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Ferromagnetism and ferroelectricity in Fe doped BaTiO3
Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Ferromagnetism and ferroelectricity in Fe doped BaTiO3 Bipul Deka, S. Ravi n, A. Perumal, D. Pamu Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781039, Assam
art ic l e i nf o
Keywords: Perovskites Multiferroics Ferromagnetism Ferroelectricty Rietveld refinement dielectric and magnetic properties
a b s t r a c t We report the investigation of crystal structure, magnetic and dielectric properties of BaTi1 xFexO3 samples for x ¼0.0–0.3. The parent compound is found to crystallize in tetragonal structure while Fe doped samples are found to crystallize in the mixture of tetragonal and hexagonal phases but they are free from any impurity phase. Room temperature ferromagnetism with the transition temperature (Tc) of 462 K was observed for x ¼ 0.3 sample. Fe doped samples exhibit ferroelectric transition with transition temperature (TcF) in the range of 390 K for x ¼0.0–312 K for x ¼0.2. The dielectric constant, ε0 is found to decrease with the increase in doping concentrations. & 2014 Elsevier B.V. All rights reserved.
1. Introduction Multiferroism is a phenomenon of occurrence of two or more ferroic orderings such as ferroelectricty, ferromagnetism, ferroelasticity etc. in a single material [1–3]. The recent interest in multiferroics is due to magneto-electric effect and its application in spintronic devices [3,4]. Here, one can tune magnetization (dielectric polarization) by applying electric field (magnetic field) and it leads to the development of energy efficient and multifunctional devices [4]. Naturally occurring very few single phase compounds act as multiferroics and such compounds exhibit low transition temperature and are often difficult to synthesize in single phase form [3]. So, multiferroics continue to be research interest of material scientists. Perovskites compounds with ABO3 type structure are being studied extensively to induce multiferroicity [5]. BaTiO3 is one of the materials being explored for multiferroicity due to its excellent ferroelectric properties such as high dielectric constant at room temperature and the large ferroelectric transition temperature around 400 K [6] and for the suitability of pyroelectric sensor application [7]. The feasibility of multiferroicity in transition elements (TM) doped BaTiO3 has been proposed both from experimental and theoretical studies [8,9]. From electronic structure calculation, Xu et al. [8] predicted ferromagnetism (FM) in Fe doped BaTiO3 and they have also shown experimentally that 5 at% Fe doped BaTiO3 exhibits room temperature ferroelectricty and FM with Curie temperature (Tc)E680 K. Dang et al. [10,11] reported the ferromagnetism in Fe and Mn doped BaTiO3. The observed FM was explained on the basis of electron-mediated Zener-type Ruderman–Kittel–Kasuya–Yosida interaction [8,12], formation of bound magnetic polaron [13] and double exchange interactions
n
Corresponding author. Fax: þ 91 361 2690762. E-mail address: [email protected] (S. Ravi).
of Fe þ 3–O2–Fe þ 4 [6] etc. The substitution of Ti þ 4 ions by Fe ions leads to a decrease in ferroelectric Curie temperature (TcF) and dielectric constant (ε0 ) [7,14]. It is explained in terms of the creation of oxygen vacancies which leads to the breaking of co-operative vibration of Ti–O bonds. Moreover, it is known to alter the optical properties, especially by reducing the band edge energy [15]. It is observed that even a smaller concentration of doping of transition elements (TM) in BaTiO3 gives rise to change in crystal structure from tetragonal to hexagonal [11,16,17]. The evolution of hexagonal phase in TM doped BaTiO3 and its effect on different physical properties is yet to be understood in detail. Interestingly, there are two different sites for Ti ions [18] in hexagonal phase. In an attempt to introduce FM and to study the multiferroic properties, we have prepared BaTi1 xFexO3 up to x¼0.3 and studied their structural, magnetic and dielectric properties systematically.
2. Experimental techniques BaTi1 xFexO3 (x¼ 0.0, 0.1, 0.2 and 0.3) compounds were prepared by solid state reaction route. Stoichiometric ratios of BaCO3, TiO2 and Fe2O3 of 99.9% purity were weighed and mixed under acetone medium using mortar and pestle and presintered at 900 1C for 12 h. The sintering in pellet form was carried out at 1200 1C for 36 h. X-ray diffraction (XRD) patterns were recorded using Rigaku make TTRAX III diffractometer (18 kW) by employing Cu-Kα radiation. Magnetic field (H) and temperature (T) dependences of magnetization (M) were measured using a vibrating sample magnetometer (VSM, Lakeshore, model no.7410). Temperature variation of dielectric constant was measured using an LCR meter (Wayne Kerr Electronics Pvt. Ltd., model no.:1J43100).
http://dx.doi.org/10.1016/j.physb.2014.03.069 0921-4526/& 2014 Elsevier B.V. All rights reserved.
Please cite this article as: B. Deka, et al., Physica B (2014), http://dx.doi.org/10.1016/j.physb.2014.03.069i
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3. Results and discussions Typical XRD patterns recorded for x¼ 0.0 and 0.3 samples are shown in Fig. 1. The XRD pattern of the parent compound could be refined to P4mm space group with tetragonal cell and all the observed peaks could be refined (Fig. 1(a)). On the other hand, Fe doped samples contain both tetragonal and hexagonal crystal structural phases. However, no impurity phase was detected. It is generally known that these materials are found to crystallize both in tetragonal and hexagonal crystal structure. We have refined the XRD patterns of doped samples by considering the two phase model of tetragonal (P4mm) and hexagonal (P63/mmc) crystal structures. Such refinement is shown in Fig. 1(b) for x ¼0.3 sample. The lattice parameters of x¼0.0 sample and their c/a value of 1.006 are closely comparable to the literature [6,8]. Typical lattice parameters of tetragonal phase of x¼ 0.0 and 0.2 samples are a ¼3.9914 Å, c¼ 4.0173 Å and a ¼3.9972 Å, c¼4.0122 Å, respectively. Increase in lattice parameters (especially a) and unit cell volume is observed and it can be attributed to Fe þ 3 ions (ionic radius, ri ¼ 0.645 Å) replacing the Ti þ 4 (ri ¼ 0.605 Å) ions. However, the lattice parameters of x¼0.3 sample (a ¼b ¼3.9901 Å, and c¼ 3.9957 Å) are found to be quite small compared to x ¼0.0 sample. The distinct behavior of x ¼0.3 sample is due to the presence of predominant hexagonal phase (79%) and due to the possible substitution of Ti þ 4 ions by Fe þ 4 ions (ri ¼0.585 Å). The volume percentage of hexagonal phase is found to be 10% for x ¼0.1 sample and 19% for x ¼0.2 sample. Unlike the tetragonal phase, the lattice parameters of hexagonal phase are found to decrease with the increase in doping concentration. The average crystallite size (D) was estimated by using Scherrer formula,
D ¼ ð kλ=β cos θÞ where λ ¼ 1.5406 Å for Cu-Kα radiation, β is full width at half maximum, θ is Bragg's angle and k ¼0.89 assuming the circular grains. The average crystallite size values are found to be in the range of 32–54 nm. M–H curves were recorded at room temperature and it is found that samples with x r0.2 exhibit paramagnetic behaviors. However, x¼ 0.3 sample exhibits ferromagnetic behavior as shown in Fig. 2(a). In addition to the FM behavior, a linear behavior is observed at higher applied field. This can be attributed to the presence of considerable paramagnetic or antiferromagnetic (AFM) matrix in the system. The value of saturation magnetization, Ms obtained after subtracting the linear contribution is found to be 0.03 μB/Fe ion. The small Ms value suggests the lack of long range FM ordering. Similar low Ms values at room temperature have been reported in Fe doped BaTiO3 [11,13]. However, Guo et al. [6] and Xu et al. [8] reported Ms value close to 1.0 μB/ Fe. Temperature variation of magnetization (M–T) measurements show that x ¼0.1 and 0.2 samples exhibit paramagnetic behavior down to 25 K. High temperature (300 K–700 K) M–T plot of x ¼0.3 sample is shown in Fig. 2(b), where a clear ferromagnetic transition can be seen at Tc ¼462 K. The inset of Fig. 2(b) shows the Curie–Weiss law fit to the inverse susceptibility along with the experimental data. The effective paramagnetic moment μeff was estimated from the fitted values of Curie constant, C and they are found to be 5.85, 5.10 and 6.1 μB/Fe ion for x ¼0.1, 0.2 and 0.3, respectively. The expected theoretical effective magnetic moment þ3 μth ions is 5.9 μB/Fe ion. eff by considering the substitution of Fe The results of x ¼0.1 and x¼ 0.3 samples are comparable to the theoretical value, however x¼ 0.2 sample exhibits smaller experimental value. There is a possibility that of some of the doped
Fig. 1. XRD patterns along with Rietveld refinement for (a) x¼ 0.0 and (b) x¼ 0.3 samples.
Fig. 2. (a) M–H loops measured at room temperature for x¼ 0.0 and 0.3 samples, (b) M–T plot of x¼ 0.3 sample measured under H¼ 0.2 T. Inset shows the inverse susceptibility along with Curie–Weiss law fit.
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Ti–O bonds and affects the ferroelectric interactions and dielectric constant. Such a drastic reduction in ε0 value even for 0.6 mol% of Fe was reported by Jana et al. [19].
4. Conclusions
Fig. . 3 ε/–T plot for all samples measured at 10 kHz frequency.
Fe ions are in higher valence state, say Fe þ 4 and it can give rise to reduced μeff value. However we have not determined the actual valence state of Fe ions. Magnetic interactions are possible in four different networks through oxygen ions such as (a) octahedral–octahedral Fe þ 3 interaction, (b) pentahedral–pentahedral Fe þ 3 interaction, (c) pentahedral– octahedral Fe þ 3 interaction and (d) Fe þ 3–Fe þ 4 double exchange interaction [12]. Interaction (a) leads to antiferromagnetism, while other interactions give rise to FM. The rather small Ms value observed in x¼0.3 sample and the Curie temperature obtained from Curie– Weiss law fit suggests that the sample exhibits predominant AFM interaction along with a short range of FM interaction. Even though, M–H loops and dielectric measurements have been carried out by several authors, the work on the study of magnetic transition is limited [6,8]. The observed FM Tc of 462 K is far below the Tc of possible magnetic impurity phases such as Fe3O4 (850 K) and γ-Fe2O3. So, the observed FM in x¼0.3 sample can be attributed to an intrinsic effect rather than due to any impurity phase. The dielectric properties are studied by measuring the temperature dependent dielectric constant at 10 kHz frequency as shown in Fig. 3. The parent compound BaTiO3 exhibits a clear ferroelectric transition, at TcF ¼390 K with a maximum value of dielectric constant, ε0 ¼3000. At T4700 K, a secondary rise in ε0 is observed due to the relaxation phenomenon. Fe doping by 10 at% is found to drastically reduce the ε0 value by an order of magnitude and shift the TcF to 312 K. Similar behavior of reduced TcF is observed for x ¼0.2 sample but the broad hump due to the dielectric relaxation is found to shift towards lower temperature. The observed maximum dielectric constant is comparable to the results of Ray et al. [7] and Jayanthi et al. [14]. On the other hand, the relaxation peak is found to be sharper and moved down to 425 K for x ¼0.3 sample. The drastic reduction in TcF value even for Fe concentration within 10 at% has been also reported in the literature [7,14,19]. However in most of the reports, the dielectric measurements are limited up to 500 K, where they have not observed any relaxation phenomenon. Thus the drastic reductions in TcF value upon Fe substitution can be attributed to the creation of oxygen vacancy due to Fe þ 3 ions replacing Ti þ 4 ions. Such oxygen vacancy leads to the breaking of co-operative vibration of
BaTi1 xFexO3 samples without any impurity phase are prepared for x up to 0.3. The crystal structure of the parent compound is found to be tetragonal with space group P4mm while Fe doped samples are found to crystallize in the mixture of tetragonal and hexagonal phases. The Rietveld refinement to the two phase model suggests the growth of hexagonal phase from 10 vol% for x¼ 0.1–79 vol% for x ¼0.3 sample. The lattice parameters of tetragonal phase are found to increase with Fe concentration while those of hexagonal phase are found to decrease with increase in Fe concentration. Magnetization measurements show that x ¼0.3 sample exhibits room temperature FM with Tc around 460 K while all other Fe doped samples exhibit paramagnetic behavior. The maximum ε0 value of parent compound is 3000. The temperature dependence of dielectric measurement depicts the decrease in ferroelectric transition along with sharper relaxation phenomenon with increase in Fe concentration.
Acknowledgments Authors are thankful to DST, New Delhi, India for infrastructural facilities (SR/FST/PSII-020/2009 and SR/S2/CMP-19/2006) and for the research grant, vide Ref. no. SR/S2/CMP-0078/2010. We acknowledge the CIF, IIT Guwahati for VSM facility. References [1] M. Fiebig, J. Phys. D: Appl. Phys. 38 (2005) R123. [2] L.W. Martin, S.P. Crane, Y.-H. Chu, M.B. Holcomb, M. Gajek, M. Huijben, C.-H. Yang, N. Balke, R. Ramesh, J. Phys.: Condens. Matter 20 (2008) 434220. [3] C.-W. Nan, M.I. Bichurin, S. Dong, D. Viehland, G. Srinivasan, J. Appl. Phys. 103 (2008) 031101. [4] W. Prellier, M.P. Singh, P. Murugavel, J. Phys.: Condens. Matter 17 (2005) R803. [5] D.I. Khomskii, J. Magn. Magn. Mater. 306 (2006) 1. [6] Z. Guo, L. Yang, H. Qiu, X. Zhan, J. Yin, L. Cao, Mod. Phys. Lett. 26 (2012) 1250056. [7] J. Ray, P. Hing, J. Appl. Phys. 88 (2000) 1008. [8] B. Xu, K.B. Yin, J. Lin, Y.D. Xia, X.G. Wan, J. Yin, X.J. Bai, J. Du, Z.G. Liu, Phys. Rev. B 79 (2009) 134109. [9] H. Nakayama, H. Katayama-Yoshida, Jpn. J. Appl. Phys. 40 (2001) L1355. [10] N.V. Dang, T.-L. Phan, T.D. Thanh, V.D. Lam, L.V. Hong, J. Appl. Phys. 111 (2012) 113913. [11] N.V. Dang, T.D. Thanh, L.V. Hong, V.D. Lam, T.-L. Phan, J. Appl. Phys. 110 (2011) 043914. [12] F. Lin, D. Jiang, X. Ma, W. Shi, J. Magn. Magn. Mater. 320 (2008) 691. [13] S. Ray, P. Mahadevan, S. Mandal, S.R. Krishnakumar, C.S. Kuroda, T. Sasaki, T. Taniyama, M. Itoh, Phys. Rev. B 77 (2008) 104416. [14] S. Jayanthi, T.R.N. Kutty, J. Mater. Sci: Mater. Electron 19 (2008) 615. [15] D.M. Schleich, C. Derrington, W. Godek, D. Weisberg, A. Wold, Mater. Res. Bull. 12 (1977) 321. [16] Ha M. Nguyen, N.V. Dang, P.-Y. Chuang, T.D. Thanh, C.-W. Hu, T.-Y. Chen, V.D. Lam, C.-H. Lee, L.V. Hong, Appl. Phys. Lett. 99 (2011) 202501. [17] D. Ginting, S.C. Yu, T.L. Phan, N.V. Dang, T.D. Thanh, V.D. Lam, J. Korean Phys. Soc. 62 (2013) 2128. [18] J. Akimoto, Y. Gotoh, Y. Oosawa, Acta Crystallogr. C50 (1994) 160. [19] A. Jana, T.K. Kundu, S.K. Pradhan, D. Chakravorty, J. Appl. Phys. 97 (2005) 0443111.
Please cite this article as: B. Deka, et al., Physica B (2014), http://dx.doi.org/10.1016/j.physb.2014.03.069i
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Ferromagnetism and ferroelectricity in Fe doped BaTiO3 Bipul Deka, S. Ravi n, A. Perumal, D. Pamu Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781039, Assam
art ic l e i nf o
Keywords: Perovskites Multiferroics Ferromagnetism Ferroelectricty Rietveld refinement dielectric and magnetic properties
a b s t r a c t We report the investigation of crystal structure, magnetic and dielectric properties of BaTi1 xFexO3 samples for x ¼0.0–0.3. The parent compound is found to crystallize in tetragonal structure while Fe doped samples are found to crystallize in the mixture of tetragonal and hexagonal phases but they are free from any impurity phase. Room temperature ferromagnetism with the transition temperature (Tc) of 462 K was observed for x ¼ 0.3 sample. Fe doped samples exhibit ferroelectric transition with transition temperature (TcF) in the range of 390 K for x ¼0.0–312 K for x ¼0.2. The dielectric constant, ε0 is found to decrease with the increase in doping concentrations. & 2014 Elsevier B.V. All rights reserved.
1. Introduction Multiferroism is a phenomenon of occurrence of two or more ferroic orderings such as ferroelectricty, ferromagnetism, ferroelasticity etc. in a single material [1–3]. The recent interest in multiferroics is due to magneto-electric effect and its application in spintronic devices [3,4]. Here, one can tune magnetization (dielectric polarization) by applying electric field (magnetic field) and it leads to the development of energy efficient and multifunctional devices [4]. Naturally occurring very few single phase compounds act as multiferroics and such compounds exhibit low transition temperature and are often difficult to synthesize in single phase form [3]. So, multiferroics continue to be research interest of material scientists. Perovskites compounds with ABO3 type structure are being studied extensively to induce multiferroicity [5]. BaTiO3 is one of the materials being explored for multiferroicity due to its excellent ferroelectric properties such as high dielectric constant at room temperature and the large ferroelectric transition temperature around 400 K [6] and for the suitability of pyroelectric sensor application [7]. The feasibility of multiferroicity in transition elements (TM) doped BaTiO3 has been proposed both from experimental and theoretical studies [8,9]. From electronic structure calculation, Xu et al. [8] predicted ferromagnetism (FM) in Fe doped BaTiO3 and they have also shown experimentally that 5 at% Fe doped BaTiO3 exhibits room temperature ferroelectricty and FM with Curie temperature (Tc)E680 K. Dang et al. [10,11] reported the ferromagnetism in Fe and Mn doped BaTiO3. The observed FM was explained on the basis of electron-mediated Zener-type Ruderman–Kittel–Kasuya–Yosida interaction [8,12], formation of bound magnetic polaron [13] and double exchange interactions
n
Corresponding author. Fax: þ 91 361 2690762. E-mail address: [email protected] (S. Ravi).
of Fe þ 3–O2–Fe þ 4 [6] etc. The substitution of Ti þ 4 ions by Fe ions leads to a decrease in ferroelectric Curie temperature (TcF) and dielectric constant (ε0 ) [7,14]. It is explained in terms of the creation of oxygen vacancies which leads to the breaking of co-operative vibration of Ti–O bonds. Moreover, it is known to alter the optical properties, especially by reducing the band edge energy [15]. It is observed that even a smaller concentration of doping of transition elements (TM) in BaTiO3 gives rise to change in crystal structure from tetragonal to hexagonal [11,16,17]. The evolution of hexagonal phase in TM doped BaTiO3 and its effect on different physical properties is yet to be understood in detail. Interestingly, there are two different sites for Ti ions [18] in hexagonal phase. In an attempt to introduce FM and to study the multiferroic properties, we have prepared BaTi1 xFexO3 up to x¼0.3 and studied their structural, magnetic and dielectric properties systematically.
2. Experimental techniques BaTi1 xFexO3 (x¼ 0.0, 0.1, 0.2 and 0.3) compounds were prepared by solid state reaction route. Stoichiometric ratios of BaCO3, TiO2 and Fe2O3 of 99.9% purity were weighed and mixed under acetone medium using mortar and pestle and presintered at 900 1C for 12 h. The sintering in pellet form was carried out at 1200 1C for 36 h. X-ray diffraction (XRD) patterns were recorded using Rigaku make TTRAX III diffractometer (18 kW) by employing Cu-Kα radiation. Magnetic field (H) and temperature (T) dependences of magnetization (M) were measured using a vibrating sample magnetometer (VSM, Lakeshore, model no.7410). Temperature variation of dielectric constant was measured using an LCR meter (Wayne Kerr Electronics Pvt. Ltd., model no.:1J43100).
http://dx.doi.org/10.1016/j.physb.2014.03.069 0921-4526/& 2014 Elsevier B.V. All rights reserved.
Please cite this article as: B. Deka, et al., Physica B (2014), http://dx.doi.org/10.1016/j.physb.2014.03.069i
B. Deka et al. / Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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3. Results and discussions Typical XRD patterns recorded for x¼ 0.0 and 0.3 samples are shown in Fig. 1. The XRD pattern of the parent compound could be refined to P4mm space group with tetragonal cell and all the observed peaks could be refined (Fig. 1(a)). On the other hand, Fe doped samples contain both tetragonal and hexagonal crystal structural phases. However, no impurity phase was detected. It is generally known that these materials are found to crystallize both in tetragonal and hexagonal crystal structure. We have refined the XRD patterns of doped samples by considering the two phase model of tetragonal (P4mm) and hexagonal (P63/mmc) crystal structures. Such refinement is shown in Fig. 1(b) for x ¼0.3 sample. The lattice parameters of x¼0.0 sample and their c/a value of 1.006 are closely comparable to the literature [6,8]. Typical lattice parameters of tetragonal phase of x¼ 0.0 and 0.2 samples are a ¼3.9914 Å, c¼ 4.0173 Å and a ¼3.9972 Å, c¼4.0122 Å, respectively. Increase in lattice parameters (especially a) and unit cell volume is observed and it can be attributed to Fe þ 3 ions (ionic radius, ri ¼ 0.645 Å) replacing the Ti þ 4 (ri ¼ 0.605 Å) ions. However, the lattice parameters of x¼0.3 sample (a ¼b ¼3.9901 Å, and c¼ 3.9957 Å) are found to be quite small compared to x ¼0.0 sample. The distinct behavior of x ¼0.3 sample is due to the presence of predominant hexagonal phase (79%) and due to the possible substitution of Ti þ 4 ions by Fe þ 4 ions (ri ¼0.585 Å). The volume percentage of hexagonal phase is found to be 10% for x ¼0.1 sample and 19% for x ¼0.2 sample. Unlike the tetragonal phase, the lattice parameters of hexagonal phase are found to decrease with the increase in doping concentration. The average crystallite size (D) was estimated by using Scherrer formula,
D ¼ ð kλ=β cos θÞ where λ ¼ 1.5406 Å for Cu-Kα radiation, β is full width at half maximum, θ is Bragg's angle and k ¼0.89 assuming the circular grains. The average crystallite size values are found to be in the range of 32–54 nm. M–H curves were recorded at room temperature and it is found that samples with x r0.2 exhibit paramagnetic behaviors. However, x¼ 0.3 sample exhibits ferromagnetic behavior as shown in Fig. 2(a). In addition to the FM behavior, a linear behavior is observed at higher applied field. This can be attributed to the presence of considerable paramagnetic or antiferromagnetic (AFM) matrix in the system. The value of saturation magnetization, Ms obtained after subtracting the linear contribution is found to be 0.03 μB/Fe ion. The small Ms value suggests the lack of long range FM ordering. Similar low Ms values at room temperature have been reported in Fe doped BaTiO3 [11,13]. However, Guo et al. [6] and Xu et al. [8] reported Ms value close to 1.0 μB/ Fe. Temperature variation of magnetization (M–T) measurements show that x ¼0.1 and 0.2 samples exhibit paramagnetic behavior down to 25 K. High temperature (300 K–700 K) M–T plot of x ¼0.3 sample is shown in Fig. 2(b), where a clear ferromagnetic transition can be seen at Tc ¼462 K. The inset of Fig. 2(b) shows the Curie–Weiss law fit to the inverse susceptibility along with the experimental data. The effective paramagnetic moment μeff was estimated from the fitted values of Curie constant, C and they are found to be 5.85, 5.10 and 6.1 μB/Fe ion for x ¼0.1, 0.2 and 0.3, respectively. The expected theoretical effective magnetic moment þ3 μth ions is 5.9 μB/Fe ion. eff by considering the substitution of Fe The results of x ¼0.1 and x¼ 0.3 samples are comparable to the theoretical value, however x¼ 0.2 sample exhibits smaller experimental value. There is a possibility that of some of the doped
Fig. 1. XRD patterns along with Rietveld refinement for (a) x¼ 0.0 and (b) x¼ 0.3 samples.
Fig. 2. (a) M–H loops measured at room temperature for x¼ 0.0 and 0.3 samples, (b) M–T plot of x¼ 0.3 sample measured under H¼ 0.2 T. Inset shows the inverse susceptibility along with Curie–Weiss law fit.
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Ti–O bonds and affects the ferroelectric interactions and dielectric constant. Such a drastic reduction in ε0 value even for 0.6 mol% of Fe was reported by Jana et al. [19].
4. Conclusions
Fig. . 3 ε/–T plot for all samples measured at 10 kHz frequency.
Fe ions are in higher valence state, say Fe þ 4 and it can give rise to reduced μeff value. However we have not determined the actual valence state of Fe ions. Magnetic interactions are possible in four different networks through oxygen ions such as (a) octahedral–octahedral Fe þ 3 interaction, (b) pentahedral–pentahedral Fe þ 3 interaction, (c) pentahedral– octahedral Fe þ 3 interaction and (d) Fe þ 3–Fe þ 4 double exchange interaction [12]. Interaction (a) leads to antiferromagnetism, while other interactions give rise to FM. The rather small Ms value observed in x¼0.3 sample and the Curie temperature obtained from Curie– Weiss law fit suggests that the sample exhibits predominant AFM interaction along with a short range of FM interaction. Even though, M–H loops and dielectric measurements have been carried out by several authors, the work on the study of magnetic transition is limited [6,8]. The observed FM Tc of 462 K is far below the Tc of possible magnetic impurity phases such as Fe3O4 (850 K) and γ-Fe2O3. So, the observed FM in x¼0.3 sample can be attributed to an intrinsic effect rather than due to any impurity phase. The dielectric properties are studied by measuring the temperature dependent dielectric constant at 10 kHz frequency as shown in Fig. 3. The parent compound BaTiO3 exhibits a clear ferroelectric transition, at TcF ¼390 K with a maximum value of dielectric constant, ε0 ¼3000. At T4700 K, a secondary rise in ε0 is observed due to the relaxation phenomenon. Fe doping by 10 at% is found to drastically reduce the ε0 value by an order of magnitude and shift the TcF to 312 K. Similar behavior of reduced TcF is observed for x ¼0.2 sample but the broad hump due to the dielectric relaxation is found to shift towards lower temperature. The observed maximum dielectric constant is comparable to the results of Ray et al. [7] and Jayanthi et al. [14]. On the other hand, the relaxation peak is found to be sharper and moved down to 425 K for x ¼0.3 sample. The drastic reduction in TcF value even for Fe concentration within 10 at% has been also reported in the literature [7,14,19]. However in most of the reports, the dielectric measurements are limited up to 500 K, where they have not observed any relaxation phenomenon. Thus the drastic reductions in TcF value upon Fe substitution can be attributed to the creation of oxygen vacancy due to Fe þ 3 ions replacing Ti þ 4 ions. Such oxygen vacancy leads to the breaking of co-operative vibration of
BaTi1 xFexO3 samples without any impurity phase are prepared for x up to 0.3. The crystal structure of the parent compound is found to be tetragonal with space group P4mm while Fe doped samples are found to crystallize in the mixture of tetragonal and hexagonal phases. The Rietveld refinement to the two phase model suggests the growth of hexagonal phase from 10 vol% for x¼ 0.1–79 vol% for x ¼0.3 sample. The lattice parameters of tetragonal phase are found to increase with Fe concentration while those of hexagonal phase are found to decrease with increase in Fe concentration. Magnetization measurements show that x ¼0.3 sample exhibits room temperature FM with Tc around 460 K while all other Fe doped samples exhibit paramagnetic behavior. The maximum ε0 value of parent compound is 3000. The temperature dependence of dielectric measurement depicts the decrease in ferroelectric transition along with sharper relaxation phenomenon with increase in Fe concentration.
Acknowledgments Authors are thankful to DST, New Delhi, India for infrastructural facilities (SR/FST/PSII-020/2009 and SR/S2/CMP-19/2006) and for the research grant, vide Ref. no. SR/S2/CMP-0078/2010. We acknowledge the CIF, IIT Guwahati for VSM facility. References [1] M. Fiebig, J. Phys. D: Appl. Phys. 38 (2005) R123. [2] L.W. Martin, S.P. Crane, Y.-H. Chu, M.B. Holcomb, M. Gajek, M. Huijben, C.-H. Yang, N. Balke, R. Ramesh, J. Phys.: Condens. Matter 20 (2008) 434220. [3] C.-W. Nan, M.I. Bichurin, S. Dong, D. Viehland, G. Srinivasan, J. Appl. Phys. 103 (2008) 031101. [4] W. Prellier, M.P. Singh, P. Murugavel, J. Phys.: Condens. Matter 17 (2005) R803. [5] D.I. Khomskii, J. Magn. Magn. Mater. 306 (2006) 1. [6] Z. Guo, L. Yang, H. Qiu, X. Zhan, J. Yin, L. Cao, Mod. Phys. Lett. 26 (2012) 1250056. [7] J. Ray, P. Hing, J. Appl. Phys. 88 (2000) 1008. [8] B. Xu, K.B. Yin, J. Lin, Y.D. Xia, X.G. Wan, J. Yin, X.J. Bai, J. Du, Z.G. Liu, Phys. Rev. B 79 (2009) 134109. [9] H. Nakayama, H. Katayama-Yoshida, Jpn. J. Appl. Phys. 40 (2001) L1355. [10] N.V. Dang, T.-L. Phan, T.D. Thanh, V.D. Lam, L.V. Hong, J. Appl. Phys. 111 (2012) 113913. [11] N.V. Dang, T.D. Thanh, L.V. Hong, V.D. Lam, T.-L. Phan, J. Appl. Phys. 110 (2011) 043914. [12] F. Lin, D. Jiang, X. Ma, W. Shi, J. Magn. Magn. Mater. 320 (2008) 691. [13] S. Ray, P. Mahadevan, S. Mandal, S.R. Krishnakumar, C.S. Kuroda, T. Sasaki, T. Taniyama, M. Itoh, Phys. Rev. B 77 (2008) 104416. [14] S. Jayanthi, T.R.N. Kutty, J. Mater. Sci: Mater. Electron 19 (2008) 615. [15] D.M. Schleich, C. Derrington, W. Godek, D. Weisberg, A. Wold, Mater. Res. Bull. 12 (1977) 321. [16] Ha M. Nguyen, N.V. Dang, P.-Y. Chuang, T.D. Thanh, C.-W. Hu, T.-Y. Chen, V.D. Lam, C.-H. Lee, L.V. Hong, Appl. Phys. Lett. 99 (2011) 202501. [17] D. Ginting, S.C. Yu, T.L. Phan, N.V. Dang, T.D. Thanh, V.D. Lam, J. Korean Phys. Soc. 62 (2013) 2128. [18] J. Akimoto, Y. Gotoh, Y. Oosawa, Acta Crystallogr. C50 (1994) 160. [19] A. Jana, T.K. Kundu, S.K. Pradhan, D. Chakravorty, J. Appl. Phys. 97 (2005) 0443111.
Please cite this article as: B. Deka, et al., Physica B (2014), http://dx.doi.org/10.1016/j.physb.2014.03.069i