Impedance spectroscopy and ferromagnetic properties of Bi0.8Gd0.2FeO3 multiferroics

Journal of Magnetism and Magnetic Materials 435 (2017) 154–161

Contents lists available at ScienceDirect

Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm

Impedance spectroscopy and ferromagnetic properties of Bi0.8Gd0.2FeO3 multiferroics Yahui Tian a, Fei Xue b, Qiuyun Fu a,⇑, Dongxiang Zhou a, Yunxiang Hu a, Ling Zhou a, Zhiping Zheng a, Zengnian Xin a a b

School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, PR China Center of Collaboration and Innovation, Jiangxi University of Technology, Nanchang, Jiangxi 330098, PR China

a r t i c l e

i n f o

Article history: Received 9 September 2016 Received in revised form 19 November 2016 Accepted 11 March 2017 Available online 29 March 2017

a b s t r a c t Multiferroic Bi0.8Gd0.2FeO3 (BGFO) ceramics were prepared by a rapid-liquid phase sintering process. BGFO ceramics can be sintered at a sintering temperature range of 875 °C–940 °C and shown a pure orthorhombic (space group, Pnma) structure. The crystal symmetry and lattice parameters were determined from the Rietveld analysis for the experimental data. BGFO ceramics sintered at 900 °C exhibited high theoretical relative density (
98%), strong ferroelectricity and good magnetism. BGFO ceramics exhibited the similar dielectric relaxation properties to the typical relaxor ferroelectrics. The role of oxygen vacancies at high temperature in dielectric and ac conductivity behavior was also discussed. The diffusing of structure defects between the grain and grain boundary was established using Impedance Spectroscopy (IS). Ó 2017 Elsevier B.V. All rights reserved.

1. Introduction The coupling of magnetism and ferroelectricity in multiferroics is defined as magnetoelectric effect, which opens up the possibilities of designing novel applications for multiferroics in sensors, transducers, memory devices, actuators, magnetoelectrics, spintronics, and photovoltaic devices etc [1,2]. As a promising multiferroic material, BiFeO3 (BFO) has been attracting enormous attention, its ferroelectricity stems from the 6s2 lone pair of Bi3+, and the magnetism originates in the partially filled 3d orbitals of the Fe3+ ions [3–6]. However, some challenges in BFO need to overcome, such as, metastable in air, very prone to show parasitic phases and impurities, pure electric insulation, and very weak macroscopic ferromagnetic moment (ca. 106 lB per Fe ion) [5]. To suppress the formation of impurities, improve the electric insulation, and enhance the magnetoelectric characteristics in BFO, many strategies have been proposed, including rapid liquid-phase sintering, air quenching, and element substitution [6,7–13]. Among them, rapid-liquid-phase sintering process is a good method to improve the electric insulation and suppress the formation of impurity phases. While, substituting Bi3+ ions with lanthanide ions is an

⇑ Corresponding author. E-mail address: [email protected] (Q. Fu). http://dx.doi.org/10.1016/j.jmmm.2017.03.024 0304-8853/Ó 2017 Elsevier B.V. All rights reserved.

effectively method to improve the magnetization [14–16], and the origin of spontaneous magnetization in lanthanides doped BFO has been discussed in terms of a composition driven structural evolution [6,7]. For the selected Gd3+ elements in this letter, the ionic radii in 8-fold coordination are quite small (1.053 Å) and the effect magnetic moment (7.94 lB) is quite high comparing to other lanthanide elements. Besides, at a proper concentration (x = 0.20), Bi1xGdxFeO3 ceramics reveal a structure evolution from rhombohedral phase (for BFO ceramics) to orthorhombic phase [14,17]. Thus, we expect for a high magnetization in Gd3+ doped BFO ceramics. Meanwhile, GdFeO3 is a very interesting compound with antiferromagnetic transition at TN
661 K, which is typically used for terahertz sensors, frequency tunable terahertz lasers, and magneto-optical data storage [18–21]. Thus, with the introduction of Gd3+ magnetic ions into BFO, a number of attractive physical phenomena will be found. However, there are few reports on the dielectric relaxation, ac conductivity, impedance behavior, and ferromagnetic properties for Bi1xGdxFeO3 system. Thus, in this work, we studied the orthorhombic Bi0.8Gd0.2FeO3 (BGFO) ceramics. BGFO ceramics were prepared by rapid-liquid phase sintering, and their structure, impedance, ac conductivity, and ferromagnetic behaviors were investigated. It was found that BGFO ceramics sintered at 900 °C exhibited good electric insulation, and appeared to be a typical relaxor ferroelectric. The calculated activation energy for dielectric relaxation and conduction

Y. Tian et al. / Journal of Magnetism and Magnetic Materials 435 (2017) 154–161

revealed the second-ionization of oxygen vacancies. Through impedance spectroscopy (IS), the heterogeneity in the electrical microstructure of the ceramics was studied, which leads to high dielectric constant in BGFO ceramics.

2. Experimental Ceramic samples of Bi0.8Gd0.2FeO3 (BGFO) were prepared by rapid-liquid phase sintering process using high-purity Bi2O3, Gd2O3, and Fe2O3 regents. The starting materials were weighed according to the stoichiometric ratio of BGFO, and first mixed thoroughly in alcohol using zirconia for 6 h. After drying at 80 °C for 12 h, these mixtures were calcined at 800 °C for 2 h. The obtained mixtures were ball-milled again for 6 h and mixed thoroughly with a polyvinyl alcohol binder solution. The powders were then pressed into 10 mm diameter and 0.8 mm thickness of discs at 35 MPa. These discs were sintered at 825 °C–960 °C for 2 h in air with a heating rate of 600 °C /h and directly cooled in the air to synthesize BGFO ceramics. Finally, the ceramic samples were polished to a thickness of
0.3 mm, and the silver electrodes were fired on both surfaces of the sintered ceramics at 550 °C for 30 min. The crystalline structure of the ceramic samples was determined using X-ray diffraction (XRD) analysis with CuKa radiation (Empyrean, PANalytical B.V.). The measured XRD patterns were analyzed by the Rietveld method using the software named General Structure Analysis System (GSAS). The microstructures were observed using a field-emission scanning electron microscope (FESEM, Nova NonoSEM 450). The density q was measured by the Archimedes method. The leakage current density J and ferroelectric hysteresis (PE) loops were measured at room temperature using a ferroelectric measuring system (Multiferroic, Radiant Technologies, Inc.). Impedance spectroscopy was performed using an Agilent 4294A impedance analyzer and VDMS-2000 measuring system. The magnetic hysteresis loop was measured by vibrating sample magnetometer (VSM, Lakeshore 7400).

155

3. Results and discussion The dependence of the X-ray diffraction (XRD) patterns of the Bi0.8Gd0.2FeO3 (BGFO) ceramic samples on sintering temperature (Ts) are shown in Fig. 1. From Fig. 1(a), all the samples show an orthorhombic structure with space group Pnma (JCPDS number of 71-4394). At lower Ts (<875 °C), there are impurity phases identified as Bi2Fe4O9 (JCPDS number of 20–0836) and Bi25FeO39 (JCPDS number of 78–1543). Besides, at higher Ts (960 °C), small trace of Bi2Fe4O9 impurity phases was also detected. Meanwhile, the peak of (121) increased obviously with the increasing in Ts (Fig. 1(b)), which means the increment of the grain size. In order to further analyze the effect of sintering temperature (TS) on the structure of BGFO ceramics, Rietveld method is used to refine the lattice parameters (a, b, c, and V), which bases on the orthorhombic structure. The TS dependences of lattice parameters and fitting parameters (Rwp, Rp and v2) for BGFO ceramics derived from the refinement are summarized in Table 1. As can be seen from Fig. 2, small values of reliability Rwp of 3.36%–7.61% ( 15%) and goodness-of-fit parameters v2 of 0.4072–1.774 (<2) are obtained for all the refinement, which means a good matching between the experimental and theoretical spectra. The refined lattice parameters b, c, and V decrease from 7.8243 Å/3.4363 Å/239.30 Å3 to 7.8057 Å/5.4309 Å/238.79 Å3, while parameter a increases from 5.6260 Å to 5.6330 Å with TS increasing from 825 °C to 960 °C. Fig. 3(a)–(c) shows the field emission scanning electron microscope (FESEM) images of the BGFO ceramics sintered at 850 °C, 900 °C, and 960 °C. As can be seen, BGFO ceramics sintered at 900 °C have the least porosities, while best crystallization. The grain size for BGFO ceramics is about 1–2 lm, and has the increasing trend with the increase of TS. The relative density qr of BGFO ceramics is measured using the Archimedes’ method, and the dependence of qr on the sintering temperature is shown in Fig. 3 (d). A highest qr (8.26  103 kg/m3) is achieved for BGFO ceramics sintered at 900 °C, which reaches almost 98% of the theoretical

Fig. 1. XRD patterns of Bi0.8Gd0.2FeO3 ceramics sintered at 825–960 °C.

156

Y. Tian et al. / Journal of Magnetism and Magnetic Materials 435 (2017) 154–161

Table 1 Lattice parameters (a, b, c and V) and fitting parameters (Rwp, Rp and v2) of BGFO ceramics sintered at 825 °C960 °C. Lattice parameters

R factors (%) 3

TS

a (Å)

b (Å)

c (Å)

V (Å )

Rwp

Rp

v2

825 °C 850 °C 875 °C 900 °C 920 °C 940 °C 960 °C

5.6260 5.6300 5.6307 5.6312 5.6311 5.6315 5.6330

7.8243 7.8109 7.8098 7.8091 7.8093 7.8084 7.8057

5.4363 5.4345 5.4334 5.4332 5.4330 5.4326 5.4309

239.30 238.98 238.93 238.92 238.92 238.89 238.79

7.61 4.85 3.36 4.22 3.50 3.71 4.14

4.89 3.32 2.60 3.07 2.56 2.64 2.89

1.7740 1.2340 0.8547 0.9734 0.8072 0.8833 0.9960

Fig. 2. Observed (hollow circles), calculated (solid line), and difference (solid line at the bottom) XRD patterns for Bi0.8Gd0.2FeO3 ceramics sintered at 825–960 °C. Bragg reflections are indicated by ticks.

density (qr = 8.43  103 kg/m3). It is evident from Figs. 1 and 2 that the BGFO ceramics can be sintered at the temperature range of 875 °C940 °C. Fig. 4(a) shows hysteresis loop (PE) for the BGFO ceramics sintered at 900 °C. As can be seen, the BGFO ceramics sintered at 900 °C show the best polarization properties, possessing a

maximum polarization of 4 lC/cm2 and remanent polarization of 1.7 lC/cm2. The measured polarization values are quite higher than that of pure and other elements doped BFO ceramics, ascribing to the high relative density (98%) and low leakage current density (
2  10–6 A/cm2) in BGFO ceramics. For pure and other doped BFO ceramics, it was difficult to measure the ferroelectric loops

Y. Tian et al. / Journal of Magnetism and Magnetic Materials 435 (2017) 154–161

157

Fig. 3. Cross-sectional FSEM micrograph of Bi0.8Gd0.2FeO3 ceramics sintered at (a) 850 °C, (b) 900 °C and (c) 960 °C. (d) Theoretical relative density of Bi0.8Gd0.2FeO3 ceramics sintered at 825–960 °C.

Fig. 4. (a) Hysteresis loop (P–E) and (c) magnetization vs magnetic field curves of Bi0.8Gd0.2FeO3 ceramics sintered at 900 °C. The inset of (a) shows the leakage current density (J–E).

under high applied fields, and leakage P(E) hysteresis loops are often reported [7,14]. Fig. 4(b) shows the magnetization data for the BGFO ceramics sintered at 900 °C. From the Fig. 4(b), the BGFO ceramics sintered at 900 °C show large coercivity (Hc) with 490 Oe, remanent magnetization with 40 memu/g and maximum magnetization with 0.4 emu/g. By comparison, the reported pure BFO ceramics present typical antiferromagnets without any Mr and with zero Hc [14,16]. The improved Mr in BGFO ceramics can be explained as: (a) The

increase in the induced magnetic moment, caused by the structure evolution (Table 1); (b) the additional magnetic moments introduced by Gd3+ ions (7.94 lB). [13,22,23]. Since the BGFO ceramics sintered at 900 °C possessed high density and good electric, magnetic properties, we studied their dielectric properties in details. Fig. 5(a) shows the dielectric constant (er) versus temperature (T) for the BGFO ceramics sintered at 900 °C with different frequencies. As can be seen, a very diffuse dielectric pattern is observed in the temperature range of 522 K–663 K, with

158

Y. Tian et al. / Journal of Magnetism and Magnetic Materials 435 (2017) 154–161

Fig. 5. (a) Temperature dependences of er at 1 kHz2 MHz, (b) plots of ln (1/er–1/em) versus ln (T–Tm) at 1 kHz2 MHz for Bi0.8Gd0.2FeO3 ceramics sintered at 900 °C.

a very high dielectric constant peak (on the order of 103), which is attributed to the Currie transition. The phase transition temperature (Tm) is corresponded to the dielectric constant peak (em), which increases significantly with the increase of frequency. We also found the same phenomenon in other relaxor ferroelectrics, such as Pb(Fe0.66W0.33)0.80Ti0.20O3 (PFWT) [24]. The degree of the diffuseness c (1  c  2) for the phase transition can be described by (1/e–1/em) = C–1(T–Tm)c [25], where C is the Curie-like constant. Theoretically, c = 1 and c = 2 stand for normal ferroelectric and ideal relaxor ferroelectric, respectively. Fig. 5 (b) shows the curve of ln(1/e–1/em) as a function of ln(T–Tm) for the BGFO ceramics sintered at 900 °C in the frequency range of 1 kHz– 2 MHz, and the fitting results are summarized in Table 2. As can be seen, the obtained c (1.7–1.9) is close to 2, which match well with those of typical relaxor ferroelectric, implying relaxor character of BGFO ceramics [26,27]. To explain the diffusive phase transition in relaxor ferroelectrics, a variety of theoretical models were proposed, and the most cited is the compositional fluctuation of

Table 2 Tm, em, c and C, of the Bi0.8Gd0.2FeO3 ceramics sintered at 900 °C. Frequency

Tm (K)

em

c

C (K)

1 kHz 10 kHz 100 kHz 500 kHz 1 MHz 2 MHz

522 557 602 634 648 663

3869 2695 1823 1158 882 650

1.708 ± 0.007 1.828 ± 0.011 1.872 ± 0.005 1.949 ± 0.027 1.924 ± 0.037 1.875 ± 0.038

2.51  107 2.34  107 8.57  106 9.73  106 7.90  106 6.65  106

Smolenskii’s [28,29]. According to the theory of Smolenskii [29], the relaxor behavior in BGFO ceramics comes from the disorder distributions of Bi3+ and Gd3+ (or Fe2+ and Fe3+) ions on the crystallographically equivalent sites. These disorder distributions causes for different polar microregions with different Currie temperatures, leading to the origin of relaxor behavior in the BGFO crystal, and Tm is the average Currie temperature [24]. In order to further analyze the conduction process in these samples, complex impedance (Z = Z0  jZ00 ) and ac conductivity (rac) are presented. Fig. 6(a) plots the frequency dependence of the normalized imaginary part (Z00 /Z00 max) of the impedance for BGFO ceramics sintered at 900 °C. As can be seen, a single Debye-like peak frequency (fmax) was observed, and we define the inverse of fmax as relaxation time (s = 1/2pfmax). Fig. 6(b) shows the plots of ln(s) vs 1/T, and the value of s can be derived from the Arrhenius equation s = s0exp(ER/kBT) [30], where, s0 and kB are constant, ER is the activation energy. The ER and s0 values of the dielectric relaxation are calculated to be 1.059 eV and 1.08  10–15 s, respectively. The calculated ER value for BGFO ceramics suggests the involvement of oxygen vacancies V 0 (the activation energy for V 0 is 0.7–1.1 eV) at high temperatures [31]. Fig. 7(a) shows the ac conductivity (rac) versus frequency for the BGFO samples sintered at 900 °C. The value of rac can be derived from the following equation [32]:

rac ¼

Z0 02

Z þ Z 002



t ¼ xe0 e00 ¼ xe0 er tan d; A

ð1Þ

Where, t and A are the thickness and area of BGFO samples, respectively, and x is the angular frequency (x = 2pf). AC conductivity consists two terms [33–35]

Fig. 6. (a) Frequency dependence of imaginary part (Z00 /Z00 max) of impedance, (b) inverse of the peak frequency (relaxation time s) as a function of temperature.

159

Y. Tian et al. / Journal of Magnetism and Magnetic Materials 435 (2017) 154–161

Fig. 7. (a) ac conductivity plotted for Bi0.8Gd0.2FeO3 ceramics sintered at 900 °C, as a function of frequency at varying temperatures in the range of 300600 °C, (b) dc conductivity vs 1/T plots, and (c) the exponent value (n) obtained from Jonscher power law.

rac ðTÞ ¼ r1 ðTÞ þ r2 ðTÞ

ð2Þ

The first term is frequency-independent conductivity, called dc conductivity rdc, which is only affected by temperature. The fitted rdc values at different temperature for the BGFO ceramics sintered at 900 °C follow the Arrhenius law Arrhenius law rdc (T) = r0exp (Ea/kBT) = r0exp(Ea/kBT) [31], where kB and r0 are constant, Ea is the activation energy of conduction. As shown in Fig. 7(b), two linear relations can be fitted in low- and high-temperature regions using Eq. (1), indicating different conduction mechanisms over different temperature ranges, and the division temperature is
440 K. The calculated Ea at low temperature region is 0.134–0.313 eV, which is too low to be the mobility of ionic (e.g. oxygen vacancies at 0.7–1.1 eV) in the space charge regions, but is electronic [31,36,37]. While, the calculated Ea = 0.7–0.9 at high temperature region (440 K) is comparable to the activation energy of dielectric relaxation ER. So, at higher temperature, the increasing of dc conductivity is contributed from the motivation of oxygen vacancies. The second term is the temperature and frequency dependent ac conductivity, which is related to the dielectric relaxation caused by the localized charge carriers and following a power law r2(x, T) = B(T)xn(T) [35], where B is a parameter having unit of conductivity and n is frequency exponent. The value of n in BGFO samples obtained from the second term decreases from 0.76 to 0.23 with the increase in temperature range 300–380 K and increases from 0.23 to 1.49 with increase in temperature in the range 380–600 K (Fig. 3(c)). As been reported, a minimum frequency exponent n followed by an increase with rising temperature can be explained by OLPT (Overlapping Large Polaron Tunneling) model. [34,35] Fig. 8 shows the Z0 –Z00 plots of BGFO ceramics sintered at 900 °C. From Fig. 8, two well resolved semicircles are obtained in the whole frequency range for BGFO ceramics at 480 K and 560 K,

showing Maxwell-Wagner (M-W) type of relaxation behavior. And the contribution of the impedance at the two semicircles is arisen from the grains and grain boundaries, respectively. As been reported, this behavior could be well modeled only by invoking an equivalent circuit consisting of a series of two constant phase elements (CPE) connected in parallel to the resistors (shown in the inset of Fig. 8(a)) [38]. The frequency dependent capacitance associated with a CPE element is given by C(x) = [A(jx)–a] [35], where, A is a constant and a is the distribution of relaxation times. Accordingly, a = 0 represents an idea capacitor behavior and a values > 0 are attributed to the capacitance dispersion owing to the electric heterogeneity in the sample or due to the electrode polarization behavior (called M-W effect). And the series resistor (RS) in the inset of Fig. 8(a) is added to the circuit to account for the nonzero intercept on the real axis of the impedance plot. The expression for Z⁄ that used to model this impedance behavior is given as follows [39]: 00

Z  ðxÞ ¼ Z 0 ðxÞ  jZ ðxÞ ¼ 1þjxRggC g ðxÞ þ 1þjxR gbC R

R

gb gb ðxÞ

¼ 1þA

Rg 1 ðjxÞ

ð1a1 Þ

Rg

þ 1þA

Rgb 2 ðjxÞ

ð1a2 Þ

ð3Þ

Rgb

where, Rg and Rgb are grain and grain boundary resistances, respectively. An impedance analysis software named Z-View (demo version 3.1) was used to model the impedance spectra based on Eq. (3) and the capacitances (C1, C2), resistances (R1, R2), and the exponent values (a1, a2) were directly obtained from the fit. The fitting results for the impedance spectrum are summarized in Table 3. As can be seen, the grain and grain boundary resistances decrease and correspond to each other with the rising temperature. It means that the oxygen vacancies are ionized and motivate with the rising temperature, and resulting to the diffusing of structure defects (V 0 ) between the grain and grain boundary.

160

Y. Tian et al. / Journal of Magnetism and Magnetic Materials 435 (2017) 154–161

Fig. 8. (a) and (b) Impedance spectroscopy (Z0 –Z00 ) for Bi0.8Gd0.2FeO3 ceramics sintered at 900 °C, measured at 480 K and 560 K, respectively. Inset of (a) shows an equivalent circuit for the samples. Inset of (b) shows the temperature dependence of resistivity of the samples with the grains and grain boundaries.

Table 3 The capacitances (Cg, Cgb), resistances (Rg, Rgb), and the exponent values (a1, a2) were calculated from impedance spectra (Z0 Z00 ) for the Bi0.8Gd0.2FeO3 ceramics sintered at 900 °C. Temperature

Cg (nC)

Cgb (nC)

Rg (kX)

Rgb (kX)

a1

a2

v2 (%)

480 K 500 K 520 K 540 K 560 K

0.204 0.145 0.202 0.191 0.899

12.26 11.12 7.20 13.7 4.83

2.77 1.40 1.01 0.56 0.474

76.00 27.55 10.62 4.82 2.03

0.01 0 0.03 0.02 0.11

0.22 0.20 0.14 0.19 0.09

0.79 0.75 0.86 0.90 0.36

4. Conclusions The single-phase BGFO multiferroic ceramics were prepared by a rapid-liquid phase sintering process and the microstructure, impedance, ac conductivity, and ferromagnetic properties were studied. The XRD results show that Gd3+ has successfully diffused into the BiFeO3 lattices and an orthorhombic phase of BGFO is obtained. At the sintering temperature of 900 °C, BGFO ceramics exhibited high density, high resistivity, strong ferroelectricity, improved ferromagnetism. An activation energy value of 1.059 eV was reached for dielectric relaxation. For BGFO sintered at 900 °C, the conduction is dominated by the mobility of electronic in the space charge regions at low temperatures, while the motion of oxygen vacancies at high temperatures. Acknowledgments This work was supported by National Natural Science Foundation of China (Grant No. 61571203), National Key R&D Program of China, and National High Technology Research and Development Program of China (863 Program No. 2013AA030903). The authors acknowledge the assistance by the Analytical and Testing Center of Huazhong University of Science and Technology. References [1] M. Fiebig, Revival of the magnetoelectric effect, ChemInform 36 (2005) R123– R152. [2] M. Gajek, H. Bea, K. Bouzehouane, Spintronics with multiferroics, J. Phys.: Condens. Matter 20 (2008), 726726. [3] Y.H. Lin, Q.H. Jiang, Y. Wang, C.W. Nan, L. Chen, J. Yu, Enhancement of ferromagnetic properties in BiFeO3 polycrystalline ceramic by La doping, Appl. Phys. Lett. 90 (2007), 1725073.

[4] K.F. Wang, J.M. Liu, Z.F. Ren, Multiferroicity: the coupling between magnetic and polarization orders, Adv. Phys. 58 (2009) 321–448. [5] G. Catalan, J.F. Scott, Physics and applications of bismuth ferrite, Adv. Mater. 21 (2009) 2463–2485. [6] G.L. Yuan, S. Wing, J.M. Liu, Z.G. Liu, Structural transformation and ferroelectromagnetic behavior in single-phase Bi1xNdxFeO3 multiferroic ceramics, Appl. Phys. Lett. 89 (2006), 0529053. [7] V. Koval, I. Skorvanek, M. Reece, L. Mitoseriu, H. Yan, Effect of dysprosium substitution on crystal structure and physical properties of multiferroic BiFeO3 ceramics, J. Eur. Ceram. Soc. 34 (2014) 641–651. [8] V. Singh, S. Sharma, M. Kumar, R.K. Kotnala, R.K. Dwivedi, Structural transition, magnetic and optical properties of Pr and Ti co-doped BiFeO3 ceramics, J. Magn. Magn. Mater. 349 (2014) 264–267. [9] R. Das, K. Mandal, Magnetic, ferroelectric and magnetoelectric properties of Ba-doped BiFeO3, J. Magn. Magn. Mater. 324 (2012) 1913–1918. [10] S.T. Zhang, M.H. Lu, D. Wu, Y.F. Chen, N.B. Ming, Larger polarization and weak ferromagnetism in quenched BiFeO3 ceramics with a distorted rhombohedral crystal structure, Appl. Phys. Lett. 87 (2005), 2629073. [11] K.S. Nalwa, A. Garg, Phase evolution, magnetic and electrical properties in Smdoped bismuth ferrite, J. Appl. Phys. 103 (2008) 044101–44106. [12] V.R. Palkar, D.C. Kundaliya, S.K. Malik, S. Bhattacharya, Magnetoelectricity at room temperature in the Bi0.9-xTbxLa0.1FeO3 system, Phys. Rev. B Condens. Matter 69 (2004) 212102–212113. [13] D.H. Wang, W.C. Goh, M. Ning, C.K. Ong, Effect of Ba doping on magnetic, ferroelectric, and magnetoelectric properties in mutiferroic BiFeO3 at room temperature, Appl. Phys. Lett. 88 (2006) 212907–212913. [14] F. Xue, Q.Y. Fu, D.X. Zhou, Y.H. Tian, Y.X. Hu, Z.P. Zheng, L. Zhou, Properties of Bi0.8Ln0.2FeO3 (Ln=La, Gd, Ho) multiferroic ceramics, Ceram. Int. 41 (2015) 14718–14726. [15] H.F. Zhou, Z.L. Hou, L.B. Kong, H.B. Jin, M.S. Cao, X. Qi, Enhanced magnetization and improved leakage in Er-doped BiFeO3 nanoparticles, Phys. Status Solidi 210 (2013) 809–813. [16] X.Q. Zhang, Y. Sui, X.J. Wang, Y. Wang, Z. Wang, Effect of Eu substitution on the crystal structure and multiferroic properties of BiFeO3, J. Alloys Compd. 507 (2010) 157–161. [17] V.A. Khomchenko, D.A. Kiselev, I.K. Bdikin, V.V. Shvartsman, P. Borisov, W. Kleemann, J.M. Vieira, A.L. Kholkin, Crystal structure and multiferroic properties of Gd-substituted BiFeO3, Appl. Phys. Lett. 93 (2008), 2629053. [18] F. Söderlind, M.A. Fortin, R.M. Petoral, A. Klasson, T. Veres, M. Engström, K. Uvdal, P.O. Käll, Colloidal synthesis and characterization of ultrasmall perovskite GdFeO3 nanocrystals, Nanotechnology 19 (2008) 085608–85615.

Y. Tian et al. / Journal of Magnetism and Magnetic Materials 435 (2017) 154–161 [19] A. Bashir, M. Ikram, R. Kumar, P.N. Lisboa-Filho, Structural, electronic structure and magnetic studies of GdFe1xNixO3, X  0.5, J. Alloys Compd. 521 (2012) 183–188. [20] F. Söderlind, L. Selegård, P. Nordblad, K. Uvdal, P.O. Käll, Sol-gel synthesis and characterization of polycrystalline GdFeO3 and Gd3Fe5O12 thin films, J. Sol-Gel Sci. Technol. 49 (2009) 253–259. [21] L. Banwari, K.K. Bamzai, P.N. Kotru, B.M. Wanklyn, Microhardness, fracture mechanism and dielectric behaviour of flux-grown GdFeO3 single crystals, Mater. Chem. Phys. 85 (2004) 353–356. [22] V.A. Khomchenko, D.A. Kiselev, J.M. Vieira, L. Jian, A.L. Kholkin, A.M.L. Lopes, Y. G. Pogorelov, J.P. Araujo, M. Maglione, Effect of diamagnetic Ca, Sr, Pb, and Ba substitution on the crystal structure and multiferroic properties of the BiFeO3 perovskite, J. Appl. Phys. 103 (2008) 024105–24106. [23] Y. Wang, C.W. Nan, Enhanced ferroelectricity in Ti-doped multiferroic BiFeO3 thin films, Appl. Phys. Lett. 89 (2006), 0529033. [24] A. Kumar, I. Rivera, R.S. Katiyar, J.F. Scott, Multiferroic Pb(Fe0.66W0.33)0.80Ti0.2O3 thin film: a room-temperature relaxor ferroelectric and weak ferromagnetic, Appl. Phys. Lett. 92 (2008) 132913–132923. [25] Y. Wan, Y. Li, Q. Li, W. Zhou, Q.J. Zheng, X.C. Wu, C.G. Xu, B.P. Zhu, D.M. Lin, Microstructure, ferroelectric, piezoelectric, and ferromagnetic properties of Scmodified BiFeO3–BaTiO3 multiferroic ceramics with MnO2 addition, J. Am. Ceram. Soc. (2014) 1–10. [26] A.A. Bokov, Z.G. Ye, Recent Progress in relaxor ferroelectrics with perovskite structure, J. Mater. Sci. 41 (2006) 31–52. [27] G.A. Samara, The relaxational properties of compositionally disordered ABO3 perovskites, J. Phys.: Condens. Matter 15 (2003) R367–R411. [28] N. Ichinose, Aging properties of Pb(Mg1/3Nb2/3)O3 based relaxor ferroelectrics, Ferroelectrics 203 (1997) 187–199.

161

[29] G.A. Smolenskii, A.I. Agranovskaya, Dielectric polarization of a number of complex compounds, Sov. Phys. Solid State 1 (1959) 1429–1437. [30] X. Lv, X.Q. Liu, H.J. Zhao, W.Z. Yang, X.M. Chen, Dielectric and Magnetic Properties of Sr(Fe1/2Ta1/2)O3 Complex Perovskite Ceramics, J. Am. Ceram. Soc. 96 (2013) 1188–1192. [31] J. Wu, J. Wang, Ferroelectric and impedance behavior of La- and Ti-codoped BiFeO3 thin films, J. Am. Ceram. Soc. 93 (2010) 2795–2803. [32] Y.H. Lin, Q.H. Jiang, Y. Wang, C.W. Nan, L. Chen, J. Yu, Enhancement of ferromagnetic properties in BiFeO3 polycrystalline ceramic by La doping, Appl. Phys. Lett. 90 (2007), 172507–3. [33] L. Zhang, Electrode and grain-boundary effects on the conductivity of CaCu3Ti4O12, Appl. Phys. Lett. 87 (2005), 022907–3. [34] J. Kolte, A.S. Daryapurkar, D.D. Gulwade, P. Gopalan, Microwave sintered Bi0.90La0.10Fe0.95Mn0.05O3 nanocrystalline ceramics: impedance and modulus spectroscopy, Ceram. Int. 42 (2016) 12914–12921. [35] E.V. Gopalan, K.A. Malini, S. Saravanan, D.S. Kumar, Y. Yoshida, M.R. Anantharaman, Evidence for polaron conduction in nanostructured manganese ferrite, J. Phys. D Appl. Phys. 41 (2008) 185005–185009. [36] S.A. Long, R.N. Blumenthal, Ti-rich nonstoichiometric BaTiO3: II, analysis of Defect Structure, J. Am. Ceram. Soc. 54 (1971) 577–583. [37] A. Chen, Y. Zhi, L.E. Cross, Oxygen-vacancy-related low-frequency dielectric relaxation and electrical conduction in Bi: SrTiO3, Phys. Rev. B 62 (2000) 228– 236. [38] K.R.S. Preethi Meher, K.B.R. Varma, Colossal dielectric behavior of semiconducting Sr2TiMnO6 ceramics, J. Appl. Phys. 105 (2009) 034113–34118. [39] L. Qiao, X.F. Bi, Evaluation of magnetoelectric coupling in a BaTiO3–Ni composite ferroic film by impedance spectroscopy, Appl. Phys. Lett. 92 (2008) 214101–214103.