Structural, dielectric and magnetic studies of multiferroics Bi1-xSrxFeO3 (x = 0.1, 0.2, 0.3, 0.4, and 0.5)

Solid State Sciences 72 (2017) 5e9

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Structural, dielectric and magnetic studies of multiferroics Bi1-xSrxFeO3 (x ¼ 0.1, 0.2, 0.3, 0.4, and 0.5) Jaiparkash a, *, R.S. Chauhan b, Ravi Kumar c, Pawan Kumar d, Anil Kumar e a

Department of Physics, Manav Rachna University, Faridabad, 121 001, India Department of Physics, RBS College, Agra, 282 002, India c National Institute of Technology, Hamirpur, 177 005, India d Department of Physics, National Institute of Technology, Rourkela, 769 008, India e Department of Physics, Govt. College for Women, Bawani Khera, Bhiwani, 127 032, India b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 April 2017 Received in revised form 28 July 2017 Accepted 1 August 2017 Available online 4 August 2017

A series of Bi1-xSrxFeO3 (x ¼ 0.1, 0.2, 0.3, 0.4 and 0.5) multiferroic samples have been prepared using rapid two stage solid state reaction method in order to study the effect of different concentrations of strontium on the crystal structure, dielectric and magnetic properties. Structural analysis has been performed using x-ray diffraction (XRD) measurements. Rietveld refined XRD data confirms that all the samples are of single phase, having hexagonal structure with R3c space group. Unit cell volume decreases with increase in Sr2þ concentration. Dielectric measurements have been performed in the temperature range of 295 e520 K at selected frequencies in the range 100e900 kHz which show weak frequency dispersion. Activation energies are calculated for all the samples. Considerable enhancement in magnetic properties has been observed particularly for 40% and 50% doped samples. Further, exchange bias in hysteresis loop of 50% doped sample has also been observed. © 2017 Elsevier Masson SAS. All rights reserved.

Keywords: Multiferroics Canted antiferromagnetism Dielectric modulus Activation energy Exchange bias

1. Introduction Multiferroic materials (e.g. BiFeO3, BiMnO3 and YMnO3) have awestruck substantial research concentration due to the coexistence of ferroelectricity and magnetism [1]. These materials have excellent scope for use in multiple state memory elements and magnetic field sensors. But, the intricacy with most of the multiferroics is that they tend to have low temperature of magnetic transitions. However, the single-phase BiFeO3(BFO) ceramic exhibit better application potential by virtue of its higher ferroelectric curie temperature (Tc) of ~1103 K and antiferromagnetic Neel temperature (TN) of ~643 K [2]. In fact, the single phase bulk BiFeO3 exhibits a rhombohedral distorted perovskite structure with space group R3c, where all ions along the (111)c direction are displaced relative to the ideal centrosymmetric positions and oxygen octahedra rotate alternately clockwise and counter-clockwise about this (111)c direction. In this material, stereochemically active Bi3þ lone pair creates off-centre distortion resulting ferroelectric polarization, while partially filled 3d-orbitals of Fe3þ ions cause G-type

* Corresponding author. E-mail address: [email protected] (Jaiparkash). http://dx.doi.org/10.1016/j.solidstatesciences.2017.08.001 1293-2558/© 2017 Elsevier Masson SAS. All rights reserved.

antiferromagnetic ordering due to super exchange interaction [3]. Large leakage current and antiferromagnetic characters are big barriers in its practical applications [4,5]. In order to get rid of these barriers, some researchers have attempted doping at A-site by substituting Bi3þ with La3þ, Nd3þ, Sm3þ or Ba2þ and reported saturated ferroelectric or ferromagnetic hysteresis loops [6,7]. Another research group [8] has also observed an improved magnetic property by substituting partial Bi3þ with La3þ and Fe3þ with Mn4þ, respectively. Some researchers [9,10] have attempted heterovalent doping by Ca2þ, Sr2þ, Pb2þ and Ba2þ at Bi-site of BFO only up to 30% doping. They have shown that doping with Ca2þ and Sr2þ increases the conductivity and thus reducing the ferroelectric properties while Pb2þ maintains the parental character. The magnetization is found to increase with the increase in ionic radius of doped ion. Heterovalent Ca2þ e doping at Bi3þ- site may create either oxygen vacancies or there may be the conversion of Fe3þ to Fe4þ causing a change in its multiferroic properties. No increase in magnetization has been observed for Ca2þ and Sr2þ doped samples up to 30% doping level [9,10]. We also have reported the effect of Ca2þ and Ba2þ on the structural, dielectric and magnetic properties of BiFeO3 in our earlier studies [11,12]. Keeping in mind the above facts, Bi1-xSrxFeO3 (x ¼ 0.1, 0.2, 0.3, 0.4, and 0.5) samples have been synthesized; further their structural, dielectric and magnetic

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properties have been studied. An improvement in magnetic and dielectric properties of BiFeO3 has been noticed with doping. Further, exchange bias effect in case of 50% doped sample has been observed which has not been reported earlier to the best of our knowledge. 2. Experimental details Bulk samples of Bi1-xSrxFeO3 (x ¼ 0.1, 0.2, 0.3, 0.4, and 0.5) were synthesized using rapid two stage solid state reaction method rather than conventional solid state reaction method to inhibit the unwanted effects created due to volatilization of Bi. The stoichiometric sky-scraping purity amounts of Bi2O3, Fe2O3 and SrCO3 were ground in agate mortar more or less for two hours to make fine powder. After that the powder was pressed to a pellet (10 mm diameter) form and heated at 850  C for half an hour in first stage. After this the pellets were crushed and ground for an hour. These were again heated for ten minutes at 950  C after pressing to a pellet form. In both the stages, the samples were quenched by removing them from the furnace immediately after sintering. In order to examine the phase of the samples, powder x-ray diffraction patterns were collected at room temperature using Phillips X e ray diffractometer with Cu Ka radiation. Dielectric measurements were performed using Agilent 4285A (75 kHz-30 MHz) precision LCR meter in the temperature range of 295e520 K at selected frequencies in the range 100e900 kHz. Field dependent magnetization measurements of the samples have been performed using vibrating sample magnetometer (VSM) at room temperature. 3. Results and discussion 3.1. X e ray diffraction studies X-ray diffraction (XRD) patterns of Bi1-x Srx FeO3 (x ¼ 0.1, 0.2, 0.3, 0.4 & 0.5) samples were collected at room temperature. XRD data was additionally analyzed with Rietveld refinement using FULLPROF code to expose whether materials formed are of single phase in nature or not. Fig. 1 shows the XRD pattern along with the fitted curve and difference line. A good agreement between observed and calculated data suggests that Bi1-xSrxFeO3 samples are in single phase. These were indexed to hexagonal unit cell with R3c space group. In Bi1-xSrxFeO3, Bi/Sr is associated with Wyckoff position 6a (0, 0, z), Fe is at position 6a (0, 0, 0) and O is at 18b (x, y, z). All the parameters related to unit cell and refinements are listed in Table. It is clearly evident from the tabulated parameters that with

Fig. 1. X-ray diffraction pattern for the samples Bi1-xSrxFeO3(x ¼ 0.1,0.2,0.3,0.4 and 0.5) collected at 300 K.

increased doping, there is a reduction in the volume of the unit cell. It could be due to comparable ionic radius of Sr2þ and Bi3þ and due to creation of oxygen vacancies to accommodate charge imbalance [12,13,and14]. We also observe from the XRD pattern that Bragg reflections of all the samples are quite sharp. But, with the increase in the value of x from 0.1 to 0.5, a reduction in the intensities of all peaks except the most intense peak can be observed suggesting a lattice distortion in the samples due to Sr2þ doping [13]. Volatile nature of Bi2O3 might be responsible for the poor crystallization of these samples [15]. Also the average crystallite size is increasing with the increase in doping up to 40% doping which indicates that replacement of volatile Bi3þ with Sr2þ is making the crystal growth easy. Unlike a number of publications on heterovalent doped BiFeO3 [14,16], no structural phase transition has been observed even up to 50% doping. 3.2. Dielectric properties The temperature dependency (295e520 K) of dielectric constant (ε0 ) and loss tangent (tand) for x ¼ 0.1, 0.2, 0.3, 0.4 and 0.5 at selected frequencies has been displayed by Figs. 2 and 3 ε0 and tand increase with rising temperature at a specific frequency for all samples. Because of Sr2þ doping, there may be the formation of oxygen vacancies resulting in a distorted system. Thus the ordinary model of band conduction may be replaced by localized sites, which are surrounded by very high potential wells which cannot be surpassed by electrons. Dielectric polarization is the result of creation of dipoles at these localized hopping sites. Thus the phenomenon of localized charge hopping between spatially variable potentials may result in conduction as well as dipolar effects [11]. Also, ε0 decreases as the doping level increases from 10% to 30% (125 for 10%, 120.885 for 20%, 98.463 for 30%) at 450 K temperature (see Fig. 7). The value of ε0 starts increasing (194.962 for 40% and 336.736 for 50%) for further increasing the doping concentration at the same temperature. Although, the dielectric behaviour of the BiFeO3 system is very hard to understand but it can be described qualitatively as; replacing Bi3þ ions (having lone pair) by Sr2þ ions (not having lone pair) will reduce the off centre distortion on one hand resulting a decrease in dielectric constant as Bi3þ ions are responsible for ferroelectricity in these systems [19]. On the other hand, it is creating oxygen vacancies resulting in the collapse of oxygen octahedra (FeO6). These vacancies will induce hopping as well as dipoles resulting an enhancement in ε0 as well as tand. But the two processes might not be taking place at the same pace. As a result,

Fig.

2. Temperature dependence of dielectric constant ¼ 0.1,0.2,0.3,0.4 and 0.5) at selected frequencies.

xSrxFeO3(x

(ε0 )

for

Bi1-

Jaiparkash et al. / Solid State Sciences 72 (2017) 5e9

Fig.

3. Temperature dependence of loss tangent ¼ 0.1,0.2,0.3,0.4 and 0.5) at selected frequencies.

(tand)

for

7

Bi1-

xSrxFeO3(x

Fig. 6. The semi-log plot of relaxation frequency as a function of 1/T for Bi1¼ 0.1,0.2,0.3,0.4 and 0.5).

xSrxFeO3(x

Fig. 4. Variation of log (fε0 ) with log (f) for Bi1-xSrxFeO3(x ¼ 0.1,0.2,0.3,0.4 and 0.5).

Fig. 5. The plot of dielectric modulus (M00 ) against temperature at selected frequencies for Bi1-xSrxFeO3.

Fig. 7. Variation of ε0 with temperature for Bi1-xSrxFeO3(x ¼ 0.1,0.2,0.3,0.4 and 0.5) at 900 kHz.

there is a reduction in the dielectric constant up to 30% doping and an enhancement on further increasing the doping due to dominance of oxygen vacancy effect for these doping concentrations. The increase in temperature will increase the hopping of the charge giving rise to more conduction and dipolar effects which results in an enrichment in both dielectric constant and loss tangent. In order to confirm charge hopping, log (f ε0 ) is plotted against log f ( f is frequency). The linear behaviour in (see Fig. 4) is the signature of charge hopping inside the samples i.e. these are obeying UDR model [17]. Although there is a small deviation from linear behaviour in case of x ¼ 0.5. It is also recognized that the materials having intrinsic capacitance will only respond at such high frequency [18]. Also with the increase in frequency, there is a decrease in dielectric constant. It may be due to the fact that intra-well hopping probability of charge carriers dominates and dipoles are unable to follow the field reversals. Thus the value of ε0 decreases at high frequencies (900 kHz) attaining a value 72.298, 78.865, 63.02, 66.104 and 80.539 at room temperature for x ¼ 0.1, 0.2, 0.3, 0.4 and 0.5 respectively. Also, the values of tand are 0.00326, 0.00125, 0.00908, 0.14675 and 0.2657 at room temperature for x ¼ 0.1, 0.2,

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Jaiparkash et al. / Solid State Sciences 72 (2017) 5e9

0.3, 0.4 and 0.5 respectively. If we compare the results for x ¼ 0.3 with the earlier reports [10], then in our case the value of ε0 as well as tand is smaller. Further, the value of ε0 is slightly smaller while tand is approximately 1/9th of the value reported earlier. The small values of tand are the signature of defect free samples. The negligibly small values of tand (see Fig. 3) may project these materials as a potential candidate for multiferroic applications if anyhow ferromagnetic character is also introduced in the systems particularly at room temperature. Dielectric modulus M00 has been plotted (See Fig. 5) against temperature for getting the information about the relaxation mechanism for all the samples at selected frequencies. The nature of curves has been found to be almost same for all compositions. Peaks can be observed in the plots, which are shifting towards higher temperature with rise in frequency. The temperature at which the relaxation frequency (fr) is equal to the measuring frequency is provided by the peaks. The fr can be expressed by where fo is characteristic frequency and E is the activation energy for loss relaxation. Fig. 6 depicts the plot of ln (fr) as a function of reciprocal of temperature. The value of relaxation frequency increases with rise in temperature and verifies the above equation. The activation energies estimated from plots shown in Fig. 6 for x ¼ 0.1, 0.2, 0.3, 0.4 and 0.5 are found to be 0.947, 1.034, 0.794, 0.222 and 0.606 eV respectively. There is an irregular variation in activation energy which can be described as; Bi3þ ions are replaced by Sr2þ ions, which lead to oxygen vacancies for maintaining charge balance. Therefore, the conduction mechanism can be associated with oxygen vacancies (for low doping, up to x ¼ 0.3) [20]. At high doping concentrations (x ¼ 0.4 and 0.5), charge carriers, like electrons are responsible for conduction mechanism [21]. From imaginary part of electric modulus spectra, dielectric relaxation is observed at high temperature in samples up to x ¼ 0.3 and at low temperatures in x ¼ 0.4 and 0.5 samples. This indicates that high activation energy is required for conduction in x ¼ 0.1, 0.2 and 0.3 samples and low activation energy required for conduction in x ¼ 0.4 and 0.5 samples. This is in well agreement with the obtained activation energy values. 3.3. Magnetic properties The magnetic hysteresis loops have been measured over ±5 T at room temperature for all the samples in order to explore the

changes in magnetic properties of BiFeO3 owing to Sr e doping (Fig. 8). A clear hysteresis loop with finite remanent magnetization (Mr) and coerecive field (Hc) are observed for 40 and 50% doped samples while a negligibly small loops have been observed up to 30% doping. The value of remanent magnetization is negligibly small (0.01078 emu/g 0.01434 emu/g) upto 30% doping and it enhances considerably attaining a value of 0.14601 and 0.39042 emu/g for 40% and 50% doped samples respectively. These loops indicate the appearance of ferromagnetic character in first glance. On further analyzing these hysteresis curves, it is found that all these loops (Except in x ¼ 0.5) exhibit symmetric behaviour in both directions, horizontal as well as in vertical direction which discards the possibility of exchange bias (EB) phenomenon in the systems as we have observed in Ba doped samples [12]. In case of 50% doped sample, it is found that the loop has asymmetric behaviour in horizontal as well as vertical direction which is signature of exchange bias in this system. The value of exchange bias field has been calculated using the relation HEB ¼ Hc1þ Hc2/2 and found to be 152.402; where Hc1 and Hc2 are the coercive fields in positive and negative direction respectively [12]. The exchange bias magnetization has been calculated using the relation MEB ¼ Mr1þ Mr2/2 and found to be 0.0099; where Mr1 and Mr2 are values of remnant magnetizations in positive and negative directions respectively. The hysteresis and exchange bias effect in this sample indicates the coexistence of antiferromagnetic (AF) and ferromagnetic interactions (FM). Due to FM/AF interaction, FM spins tends to align in the field direction even when the field is reduced to negative coercive field and eventually the field overcomes the interface interaction. The upgrading in the magnetic properties of BiFeO3 could be due to three factors [22]: (i) suppression of the spiral spin structure due to small crystallite size (ii) increase in spin canting due to the lattice strain (iii) oxygen deficiency. The FWHM of XRD patterns of all samples is large i.e. the crystallite size is small which might be suppressing the spiral spin structure, enhancing the magnetization. Also, crystallite size increases as the doping is increased from 10% to 40% and decreases on further increasing the doping. Simultaneously the FeeOeFe bond angle is decreasing (See Table 1) as the doping is increased up to 40% and increases intriguingly for 50% doping attaining a value of 162 . Also, saturation in magnetization is almost achieved for 50% doped sample. As a result, the value of magnetization for the maximum measurement limit is maximum (1.076 emu/g) for 50% doped sample. Also, as Bi3þ ions are replaced by Sr2þ ions, there may be a creation of oxygen vacancies to compensate the charge imbalance resulting in the non e octahedral coordination of Fe3þ [23]. Some ferromagnetic interaction may be created due to the reduced super exchange interactions in the system which results in an increase in magnetization of the BiFeO3 system. Thus, Sr2þ doping in BFO greatly improves the magnetic properties which can be tailored by controlling Sr2þ concentration. 4. Conclusions

Fig. 8. Isothermal magnetization hysteresis for Bi1-xSrxFeO3(x ¼ 0.1,0.2,0.3,0.4 and 0.5) at 300 K.

Investigation of the crystal structure, dielectric and magnetic properties of Bi1-x SrxFeO3 (x ¼ 0.1, 0.2, 0.3, 0.4 and 0.5) has been carried out. The x e ray diffraction patterns show that all the samples are of single phase even up to 50% doping. The unit cell volume is found to decrease with the incorporation of Sr. A decrease in dielectric constant has been observed up to 30% doping level which has been explained in terms of oxygen vacancies. Small dielectric loss has been observed at room temperature. A considerable enhancement in the magnetization particularly for 50% doping with non - zero remnant magnetization has been observed with the incorporation of strontium showing a change in antiferromagnetic interaction of the parent material. Further, exchange

Jaiparkash et al. / Solid State Sciences 72 (2017) 5e9

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Table 1 Lattice and refinement parameters for Bi1-xSrxFeO3 (x ¼ 0.1, 0.2, 0.3, 0.4, 0.5). Composition (x)

Lattice Parameter (Å)

Volume of the Unit Cell (Å3)

c2

Rwp

Rexp

Average Crystallite size (nm)

Fe-O-Fe Bond angle (degree)

0.1

a ¼ 5.5725 c ¼ 13.8139 a ¼ 5.5889 c ¼ 13.7099 a ¼ 5.5899 c ¼ 13.6814 a ¼ 5.5892 c ¼ 13.6911 a ¼ 5.5842 c ¼ 13.6699

371.495

2.77

28.9

17.43

27.86

159

370.871

2.31

30.2

19.81

31.915

153.35

370.233

1.74

28.5

21.57

35.81

149.03

370.395

1.49

26.5

21.65

37.43

147

369.163

1.44

31.8

26.47

35.91

162.50

0.2 0.3 0.4 0.5

bias effect has been observed for 50% doped sample. References [1] G.L. Yuan, S.W. Or, Y.P. Wang, Z.G. Liu, J.M. Liu, Solid State Comm. 138 (2006) 76. [2] I. Sosnowska, T.P. Neumaier, E. Steichole, J. Phys. C. Solid State Phys. 15 (1982) 4835. [3] G.L. Yuan, K.Z. Baba -Kishi, J.M. Liu, S. Wing Or, Y.P. Wang, Z.G. Liu, J. Am. Ceram. Soc. 89 (2006) 3136. [4] A.K. Pradhan, K. Zhang, D. Hunter, J.B. Dadson, G.B. Loiutts, P. Bhattacharya, R. Katiyar, J. Zhang, D.J. Sellmyer, U.N. Roy, Y. Cui, A. Burger, J. Appl. Phys. 97 (2005) 093903. [5] M.M. Kumar, V.R. Palkar, Appl. Phys. Lett. 76 (2000) 2764. [6] G.L. Yuan, S.W. Or, J. Appl. Phys. 100 (2006) 024109. [7] D.H. Wang, W.C. Goh, M. Ning, C.K. Ong, Appl. Phys. Lett. 88 (2006) 212907. [8] V.R. Palkar, D.C. Kundaliya, S.K. Malik, J. Appl. Phys. 93 (2003) 4337. [9] V.A. Khomchenko, D.A. Kiselev, J.M. Vieira, A.L. Kholkin, M.A. Sa, Y.G. Pogorelov, Appl. Phys. Lett. 90 (2007) 242901. [10] V.A. Khomchenko, D.A. Kiselev, J.M. Vieira, Li Jian, A.L. Kholkin, A.M. L.Lopes, Y.G. Pogorelov, J.P. Araujo, M. Maglione, J. Appl. Phys. 103 (2008) 024105.

[11] Jaiparkash, Yogesh Kumar, R.S. Chauhan, Ravi Kumar, Solid State Sci. 13 (2011) 1869. [12] Jaiparkash, R.S. Chauhan, Ravi Kumar, Yogesh Kumar, N. Vijayan, J. Alloys Compd. 598 (2014) 248. [13] Deepti Kothari, V. Raghavendra Reddy, Ajay Gupta, Vasant Sathe, A. Banerjee, Appl. Phys. Lett. 91 (2007) 202505. [14] J. Li, Y. Duan, H. He, D. Song, J. Alloys Compd. 315 (2001) 259. [15] Benfang Yu, Meiya Li, Jun Liu, Dongyun Guo, Ling Pei, Xingzhong Zhao, J. Phys. D. Appl. Phys. 41 (2008) 065003. [16] G.L. Yuan, S.W. Or, J.M. Liu, Z.G. Liu, Appl. Phys. Lett. 89 (2006) 052905. [17] A.K. Jonscher, J. Phys. D. Appl. Phys. 32 (1999) R57. [18] G. Anjum, Ravi Kumar, S. Mollah, D.K. Shukla, Shalendra, C.G. Lee, J. Appl. Phys. 107 (2010), 103916-103916-9. [19] A.R. Makhdoom, M.J. Akhtar, M.A. Rafiq, M.M. Hassan, Ceram. Int. 38 (2012) 3829. [20] Pragya Pandit, S. Satapathy, P.K. Gupta, Phys. B 406 (2011) 2669. [21] Jayant Kolte, Paresh H. Salame, A.S. Daryapurkar, P. Gopalan, AIP Adv. 5 (2015) 097164. [22] R. Mazumder, P. Sujatha Devi, Dipten Bhattacharya, a P. Choudhury, A. Sen, M. Raja, Appl. Phys. Lett. 91 (2007) 062510. [23] Chou Yang, Jiang Ji-Sen, Qian Fang-Zhen, Jiang Dong-Mei, Wang Chun-Mei, Zhang Wei-Guo, J. Alloys Compd. 507 (2010) 29.