7 July 2015 Received in revised form 2 September 2015 Accepted 4 September 2015 Available online 5 September 2015
KBiFe2O5 thin films grown on quartz substrates by a chemical solution deposition method have been investigated by means of optical and magnetic measurements. The effects of synthesized temperature and the molar ratios of K/Bi on the phase formation of KBiFe2O5 thin film were assessed. Microstructural characterizations using X-ray diffraction and scanning electron microscopy indicate that the sample annealed at 750 °C shows better crystallinity. Optical transmittance measurement and magnetization curves show optical and magnetic tunability of KBiFe2O5 thin film with increasing growth temperature, respectively, where the possible physical mechanisms were provided and discussed. These results are helpful in the deeper understanding of relation between crystal structure and physics in perovskite-like oxides and show the potential role, such materials can play, in solar-energy devices and multiferroic applications. & 2015 Elsevier B.V. All rights reserved.
Keywords: Multiferroics KBiFe2O5 Magnetic materials Sol–gel preparation Thin films
1. Introduction In recent years, multiferroics are regarded as an intelligent material owing to its simultaneous effects of ferroelectricity and ferromagnetism [1–2]. A lot of efforts on multiferroic materials concentrate on ABO3 perovskite structures, which is composed of a three-dimensional framework of corner-sharing BO6 octahedra [3– 5]. But, it is a major challenge for obtaining multiferroics that two order parameters are both either strong at any temperature or nonzero at room temperature and above. To address the above issues, some studies on brownmillerite structure A2B2O5 provide an alternative approach to prepare multiferroic materials due to its characteristic oxygen-deficient perovskite structure [6]. A2B2O5 is composed of perovskite-like three-dimensional framework of corner-sharing BO6 octahedra alternating with slabs containing rows of corner-sharing BO4 tetrahedra, formed by the deficiency of oxygen during the formation of the structure [7–8]. As indicated in the literature [9], the coexistence of electric and magnetic properties could be achieved in KBiFe2O5 crystal structure which contains tetrahedral Fe3 þ in a n
Corresponding author. E-mail address: [email protected] (P. Yang).
http://dx.doi.org/10.1016/j.matlet.2015.09.018 0167-577X/& 2015 Elsevier B.V. All rights reserved.
[Fe2O3] block that alternates with a [(K, Bi)O2] block. In addition, this new multiferroic KBiFe2O5 features a narrow band gap (Eg) without losing the useful ferroelectricity, which is attributed to its smaller coordination number and the inverted t2g/eg orbitals of tetrahedra with lack of inversion symmetry. Compared with the nanopowders, thin films occupy more important position in the field of actual device preparations. However, experiments on KBiFe2O5 thin films have not been conducted yet. In this paper, KBiFe2O5 thin films were deposited on the quartz substrates by a chemical solution deposition method. The microstructure, surface morphologies, optical and magnetic properties were analyzed. The effects of growth temperature and the ration of K/Bi on the phase formation of KBiFe2O5 thin film as well as the physical properties were further discussed. We hope that our work will inspire more experimental explorations on KBiFe2O5 as a candidate material for use in multiferroic and solar-energy applications.
2. Experimental section The multiferroic KBiFe2O5 films were deposited on quartz substrates by a sol–gel process. Analytically pure C2H3KO2, Bi (NO3)3 5H2Oand Fe (NO3)3 9H2O were used as starting materials.
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The starting materials were dissolved in CH3COOH and C2H6O2 mixture and the concentration of the final solution was adjusted to 0.25 mol/L. The precursor solution was spin-coated on quartz substrate at 6000 rpm for 30 s. The thermal treatments of thin films were carried out in a rapid thermal processor. The spincoating and thermal treatments were repeated several times until the thicknesses of thin films are 200 nm. The crystalline structure of the KBiFe2O5 films was investigated by X-ray diffraction (XRD, Bruker D8 Advance) with Cu-Kα radiation source. The surface morphologies were characterized by scanning electron microscopy (SEM, Philips XL30FEG). The magnetic properties of the samples were investigated by physical property measurement system (PPMS-9, Quantum Design). The optical properties were characterized by transmittance spectra (PerkinElmer UV/VIS Lambda 2 S). All measurements were performed at room temperature.
3. Results and discussion Room-temperature XRD patterns of the as-prepared samples at different annealing temperatures are shown in Fig. 1(a). These XRD data show the films to be consistent with a polycrystalline like-perovskite structure, which is similar to Zhang's results [9], at least within the detection limits of the instrument. In Fig. 1(a), a slight impurity peaks marked by special symbol have been observed in KBiFe2O5 film annealed at 650 °C, which corresponds to K2FeO4. The absence of impurity phases for samples at 750 and 850 °C indicates that appropriately increased annealing temperature can improve the crystallinity of KBiFe2O5. Moreover, it is notable that the sample exhibited a phase transition behavior with the increase of the temperature from 750 °C to 850 °C, which may be attributed to the strong interface effects [10]. Fig. 1 (a)–(c) show SEM images of the thin films with smooth surfaces. The films annealed at 650 and 750 °C are well crystallized since favorable grains can be identified clearly. However, the surface morphology for the sample at 850 °C is ambiguous possibly because the surface is too smooth to be observed visible grain boundary. To confirm the effects of chemical composition of the KBiFe2O5 on its crystal-structure and surface morphology, the XRD patterns
and SEM images of the samples with different molar ratios of K/Bi were shown in Fig. 2. From the XRD results, the thin films were well crystallized but a weak impurity peak at 2θ ¼25° exists in the composition with K/Bi ¼ 0.95 and K/Bi ¼1.00, then disappears gradually with increasing K content, which might originates from the stronger volatility of element K than Bi. The EDX analysis results of the KBiFe2O5 thin films are summarized in inset table of Fig. 2a. It can be seen that chemical composition of sample K/ Bi¼1.05 is most close to the standard molar ratio 1:1:2 for Bi:K:Fe (in at%), which agrees with the above XRD analysis. These results show that annealing temperature 750 °C and molar ratio K/Bi ¼1:05 are the optimum condition for forming the pure KBiFe2O5 thin film in our experiment. SEM surface morphologies of three different compositions appear to be the same, indicating that the effect of K/Bi ratio is inferior to that growth temperature on the crystal growth. Fig. 3(a) shows the optical transmittance spectra of the KBiFe2O5 thin films in the wavelength range from 200 to 1500 nm. The spectra also indicates that the absorption edge is around 500 nm and the absorption edges of KBiFe2O5 films show a blue shift with the increasing growth temperature. The corresponding optical band-gaps can be estimated from the tangent lines in the plot of the Tauc’s law (αhv)2 versus hv for the direct band-gap material, where α is the absorption coefficient and hν is photon energy, as presented in Fig. 3(b) [11]. The slope of the linear part suggests the band gap of KBiFe2O5 is estimated to be 2.43, 2.54, and 2.57 eV for growth temperature 650, 750 and 850 °C, respectively. In the fundamental absorption edges region, the absorption is due to the band-to-band transition from the top of valence band to the bottom of conduction band directly. The valence band and conduction band of KBiFe2O5 are composed of O 2 p and Fe 3d (egt2g) states, respectively [9]. Compared with BiFeO3[12], the KBiFe2O5 film presents a sharply lower band gap, which is attributed to a distorted crystal field at the tetrahedral environment leading to the further splitting of the t2g and eg and their orbital inversions [9]. Finally, the measured magnetization versus magnetic field (M– H) curves of the samples at room temperature, are shown in Fig. 4. In crystal-structure of KBiFe2O5, the antiparallel array of nearest neighbor Fe3 þ spins give rise to net magnetic moments. Compared with the KBiFe2O5 powders [9], thin films exhibit larger
Fig. 1. (a) XRD patterns of the KBiFe2O5 thin films with annealing temperature of 650, 750 and 850 °C, respectively; and (b)–(d) SEM micrographs corresponding to the samples.
X. Zhai et al. / Materials Letters 161 (2015) 423–426
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Fig. 2. (a) XRD patterns of the KBiFe2O5 thin films with different molar ratios of K/Bi; (b)-(d) SEM surface morphologies corresponding to the films, respectively. The inset table of (a) illustrates the composition ratios of the KBiFe2O5 thin films with different molar ratios of K/Bi at annealing temperature of 750 °C.
Fig. 3. (a) Optical transmittance spectra of the as-grown KBiFe2O5 thin films. (b) Plots of (αhν)2 versus hν for KBiFe2O5 thin films with annealing temperature of 650–850 °C.
magnetization and clearer hysteresis loops at room temperature. It may be attributed to the strong interface strain-induced more structure distortion [13]. All films exhibit weak ferromagnetic behavior which is further improved by modifying the growth temperature. Only the M–H curves corresponding to 750 °C show the saturation tendency, and the saturation magnetization (Ms), 1.1 emu/cm3, has been also observed. The observed remanent magnetization (Mr) and coercive filed (Hc) versus growth temperature (T) relation shown in Fig. 4(d) can be explained considering the modification of the short-range G-type antiferromagnetic order [14], namely, Fe3 þ may have three different spin state configuration; high spin (HS, S ¼5/2, t2g3eg2), intermediate spin (IS, S¼ 3/2, t2g4eg1) and low spin (LS, S ¼1/2, t2g5eg°). The comparable crystal field splitting energy and spin–spin exchange splitting energy leads to spin state transition. The thin film annealed at 750 °C shows a better hysteresis loop due to Fe3 þ HS state in KBiFe2O5, which further confirms that 750 °C is the most suitable temperature to synthesize high-performance KBiFe2O5 films.
4. Conclusions In summary, KBiFe2O5 thin films have been grown on quartz substrates by a chemical solution deposition method. The effects of growth temperature and the molar ratios of K/Bi on the phase formation of KBiFe2O5 thin film have also been studied. Optical transmittance measurement and magnetization curves show optical and magnetic tunability of KBiFe2O5 thin film with increasing annealing temperature, respectively. These results are helpful in the deeper understanding of relation between crystal structure and physics in perovskite-like oxides and show such materials have promising applications in solar-energy devices and multiferroics.
Acknowledgments This work was supported by the National Natural Science Foundation of China (61474045), the State Key Basic Research Program of China (2013CB922300).
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X. Zhai et al. / Materials Letters 161 (2015) 423–426
Fig. 4. (a)–(c) The magnetization hysteresis (M–H) loops of the KBiFe2O5 films. (d) Mr and Hc as functions of thin film annealing temperature.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
W. Eerenstein, N.D. Mathur, J.F. Scott, Nature 442 (2009) 759. S.W. Cheong, M. Mostovoy, Nat. Mater. 6 (2007) 13. L.J. Zhai, H.Y. Wang, Eur. Phys. J. B 87 (2014) 250. M. Staruch, L. Kuna, A. McDannald, M. Jain, J. Magn. Magn. Mater. 377 (2015) 117–120. L. Tan, L. Yang, X. Gu, W. Jin, L. Zhang, N. Xu, J. Membr. Sci. 230 (2004) 21–27. S. Shin, M. Yonemura, H. Ikawa, Mater. Res. Bull. 13 (1978) 1017. J.B. Goodenough, J.E. Ruiz-Diaz, Y.S. Zhen, Solid State Ion. 44 (1990) 21. S. Tanasescu, N.D. Totir, D.I. Marchidan, Solid State Ion. 134 (2000) 265–270. G.H. Zhang, H. Wu, G.B. Li, Q.Z. Huang, C.Y. Yang, F.Q. Huang, F.H. Liao, J.H. Lin,
Sci. Rep. 3 (2013) 1265. [10] W.L. Zhou, H.M. Deng, L. Yu, P.X. Yang, J.H. Chu, J. Appl. Phys. 117 (2015) 194102. [11] J. Tauc, R. Grigorovici, A. Vancu, Phys. Status Solidi 15 (1966) 627–637. [12] W.L. Zhou, H.M. Deng, H.Y. Cao, J. He, J. Liu, P.X. Yang, J.H. Chu, Mater. Lett. 44 (2015) 94. [13] R.J. Zeches, M.D. Rossell, J.X. Zhang, A.J. Hatt, Q. He, C.H. Yang, A. Kumar, C. H. Wang, A. Melville, C. Adamo, N.A. Spaldin, L.W. Martin, R. Ramesh, Science 326 (2009) 977–980. [14] M. Haruta, H. Kurata, K. Matsumoto, S. Inoue, Y. Shimakawa, S. Isoda, J. Appl. Phys. 110 (2011) 033708.