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Influence of Zr substitution on structure, electrical and magnetic properties of Bi0.9Ho0.1FeO3 ceramics
Results in Physics 14 (2019) 102489
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Influence of Zr substitution on structure, electrical and magnetic properties of Bi0.9Ho0.1FeO3 ceramics Guannan Lia, Haoting Zhaoa, Ruxia Yangb, Jianfeng Tanga, Chunmei Lia, Yuming Lub, a b
T
⁎
School of Materials and Energy, Southwest University, Chongqing 400715, China School of Physical Science and Technology, Southwest University, Chongqing 400715, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Multiferroic materials Ferroelectric Ferromagnetic Bismuth ferrite
Polycrystalline Bi0.9Ho0.1Fe1-xZrxO3 ceramics have been synthesized by standard solid state sintering method. The influences of Zr doping on structure, dielectric, ferroelectric and magnetic properties were investigated. The results of the XRD patterns suggest that all the samples have a rhombohedral structure (space group R3c). Structure refinement of the XRD patterns shows that the lattice parameters are increased as the Zr4+ concentration increasing. The average grain size of the samples decreases with Zr doping due to the suppressed of grain growth caused by Zr substitution. The dielectric loss is reduced and the remnant polarization (Pr) is decreased with increasing Zr concentration. As Zr doping level increases, the weak ferromagnetism becomes more obvious, and the remnant magnetization gradually increases and reaches the maximum value at x = 0.04.
Introduction Materials those exhibit two or more properties of ferroelectricity, ferromagnetism and ferroelasticity, also known as multiferroic materials, have attracted intense attention during the last two decades [1,2]. The most promising advantage of the occurrence of multiferroic could be utilized to control the magnetization vector by the external electric or control the polarization vector by the external magnetic field [3]. Because of the extraordinary performance, multiferroics have been applied widely in the areas of information storage, spintronic devices and sensors [4–6]. BiFeO3 (BFO) is one of the potential candidates for its high Curie temperature (1100 K) and high Néel temperature (640 K) [6,7]. It exhibits a rhombohedral perovskite (ABO3) structure with space group R3c and G-type antiferromagnetism. The theoretical calculation shows that the spontaneous polarization for BiFeO3 will reach 90–100 μC/cm2 [8]. Though scholars have done many studies on BiFeO3 ceramics, the gap still exists between the experimental and theoretical values. Beyond that, there are also some inherent problems, such as high leakage current, low remnant polarization and weak magnetoelectric coupling, which may restrict the applications of BiFeO3. In recent years, the site-engineering concept has been widely adopted to improve the multiferroic properties of BiFeO3. The results indicate that rare earth ions (such as Nd3+, La3+, Gd3+, Pr3+, Eu3+,Tb3+ [9–13]) or transition metals (such as Nb5+, Zr4+,
⁎
Ti4+,Mn3+, Co3+, Cr3+ [14–16]) substitution in A or B site of BiFeO3 could suppress the spiral spin magnetic structure and oxygen vacations, thus the macroscopic magnetism appears and the leakage current is reduced. Further investigations show that the co-doping in both A and B sites can effectively improve the ferroelectric and magnetic properties [17,18]. Wang et al. have Nd and Nb co-doped in BiFeO3 and found that the doped sample has a phase transition from R3c to Pbam. The antiferromagnetic nature in BiFeO3 eventually evolves into a state of weak ferromagnetism due to the decrease of particle size and structure transition [19]. Pradhan et al. found that both the electric polarization and magnetization properties were enhanced in the Bi1-xHoxFeO3 [20]. Muneeswaran et al. have synthesized Bi1-xHoxFeO3 (x = 0.0–0.2) and found a phase transition from R3c (x = 0.0, 0.05 and 0.1) to Pnma (x = 0.15, 0.18 and 0.2). The leakage current is reduced dramatically by Ho doping and the values of remnant polarization reached the maximum with x = 0.1 [21]. Song et al. have synthesized Bi1-xHoxFeO3 (x = 0.0, 0.05 and 0.1) using the rapid liquid phase sintering method [22]. Both of the ferroelectricity and the dielectric constant of the BiFeO3 sample were enhanced with Ho3+ doping. When x = 0.1, the remnant polarization and remnant magnetization reached the maximum value. Xue et al. found that Ho doping BiFeO3 has effectively decreased the leakage current and increased magnetization [23]. Kumar et al. reported the enhancement of magnetization and magnetodielectric properties for the La and Zr co-doped BiFeO3 [24]. However, the structure, dielectric properties, magnetic and ferroelectric
Corresponding author. E-mail address: [email protected] (Y. Lu).
https://doi.org/10.1016/j.rinp.2019.102489 Received 23 April 2019; Received in revised form 9 June 2019; Accepted 28 June 2019 Available online 02 July 2019 2211-3797/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Results in Physics 14 (2019) 102489
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properties of Ho and Zr co-doped have not been investigated systematically. Because the valence of Zr4+ is higher than that of Fe3+, it is expected that the high-valence Zr4+ substitution for Fe3+ could fill the oxygen vacancies induced by the volatility of the Bi element. The Zr doping could also break the spiral spin magnetic structure and improve the magnetic properties of the Bi0.9Ho0.1FeO3 ceramics. Therefore, in this work, we chose the Ho and Zr ions to substitute for the Bi and Fe ions in the BiFeO3 ceramics by the conventional solid-state sintering method. The effects of doping on structure, morphology, multiferroic properties and dielectric properties are investigated. Experimental Polycrystalline of Bi0.9Ho0.1Fe1-xZrxO3 (BHFZO, x = 0–0.05) samples were prepared using the conventional solid-state sintering method. Raw materials of Bi2O3 (99.9%), Ho2O3 (99.99%), Fe2O3 (99.99%) and ZrO2 (99.99%) were weighed in stoichiometry and then mixed, ground for 1 h using a ball mill (Mini-Mill Pulverisette 23, Fritsch). Dried powders were calcined at 800 °C for 5 h after being ground. After calcination, the mixtures were pressed into tablets and sintered again in the air at 870 °C. Then, the samples were reground, pressed into pellets and sintered at the same temperature for another 4 h, and finally furnace-cooled to room temperature. The crystal structure was studied by an X-ray diffractometer (XRD, 6100, Shimadzu, Japan) using Cu Kα radiation (λ = 1.54056 Å). Data were collected in the 2θ range of 20° to 80° with a scanning speed of 2°/ min. The microstructures of the surfaces for the samples were observed by scanning electron microscope (JSM 6610, Jeol, Japan). For electrical properties measurements, the Ag paste was applied onto both sides of the sintered tablets, and then the tablets were sintered at 600 °C for 10 min to make electrodes. The temperature-dependent dielectric properties at 100 Hz and 1 MHz were measured from room temperature to 480 °C by an LCR meter (E4980AL, Keysight, USA). To investigate the ferroelectric behavior of the ceramics, polarization-electric field (P-E) hysteresis loops were measured using a ferroelectric analyzer (TF analyzer 2000E, aixACCT, Germany). Magnetic characterization was performed using a Physical Property Measurement System (PPMS, Quantum Design, USA).
Fig. 2. Plot of observed and calculated XRD patterns for Bi0.9Ho0.1FeO3. The short bars indicate the positions of Bragg reflections. The difference plot, I (obs)-I(cal), is shown in the lower part of the figure.
crystal structure. Some small traces of secondary phases Bi2Fe4O9 (Fe rich phase) are also observed in all the XRD patterns around 28° (asterisk in Fig. 1). To obtain the detailed crystal structure information of all the samples, the collected XRD patterns are refined by the standard Rietveld refinement with the Fullprof program. The results of the Rietveld refinement show that all the secondary phases are within the error limit of less than 3% relative to the main Bi0.9Ho0.1Fe1-xZrxO3 phase. The structure and other physical properties of the samples are affected scarcely by the impurity phase [25,26]. The observed, calculated, reflection positions and the different patterns of x = 0 are shown in Fig. 2. The calculated patterns are in good agreement with the observed experimental data indicating that the results are reliable. The similar results are obtained for all the other samples. Table 1 lists all the data obtained from the Rietveld refinement for the samples such as lattice parameters a, b and c, c/a ratio, unit cell volume and adjustment parameters. The quality of the refinement is quantified by the Rwp showed in Table 1. It is obvious that as the Zr concentration increasing, the lattice parameters a and c increased while the c/a ratio decreased. This is because that the ionic radius of Zr4+ (0.72 Å) is larger than that of Fe3+ (0.64 Å). The variation of the lattice parameters also suggests that the Zr4+ was successfully doped into the Bi0.9Ho0.1FeO3 lattice. Fig. 3 displays the scanning electron micrograph (SEM) images of the Bi0.9Ho0.1Fe1-xZrxO3 ceramics for x = 0–0.05, respectively. The results of the SEM show that all the samples have even grain size with low porosity, which suggests that the microstructure of the samples is relatively dense. The values of bulk density are 7.885, 7.508, 7.475, 7.491, 7.489 and 7.464 g/cm3, respectively. With the Zr4+ addition, more pores appear, which leads to a decrease in bulk density. The inset shows an enlarged image of the SEM for each sample [27]. For the Bi0.9Ho0.1FeO3 ceramic, the average grain size is about 5.3 μm. It is observed that the average grain size for the doped samples decreases to 1.5–1.9 μm. The same phenomenon was observed in Sm doped BFO and Eu, Sr co-doped BFO ceramics [28,29]. It means that the Zr substitution
Results and discussion Fig. 1 exhibits the XRD patterns of polycrystalline Bi0.9Ho0.1Fe1-xZrxO3 (x = 0–0.05) ceramics at room temperature. The XRD patterns of Bi0.9Ho0.1Fe1-xZrxO3 (x = 0–0.05) can be ascribed to a rhombohedral structure having space group R3c according to the earlier reported [18]. It is shown that the Zr doping has not distorted the
Table 1 The lattice parameters obtained by Rietveld refinement for Bi0.9Ho0.1Fe1-xZrxO3 (x = 0–0.05).
a (Å) c (Å) c/a V (Å3) Rwp
Fig. 1. XRD patterns of Bi0.9Ho0.1Fe1-xZrxO3 (x = 0–0.05). 2
x=0
x = 0.01
x = 0.02
x = 0.03
x = 0.04
x = 0.05
5.5713(7) 13.839(2) 2.4839 372.03(8) 13.6
5.5739(3) 13.845(1) 2.4838 372.52(4) 13.5
5.5746(7) 13.845(2) 2.4835 372.61(9) 13.2
5.5753(7) 13.846(2) 2.4834 372.75(8) 12.3
5.5768(7) 13.847(2) 2.4829 372.95(23) 13.0
5.5779(8) 13.849(2) 2.4828 373.16(9) 13.3
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Fig. 3. SEM images of Bi0.9Ho0.1Fe1-xZrxO3, (a) x = 0, (b) x = 0.01, (c) x = 0.02, (d) x = 0.03, (e) x = 0.04 and (f) x = 0.05.
values of x = 0.01, 0.02, 0.03, 0.04 and 0.05 ceramics are 4.146 μC/ cm2 and 64.9 kV/cm, 3.351 μC/cm2 and 54.4 kV/cm, 3.494 μC/cm2 and 53.2 kV/cm, 3.221 μC/cm2 and 49.4 kV/cm and 2.316 μC/cm2 and 48.1 kV/cm under 40 kV/cm applied field, respectively. With the increase of the Zr concentration, the Pr decreases but the Ec increases at first and then decreases. In general, the rounded feature shape is always related to the leakage current. With the Zr concentration increasing, the shape of the P-E loops gets a little slimmer except for x = 0.01. For the other doped ceramics, the slimmer shapes indicate an improvement in the leakage current which can be proved in Fig. 6. The slight decrease of the Pr value might be attributed to the poor microstructural features which have more porosities with Zr doping [31,32]. Another possible reason might be attributed to the distortion of the expansive lattice which confirmed by the XRD refinements. As the lattice volume increasing and the ratio of c/a decreasing, the decrement is found in the
can suppress grain growth. The decrease of grain size for the doped samples may be attributed to the suppression of oxygen vacancy concentration. When the Zr4+ substitutes the Fe3+, the difference in the ionic valence between Zr4+ and Fe3+ requires charge compensation. The extra electrons will fill the oxygen vacancies which makes the motion of oxygen ion slower, thus the grain growth is suppressed. To study the ferroelectric behaviors of the samples, the polarization versus electric field (P-E) hysteresis loops of Bi0.9Ho0.1Fe1-xZrxO3 (x = 0–0.05) are shown in Fig. 4. The measurements were carried out at room temperature and a frequency of 10 Hz. Under the 40 kV/cm applied field, the breakdown field is not achieved and the saturated polarization loop is also not obtained for all the ceramics, which is the typical feature of the BFO ceramic [30]. As shown in Fig. 4, the remnant polarization value (2Pr) and the coercive field (2Ec) are 7.87 μC/cm2 and 56.2 kV/cm for the sample without Zr doping. The 2Pr and 2Ec
Fig. 4. Polarization versus electric field (P-E) hysteresis loops of Bi0.9Ho0.1Fe1-xZrxO3 (x = 0–0.05) at room temperature. 3
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Fig. 5. Dielectric constant (εr) versus temperature curves for Bi0.9Ho0.1Fe1-xZrxO3 (x = 0–0.05), (a) 100 Hz and (b) 1 MHz.
transition from Fe3+ to Fe2+ [34]. The conductivity of the dielectric materials increases with the oxygen vacancy concentration increasing, which results in the lower dielectric constant and higher dielectric loss. The doping Zr4+ can provide extra electron to fill the oxygen vacancies. Consequently, the lower concentration of oxygen vacancies is reached. Another reason might be attributed to the change of the microstructure. The decrease in the grain size of the ceramics increases the grain boundary resistance and reduces the dielectric loss [37]. The leakage current behavior is a crucial factor in understanding the ferroelectric properties of the samples. Fig. 6 shows the leakage current density log (J) versus the applied electric field (E) of all the samples measured at room temperature. Obviously, Zr doping greatly reduces the leakage current density, and the smallest leakage current density is obtained in the ceramic of x = 0.05. This feature manifests that the Zr4+ doping can improve the ferroelectric properties of the Bi0.9Ho0.1Fe1-xZrxO3 system. The extra electrons of Zr4+ can fill the oxygen vacancies. Previous studies show that the origin of the high leakage current related to the oxygen vacancies [29,38], which can generate deep-trap energy levels in the band gap for activated electrons to be mobile. Therefore, as the Zr4+ content increasing, the concentration of the oxygen vacancies is reduced and then the leakage current density is decreased. The room temperature magnetic hysteresis loops of all the ceramics are shown in Fig. 7. In general, undoped BiFeO3 shows antiferromagnetic behavior with G type spin ordering below Néel temperature. The G-type magnetic structure is modulated by a cycloidal spiral with a period of 62 nm, which leads to the cancellation of macroscopic magnetization [5,39]. Whenever this spiral structure is damaged, the magnetization will be induced in the materials. The M−H
Fig. 6. Room temperature leakage current density as a function of applied electric field of Bi0.9Ho0.1Fe1-xZrxO3 (x = 0–0.05).
intrinsic polarization which is dependent on ferroelectric off-centering distortion [33]. Fig. 5 shows the temperature dependence of the dielectric constant εr and the dielectric loss tanδ of the Bi0.9Ho0.1Fe1-xZrxO3 (x = 0–0.05) ceramics at 100 Hz and 1 MHz. A distinct decrease in the dielectric constant is observed as the frequency increasing. This could be attributed to the different polarization mechanisms that include electronic displacement, ionic displacement, turning-direction and space charge polarization at different frequencies. At low frequencies, all the polarization mechanisms are effective to the dielectric constant while only the electronic placement polarization is effective at higher frequencies [27,34,35]. An evident increase in dielectric constant can be observed between 300 and 400 °C, which is just located in the range of Néel temperature [26]. The Zr doping changed the lattice parameters of the Bi0.9Ho0.1Fe1-xZrxO3 ceramics, which results in a change in relative location between Fe3+ and O2−. Consequently, the relative location of positive and negative charge center changes. Since the magnetic and dielectric properties are related to the Fe-O bond, the sudden increased in dielectric constant may signify indirectly the coupling between polarization and magnetization [22,36]. The maximum of dielectric constant shifts toward higher temperatures with increasing measurement frequency, reflecting a typical dielectric relaxation behavior. In Fig. 5(b), the dielectric constant of x = 0 sample rises to its maximum value at Néel temperature and then decreases and again increases up to the measured temperature range. It is further observed that the dielectric constant increases gradually with x increasing at 100 Hz frequency. In the inset of Fig. 5 (a) and (b), the dielectric loss shows an evident decrease with Zr doping. This might be attributed to the charge compensation mechanism. According to previous reports, the oxygen vacancy can be produced by the volatilization of Bismuth and the
Fig. 7. Room temperature magnetic hysteresis loops of Bi0.9Ho0.1Fe1-xZrxO3 (x = 0–0.05). 4
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loops of all the samples are not saturated under the magnetic field of 30,000 Oe, indicating the intrinsic antiferromagnetic nature of the samples. In the doped ceramics, the Zr4+ replaces Fe3+ at Fe-site generating an unbalance antiferromagnetic structure. Therefore, all the samples exhibit a weak ferromagnetic as shown in Fig. 7. The increasing of the hysteresis loops area in Zr doped samples confirms that the magnetic properties are improved by Zr4+ doping. The inset in Fig. 7 is the enlarged view of part of M-H loops with the Magnetic field from −5000 to 5000 Oe. For x = 0.00–0.05, the remnant magnetization Mr are 0.0510, 0.0586, 0.0655, 0.0661, 0.0747 and 0.0584 emu/g, respectively. The remnant magnetization of Bi0.9Ho0.1Fe1-xZrxO3 samples increases gradually as x increasing (x ≤ 0.04) and reaches maximum value for x = 0.04. The enhancement in remnant magnetization may be caused by the structure distortion, which damaged the spiral structure by increasing Zr doping content. Another reason might be that the magnetization is sensitive to the particle size. The surface to volume ratio becomes larger with decreasing particle size. At the particle surface, the long-range antiferromagnetic order is interrupted resulting in the surface induced weak ferromagnetism [29,40]. For x = 0.05, the Mr decreases obviously. This might be attributed to the pores between grains. More pores in the ceramics indicate the lower densification which leads to an easier magnetization switching for x = 0.05.
[10] [11]
[12] [13]
[14]
[15] [16]
[17]
[18]
[19]
[20]
Conclusions
[21]
In summary, Ho and Zr co-doped Bi0.9Ho0.1Fe1-xZrxO3 (x = 0–0.05) ceramics were synthesized using a conventional solid-state sintering method. The results of the XRD patterns suggest that all the samples have a rhombohedral structure (space group R3c). By increasing Zr doping concentration, the lattice parameters increased due to the larger radius of Zr4+ than that of Fe3+. The SEM images reveal that the average grain size changes in the trend of small with Zr doping. The Zr4+ doping at Fe3+ site reduces the dielectric loss. The improved dielectric property in Zr4+ doped samples is attributed to the suppression of the oxygen vacancies. For the same reason, in the Zr doped samples, the leakage current density is also reduced significantly and the shapes of the P-E loops get a little slimmer. As Zr4+ doping level increasing, the weak ferromagnetism becomes more visible, and the remnant magnetization gradually increases and reaches the maximum value at x = 0.04, accompanied by the decreasing in remnant ferroelectric polarization.
[22]
[23] [24]
[25]
[26] [27]
[28]
[29]
Acknowledgments
[30] [31]
This work was supported by the Fundamental Research Funds for the Central Universities, China, under Grants Nos. XDJK2019C003 and XDJK2019C111 and National Natural Science Foundation of China (Grant No. 51501159, 51802270 and 51601153).
[32]
[33]
References [34] [1] Eerenstein W, Mathur ND, Scott JF. Multiferroic and magnetoelectric materials. Nature 2006;442:759. [2] Bibes M. Nanoferronics is a winning combination. Nat Mater 2012;11:354–7. [3] Wu SM, Cybart SA, Yu P, Rossell MD, Zhang JX, Ramesh R, et al. Reversible electric control of exchange bias in a multiferroic field-effect device. Nat Mater 2010;9:756–61. [4] Jarillo-Herrero P, Sapmaz S, Dekker C, Kouwenhoven LP, van der Zant HSJ. Electron-hole symmetry in a semiconducting carbon nanotube quantum dot. Nature 2004;429:389–92. [5] Catalan G, Scott JF. Physics and Applications of Bismuth Ferrite. Adv Mater 2009;21:2463–85. [6] Ramesh R, Spaldin NA. Multiferroics: progress and prospects in thin films. Nat Mater 2007;6:21–9. [7] Wang J, Neaton JB, Zheng H, Nagarajan V, Ogale SB, Liu B, et al. Epitaxial BiFeO3 multiferroic thin film heterostructures. Science 2003;299:1719–22. [8] Neaton JB, Ederer C, Waghmare UV, Spaldin NA, Rabe KM. First-principles study of spontaneous polarization in multiferroic BiFeO3. Phys Rev B 2005;71:8. [9] Iorgu AI, Maxim F, Matei C, Ferreira LP, Ferreira P, Cruz MM, et al. Fast synthesis of
[35]
[36] [37]
[38]
[39]
[40]
5
rare-earth (Pr3+, Sm3+, Eu3+ and Gd3+) doped bismuth ferrite powders with enhanced magnetic properties. J Alloys Compd 2015;629:62–8. Zhang J, Wu YJ, Chen XK, Chen XJ. Structural evolution and magnetization enhancement of Bi1−xTbxFeO3. J Phys Chem Solids 2013;74:849–53. Carvalho TT, Fernandes JRA, Perez de la Cruz J, Vidal JV, Sobolev NA, Figueiras F, Agostinho Moreira J, Tavares PB, et al. Room temperature structure and multiferroic properties in Bi0.7La0.3FeO3 ceramics. J Alloys Compd 2013;554:97–103. Wu YJ, Chen XK, Zhang J, Chen XJ. Magnetic enhancement across a ferroelectric–paraelectric phase boundary in Bi1−xSmxFeO3. Phys B 2013;411:106–9. Wu YJ, Chen XK, Zhang J, Chen XJ. Magnetic enhancement across a ferroelectric–antiferroelectric phase boundary in Bi1−xNdxFeO3. J Appl Phys 2012;111:053927. Godara S, Sinha N, Kumar B. Study the influence of Nd and Co/Cr co-substitutions on structural, electrical and magnetic properties of BiFeO3 nanoparticles. Ceram Int 2016;42:1782–90. Khomchenko VA, Paixão JA. Effect of Nb doping on the morphology and multiferroic behavior of Bi0.9La0.1FeO3 ceramics. Mater Lett 2016;169:180–4. Gowrishankar M, Babu DR, Madeswaran S. Effect of Gd–Ti co-substitution on structural, magnetic and electrical properties of multiferroic BiFeO3. J Magn Magn Mater 2016;418:54–61. Shankar S, Kumar M, Kumar S, Thakur OP, Ghosh AK. Enhanced multiferroic properties and magneto-dielectric effect analysis of La/Co modified BiFeO3. J Alloys Compd 2017;694:715–20. Vashisth BK, Bangruwa JS, Beniwal A, Gairola SP, Kumar A, Singh N, et al. Modified ferroelectric/magnetic and leakage current density properties of Co and Sm codoped bismuth ferrites. J Alloys Compd 2017;698:699–705. Wang T, Xu T, Gao S, Song SH. Effect of Nd and Nb co-doping on the structural, magnetic and optical properties of multiferroic BiFeO3 nanoparticles prepared by sol-gel method. Ceram Int 2017;43:4489–95. Pradhan SK, Das J, Rout PP, Mohanta VR, Das SK, Samantray S, et al. Effect of holmium substitution for the improvement of multiferroic properties of BiFeO3. J Phys Chem Solids 2010;71:1557–64. Muneeswaran M, Lee SH, Kim DH, Jung BS, Chang SH, Jang J-W, et al. Structural, vibrational, and enhanced magneto-electric coupling in Ho-substituted BiFeO3. J Alloys Compd 2018;750:276–85. Song GL, Song YC, Su J, Song XH, Zhang N, Wang TX, et al. Crystal structure refinement, ferroelectric and ferromagnetic properties of Ho3+ modified BiFeO3 multiferroic material. J Alloys Compd 2017;696:503–9. Xue F, Fu Q, Zhou D, Tian Y, Hu Y, Zheng Z, et al. Properties of Bi0.8Ln0.2FeO3 (Ln=La, Gd, Ho) multiferroic ceramics. Ceram Int 2015;41:14718–26. Kumar A, Yadav KL, Rani J. Low temperature step magnetization and magnetodielectric study in Bi0.95La0.05Fe1−xZrxO3 ceramics. Mater Chem Phys 2012;134:430–4. Acharya S, Sutradhar S, Mandal J, Mukhopadhyay K, Deb AK, Chakrabarti PK. Sol–gel derived nanocrystalline multiferroic BiFeO3 and R3+ (R=Er and Tm) doped therein: Magnetic phase transitions and enhancement of magnetic properties. J Magn Magn Mater 2012;324:4209–18. Barbar SK, Jangid S, Roy M, Chou FC. Synthesis, structural and electrical properties of Bi1−xDyxFeO3 multiferroic ceramics. Ceram Int 2013;39:5359–63. Kumar A, Sharma P, Yang W, Shen J, Varshney D, Li Q. Effect of La and Ni substitution on structure, dielectric and ferroelectric properties of BiFeO3 ceramics. Ceram Int 2016;42:14805–12. Li Q, Bao S, Liu Y, Li Y, Jing Y, Li J. Influence of lightly Sm-substitution on crystal structure, magnetic and dielectric properties of BiFeO3 ceramics. J Alloys Compd 2016;682:672–8. Mao W, Chen W, Wang X, Zhu Y, Ma Y, Xue H, et al. Li Xa, Huang W. Influence of Eu and Sr co-substitution on multiferroic properties of BiFeO3. Ceram Int 2016;42:12838–42. Dawber M, Rabe KM, Scott JF. Physics of thin-film ferroelectric oxides. Rev Mod Phys 2005;77:1083–130. Raghavan CM, Kim JW, Kim SS, Viehland D. Effects of Ho and Ti doping on structural and electrical properties of BiFeO3 thin films. J Am Ceram Soc 2014;97:235–40. Raghavan CM, Kim JW, Kim SS. Studies on structural and electrical properties of transition metal ions (Mn, Co and Cu) co-doped Bi0.9Dy0.1FeO3 thin films. J Sol-Gel Sci Technol 2013;67:486–91. Wei J, Liu Y, Bai X, Li C, Liu Y, Xu Z, et al. Crystal structure, leakage conduction mechanism evolution and enhanced multiferroic properties in Y-doped BiFeO3 ceramics. Ceram Int 2016;42:13395–403. Saxena P, Kumar A, Sharma P, Varshney D. Improved dielectric and ferroelectric properties of dual-site substituted rhombohedral structured BiFeO3 multiferroics. J Alloys Compd 2016;682:418–23. Verma V, Beniwal A, Ohlan A, Tripathi R. Structural, magnetic and ferroelectric properties of Pr doped multiferroics bismuth ferrites. J Magn Magn Mater 2015;394:385–90. Mazumder R, Sujatha Devi P, Bhattacharya D, Choudhury P, Sen A, Raja M. Ferromagnetism in nanoscale BiFeO3. Appl Phys Lett 2007;91:062510. Mukherjee S, Gupta R, Garg A, Bansal V, Bhargava S. Influence of Zr doping on the structure and ferroelectric properties of BiFeO3 thin films. J Appl Phys 2010;107:123535. Qi XD, Dho J, Tomov R, Blamire MG, MacManus-Driscoll JL. Greatly reduced leakage current and conduction mechanism in aliovalent-ion-doped BiFeO3. Appl Phys Lett 2005;86:3. Ruette B, Zvyagin S, Pyatakov AP, Bush A, Li JF, Belotelov VI, et al. Magnetic-fieldinduced phase transition in BiFeO3 observed by high-field electron spin resonance: cycloidal to homogeneous spin order. Phys Rev B 2004;69:7. Jia DC, Xu JH, Ke H, Wang W, Zhou Y. Structure and multiferroic properties of BiFeO3 powders. J Eur Ceram Soc 2009;29:3099–103.
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Influence of Zr substitution on structure, electrical and magnetic properties of Bi0.9Ho0.1FeO3 ceramics Guannan Lia, Haoting Zhaoa, Ruxia Yangb, Jianfeng Tanga, Chunmei Lia, Yuming Lub, a b
T
⁎
School of Materials and Energy, Southwest University, Chongqing 400715, China School of Physical Science and Technology, Southwest University, Chongqing 400715, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Multiferroic materials Ferroelectric Ferromagnetic Bismuth ferrite
Polycrystalline Bi0.9Ho0.1Fe1-xZrxO3 ceramics have been synthesized by standard solid state sintering method. The influences of Zr doping on structure, dielectric, ferroelectric and magnetic properties were investigated. The results of the XRD patterns suggest that all the samples have a rhombohedral structure (space group R3c). Structure refinement of the XRD patterns shows that the lattice parameters are increased as the Zr4+ concentration increasing. The average grain size of the samples decreases with Zr doping due to the suppressed of grain growth caused by Zr substitution. The dielectric loss is reduced and the remnant polarization (Pr) is decreased with increasing Zr concentration. As Zr doping level increases, the weak ferromagnetism becomes more obvious, and the remnant magnetization gradually increases and reaches the maximum value at x = 0.04.
Introduction Materials those exhibit two or more properties of ferroelectricity, ferromagnetism and ferroelasticity, also known as multiferroic materials, have attracted intense attention during the last two decades [1,2]. The most promising advantage of the occurrence of multiferroic could be utilized to control the magnetization vector by the external electric or control the polarization vector by the external magnetic field [3]. Because of the extraordinary performance, multiferroics have been applied widely in the areas of information storage, spintronic devices and sensors [4–6]. BiFeO3 (BFO) is one of the potential candidates for its high Curie temperature (1100 K) and high Néel temperature (640 K) [6,7]. It exhibits a rhombohedral perovskite (ABO3) structure with space group R3c and G-type antiferromagnetism. The theoretical calculation shows that the spontaneous polarization for BiFeO3 will reach 90–100 μC/cm2 [8]. Though scholars have done many studies on BiFeO3 ceramics, the gap still exists between the experimental and theoretical values. Beyond that, there are also some inherent problems, such as high leakage current, low remnant polarization and weak magnetoelectric coupling, which may restrict the applications of BiFeO3. In recent years, the site-engineering concept has been widely adopted to improve the multiferroic properties of BiFeO3. The results indicate that rare earth ions (such as Nd3+, La3+, Gd3+, Pr3+, Eu3+,Tb3+ [9–13]) or transition metals (such as Nb5+, Zr4+,
⁎
Ti4+,Mn3+, Co3+, Cr3+ [14–16]) substitution in A or B site of BiFeO3 could suppress the spiral spin magnetic structure and oxygen vacations, thus the macroscopic magnetism appears and the leakage current is reduced. Further investigations show that the co-doping in both A and B sites can effectively improve the ferroelectric and magnetic properties [17,18]. Wang et al. have Nd and Nb co-doped in BiFeO3 and found that the doped sample has a phase transition from R3c to Pbam. The antiferromagnetic nature in BiFeO3 eventually evolves into a state of weak ferromagnetism due to the decrease of particle size and structure transition [19]. Pradhan et al. found that both the electric polarization and magnetization properties were enhanced in the Bi1-xHoxFeO3 [20]. Muneeswaran et al. have synthesized Bi1-xHoxFeO3 (x = 0.0–0.2) and found a phase transition from R3c (x = 0.0, 0.05 and 0.1) to Pnma (x = 0.15, 0.18 and 0.2). The leakage current is reduced dramatically by Ho doping and the values of remnant polarization reached the maximum with x = 0.1 [21]. Song et al. have synthesized Bi1-xHoxFeO3 (x = 0.0, 0.05 and 0.1) using the rapid liquid phase sintering method [22]. Both of the ferroelectricity and the dielectric constant of the BiFeO3 sample were enhanced with Ho3+ doping. When x = 0.1, the remnant polarization and remnant magnetization reached the maximum value. Xue et al. found that Ho doping BiFeO3 has effectively decreased the leakage current and increased magnetization [23]. Kumar et al. reported the enhancement of magnetization and magnetodielectric properties for the La and Zr co-doped BiFeO3 [24]. However, the structure, dielectric properties, magnetic and ferroelectric
Corresponding author. E-mail address: [email protected] (Y. Lu).
https://doi.org/10.1016/j.rinp.2019.102489 Received 23 April 2019; Received in revised form 9 June 2019; Accepted 28 June 2019 Available online 02 July 2019 2211-3797/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
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properties of Ho and Zr co-doped have not been investigated systematically. Because the valence of Zr4+ is higher than that of Fe3+, it is expected that the high-valence Zr4+ substitution for Fe3+ could fill the oxygen vacancies induced by the volatility of the Bi element. The Zr doping could also break the spiral spin magnetic structure and improve the magnetic properties of the Bi0.9Ho0.1FeO3 ceramics. Therefore, in this work, we chose the Ho and Zr ions to substitute for the Bi and Fe ions in the BiFeO3 ceramics by the conventional solid-state sintering method. The effects of doping on structure, morphology, multiferroic properties and dielectric properties are investigated. Experimental Polycrystalline of Bi0.9Ho0.1Fe1-xZrxO3 (BHFZO, x = 0–0.05) samples were prepared using the conventional solid-state sintering method. Raw materials of Bi2O3 (99.9%), Ho2O3 (99.99%), Fe2O3 (99.99%) and ZrO2 (99.99%) were weighed in stoichiometry and then mixed, ground for 1 h using a ball mill (Mini-Mill Pulverisette 23, Fritsch). Dried powders were calcined at 800 °C for 5 h after being ground. After calcination, the mixtures were pressed into tablets and sintered again in the air at 870 °C. Then, the samples were reground, pressed into pellets and sintered at the same temperature for another 4 h, and finally furnace-cooled to room temperature. The crystal structure was studied by an X-ray diffractometer (XRD, 6100, Shimadzu, Japan) using Cu Kα radiation (λ = 1.54056 Å). Data were collected in the 2θ range of 20° to 80° with a scanning speed of 2°/ min. The microstructures of the surfaces for the samples were observed by scanning electron microscope (JSM 6610, Jeol, Japan). For electrical properties measurements, the Ag paste was applied onto both sides of the sintered tablets, and then the tablets were sintered at 600 °C for 10 min to make electrodes. The temperature-dependent dielectric properties at 100 Hz and 1 MHz were measured from room temperature to 480 °C by an LCR meter (E4980AL, Keysight, USA). To investigate the ferroelectric behavior of the ceramics, polarization-electric field (P-E) hysteresis loops were measured using a ferroelectric analyzer (TF analyzer 2000E, aixACCT, Germany). Magnetic characterization was performed using a Physical Property Measurement System (PPMS, Quantum Design, USA).
Fig. 2. Plot of observed and calculated XRD patterns for Bi0.9Ho0.1FeO3. The short bars indicate the positions of Bragg reflections. The difference plot, I (obs)-I(cal), is shown in the lower part of the figure.
crystal structure. Some small traces of secondary phases Bi2Fe4O9 (Fe rich phase) are also observed in all the XRD patterns around 28° (asterisk in Fig. 1). To obtain the detailed crystal structure information of all the samples, the collected XRD patterns are refined by the standard Rietveld refinement with the Fullprof program. The results of the Rietveld refinement show that all the secondary phases are within the error limit of less than 3% relative to the main Bi0.9Ho0.1Fe1-xZrxO3 phase. The structure and other physical properties of the samples are affected scarcely by the impurity phase [25,26]. The observed, calculated, reflection positions and the different patterns of x = 0 are shown in Fig. 2. The calculated patterns are in good agreement with the observed experimental data indicating that the results are reliable. The similar results are obtained for all the other samples. Table 1 lists all the data obtained from the Rietveld refinement for the samples such as lattice parameters a, b and c, c/a ratio, unit cell volume and adjustment parameters. The quality of the refinement is quantified by the Rwp showed in Table 1. It is obvious that as the Zr concentration increasing, the lattice parameters a and c increased while the c/a ratio decreased. This is because that the ionic radius of Zr4+ (0.72 Å) is larger than that of Fe3+ (0.64 Å). The variation of the lattice parameters also suggests that the Zr4+ was successfully doped into the Bi0.9Ho0.1FeO3 lattice. Fig. 3 displays the scanning electron micrograph (SEM) images of the Bi0.9Ho0.1Fe1-xZrxO3 ceramics for x = 0–0.05, respectively. The results of the SEM show that all the samples have even grain size with low porosity, which suggests that the microstructure of the samples is relatively dense. The values of bulk density are 7.885, 7.508, 7.475, 7.491, 7.489 and 7.464 g/cm3, respectively. With the Zr4+ addition, more pores appear, which leads to a decrease in bulk density. The inset shows an enlarged image of the SEM for each sample [27]. For the Bi0.9Ho0.1FeO3 ceramic, the average grain size is about 5.3 μm. It is observed that the average grain size for the doped samples decreases to 1.5–1.9 μm. The same phenomenon was observed in Sm doped BFO and Eu, Sr co-doped BFO ceramics [28,29]. It means that the Zr substitution
Results and discussion Fig. 1 exhibits the XRD patterns of polycrystalline Bi0.9Ho0.1Fe1-xZrxO3 (x = 0–0.05) ceramics at room temperature. The XRD patterns of Bi0.9Ho0.1Fe1-xZrxO3 (x = 0–0.05) can be ascribed to a rhombohedral structure having space group R3c according to the earlier reported [18]. It is shown that the Zr doping has not distorted the
Table 1 The lattice parameters obtained by Rietveld refinement for Bi0.9Ho0.1Fe1-xZrxO3 (x = 0–0.05).
a (Å) c (Å) c/a V (Å3) Rwp
Fig. 1. XRD patterns of Bi0.9Ho0.1Fe1-xZrxO3 (x = 0–0.05). 2
x=0
x = 0.01
x = 0.02
x = 0.03
x = 0.04
x = 0.05
5.5713(7) 13.839(2) 2.4839 372.03(8) 13.6
5.5739(3) 13.845(1) 2.4838 372.52(4) 13.5
5.5746(7) 13.845(2) 2.4835 372.61(9) 13.2
5.5753(7) 13.846(2) 2.4834 372.75(8) 12.3
5.5768(7) 13.847(2) 2.4829 372.95(23) 13.0
5.5779(8) 13.849(2) 2.4828 373.16(9) 13.3
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Fig. 3. SEM images of Bi0.9Ho0.1Fe1-xZrxO3, (a) x = 0, (b) x = 0.01, (c) x = 0.02, (d) x = 0.03, (e) x = 0.04 and (f) x = 0.05.
values of x = 0.01, 0.02, 0.03, 0.04 and 0.05 ceramics are 4.146 μC/ cm2 and 64.9 kV/cm, 3.351 μC/cm2 and 54.4 kV/cm, 3.494 μC/cm2 and 53.2 kV/cm, 3.221 μC/cm2 and 49.4 kV/cm and 2.316 μC/cm2 and 48.1 kV/cm under 40 kV/cm applied field, respectively. With the increase of the Zr concentration, the Pr decreases but the Ec increases at first and then decreases. In general, the rounded feature shape is always related to the leakage current. With the Zr concentration increasing, the shape of the P-E loops gets a little slimmer except for x = 0.01. For the other doped ceramics, the slimmer shapes indicate an improvement in the leakage current which can be proved in Fig. 6. The slight decrease of the Pr value might be attributed to the poor microstructural features which have more porosities with Zr doping [31,32]. Another possible reason might be attributed to the distortion of the expansive lattice which confirmed by the XRD refinements. As the lattice volume increasing and the ratio of c/a decreasing, the decrement is found in the
can suppress grain growth. The decrease of grain size for the doped samples may be attributed to the suppression of oxygen vacancy concentration. When the Zr4+ substitutes the Fe3+, the difference in the ionic valence between Zr4+ and Fe3+ requires charge compensation. The extra electrons will fill the oxygen vacancies which makes the motion of oxygen ion slower, thus the grain growth is suppressed. To study the ferroelectric behaviors of the samples, the polarization versus electric field (P-E) hysteresis loops of Bi0.9Ho0.1Fe1-xZrxO3 (x = 0–0.05) are shown in Fig. 4. The measurements were carried out at room temperature and a frequency of 10 Hz. Under the 40 kV/cm applied field, the breakdown field is not achieved and the saturated polarization loop is also not obtained for all the ceramics, which is the typical feature of the BFO ceramic [30]. As shown in Fig. 4, the remnant polarization value (2Pr) and the coercive field (2Ec) are 7.87 μC/cm2 and 56.2 kV/cm for the sample without Zr doping. The 2Pr and 2Ec
Fig. 4. Polarization versus electric field (P-E) hysteresis loops of Bi0.9Ho0.1Fe1-xZrxO3 (x = 0–0.05) at room temperature. 3
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Fig. 5. Dielectric constant (εr) versus temperature curves for Bi0.9Ho0.1Fe1-xZrxO3 (x = 0–0.05), (a) 100 Hz and (b) 1 MHz.
transition from Fe3+ to Fe2+ [34]. The conductivity of the dielectric materials increases with the oxygen vacancy concentration increasing, which results in the lower dielectric constant and higher dielectric loss. The doping Zr4+ can provide extra electron to fill the oxygen vacancies. Consequently, the lower concentration of oxygen vacancies is reached. Another reason might be attributed to the change of the microstructure. The decrease in the grain size of the ceramics increases the grain boundary resistance and reduces the dielectric loss [37]. The leakage current behavior is a crucial factor in understanding the ferroelectric properties of the samples. Fig. 6 shows the leakage current density log (J) versus the applied electric field (E) of all the samples measured at room temperature. Obviously, Zr doping greatly reduces the leakage current density, and the smallest leakage current density is obtained in the ceramic of x = 0.05. This feature manifests that the Zr4+ doping can improve the ferroelectric properties of the Bi0.9Ho0.1Fe1-xZrxO3 system. The extra electrons of Zr4+ can fill the oxygen vacancies. Previous studies show that the origin of the high leakage current related to the oxygen vacancies [29,38], which can generate deep-trap energy levels in the band gap for activated electrons to be mobile. Therefore, as the Zr4+ content increasing, the concentration of the oxygen vacancies is reduced and then the leakage current density is decreased. The room temperature magnetic hysteresis loops of all the ceramics are shown in Fig. 7. In general, undoped BiFeO3 shows antiferromagnetic behavior with G type spin ordering below Néel temperature. The G-type magnetic structure is modulated by a cycloidal spiral with a period of 62 nm, which leads to the cancellation of macroscopic magnetization [5,39]. Whenever this spiral structure is damaged, the magnetization will be induced in the materials. The M−H
Fig. 6. Room temperature leakage current density as a function of applied electric field of Bi0.9Ho0.1Fe1-xZrxO3 (x = 0–0.05).
intrinsic polarization which is dependent on ferroelectric off-centering distortion [33]. Fig. 5 shows the temperature dependence of the dielectric constant εr and the dielectric loss tanδ of the Bi0.9Ho0.1Fe1-xZrxO3 (x = 0–0.05) ceramics at 100 Hz and 1 MHz. A distinct decrease in the dielectric constant is observed as the frequency increasing. This could be attributed to the different polarization mechanisms that include electronic displacement, ionic displacement, turning-direction and space charge polarization at different frequencies. At low frequencies, all the polarization mechanisms are effective to the dielectric constant while only the electronic placement polarization is effective at higher frequencies [27,34,35]. An evident increase in dielectric constant can be observed between 300 and 400 °C, which is just located in the range of Néel temperature [26]. The Zr doping changed the lattice parameters of the Bi0.9Ho0.1Fe1-xZrxO3 ceramics, which results in a change in relative location between Fe3+ and O2−. Consequently, the relative location of positive and negative charge center changes. Since the magnetic and dielectric properties are related to the Fe-O bond, the sudden increased in dielectric constant may signify indirectly the coupling between polarization and magnetization [22,36]. The maximum of dielectric constant shifts toward higher temperatures with increasing measurement frequency, reflecting a typical dielectric relaxation behavior. In Fig. 5(b), the dielectric constant of x = 0 sample rises to its maximum value at Néel temperature and then decreases and again increases up to the measured temperature range. It is further observed that the dielectric constant increases gradually with x increasing at 100 Hz frequency. In the inset of Fig. 5 (a) and (b), the dielectric loss shows an evident decrease with Zr doping. This might be attributed to the charge compensation mechanism. According to previous reports, the oxygen vacancy can be produced by the volatilization of Bismuth and the
Fig. 7. Room temperature magnetic hysteresis loops of Bi0.9Ho0.1Fe1-xZrxO3 (x = 0–0.05). 4
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loops of all the samples are not saturated under the magnetic field of 30,000 Oe, indicating the intrinsic antiferromagnetic nature of the samples. In the doped ceramics, the Zr4+ replaces Fe3+ at Fe-site generating an unbalance antiferromagnetic structure. Therefore, all the samples exhibit a weak ferromagnetic as shown in Fig. 7. The increasing of the hysteresis loops area in Zr doped samples confirms that the magnetic properties are improved by Zr4+ doping. The inset in Fig. 7 is the enlarged view of part of M-H loops with the Magnetic field from −5000 to 5000 Oe. For x = 0.00–0.05, the remnant magnetization Mr are 0.0510, 0.0586, 0.0655, 0.0661, 0.0747 and 0.0584 emu/g, respectively. The remnant magnetization of Bi0.9Ho0.1Fe1-xZrxO3 samples increases gradually as x increasing (x ≤ 0.04) and reaches maximum value for x = 0.04. The enhancement in remnant magnetization may be caused by the structure distortion, which damaged the spiral structure by increasing Zr doping content. Another reason might be that the magnetization is sensitive to the particle size. The surface to volume ratio becomes larger with decreasing particle size. At the particle surface, the long-range antiferromagnetic order is interrupted resulting in the surface induced weak ferromagnetism [29,40]. For x = 0.05, the Mr decreases obviously. This might be attributed to the pores between grains. More pores in the ceramics indicate the lower densification which leads to an easier magnetization switching for x = 0.05.
[10] [11]
[12] [13]
[14]
[15] [16]
[17]
[18]
[19]
[20]
Conclusions
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In summary, Ho and Zr co-doped Bi0.9Ho0.1Fe1-xZrxO3 (x = 0–0.05) ceramics were synthesized using a conventional solid-state sintering method. The results of the XRD patterns suggest that all the samples have a rhombohedral structure (space group R3c). By increasing Zr doping concentration, the lattice parameters increased due to the larger radius of Zr4+ than that of Fe3+. The SEM images reveal that the average grain size changes in the trend of small with Zr doping. The Zr4+ doping at Fe3+ site reduces the dielectric loss. The improved dielectric property in Zr4+ doped samples is attributed to the suppression of the oxygen vacancies. For the same reason, in the Zr doped samples, the leakage current density is also reduced significantly and the shapes of the P-E loops get a little slimmer. As Zr4+ doping level increasing, the weak ferromagnetism becomes more visible, and the remnant magnetization gradually increases and reaches the maximum value at x = 0.04, accompanied by the decreasing in remnant ferroelectric polarization.
[22]
[23] [24]
[25]
[26] [27]
[28]
[29]
Acknowledgments
[30] [31]
This work was supported by the Fundamental Research Funds for the Central Universities, China, under Grants Nos. XDJK2019C003 and XDJK2019C111 and National Natural Science Foundation of China (Grant No. 51501159, 51802270 and 51601153).
[32]
[33]
References [34] [1] Eerenstein W, Mathur ND, Scott JF. Multiferroic and magnetoelectric materials. Nature 2006;442:759. [2] Bibes M. Nanoferronics is a winning combination. Nat Mater 2012;11:354–7. [3] Wu SM, Cybart SA, Yu P, Rossell MD, Zhang JX, Ramesh R, et al. Reversible electric control of exchange bias in a multiferroic field-effect device. Nat Mater 2010;9:756–61. [4] Jarillo-Herrero P, Sapmaz S, Dekker C, Kouwenhoven LP, van der Zant HSJ. Electron-hole symmetry in a semiconducting carbon nanotube quantum dot. Nature 2004;429:389–92. [5] Catalan G, Scott JF. Physics and Applications of Bismuth Ferrite. Adv Mater 2009;21:2463–85. [6] Ramesh R, Spaldin NA. Multiferroics: progress and prospects in thin films. Nat Mater 2007;6:21–9. [7] Wang J, Neaton JB, Zheng H, Nagarajan V, Ogale SB, Liu B, et al. Epitaxial BiFeO3 multiferroic thin film heterostructures. Science 2003;299:1719–22. [8] Neaton JB, Ederer C, Waghmare UV, Spaldin NA, Rabe KM. First-principles study of spontaneous polarization in multiferroic BiFeO3. Phys Rev B 2005;71:8. [9] Iorgu AI, Maxim F, Matei C, Ferreira LP, Ferreira P, Cruz MM, et al. Fast synthesis of
[35]
[36] [37]
[38]
[39]
[40]
5
rare-earth (Pr3+, Sm3+, Eu3+ and Gd3+) doped bismuth ferrite powders with enhanced magnetic properties. J Alloys Compd 2015;629:62–8. Zhang J, Wu YJ, Chen XK, Chen XJ. Structural evolution and magnetization enhancement of Bi1−xTbxFeO3. J Phys Chem Solids 2013;74:849–53. Carvalho TT, Fernandes JRA, Perez de la Cruz J, Vidal JV, Sobolev NA, Figueiras F, Agostinho Moreira J, Tavares PB, et al. Room temperature structure and multiferroic properties in Bi0.7La0.3FeO3 ceramics. J Alloys Compd 2013;554:97–103. Wu YJ, Chen XK, Zhang J, Chen XJ. Magnetic enhancement across a ferroelectric–paraelectric phase boundary in Bi1−xSmxFeO3. Phys B 2013;411:106–9. Wu YJ, Chen XK, Zhang J, Chen XJ. Magnetic enhancement across a ferroelectric–antiferroelectric phase boundary in Bi1−xNdxFeO3. J Appl Phys 2012;111:053927. Godara S, Sinha N, Kumar B. Study the influence of Nd and Co/Cr co-substitutions on structural, electrical and magnetic properties of BiFeO3 nanoparticles. Ceram Int 2016;42:1782–90. Khomchenko VA, Paixão JA. Effect of Nb doping on the morphology and multiferroic behavior of Bi0.9La0.1FeO3 ceramics. Mater Lett 2016;169:180–4. Gowrishankar M, Babu DR, Madeswaran S. Effect of Gd–Ti co-substitution on structural, magnetic and electrical properties of multiferroic BiFeO3. J Magn Magn Mater 2016;418:54–61. Shankar S, Kumar M, Kumar S, Thakur OP, Ghosh AK. Enhanced multiferroic properties and magneto-dielectric effect analysis of La/Co modified BiFeO3. J Alloys Compd 2017;694:715–20. Vashisth BK, Bangruwa JS, Beniwal A, Gairola SP, Kumar A, Singh N, et al. Modified ferroelectric/magnetic and leakage current density properties of Co and Sm codoped bismuth ferrites. J Alloys Compd 2017;698:699–705. Wang T, Xu T, Gao S, Song SH. Effect of Nd and Nb co-doping on the structural, magnetic and optical properties of multiferroic BiFeO3 nanoparticles prepared by sol-gel method. Ceram Int 2017;43:4489–95. Pradhan SK, Das J, Rout PP, Mohanta VR, Das SK, Samantray S, et al. Effect of holmium substitution for the improvement of multiferroic properties of BiFeO3. J Phys Chem Solids 2010;71:1557–64. Muneeswaran M, Lee SH, Kim DH, Jung BS, Chang SH, Jang J-W, et al. Structural, vibrational, and enhanced magneto-electric coupling in Ho-substituted BiFeO3. J Alloys Compd 2018;750:276–85. Song GL, Song YC, Su J, Song XH, Zhang N, Wang TX, et al. Crystal structure refinement, ferroelectric and ferromagnetic properties of Ho3+ modified BiFeO3 multiferroic material. J Alloys Compd 2017;696:503–9. Xue F, Fu Q, Zhou D, Tian Y, Hu Y, Zheng Z, et al. Properties of Bi0.8Ln0.2FeO3 (Ln=La, Gd, Ho) multiferroic ceramics. Ceram Int 2015;41:14718–26. Kumar A, Yadav KL, Rani J. Low temperature step magnetization and magnetodielectric study in Bi0.95La0.05Fe1−xZrxO3 ceramics. Mater Chem Phys 2012;134:430–4. Acharya S, Sutradhar S, Mandal J, Mukhopadhyay K, Deb AK, Chakrabarti PK. Sol–gel derived nanocrystalline multiferroic BiFeO3 and R3+ (R=Er and Tm) doped therein: Magnetic phase transitions and enhancement of magnetic properties. J Magn Magn Mater 2012;324:4209–18. Barbar SK, Jangid S, Roy M, Chou FC. Synthesis, structural and electrical properties of Bi1−xDyxFeO3 multiferroic ceramics. Ceram Int 2013;39:5359–63. Kumar A, Sharma P, Yang W, Shen J, Varshney D, Li Q. Effect of La and Ni substitution on structure, dielectric and ferroelectric properties of BiFeO3 ceramics. Ceram Int 2016;42:14805–12. Li Q, Bao S, Liu Y, Li Y, Jing Y, Li J. Influence of lightly Sm-substitution on crystal structure, magnetic and dielectric properties of BiFeO3 ceramics. J Alloys Compd 2016;682:672–8. Mao W, Chen W, Wang X, Zhu Y, Ma Y, Xue H, et al. Li Xa, Huang W. Influence of Eu and Sr co-substitution on multiferroic properties of BiFeO3. Ceram Int 2016;42:12838–42. Dawber M, Rabe KM, Scott JF. Physics of thin-film ferroelectric oxides. Rev Mod Phys 2005;77:1083–130. Raghavan CM, Kim JW, Kim SS, Viehland D. Effects of Ho and Ti doping on structural and electrical properties of BiFeO3 thin films. J Am Ceram Soc 2014;97:235–40. Raghavan CM, Kim JW, Kim SS. Studies on structural and electrical properties of transition metal ions (Mn, Co and Cu) co-doped Bi0.9Dy0.1FeO3 thin films. J Sol-Gel Sci Technol 2013;67:486–91. Wei J, Liu Y, Bai X, Li C, Liu Y, Xu Z, et al. Crystal structure, leakage conduction mechanism evolution and enhanced multiferroic properties in Y-doped BiFeO3 ceramics. Ceram Int 2016;42:13395–403. Saxena P, Kumar A, Sharma P, Varshney D. Improved dielectric and ferroelectric properties of dual-site substituted rhombohedral structured BiFeO3 multiferroics. J Alloys Compd 2016;682:418–23. Verma V, Beniwal A, Ohlan A, Tripathi R. Structural, magnetic and ferroelectric properties of Pr doped multiferroics bismuth ferrites. J Magn Magn Mater 2015;394:385–90. Mazumder R, Sujatha Devi P, Bhattacharya D, Choudhury P, Sen A, Raja M. Ferromagnetism in nanoscale BiFeO3. Appl Phys Lett 2007;91:062510. Mukherjee S, Gupta R, Garg A, Bansal V, Bhargava S. Influence of Zr doping on the structure and ferroelectric properties of BiFeO3 thin films. J Appl Phys 2010;107:123535. Qi XD, Dho J, Tomov R, Blamire MG, MacManus-Driscoll JL. Greatly reduced leakage current and conduction mechanism in aliovalent-ion-doped BiFeO3. Appl Phys Lett 2005;86:3. Ruette B, Zvyagin S, Pyatakov AP, Bush A, Li JF, Belotelov VI, et al. Magnetic-fieldinduced phase transition in BiFeO3 observed by high-field electron spin resonance: cycloidal to homogeneous spin order. Phys Rev B 2004;69:7. Jia DC, Xu JH, Ke H, Wang W, Zhou Y. Structure and multiferroic properties of BiFeO3 powders. J Eur Ceram Soc 2009;29:3099–103.