Dielectric and Magnetic Behavior of Sol-Gel Grown BiFeO3 Multiferroic A.B. Ravalia†, M.V. Vagadia†, U.D. Khachar†, R.R. Doshi†, P.S. Solanki†, B.T. Savalia!, N.A. Shah* and D.G. Kuberkar† †
Department of Physics, Saurashtra University, Rajkot - 360 005, India Department of Electronics, Saurashtra University, Rajkot - 360 005, India ! Shree Sardar Patel Vidyamandir, Rajkot - 360 005, India + E-mail:
[email protected]
*
Abstract. We report the dielectric and magnetic properties of BiFeO 3 (BFO) multiferroic synthesized by modified solgel route. Frequency dependent dielectric constant and loss studies show higher value at lower frequencies while temperature dependent dielectric behaviour show higher value of dielectric constant above 250K. Low magnetization value is exhibited by the BFO sample studied. DTA study shows the phase transition at 340 C and 816 C. Keywords: Multiferroics, sol-gel, dielectric PACS: 75.85.+t, 81.20.Fw, 77.22. Px, 75.40.-s
INTRODUCTION EXPERIMENTAL Multiferroics are known to exhibit ferromagnetic and ferroelectric properties simultaneously as well as coupling between them. BiFeO3 (BFO) multiferroic possesses high ferroelectric transition temperature (TC) and antiferromagnetic Neel temperature (TN) suitable for potential applications in functional devices [1]. BFO displays co-existence of ferroelectricity (TC ~ 816 – 845 C) and antiferromagnetism (TN ~ 375 C) over a wide temperature range. Prolonged sintering of BFO bulk at elevated temperature results in the oxygen stoichiometry deviation and valance fluctuations between of Fe+3 and Fe+2 leading to high conductivity making it difficult to observe ferroelectric hysteresis loop at room temperature (RT). Observation of P–E loop in BFO at 80 K has been reported by Teague et al [2]. It is also reported that the enhancement in magnetization in BFO thin films is thickness dependent [1]. Ferroelectric hysteresis loop at RT has been reported recently in BFO synthesized by rapid thermal process. [3]. In this communication, the results of the structural, dielectric and magnetic properties of BFO synthesized by modified sol-gel route have been discussed in the context of role of synthesis parameters in modifying these properties.
Modified sol-gel method has been employed to prepare BiFeO3 (BFO) multiferroics. Bismuth nitrate pentahydrate [Bi (NO3)3 5H2O] and iron nitrate nonahydrate [Fe (NO3)3 9H2 O] were dissolved in acetic acid and double distilled water with 1:1 ratio resulting in light blackish red solution. The solution was stirred constantly for 6 hrs between 70 C to 150 C and then heated on hot plate for 24 hrs. Heating of the solution at elevated temperature results in dark brownish powder of BFO. The pellets made under 4 Tonn pressure were sintered initially at 600 C for 12 hrs and finally at 800 C for 12 hrs. XRD measurements were carried out to determine structure and phase purity. SEM micrographs were obtained to understand the microstructure. Differential thermal analysis (DTA) was carried out up to 850 C to understand the phase transition, if any, in BFO. Frequency and temperature dependent dielectric constant and loss were measured using Agilent L-C-R meter with Lakeshore temperature controller.
RESULTS AND DISCUSSION XRD pattern of BFO obtained at RT show the single phasic nature of the sample crystallizing in hexagonal structure having R3c (no. 161) space group
Solid State Physics, Proceedings of the 55th DAE Solid State Physics Symposium 2010 AIP Conf. Proc. 1349, 1143-1144 (2011); doi: 10.1063/1.3606267 © 2011 American Institute of Physics 978-0-7354-0905-7/$30.00
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(Fig. 1). The crystallite size determined from XRD is ~35nm. SEM micrograph (not shown here) shows well developed grains having compact arrangement with an average grain size ~3-10µm. DTA analysis shows the phase transitions at 340 C and 816 C (Inset Fig. 1) which corresponds to the reported phase transition (TN and TC ) for BFO [4]. Frequency and temperature dependent dielectric measurements were carried out in the frequency and temperature range of 75KHz to 5MHz and 140K to 400K respectively.
Figure (4) shows M vs. H plot of sol- gel grown BFO collected at RT. Only a linear ferromagnetic hysteresis loop is observed without saturation in magnetization value up to 1T. This linear M-H loop differentiates from the paramagnetic material based on the observed antiferromagnetic phase transition in BFO. A similar magnetization character is also reported earlier [3, 5].
FIGURE 4. M vs. H curve of BFO at room temperature
CONCLUSION
FIGURE 1. XRD pattern of modified sol–gel grown BFO and inset figure shows DTA curve of BFO
In summary, single phase BFO has been synthesized by modified sol-gel route. Synthesis technique and sintering method play a major role in attributing ferroelectric property. The temperature and frequency dependent dielectric constant and loss indicate good dielectric behavior and M-H loop also supports the weak ferromagnetism at RT in the BFO. The non observation in P - E hysteresis loop may be attributed to the high leakage current in sol-gel grown BFO.
100 90 80 70 60 50 40 30 20 10 0
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Figure (2) shows the variation in dielectric constant (ε’) and loss (tanδ) with frequency. With the increases in frequency, ε’ and tanδ decreases with highest value of ε’ at lower frequencies which may be attributed to higher relaxation time [4, 5].
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ACKNOWLEDGMENTS
f (MHz) FIGURE 2. (colour online) Variation in ε’ and tanδ with respect to frequency at room temperature of BFO
This work is supported by IUAC, New Delhi UFUR research project of DGK. Authors thank Dr. K. Asokan, IUAC for dielectric measurement and Prof. Venugopal, Udaipur for magnetization studies.
14 12 10 8 6 4 2
0.5MHz 0.7MHz 0.9MHz 1.0MHz
0.8 0.6 tan
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Temperature dependent dielectric behavior of BFO at 0.5, 0.7, 0.9, 1.0 MHz frequencies is shown in Figure 3. Dielectric constant and loss values rapidly increase above 250K. The high values of tanδ at RT indicate that, the sintering process plays a key role in the dielectric properties. The higher loss generally originates from the higher conductivity, resulting higher leakage current which may arise due to fluctuations in Fe+2 and Fe+3 during long sintering process. This possess difficulty in the observation of P-E hysteresis loop in sol-gel grown BFO [6].
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REFERENCES 1. J. Wang, B.J. Neaton, H. Zheng, V. Nagarajan, S.B. Ogale, Science 299,1719-172 (2003). 2. J. R. Teague, R.Gerson, and W. J. James, Solid State Commun. 8,1073-1074 (1970). 3. Y P Wang, G L Yuan, X Y Chen, J-M Liu and Z G Liu, J. Phy. D: App. Phy. 39, 2019-2023 (2006). 4. M. Mahesh Kumar, V. R. Palkar, K. Srinivas and S.V. Suryanarayana, App. Phy. Lett. 76,(19)2764-2766 (2000). 5. V. R. Palkar, Darshan C. Kundaliya, S.K. Malik and S. Bhattacharya, Phy. Rev. B. 69, 212102 (2004). 6. A.K. Pradhan, K. Zhang, D. Hunter, J. Appl. Phys. 97, 093903-093904 (2005).
0.5 MHz 0.7 MHz 0.9 MHz 1.0 MHz
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150 200 250 300 350 400 T (K)
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FIGURE 3. (colour online) Temperature dependent (a) Dielectric constant and (b) Loss of BFO
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