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Multiferroic magnetoelectric coupling and relaxor ferroelectric behavior in 0.7BiFeO3–0.3BaTiO3 nanocrystals
Solid State Communications 151 (2011) 920–923
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Multiferroic magnetoelectric coupling and relaxor ferroelectric behavior in 0.7BiFeO3 –0.3BaTiO3 nanocrystals Kuldeep Chand Verma a,∗ , R.K. Kotnala b a
Department of Physics, Eternal University, Baru Sahib, (H.P.) 173101, India
b
National Physical Laboratory, New Delhi-110012, India
article
info
Article history: Received 15 February 2011 Received in revised form 25 February 2011 Accepted 11 April 2011 by P. Sheng Available online 18 April 2011 Keywords: A. Nanostructures B. Chemical synthesis C. X-ray scattering D. Magnetoelectric coupling
abstract The structural, microstructural, polarization, magnetization, dielectric constant, and relaxor characteristics of 0.7BiFeO3 –0.3BaTiO3 (BF–BT) nanocrystals have been studied. BF–BT nanocrystals were prepared by a chemical route using polyvinyl alcohol as surfactant. The phase structure is confirmed by X-ray diffraction and average particle size by transmission and scanning electron microscopy. The magnetoelectric coupling is studied by polarization hysteresis loops under the influence of applied magnetic field and the phase transition anomaly. The diffuse phase transition is studied by modified Curie–Weiss law and relaxor characteristics by Vogel–Fulcher relation. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Currently, there is considerable interest in multiferroic (MF) materials of magnetoelectric coupling (ME) due to their potential applications in novel data storage and actuator devices where the magnetization could be altered by an electric field and electric polarization through a magnetic field [1–4]. In this context, BiFeO3 (BFO) is one of the most extensively studied MF magnetoelectric compound in which Bi 6s lone pair electrons are believed to be responsible for ferroelectricity [1–3] while partially filled ‘d’ orbitals of Fe lead to magnetic ordering. MF BFO with space group R3c has a high ferroelectric Curie temperature (TC = 820–850 °C) and a high Néel temperature (TN = 310– 370 °C), accordingly possessing simultaneous ferroelectric and antiferromagnetic orderings below TN [5,6]. It raises the possibility of potential devices based on ME coupling operating at the room temperature. It shows G-type antiferromagnetic spin configuration along the [001] h direction with a long range cycloidal spin structure incommensurate with the lattice along the [110] h direction of the hexagonal unit cell of the rhombohedral structure below ∼370 °C [5,7]. This long period modulated magnetic structure leads to cancelation of net macroscopic magnetization and hence inhibits the observation of a linear ME effect [8].
∗
Corresponding author. Tel.: +91 9418941286; fax: +91 01799276006. E-mail address: [email protected] (K.C. Verma).
0038-1098/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2011.04.012
The coupling of the antiferromagnetic and ferroelectric order parameters in BFO has recently been demonstrated under electric poling, where ferroelectric domain reorientation induces rotation of the spin spiral as well [9]. The incommensurate cycloidal spin structure can be suppressed with the application of high magnetic fields [10], and probably by chemical substitutions too [1–3]. ME coupling produced by the electric switching of magnetizations or magnetic switching of polarizations, i.e., switching from +M to −M with electric field E has been examined, or conversely from +Pr to −Pr with magnetic field H, in MF magnetoelectrics [11,12]. Also, the observation of an anomaly of the dielectric constant at the magnetic transition is taken as an unambiguous evidence for magnetoelectric coupling of intrinsic multiferroic origin [1–3]. Ferroelectric materials possessing polar microscopic domains show a sharp phase transition at the Curie temperature, whereas relaxors pass through different dynamical states [13,14]. However, the magnetoelectric properties of the system with mesoscopic order such as in relaxors are of technological importance. Since, the fabrication of pure BFO nanocrystals without impurity phases is generally difficult due to the formation of a Bi-deficient second phase such as Bi2 Fe4 O9 leading to poor electrical properties [15]. The involved reduction of Fe3+ to Fe2+ might affect the low electrical resistivity of BFO MF [16]. The formation of a solid solution between BFO and ferroelectric ABO3 compounds has been attractive as a means of improving structural stability and several other properties [11–14]. A (1 − x)BiFeO3 –xBaTiO3 system is
K.C. Verma, R.K. Kotnala / Solid State Communications 151 (2011) 920–923
921
expected to provide the desired structural stabilization, and its electrical properties. In this paper, we present the results of structure, micrographs, magnetization, dielectric behavior, electrical polarization under the influence of magnetic field (H ) and the relaxor characteristics of 0.7BiFeO3 –0.3BaTiO3 (BF–BT) nanocrystals. The 30% BaTiO3 substitution not only reduces the ferroelectric transition temperature of BFO, which enabled us to observe the high-temperature cubic phase but also led to the synthesis of nearly phase pure samples containing only the rhombohedral phase in the R3c space group. 2. Experimental details BF–BT nanocrystals were prepared by a chemical route using polyvinyl alcohol (PVA) as a surfactant. Bismuth chloride, barium acetate, iron chloride, tetra-n-butyl orthotitanate and PVA were used as the starting materials. A detailed procedure for material synthesis is given elsewhere [17]. The BF–BT powder was annealed at 650 °C for 2 h. The phase structure was analyzed by Xray diffraction (XRD) by using X-Pert PRO system, and the microstructure by transmission/scanning electron microscopy (TEM/SEM) by using Hitachi H-7500/JEOL JSM6100, respectively. The magnetization without field cooling was measured on the powder specimen of BF–BT by using VSM-735. The electrical measurements were carried out on BF–BT pellet specimen sintered at 1000 °C/5 h. For these measurements, Ag contact was deposited on both the surfaces of the pellet as top and bottom electrodes. The dielectric properties in the frequency range 100 kHz–10 MHz were studied in the temperature range of 298–725 K using an impedance analyzer (Wayne Kerr 6500B). Polarization under the influence of magnetic field was measured using Radiant Technologies ferroelectric test system.
Fig. 1. XRD pattern of BF–BT nanocrystals annealed at 650 °C for 2 h. Fig. 1(a) shows TEM image and SEM image (inset).
3. Results and discussion Fig. 1 show the XRD pattern of BF–BT nanocrystals annealed at 650 °C/2 h. It is clear that the BF–BT nanocrystals are highly crystallized and all diffraction peaks can be perfectly indexed (using ASTM card) for rhombohedrally distorted perovskite of R3c space group. Crystallization in perovskite BF–BT is attributable to the stabilization of the perovskite phase by the formation of a solid solution with BaTiO3 , because of the formation of Bi2 Fe4 O9 in pure BFO. The average particles size using Scherer’s relation of XRD peak is 27 nm. The average particles size from TEM image is 26 nm [Fig. 1(a)] and from SEM image [inset of Fig. 1(a)] it is 29 nm. Fig. 2 shows room-temperature measurement of polarization vs. applied electric field (P–E) under the influence of magnetic field (H = 0 − 0.6 T) and a recovery hysteresis after the removal of H for BF–BT pellet specimen. Without applying H, values of Pmax = 24.80, Pr = 15.13 µC cm−2 and Ec = 53.6 kV cm−1 are observed. These values of polarization are quite larger than they are observed in pure BFO [15,16] because the formation of Bi2 Fe4 O9 of low electrical resistivity. However the doping of BT is located at Fe site and acts as acceptors for improving the electrical properties of BF–BT nanocrystals. In this case of an atmosphere with a low oxygen concentration, since doped BT as an acceptor works effectively in the charge compensation of reduced Fe2+ ions at the Fe site of BF–BT, the amount of oxygen vacancies might increase in the crystal lattice. This reaction (the reduction of the Fe ion) is expressed as follows (1): × O× o + 2FeFe →
1
2
O2 + 2Fe′Fe + V¨o .
(1)
BT is substituted at the Fe3+ site might be accompanied by the formation of V¨O . From the above data, it is found that the O2
Fig. 2. P–E hysteresis under the application of an external magnetic field from 0 to 0.6 T for BF–BT nanocrystals. (a) P ∼ 0 at 0.6 T, showing small polarization values with lossy hysteresis assuming a contribution from the electrode. (b) The variation of Pr and Ec with applied H.
concentration and activity during the crystallization are important in fabricating of BF–BT nanocrystals with good electrical properties. Switching from +Pr to −Pr by E, and magnetic switching from +Pr to zero at 0.6 T have been observed. The switching of P with H has significance in detector applications [11]. Fig. 1(a) shows a low polarization response with lossy hysteresis of the BF–BT specimen at 0.6 T, which may be caused by the electrode, had also been observed by Kumar et al. [11]. This suggests that the switching of |P | to zero appeared at a value of H quite close to the field of magnetic saturation, H ≥ 0.6 T in the M–H loop [Fig. 3(a)]. The analysis indicates switching of polarization (Pr ∼ 0) near H = 0.6 T. The ferroelectric properties strongly depend on the external magnetic field H of ME multiferroics. A continuous increase in the polarization P is seen with an increase of H (Fig. 2), this is an indicative of the coupling between the order parameters: polarization and magnetization. When a magnetic field is applied to a magnetoelectric material, the material is strained. This strain induces a stress on the piezoelectrics (all ferroelectrics are piezoelectrics), which generates the electric field. This field can orient the ferroelectric domains, leading to an increase in polarization value. A similar increase of the saturation polarization with an increase of the magnetic field is observed in Tb doped BFO by Palkar et al. [18]. A continuous increase in the polarization Pr is seen with an increase of H up to 0.3 T and reduced abruptly after 0.4 T, and approached zero at 0.6 T as shown in Fig. 2(b), by considering the effect of H,
922
K.C. Verma, R.K. Kotnala / Solid State Communications 151 (2011) 920–923
Fig. 4. Plot of log(1/ε − 1/εm ) versus log(T − Tm ) at 1 MHz by a modified Curie–Weiss law, and inset shows VF characteristics.
The relaxor ferroelectric is confirmed by Vogel–Fulcher relationship [22] described as (3): Fig. 3. Temperature dependence of the ε and tan δ without the influence of a magnetic field, and magnetic susceptibility (χ ) measured at 500 Oe (inset (a) show M–H curve, and (b) variation of Tm with frequency) of BF–BT nanocrystals.
on Pr and Ec . Thus, the ferroelectric hysteresis flops under the application of external magnetic field (≥0.6 T), revealing a strong ME coupling in BF–BT system. The Fig. 3(a) show the M–H hysteresis loop of BF–BT nanocrystals measured at room temperature without field cooling, indicating a strong ferromagnetic nature. The ME coupling due to intrinsic effect of BF–BT nanocrystals is also indicated by the anomaly in the phase transition temperatures of electric and magnetic behavior. Fig. 3 shows the temperature dependence of the inverse of DC magnetization at H = 500 Oe in the temperature range 298–725 K for BF–BT nanocrystals. The extrapolation of the straight line fit to the high-temperature region gives a TN of 641 K. We observe an anomaly in the temperature dependable relative permittivity, ε(T ) by observing peaks near 641 K correspond to the TN in χ −1 (T ) curve as shown in Fig. 3. However, the ferroelectric phase transition temperature of parent BFO (820–850 °C) is reduced by the doping of BT (Curie temperature of BT is 120 °C [19]). Above observation in anomaly in the phase transition confirms the magnetoelectric coupling of intrinsic multiferroic origin, which can be correlated to the inverse of the Dsyaloshinskii–Moriya-type interaction occurring in complex magnetic structures such as noncollinear canted antiferromagnets, where the canted spin polarizes the off-center orbital through electron–lattice interaction [20]. Fig. 3 also shows frequency dispersion of ε and tan δ with a peak temperature Tm observed, representing the dielectric relaxor behavior. To find relaxor BF–BT nanocrystals, Fig. 3(b) shows the variation of Tm with frequency. The diffuse phase transition (DPT) feature of the BF–BT nanocrystals was fitted using the modified Curie–Weiss law [21] described as (2): 1
ε
−
1
εm
=
(T − Tm )γ C1
,
(2)
which defines the diffuseness of the relaxor phase transition, where γ (1 ≤ γ ≤ 2) is the degree of relaxation called dielectric diffusivity and C1 is assumed to be constant. When γ = 1, the material follows an ideal Curie–Weiss law and γ = 2 corresponds to DPT characteristics. Fig. 4 shows the plot of log(1/ε − 1/εm ) as function of log(T − Tm ) at 1 MHz, γ = 1.82 for BF–BT is obtained by linear fitting to the experimental data.
[ f = f0 exp
] −E a , kB (Tm − TVF )
(3)
which explained a freezing process of polar nanoregions (PNRs), where f is the measured frequency, f0 is a preexponential function, Ea is an activation energy, kB is the Boltzmann constant, Tm is the temperature corresponding to the dielectric maxima, and TVF is the characteristic Vogel–Fulcher freezing temperature. A good linear fit to ln f versus 1/(Tm − TVF ) is obtained in the frequency range of f = 100 kHz to 10 MHz as shown in Fig. 4(inset) for BF–BT specimen, suggesting VF -type dielectric relaxation. The fitting parameters, f0 = 0.55 × 1012 Hz, Ea = 0.039 eV, and TVF = 607 K are extracted. This relaxation process is quite similar to the relaxation process of nanodomains in normal relaxor materials [23]. 4. Conclusions In summary, the nanocrystals of BF–BT MF have been prepared by a chemical synthesis route at low temperature of crystallization (∼650 °C). The XRD pattern show rhombohedrally distorted perovskite phase of BF–BT MF. The average particles size from TEM 26 nm and from SEM 29 nm is measured. The ferroelectric hysteresis under the application of an external magnetic field and the flop near 0.6 T reveal a strong ME coupling in BF–BT MF. The intrinsic ME coupling of MF origin by anomaly in phase transition temperature is observed. The DPT results and the good linear fit of the VF relation show relaxor characteristics. References [1] W. Eerenstein, N.D. Mathur, J.F. Scott, Nature 442 (2006) 759. [2] A. Singh, A. Gupta, R. Chatterjee, Appl. Phys. Lett. 93 (2008) 022902. [3] W. Chen, S. Shannigrahi, X.F. Chen, Z.H. Wang, W. Zhu, O.K. Tana, Solid State Commun. 150 (2010) 271. [4] N.A. Hill, J. Phys. Chem. B 104 (2000) 6694. [5] I. Sosnowska, T.P. Neumaier, E. Steichele, J. Phys. C. Solid State Phys. 15 (1982) 4835. [6] C. Blaauw, F. Woude, J. Phys. C. Solid State Phys. 6 (1973) 1422. [7] W. Kaczmarek, Z. Pajak, M. Polomska, Solid State Commun. 17 (1975) 807. [8] W.M. Zhu, H.Y. Guo, Z.G. Ye, Phys. Rev. B 78 (2008) 014401. [9] D. Lebeugle, D. Colson, A. Forget, M. Viret, A.M. Bataille, A. Gukasov, Phys. Rev. Lett. 100 (2008) 227602. [10] B. Ruette, S. Zvyagin, A.P. Pyatakov, A. Bush, J.F. Li, V.I. Belotelov, A.K. Zvezdin, D. Viehland, Phys. Rev. B 69 (2004) 064114. [11] A. Kumar, G.L. Sharma, R.S. Katiyar, R. Pirc, R. Blinc, J.F. Scott, J. Phys:Condens. Matter 21 (2009) 382204. [12] V.R. Palkar, S.K. Malic, Solid State Commun. 134 (2005) 783. [13] A.A. Bokov, Z.G. Ye, J. Mater. Sci. 41 (2006) 31.
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S. Gupta, R.S. Katiyar, R. Guo, A.S. Bhalla, J. Raman Spectrosc. 31 (2000) 921. F. Tyholdt, S. Jorgensen, H. Fjellvag, A.E. Gunnaes, J. Mater. Res. 20 (2005) 2127. Y. Wang, Q.-H. Jiang, H.C. He, C.W. Nan, Appl. Phys. Lett. 88 (2006) 142503. K.C. Verma, R.K. Kotnala, N.S. Negi, Appl. Phys. Lett. 92 (2008) 152902. V.R. Palkar, D.C. Kundaliya, S.K. Malik, S. Bhattacharya, Phys. Rev. B 69 (2004) 212102.
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[19] A. Jana, T.K. Kundu, S.K. Pradhan, D. Chakravorty, J. Appl. Phys. 97 (2005) 044311. [20] S.W. Cheong, M. Mostovoy, Nature (London) 6 (2007) 13. [21] Y. Zhi, A. Chen, R.Y. Guo, A.S. Bhalla, J. Appl. Phys. 92 (2002) 2655. [22] R Pirc, R. Blinc, Phys. Rev. B 76 (2007) 020101. [23] A. Kumar, I. Rivera, R.S. Katiyar, J.F. Scott, Appl. Phys. Lett. 92 (2008) 132913.
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Multiferroic magnetoelectric coupling and relaxor ferroelectric behavior in 0.7BiFeO3 –0.3BaTiO3 nanocrystals Kuldeep Chand Verma a,∗ , R.K. Kotnala b a
Department of Physics, Eternal University, Baru Sahib, (H.P.) 173101, India
b
National Physical Laboratory, New Delhi-110012, India
article
info
Article history: Received 15 February 2011 Received in revised form 25 February 2011 Accepted 11 April 2011 by P. Sheng Available online 18 April 2011 Keywords: A. Nanostructures B. Chemical synthesis C. X-ray scattering D. Magnetoelectric coupling
abstract The structural, microstructural, polarization, magnetization, dielectric constant, and relaxor characteristics of 0.7BiFeO3 –0.3BaTiO3 (BF–BT) nanocrystals have been studied. BF–BT nanocrystals were prepared by a chemical route using polyvinyl alcohol as surfactant. The phase structure is confirmed by X-ray diffraction and average particle size by transmission and scanning electron microscopy. The magnetoelectric coupling is studied by polarization hysteresis loops under the influence of applied magnetic field and the phase transition anomaly. The diffuse phase transition is studied by modified Curie–Weiss law and relaxor characteristics by Vogel–Fulcher relation. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Currently, there is considerable interest in multiferroic (MF) materials of magnetoelectric coupling (ME) due to their potential applications in novel data storage and actuator devices where the magnetization could be altered by an electric field and electric polarization through a magnetic field [1–4]. In this context, BiFeO3 (BFO) is one of the most extensively studied MF magnetoelectric compound in which Bi 6s lone pair electrons are believed to be responsible for ferroelectricity [1–3] while partially filled ‘d’ orbitals of Fe lead to magnetic ordering. MF BFO with space group R3c has a high ferroelectric Curie temperature (TC = 820–850 °C) and a high Néel temperature (TN = 310– 370 °C), accordingly possessing simultaneous ferroelectric and antiferromagnetic orderings below TN [5,6]. It raises the possibility of potential devices based on ME coupling operating at the room temperature. It shows G-type antiferromagnetic spin configuration along the [001] h direction with a long range cycloidal spin structure incommensurate with the lattice along the [110] h direction of the hexagonal unit cell of the rhombohedral structure below ∼370 °C [5,7]. This long period modulated magnetic structure leads to cancelation of net macroscopic magnetization and hence inhibits the observation of a linear ME effect [8].
∗
Corresponding author. Tel.: +91 9418941286; fax: +91 01799276006. E-mail address: [email protected] (K.C. Verma).
0038-1098/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2011.04.012
The coupling of the antiferromagnetic and ferroelectric order parameters in BFO has recently been demonstrated under electric poling, where ferroelectric domain reorientation induces rotation of the spin spiral as well [9]. The incommensurate cycloidal spin structure can be suppressed with the application of high magnetic fields [10], and probably by chemical substitutions too [1–3]. ME coupling produced by the electric switching of magnetizations or magnetic switching of polarizations, i.e., switching from +M to −M with electric field E has been examined, or conversely from +Pr to −Pr with magnetic field H, in MF magnetoelectrics [11,12]. Also, the observation of an anomaly of the dielectric constant at the magnetic transition is taken as an unambiguous evidence for magnetoelectric coupling of intrinsic multiferroic origin [1–3]. Ferroelectric materials possessing polar microscopic domains show a sharp phase transition at the Curie temperature, whereas relaxors pass through different dynamical states [13,14]. However, the magnetoelectric properties of the system with mesoscopic order such as in relaxors are of technological importance. Since, the fabrication of pure BFO nanocrystals without impurity phases is generally difficult due to the formation of a Bi-deficient second phase such as Bi2 Fe4 O9 leading to poor electrical properties [15]. The involved reduction of Fe3+ to Fe2+ might affect the low electrical resistivity of BFO MF [16]. The formation of a solid solution between BFO and ferroelectric ABO3 compounds has been attractive as a means of improving structural stability and several other properties [11–14]. A (1 − x)BiFeO3 –xBaTiO3 system is
K.C. Verma, R.K. Kotnala / Solid State Communications 151 (2011) 920–923
921
expected to provide the desired structural stabilization, and its electrical properties. In this paper, we present the results of structure, micrographs, magnetization, dielectric behavior, electrical polarization under the influence of magnetic field (H ) and the relaxor characteristics of 0.7BiFeO3 –0.3BaTiO3 (BF–BT) nanocrystals. The 30% BaTiO3 substitution not only reduces the ferroelectric transition temperature of BFO, which enabled us to observe the high-temperature cubic phase but also led to the synthesis of nearly phase pure samples containing only the rhombohedral phase in the R3c space group. 2. Experimental details BF–BT nanocrystals were prepared by a chemical route using polyvinyl alcohol (PVA) as a surfactant. Bismuth chloride, barium acetate, iron chloride, tetra-n-butyl orthotitanate and PVA were used as the starting materials. A detailed procedure for material synthesis is given elsewhere [17]. The BF–BT powder was annealed at 650 °C for 2 h. The phase structure was analyzed by Xray diffraction (XRD) by using X-Pert PRO system, and the microstructure by transmission/scanning electron microscopy (TEM/SEM) by using Hitachi H-7500/JEOL JSM6100, respectively. The magnetization without field cooling was measured on the powder specimen of BF–BT by using VSM-735. The electrical measurements were carried out on BF–BT pellet specimen sintered at 1000 °C/5 h. For these measurements, Ag contact was deposited on both the surfaces of the pellet as top and bottom electrodes. The dielectric properties in the frequency range 100 kHz–10 MHz were studied in the temperature range of 298–725 K using an impedance analyzer (Wayne Kerr 6500B). Polarization under the influence of magnetic field was measured using Radiant Technologies ferroelectric test system.
Fig. 1. XRD pattern of BF–BT nanocrystals annealed at 650 °C for 2 h. Fig. 1(a) shows TEM image and SEM image (inset).
3. Results and discussion Fig. 1 show the XRD pattern of BF–BT nanocrystals annealed at 650 °C/2 h. It is clear that the BF–BT nanocrystals are highly crystallized and all diffraction peaks can be perfectly indexed (using ASTM card) for rhombohedrally distorted perovskite of R3c space group. Crystallization in perovskite BF–BT is attributable to the stabilization of the perovskite phase by the formation of a solid solution with BaTiO3 , because of the formation of Bi2 Fe4 O9 in pure BFO. The average particles size using Scherer’s relation of XRD peak is 27 nm. The average particles size from TEM image is 26 nm [Fig. 1(a)] and from SEM image [inset of Fig. 1(a)] it is 29 nm. Fig. 2 shows room-temperature measurement of polarization vs. applied electric field (P–E) under the influence of magnetic field (H = 0 − 0.6 T) and a recovery hysteresis after the removal of H for BF–BT pellet specimen. Without applying H, values of Pmax = 24.80, Pr = 15.13 µC cm−2 and Ec = 53.6 kV cm−1 are observed. These values of polarization are quite larger than they are observed in pure BFO [15,16] because the formation of Bi2 Fe4 O9 of low electrical resistivity. However the doping of BT is located at Fe site and acts as acceptors for improving the electrical properties of BF–BT nanocrystals. In this case of an atmosphere with a low oxygen concentration, since doped BT as an acceptor works effectively in the charge compensation of reduced Fe2+ ions at the Fe site of BF–BT, the amount of oxygen vacancies might increase in the crystal lattice. This reaction (the reduction of the Fe ion) is expressed as follows (1): × O× o + 2FeFe →
1
2
O2 + 2Fe′Fe + V¨o .
(1)
BT is substituted at the Fe3+ site might be accompanied by the formation of V¨O . From the above data, it is found that the O2
Fig. 2. P–E hysteresis under the application of an external magnetic field from 0 to 0.6 T for BF–BT nanocrystals. (a) P ∼ 0 at 0.6 T, showing small polarization values with lossy hysteresis assuming a contribution from the electrode. (b) The variation of Pr and Ec with applied H.
concentration and activity during the crystallization are important in fabricating of BF–BT nanocrystals with good electrical properties. Switching from +Pr to −Pr by E, and magnetic switching from +Pr to zero at 0.6 T have been observed. The switching of P with H has significance in detector applications [11]. Fig. 1(a) shows a low polarization response with lossy hysteresis of the BF–BT specimen at 0.6 T, which may be caused by the electrode, had also been observed by Kumar et al. [11]. This suggests that the switching of |P | to zero appeared at a value of H quite close to the field of magnetic saturation, H ≥ 0.6 T in the M–H loop [Fig. 3(a)]. The analysis indicates switching of polarization (Pr ∼ 0) near H = 0.6 T. The ferroelectric properties strongly depend on the external magnetic field H of ME multiferroics. A continuous increase in the polarization P is seen with an increase of H (Fig. 2), this is an indicative of the coupling between the order parameters: polarization and magnetization. When a magnetic field is applied to a magnetoelectric material, the material is strained. This strain induces a stress on the piezoelectrics (all ferroelectrics are piezoelectrics), which generates the electric field. This field can orient the ferroelectric domains, leading to an increase in polarization value. A similar increase of the saturation polarization with an increase of the magnetic field is observed in Tb doped BFO by Palkar et al. [18]. A continuous increase in the polarization Pr is seen with an increase of H up to 0.3 T and reduced abruptly after 0.4 T, and approached zero at 0.6 T as shown in Fig. 2(b), by considering the effect of H,
922
K.C. Verma, R.K. Kotnala / Solid State Communications 151 (2011) 920–923
Fig. 4. Plot of log(1/ε − 1/εm ) versus log(T − Tm ) at 1 MHz by a modified Curie–Weiss law, and inset shows VF characteristics.
The relaxor ferroelectric is confirmed by Vogel–Fulcher relationship [22] described as (3): Fig. 3. Temperature dependence of the ε and tan δ without the influence of a magnetic field, and magnetic susceptibility (χ ) measured at 500 Oe (inset (a) show M–H curve, and (b) variation of Tm with frequency) of BF–BT nanocrystals.
on Pr and Ec . Thus, the ferroelectric hysteresis flops under the application of external magnetic field (≥0.6 T), revealing a strong ME coupling in BF–BT system. The Fig. 3(a) show the M–H hysteresis loop of BF–BT nanocrystals measured at room temperature without field cooling, indicating a strong ferromagnetic nature. The ME coupling due to intrinsic effect of BF–BT nanocrystals is also indicated by the anomaly in the phase transition temperatures of electric and magnetic behavior. Fig. 3 shows the temperature dependence of the inverse of DC magnetization at H = 500 Oe in the temperature range 298–725 K for BF–BT nanocrystals. The extrapolation of the straight line fit to the high-temperature region gives a TN of 641 K. We observe an anomaly in the temperature dependable relative permittivity, ε(T ) by observing peaks near 641 K correspond to the TN in χ −1 (T ) curve as shown in Fig. 3. However, the ferroelectric phase transition temperature of parent BFO (820–850 °C) is reduced by the doping of BT (Curie temperature of BT is 120 °C [19]). Above observation in anomaly in the phase transition confirms the magnetoelectric coupling of intrinsic multiferroic origin, which can be correlated to the inverse of the Dsyaloshinskii–Moriya-type interaction occurring in complex magnetic structures such as noncollinear canted antiferromagnets, where the canted spin polarizes the off-center orbital through electron–lattice interaction [20]. Fig. 3 also shows frequency dispersion of ε and tan δ with a peak temperature Tm observed, representing the dielectric relaxor behavior. To find relaxor BF–BT nanocrystals, Fig. 3(b) shows the variation of Tm with frequency. The diffuse phase transition (DPT) feature of the BF–BT nanocrystals was fitted using the modified Curie–Weiss law [21] described as (2): 1
ε
−
1
εm
=
(T − Tm )γ C1
,
(2)
which defines the diffuseness of the relaxor phase transition, where γ (1 ≤ γ ≤ 2) is the degree of relaxation called dielectric diffusivity and C1 is assumed to be constant. When γ = 1, the material follows an ideal Curie–Weiss law and γ = 2 corresponds to DPT characteristics. Fig. 4 shows the plot of log(1/ε − 1/εm ) as function of log(T − Tm ) at 1 MHz, γ = 1.82 for BF–BT is obtained by linear fitting to the experimental data.
[ f = f0 exp
] −E a , kB (Tm − TVF )
(3)
which explained a freezing process of polar nanoregions (PNRs), where f is the measured frequency, f0 is a preexponential function, Ea is an activation energy, kB is the Boltzmann constant, Tm is the temperature corresponding to the dielectric maxima, and TVF is the characteristic Vogel–Fulcher freezing temperature. A good linear fit to ln f versus 1/(Tm − TVF ) is obtained in the frequency range of f = 100 kHz to 10 MHz as shown in Fig. 4(inset) for BF–BT specimen, suggesting VF -type dielectric relaxation. The fitting parameters, f0 = 0.55 × 1012 Hz, Ea = 0.039 eV, and TVF = 607 K are extracted. This relaxation process is quite similar to the relaxation process of nanodomains in normal relaxor materials [23]. 4. Conclusions In summary, the nanocrystals of BF–BT MF have been prepared by a chemical synthesis route at low temperature of crystallization (∼650 °C). The XRD pattern show rhombohedrally distorted perovskite phase of BF–BT MF. The average particles size from TEM 26 nm and from SEM 29 nm is measured. The ferroelectric hysteresis under the application of an external magnetic field and the flop near 0.6 T reveal a strong ME coupling in BF–BT MF. The intrinsic ME coupling of MF origin by anomaly in phase transition temperature is observed. The DPT results and the good linear fit of the VF relation show relaxor characteristics. References [1] W. Eerenstein, N.D. Mathur, J.F. Scott, Nature 442 (2006) 759. [2] A. Singh, A. Gupta, R. Chatterjee, Appl. Phys. Lett. 93 (2008) 022902. [3] W. Chen, S. Shannigrahi, X.F. Chen, Z.H. Wang, W. Zhu, O.K. Tana, Solid State Commun. 150 (2010) 271. [4] N.A. Hill, J. Phys. Chem. B 104 (2000) 6694. [5] I. Sosnowska, T.P. Neumaier, E. Steichele, J. Phys. C. Solid State Phys. 15 (1982) 4835. [6] C. Blaauw, F. Woude, J. Phys. C. Solid State Phys. 6 (1973) 1422. [7] W. Kaczmarek, Z. Pajak, M. Polomska, Solid State Commun. 17 (1975) 807. [8] W.M. Zhu, H.Y. Guo, Z.G. Ye, Phys. Rev. B 78 (2008) 014401. [9] D. Lebeugle, D. Colson, A. Forget, M. Viret, A.M. Bataille, A. Gukasov, Phys. Rev. Lett. 100 (2008) 227602. [10] B. Ruette, S. Zvyagin, A.P. Pyatakov, A. Bush, J.F. Li, V.I. Belotelov, A.K. Zvezdin, D. Viehland, Phys. Rev. B 69 (2004) 064114. [11] A. Kumar, G.L. Sharma, R.S. Katiyar, R. Pirc, R. Blinc, J.F. Scott, J. Phys:Condens. Matter 21 (2009) 382204. [12] V.R. Palkar, S.K. Malic, Solid State Commun. 134 (2005) 783. [13] A.A. Bokov, Z.G. Ye, J. Mater. Sci. 41 (2006) 31.
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