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Synthesis and study of structural, dielectric, magnetic and magnetoelectric characterization of BiFeO3–NiFe2O4 nanocomposites prepared by chemical solution method
Journal of Alloys and Compounds 585 (2014) 805–810
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Synthesis and study of structural, dielectric, magnetic and magnetoelectric characterization of BiFeO3–NiFe2O4 nanocomposites prepared by chemical solution method Hemant Singh a,b, K.L. Yadav a,⇑ a b
Department of Physics, Government Post Graduate College, Uttarkashi 249193, India Smart Materials Research Laboratory, Department of Physics, Indian Institute of Technology Roorkee, Roorkee 247667, India
a r t i c l e
i n f o
Article history: Received 8 July 2013 Received in revised form 23 September 2013 Accepted 29 September 2013 Available online 12 October 2013 Keywords: Nanocomposites Saturation magnetization Coercivity Squareness Magnetoelectric effect
a b s t r a c t Magnetoelectric (ME) nanocomposites of xNiFe2O4–(1x)BiFeO3 (x = 0.1, 0.2, 0.3, 0.4) have been prepared by chemical solution method. The samples have been calcined at various temperatures ranging from 500 to 700 °C and X-ray diffraction analysis showed phase formation of xNiFe2O4–(1x)BiFeO3 nanocomposites calcined at 500 °C. The effect of annealing temperature on structural and magnetic properties was studied. TEM shows the formation of powders of nano order size and the average crystal size was found to be around 30–70 nm. Variation of dielectric constant and dielectric loss with frequency showed dispersion in the low frequency range and an anomaly in the temperature variation of dielectric constant is an evidence of presence of magnetoelectric coupling in the prepared nanocomposites. The magnetic behavior is found to be strongly dependent upon nickel ferrite content and annealing temperature. Magnetic field-induced relative change of the dielectric constant was observed in the nanocomposites. The fractional change of magnetic field induced in dielectric constant can also be expressed by De cM2 (where c is magnetoelectric coupling coefficient). Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Increasing attention has been focused on materials exhibiting simultaneous (anti)ferroelectric, (anti)ferromagnetic, (anti)ferroelastic and ferrotoroidic behaviours [1]. Such materials are classified as multiferroic materials. Materials in which ferroelectricity and ferromagnetism occur simultaneously in the same phase and allow the coupling between the two, which is a dielectric polarization induced by an external magnetic field or a magnetization induced by an applied electric field, are known as magnetoelectric (ME) multiferroics [2,3]. After the theoretical prediction of occurrence of ME effect, ME effect in Cr2O3 was experimentally observed [4]. ME-materials can be divided into classes: single phase materials and composites. The small ME effect and low working temperatures limit the use of single phase materials. Materials with good magnetic properties can be combined with materials with ferroelectric/piezoelectric properties to acquire ME composites. Suchtelene [5] introduced the concept of product property, a suitable combination of piezomagnetic and piezoelectric can give rise to ME effect. ME effect in composites is due to induced strain in the piezomagnetic phase by the application of magnetic field that is mechanically coupled to induce stress in the piezoelectric phase ⇑ Corresponding author. Tel.: +91 1332 285744; fax: +91 1332 286662. E-mail address: [email protected] (K.L. Yadav). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.09.201
that results into the generation of induced voltage. The ME effect in composites can be written as [6]:
ME effect ¼
magnetic mechanical electrical mechanical or mechanical electrical mechanical magnetic
BiFeO3(BFO) is a promising single phase ME material having high Néel temperature (TN 643 K) and high ferroelectric Curie temperature (TC 1103 K) [7] and have been widely studied in both ceramic and thin film forms [8–11]. BFO shows weak magnetic properties, though enhanced magnetic properties can be observed in chemically subsituted films and ceramics but the enhancement is limited. This results in weak ME effect in these materials. Low magnetization of BFO based single-phase materials is due to the intrinsic spatially modulated, incommensurate cycloidal spin structure. Chemical substitution can supress or destroy the spin cycloid and thus enhance magnetization and ME effect to some extent. Another way of increasing the magnetism for the BFO based single phase materials is to form composites with suitable ferrites. Nanocomposites of ferrites (CoFe2O4, ZnFe2O4, CuFe2O4) with BFO have been synthesized to enhance ME properties [12–14]. Nickel ferrite (NiFe2O4) is a well known cubic ferrimagnetic material with soft magnetic character and both low magnetic coercivity and low electrical conductivity, which has a inverse spinel structure where Ni2+ ions occupy octahedric B-sites and
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Fe3+ ions occupy both tetrahedric A-sites and octahedric B-sites [15]. As per literature survey not much work has been done on NiFe2O4–BiFeO3 spinel – perovskite nanocomposites. In this study, a sol– gel procedure is used for the preparation of xNiFe2O4–(1x)BiFeO3 (x = 0.1, 0.2, 0.3, 0.4) spinel – perovskite nanocomposites. Hence we report the effect of addition of Nickel ferrite on the dielectric, magnetic and magnetoelectric properties of xNiFe2O4–(1x)BiFeO3 nanocomposites prepared by sol–gel method. We have also studied the effect of annealing temperature on the structural and magnetic properties of the nanocomposites. 2. Experimental Magnetoelectric spinel – perovskite nanocomposites of xNiFe2O4 – (1x)BiFeO3 with x = 0.1, 0.2, 0.3 and 0.4 were prepared using water soluble inorganic salts. Nickel nitrate Ni(NO3)2.6H2O, Ferric nitrate Fe(NO3)39H2O, and Bismuth nitrate Bi(NO3)35H2O, citric acid and ethylene glycol were used in appropriate molar proportion. The precursor solution was dried at 80 °C for 6 h to obtain the xNiFe2O4– (1x)BiFeO3 xerogel powders. The xerogel initially started to swell and filled the beaker producing a foamy precursor. Then, the xerogel powders were ground and the resulting powders were annealed at various temperatures ranging from 500 to 700 °C to obtain the desired phase. Structural characterization of the powders was carried out using X-ray powder diffraction (XRD) with Cu Ka radiation. Field effect scanning electron microscopy (FESEM) and Transmission electron microscopy (TEM) were used to observe particle morphology and average particle size. Magnetic measurements were carried out at room temperature using a vibrating sample magnetometer (VSM) with a maximum magnetic field of 10 kOe. The dielectric measurements were done on silver coated pellets using HIKOI 3532-50 LCR Hi-Tester. To study the magnetodielectric (magnetocapacitance) measurement, a Wayn Kerr 6500 high frequency LCR meter was used along with a magnet upto 10 kOe and having accuracy of 10 Oe (provided by Marine India Pvt. Ltd.).
3. Results and discussions Fig. 1(a) shows the X-ray diffraction patterns of 0.3NiFe2O4– 0.7BiFeO3 powder annealed at different temperatures ranging from 500 to 700 oC for 2 h. A close observation of the XRD pattern of the prepared nanocomposites shows that composite consisting of NiFe2O4 and BiFeO3 has formed after being annealed at 500 °C. All diffraction peaks for the materials correspond to spinel – perovskite mixed structure is observed for the powders annealed at 500 °C. As the annealing temperature increases to 600 °C, the intensity of peaks in X-ray diffraction (XRD) pattern becomes stronger and sharper, indicating that there is a steady growth in crystallite size and the powders exhibit well-established broad peaks. On further increasing the annealing temperature to 700 °C, impurity peaks starts appearing. Hence 600 °C is the appropriate temperature for complete phase formation. All XRD peaks could be identified as of BiFeO3 (JCPDS Card No. 72-2493) and NiFe2O4 (JCPDS card No. 10-325) along with some impurity peaks of Bi36Fe2O57 (JCPDS Card No. 42-0181). These impurities are always obtained during the synthesis of BiFeO3 due to its chemical kinetics [13]. Fig. 1(b) shows the X-ray diffraction patterns of the compositions of xNiFe2O4–(1x)BiFeO3 with x = 0.1, 0.2, 0.3 and 0.4 annealed at 600 °C for 2 h. The phase analysis displays the formation of spinel – perovskite mixed structure. Fig. 1(b) exhibits that the intensity of NiFe2O4 peaks become stronger and broader with the increase of NiFe2O4 content in the composites. The Xray diffraction patterns of the four compositions indicated that mixed crystalline spinel – perovskite phases have been formed in each case. The most intense peaks of BiFeO3 phase (31.8° and 32.2°) are shifted to lower angles which mean increase in lattice parameters with the increase in Ni ferrite content. This means that there is a continual structural distortion in the composites. TEM observation of nanocomposite 0.3NiFe2O4–0.7BiFeO3 powder annealed at 600 °C was carried out. TEM images in Fig. 2(a) reveal spherical well-dispersed nanoparticles of 0.3NiFe2-
Fig. 1. (a) X-ray diffraction pattern of 0.3NiFe2O4–0.7BiFeO3 annealed at various temperatures. (*) represents BiFeO3 (JCPDS Card No. 72-2493), (O) represents NiFe2O4 (JCPDS card No. 10-325) and (+) represents Bi36Fe2O57 (JCPDS Card No. 420181). (b) X-ray diffraction pattern of xNiFe2O4–(1x)BiFeO3 (x = 0.1, 0.2, 0.3 and 0.4) annealed at 600 °C.
O4–0.7BiFeO3 spinel – perovskite powders annealed at 600 °C. From the TEM image, average particle sizes can be evaluated to consist of 30–70 nm nanosized powders. Supplementary information was obtained from selected area electron diffraction pattern of 0.3NiFe2O4–0.7BiFeO3 spinel – perovskite powders calcined at 600 °C. Electron diffraction patterns confirm the phases identified by XRD analysis. As a consequence of the small crystallite size, the samples (Fig. 2(b) show diffuse diffraction spots and this ED pattern can be ascribed to reflection of the spinel phase (NiFe2O4) and perovskite phase (BiFeO3). The surface microstructures of xNiFe2O4–(1x)BiFeO3 are shown in Fig. 3(a)–(d). The morphologies of the prepared samples are dense and from the micrographs, it is clear that the grains are randomly oriented and distributed over the entire sample. Fig. 4 shows the room temperature frequency dependence of dielectric constant (e) and dielectric loss tangent (tan d) up to 1 MHz frequency. Both decrease with increase in frequency for all the compositions due to the inability of the electric dipoles to be in pace with the frequency of applied electric field for high frequencies [16]. Dielectric constant is increased and tan d value for the nanocomposite is reduced with the addition of nickel ferrite. Fig. 5 shows the temperature dependence of dielectric constant and dielectric loss tangent at different frequencies ranging from
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Fig. 2. TEM image and SAED pattern of 0.3NiFe2O4–0.7BiFeO3 powder annealed at 600 °C.
Fig. 3. FESEM images of xNiFe2O4–(1x)BiFeO3 (x = 0.1, 0.2, 0.3 and 0.4) annealed at 600 °C.
10 kHz to 1 MHz for the prepared nanocomposite system. An anomaly in the dielectric constant (e) at 330 °C for the sample with x = 0.1 is observed which is shifted to 290 °C for sample with x = 0.2, and to 390 °C for the samples with x = 0.3 and 0.4. This anomaly is consistent with earlier reports [13,17]. The observed anomaly in dielectric pattern is attributed either due to a transient interaction between oxygen ion vacancies and the Fe3+/Fe2+ ions or due to the change in magnetic ordering in BiFeO3. The Landau– Devonshire theory of phase transitions predicated this type of dielectric anomaly in magnetoelectrically ordered systems as an effect of vanishing magnetic order on electric order [18] which is a signature of magnetoelectric (ME) coupling. Magnetic properties of 0.3NiFe2O4–0.7BiFeO3 nanocomposite annealed at various temperatures are investigated at room temperature and are shown in Fig. 6(a). The sample annealed at 500 °C showed the value of saturation magnetization Ms = 12.41 emu/g, remnant magnetization Mr = 2.17 emu/g and coercive field Hc = 129 Oe. On increasing the annealing temperature saturation magnetization decreases and it is lowest for the samples annealed
at 700 °C. The high value of magnetization at low temperature is due to disordered crystal site orientation of the ion in the spinel structure. After annealing at 600 °C, ordering takes place that has resulted in lowering of magnetic moment. The inset of Fig. 6(a) shows plots of coercivity (Hc) and squareness (Mr/Ms) as a function of annealing temperature of 0.3NiFe2O4–0.7BiFeO3 nanocomposites as a function of the annealing temperature ranging from 500 to 700 °C respectively. The coercivity increases with the increase in annealing temperature from 500 to 600 °C. This can be expected as crystallite size increases with an increase in annealing temperature. But the coercivity decreases with a further increase in temperature to 600 °C. If the increase in coercivity depends only on the crystallite size then it would not decrease on a further increase in temperature above 600 °C which results in additional nanocrystallite growth. There may be magnetization pinning defects incorporated into the nanocrystallites, due to lattice mismatch between the intergrowing nanocrystallites, after the onset of significant crystallite growth at intensity of the peaks increased. All the peaks can be identified; hence it is clear that there is no intermediate
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Fig. 4. Variation of e and tand of xNiFe2O4–(1x)BiFeO3 (x = 0.1, 0.2, 0.3 and 0.4) nanocomposites annealed at 600 °C with frequency at room temperature.
phase or interphase formed in the composite. Annealing at higher temperature would reduce these defects and result in the decrease in Hc observed. Magnetic properties of the spinel – perovskite xNiFe2O4–(1x)BiFeO3 with x = 0.1, 0.2, 0.3 and 0.4, annealed at
600 °C is shown in Fig. 6(b). All the samples exhibited strong magnetic characteristics indicating the presence of ordered magnetic structure in the nanocomposites. The hysteresis is small,which shows the nanocomposites are still soft magnetic materials, similar to NiFe2O4. The plots of saturation magnetization (Ms) and coercivity (Hc) of the nanocomposites as a function NiFe2O4 concentration is shown in the inset of Fig. 6 (b). The saturation magnetization (Ms) increases while the coercivity (Hc) decreases with increasing nickel ferrite content. The increase in saturation magnetization with increasing NiFe2O4 content provides an indication that the spontaneous magnetization of the nanocomposites originates from unbalanced antiparallel spins of the ferrimagnetic NiFe2O4. The decrease in the coercivity with increasing NiFe2O4 content means that magnetization can be realized with increasing NiFe2O4 content so that the interaction of the magnetic poles on the magnetic powders becomes energetic. To investigate the coupling of electric and magnetic polarizations between NiFe2O4 and BiFeO3, the dielectric constant at frequency 1 kHz is measured as a function of the applied magnetic field and is plotted in Fig. 7(a). For multiferroic composite materials under an external magnetic field, the magnetostriction in the magnetic phase produces stresses that are transferred in the ferroelectric phase, resulting in an electric polarization via the ME effect [18,19]. As a result, the dielectric behavior is modified. The change in dielectric constant (e), e(H) – e(0)/e(0), where e(H) and e(0) denotes the dielectric constants at applied magnetic field H and zero field, is a measure of the ME response. The value of ME
Fig. 5. Dielectric constant and dielectric loss tangent for xNiFe2O4–(1x)BiFeO3 (x = 0.1, 0.2, 0.3 and 0.4) nanocomposites as a function of temperature.
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ME response of NiFe2O4–BiFeO3 nanocomposites is stronger than Tb doped BFO [18], Nb doped BiFeO3 [20], La and Nb codoped BiFeO3 [21]. In a ferroelectromagnet, the thermodynamic potential U can be written in the form
b 2
U ¼ U0 þ aP2 þ P4 PE þ a0 P2 þ
b0 4 M MH þ cP2 M2 2
where P and M are the order parameters for the polarization and the magnetization, respectively, and a, b, a0 , b0 , and c are coupling coefficients. The term representing magnetoelectric coupling, P2M2, is always allowed regardless of symmetry. Kimura et al. used this free energy expression to calculate the effect of magnetic ordering on the dielectric susceptibility and shown that the effect of magnetic ordering on the dielectric constant will be proportional to De cM2 [22]. The sign of De depends on the sign of the constant magnetoelectric interaction c, and can be either positive or negative. The variation of De/e(0) as a function of the square of magnetization (M2) is shown in Fig. 7(b) for 0.3NiFe2O4–0.7BiFeO3 nanocomposite annealed at 600 °C. Inset of Fig. 7(b) shows the variation of M2 with the applied magnetic field. It can be seen that De/e(0) changes linearly with M2. Thus De/e(0) versus M2 can also be expressed by De cM2 for 0.3NiFe2O4–0.7BiFeO3 nanocomposite. Linear fitting with this formula give the value of c 4.38 104 (emu/g)2 for 0.3NiFe2O4–0.7BiFeO3 nanocomposite. The values of magnetoelectric coupling coefficient c are 3.87 103, 5.98 104 and 2.85 104 (emu/g)2 for the xNiFe2O4–(1x)BiFeO3 with x = 0.1, 0.2 and 0.4 respectively. 4. Conclusions
Fig. 6. (a) M–H curves of 0.3NiFe2O4–0.7BiFeO3 annealed at various temperatures. Inset shows plots of coercivity (Hc) and squareness (Mr/Ms) of 0.3NiFe2O4–0.7BiFeO3 nanocomposites as a function of the annealing temperature. (b) M–H curves of xNiFe2O4–(1x)BiFeO3 (x = 0.1, 0.2, 0.3 and 0.4) annealed at 600 °C. Inset shows variation of saturation magnetization (Ms) and coercivity (Hc) of for xNiFe2O4– (1x)BiFeO3 (x = 0.1, 0.2, 0.3 and 0.4) nanocomposites as a function of the NiFe2O4 content.
response at 8 k Oe is 2.8%, 3%, 3.6% and 4.5% for the xNiFe2O4– (1x)BiFeO3 nanocomposites with x = 0.1, 0.2, 0.3 and 0.4. The
The spinel – perovskite structured nanocomposites of xNiFe2O4–(1x)BiFeO3 nanocomposites with x = 0.1, 0.2, 0.3 and 0.4 have been synthesized by chemical solution method at various temperatures ranging from 500 to 700 °C. Nanocrystalline xNiFe2O4– (1x)BiFeO3 composites show the phase formation at 600 °C annealing temperature. TEM observation showed that the average particles size is around 30–70 nm. The variation of the dielectric constant and the dielectric loss with frequency showed dispersion in the low frequency range. An anomaly in dielectric constant with temperature suggests the magnetoelectric nature of the naocomposites. The effects of annealing temperature and the NiFe2O4 content in the composites on the magnetic properties of the spinel – perovskite system have been studied in detail. The saturation
Fig. 7. (a) Variation of dielectric constant with the applied magnetic field of xNiFe2O4–(1x)BiFeO3 (x = 0.1, 0.2, 0.3 and 0.4) nanocomposites annealed at 600 °C. (b) Variation of magnetocapacitance with (magnetization)2 for 0.3NiFe2O4–0.7BiFeO3 nanocomposite annealed at 600 °C. Inset shows variation of M2 with magnetic field.
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magnetization, squareness and coercivity values have been found to vary with the composition and annealing temperature due to the crystallite growth. The saturation magnetization increases with the increase of NiFe2O4 content in the nanocomposites. Strong dependence of dielectric constant on magnetic field of the synthesized nanocomposites proves the magnetoelecgtric coupling in the nanocomposite system and magnetoelectric coupling coefficient (c) is approximated by De cM2. Acknowledgments Hemant Singh would like to acknowledge University Grants Commission, New Delhi for approving Teacher fellowship under faculty improvement programme. References [1] W. Eerenstein, N.D. Mathur, J.F. Scott, Nat. (London) 442 (2006) 759–765. [2] M. Fiebig, J. Phys. D: Appl. Phys. 38 (2005) R123–152. [3] M. Zeng, J.G. Wan, Y. Wang, H. Yu, J.M. Liu, X.P. Jiang, C.W. Nan, J. Appl. Phys. 95 (2004) 8069–8073.
[4] G.T. Rado, V.J. Folen, Phys. Rev. Lett. 7 (1961) 310–311. [5] J.V. Suchethelene, Phillips Res. Rep. 27 (1972) 28. [6] C.W. Nan, M.I. Bichurin, S. Dong, D. Viehland, G. Srinivasan, J. Appl. Phys. 103 (2008) 031101. [7] Z. Cheng, X. Wang, C.V. Kannan, K. Ozawa, H. Kimura, T. Nishida, S. Zhang, T.R. Shrout, Appl. Phys. Lett. 88 (2006) 132909. [8] P. Uniyal, K.L. Yadav, J. Phys.: Condens. Matter 21 (2009) 405901. [9] P. Uniyal, K.L. Yadav, J. Phys.: Condens. Matter 21 (2009) 012205. [10] S.K. Singh, H. Ishiwara, K. Sato, K. Maruyama, J. Appl. Phys. 102 (2007) 094109. [11] Y.H. Lee, J.M. Wu, C.H. Lai, Appl. Phys. Lett. 88 (2006) 042903. [12] X.M. Liu, S.Y. Fu, C.J. Huang, Mater. Sci. Eng. B 121 (2005) 255–260. [13] P. Uniyal, K.L. Yadav, J. Alloy Comp. 492 (2010) 406–410. [14] M. Kumar, K.L. Yadav, Mater. Lett. 61 (2007) 2089–2092. [15] N.A. Spaldin, Magnetic Materials Fundamentals and Applications, second ed., Cambridge University Press, Cambridge UK, 2011. [16] C.G. Koops, Phys. Rev. 83 (1951) 121–124. [17] R.K. Mishra, D.K. Pradhan, R.N.P. Choudhary, A. Banerjee, J. Phys.: Condens. Matter 20 (2008) 045218. [18] V.R. Palkar, D.C. Kundaliya, S.K. Malik, S. Bhattacharya, Phys. Rev. B 69 (2004) 212102. [19] S.N. Babu, J.H. Hsu, Y.S. Chen, J.G. Lin, J. Appl. Phys. 109 (2011) 07D904. [20] Y.K. Jun, W.T. Moon, C.M. Chang, H.S. Kim, H.S. Ryu, J.W. Kim, K.H. Kim, S.H. Hong, Solid State Commun. 135 (2006) 133–137. [21] H. Singh, K.L. Yadav, J. Phys.: Condens Matter. 23 (2011) 385901. [22] T. Kimura, S. Kawamoto, I. Yamada, M. Azuma, M. Takano, Y. Tokura, Phys. Rev. B 67 (2003) 180401.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Synthesis and study of structural, dielectric, magnetic and magnetoelectric characterization of BiFeO3–NiFe2O4 nanocomposites prepared by chemical solution method Hemant Singh a,b, K.L. Yadav a,⇑ a b
Department of Physics, Government Post Graduate College, Uttarkashi 249193, India Smart Materials Research Laboratory, Department of Physics, Indian Institute of Technology Roorkee, Roorkee 247667, India
a r t i c l e
i n f o
Article history: Received 8 July 2013 Received in revised form 23 September 2013 Accepted 29 September 2013 Available online 12 October 2013 Keywords: Nanocomposites Saturation magnetization Coercivity Squareness Magnetoelectric effect
a b s t r a c t Magnetoelectric (ME) nanocomposites of xNiFe2O4–(1x)BiFeO3 (x = 0.1, 0.2, 0.3, 0.4) have been prepared by chemical solution method. The samples have been calcined at various temperatures ranging from 500 to 700 °C and X-ray diffraction analysis showed phase formation of xNiFe2O4–(1x)BiFeO3 nanocomposites calcined at 500 °C. The effect of annealing temperature on structural and magnetic properties was studied. TEM shows the formation of powders of nano order size and the average crystal size was found to be around 30–70 nm. Variation of dielectric constant and dielectric loss with frequency showed dispersion in the low frequency range and an anomaly in the temperature variation of dielectric constant is an evidence of presence of magnetoelectric coupling in the prepared nanocomposites. The magnetic behavior is found to be strongly dependent upon nickel ferrite content and annealing temperature. Magnetic field-induced relative change of the dielectric constant was observed in the nanocomposites. The fractional change of magnetic field induced in dielectric constant can also be expressed by De cM2 (where c is magnetoelectric coupling coefficient). Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Increasing attention has been focused on materials exhibiting simultaneous (anti)ferroelectric, (anti)ferromagnetic, (anti)ferroelastic and ferrotoroidic behaviours [1]. Such materials are classified as multiferroic materials. Materials in which ferroelectricity and ferromagnetism occur simultaneously in the same phase and allow the coupling between the two, which is a dielectric polarization induced by an external magnetic field or a magnetization induced by an applied electric field, are known as magnetoelectric (ME) multiferroics [2,3]. After the theoretical prediction of occurrence of ME effect, ME effect in Cr2O3 was experimentally observed [4]. ME-materials can be divided into classes: single phase materials and composites. The small ME effect and low working temperatures limit the use of single phase materials. Materials with good magnetic properties can be combined with materials with ferroelectric/piezoelectric properties to acquire ME composites. Suchtelene [5] introduced the concept of product property, a suitable combination of piezomagnetic and piezoelectric can give rise to ME effect. ME effect in composites is due to induced strain in the piezomagnetic phase by the application of magnetic field that is mechanically coupled to induce stress in the piezoelectric phase ⇑ Corresponding author. Tel.: +91 1332 285744; fax: +91 1332 286662. E-mail address: [email protected] (K.L. Yadav). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.09.201
that results into the generation of induced voltage. The ME effect in composites can be written as [6]:
ME effect ¼
magnetic mechanical electrical mechanical or mechanical electrical mechanical magnetic
BiFeO3(BFO) is a promising single phase ME material having high Néel temperature (TN 643 K) and high ferroelectric Curie temperature (TC 1103 K) [7] and have been widely studied in both ceramic and thin film forms [8–11]. BFO shows weak magnetic properties, though enhanced magnetic properties can be observed in chemically subsituted films and ceramics but the enhancement is limited. This results in weak ME effect in these materials. Low magnetization of BFO based single-phase materials is due to the intrinsic spatially modulated, incommensurate cycloidal spin structure. Chemical substitution can supress or destroy the spin cycloid and thus enhance magnetization and ME effect to some extent. Another way of increasing the magnetism for the BFO based single phase materials is to form composites with suitable ferrites. Nanocomposites of ferrites (CoFe2O4, ZnFe2O4, CuFe2O4) with BFO have been synthesized to enhance ME properties [12–14]. Nickel ferrite (NiFe2O4) is a well known cubic ferrimagnetic material with soft magnetic character and both low magnetic coercivity and low electrical conductivity, which has a inverse spinel structure where Ni2+ ions occupy octahedric B-sites and
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Fe3+ ions occupy both tetrahedric A-sites and octahedric B-sites [15]. As per literature survey not much work has been done on NiFe2O4–BiFeO3 spinel – perovskite nanocomposites. In this study, a sol– gel procedure is used for the preparation of xNiFe2O4–(1x)BiFeO3 (x = 0.1, 0.2, 0.3, 0.4) spinel – perovskite nanocomposites. Hence we report the effect of addition of Nickel ferrite on the dielectric, magnetic and magnetoelectric properties of xNiFe2O4–(1x)BiFeO3 nanocomposites prepared by sol–gel method. We have also studied the effect of annealing temperature on the structural and magnetic properties of the nanocomposites. 2. Experimental Magnetoelectric spinel – perovskite nanocomposites of xNiFe2O4 – (1x)BiFeO3 with x = 0.1, 0.2, 0.3 and 0.4 were prepared using water soluble inorganic salts. Nickel nitrate Ni(NO3)2.6H2O, Ferric nitrate Fe(NO3)39H2O, and Bismuth nitrate Bi(NO3)35H2O, citric acid and ethylene glycol were used in appropriate molar proportion. The precursor solution was dried at 80 °C for 6 h to obtain the xNiFe2O4– (1x)BiFeO3 xerogel powders. The xerogel initially started to swell and filled the beaker producing a foamy precursor. Then, the xerogel powders were ground and the resulting powders were annealed at various temperatures ranging from 500 to 700 °C to obtain the desired phase. Structural characterization of the powders was carried out using X-ray powder diffraction (XRD) with Cu Ka radiation. Field effect scanning electron microscopy (FESEM) and Transmission electron microscopy (TEM) were used to observe particle morphology and average particle size. Magnetic measurements were carried out at room temperature using a vibrating sample magnetometer (VSM) with a maximum magnetic field of 10 kOe. The dielectric measurements were done on silver coated pellets using HIKOI 3532-50 LCR Hi-Tester. To study the magnetodielectric (magnetocapacitance) measurement, a Wayn Kerr 6500 high frequency LCR meter was used along with a magnet upto 10 kOe and having accuracy of 10 Oe (provided by Marine India Pvt. Ltd.).
3. Results and discussions Fig. 1(a) shows the X-ray diffraction patterns of 0.3NiFe2O4– 0.7BiFeO3 powder annealed at different temperatures ranging from 500 to 700 oC for 2 h. A close observation of the XRD pattern of the prepared nanocomposites shows that composite consisting of NiFe2O4 and BiFeO3 has formed after being annealed at 500 °C. All diffraction peaks for the materials correspond to spinel – perovskite mixed structure is observed for the powders annealed at 500 °C. As the annealing temperature increases to 600 °C, the intensity of peaks in X-ray diffraction (XRD) pattern becomes stronger and sharper, indicating that there is a steady growth in crystallite size and the powders exhibit well-established broad peaks. On further increasing the annealing temperature to 700 °C, impurity peaks starts appearing. Hence 600 °C is the appropriate temperature for complete phase formation. All XRD peaks could be identified as of BiFeO3 (JCPDS Card No. 72-2493) and NiFe2O4 (JCPDS card No. 10-325) along with some impurity peaks of Bi36Fe2O57 (JCPDS Card No. 42-0181). These impurities are always obtained during the synthesis of BiFeO3 due to its chemical kinetics [13]. Fig. 1(b) shows the X-ray diffraction patterns of the compositions of xNiFe2O4–(1x)BiFeO3 with x = 0.1, 0.2, 0.3 and 0.4 annealed at 600 °C for 2 h. The phase analysis displays the formation of spinel – perovskite mixed structure. Fig. 1(b) exhibits that the intensity of NiFe2O4 peaks become stronger and broader with the increase of NiFe2O4 content in the composites. The Xray diffraction patterns of the four compositions indicated that mixed crystalline spinel – perovskite phases have been formed in each case. The most intense peaks of BiFeO3 phase (31.8° and 32.2°) are shifted to lower angles which mean increase in lattice parameters with the increase in Ni ferrite content. This means that there is a continual structural distortion in the composites. TEM observation of nanocomposite 0.3NiFe2O4–0.7BiFeO3 powder annealed at 600 °C was carried out. TEM images in Fig. 2(a) reveal spherical well-dispersed nanoparticles of 0.3NiFe2-
Fig. 1. (a) X-ray diffraction pattern of 0.3NiFe2O4–0.7BiFeO3 annealed at various temperatures. (*) represents BiFeO3 (JCPDS Card No. 72-2493), (O) represents NiFe2O4 (JCPDS card No. 10-325) and (+) represents Bi36Fe2O57 (JCPDS Card No. 420181). (b) X-ray diffraction pattern of xNiFe2O4–(1x)BiFeO3 (x = 0.1, 0.2, 0.3 and 0.4) annealed at 600 °C.
O4–0.7BiFeO3 spinel – perovskite powders annealed at 600 °C. From the TEM image, average particle sizes can be evaluated to consist of 30–70 nm nanosized powders. Supplementary information was obtained from selected area electron diffraction pattern of 0.3NiFe2O4–0.7BiFeO3 spinel – perovskite powders calcined at 600 °C. Electron diffraction patterns confirm the phases identified by XRD analysis. As a consequence of the small crystallite size, the samples (Fig. 2(b) show diffuse diffraction spots and this ED pattern can be ascribed to reflection of the spinel phase (NiFe2O4) and perovskite phase (BiFeO3). The surface microstructures of xNiFe2O4–(1x)BiFeO3 are shown in Fig. 3(a)–(d). The morphologies of the prepared samples are dense and from the micrographs, it is clear that the grains are randomly oriented and distributed over the entire sample. Fig. 4 shows the room temperature frequency dependence of dielectric constant (e) and dielectric loss tangent (tan d) up to 1 MHz frequency. Both decrease with increase in frequency for all the compositions due to the inability of the electric dipoles to be in pace with the frequency of applied electric field for high frequencies [16]. Dielectric constant is increased and tan d value for the nanocomposite is reduced with the addition of nickel ferrite. Fig. 5 shows the temperature dependence of dielectric constant and dielectric loss tangent at different frequencies ranging from
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Fig. 2. TEM image and SAED pattern of 0.3NiFe2O4–0.7BiFeO3 powder annealed at 600 °C.
Fig. 3. FESEM images of xNiFe2O4–(1x)BiFeO3 (x = 0.1, 0.2, 0.3 and 0.4) annealed at 600 °C.
10 kHz to 1 MHz for the prepared nanocomposite system. An anomaly in the dielectric constant (e) at 330 °C for the sample with x = 0.1 is observed which is shifted to 290 °C for sample with x = 0.2, and to 390 °C for the samples with x = 0.3 and 0.4. This anomaly is consistent with earlier reports [13,17]. The observed anomaly in dielectric pattern is attributed either due to a transient interaction between oxygen ion vacancies and the Fe3+/Fe2+ ions or due to the change in magnetic ordering in BiFeO3. The Landau– Devonshire theory of phase transitions predicated this type of dielectric anomaly in magnetoelectrically ordered systems as an effect of vanishing magnetic order on electric order [18] which is a signature of magnetoelectric (ME) coupling. Magnetic properties of 0.3NiFe2O4–0.7BiFeO3 nanocomposite annealed at various temperatures are investigated at room temperature and are shown in Fig. 6(a). The sample annealed at 500 °C showed the value of saturation magnetization Ms = 12.41 emu/g, remnant magnetization Mr = 2.17 emu/g and coercive field Hc = 129 Oe. On increasing the annealing temperature saturation magnetization decreases and it is lowest for the samples annealed
at 700 °C. The high value of magnetization at low temperature is due to disordered crystal site orientation of the ion in the spinel structure. After annealing at 600 °C, ordering takes place that has resulted in lowering of magnetic moment. The inset of Fig. 6(a) shows plots of coercivity (Hc) and squareness (Mr/Ms) as a function of annealing temperature of 0.3NiFe2O4–0.7BiFeO3 nanocomposites as a function of the annealing temperature ranging from 500 to 700 °C respectively. The coercivity increases with the increase in annealing temperature from 500 to 600 °C. This can be expected as crystallite size increases with an increase in annealing temperature. But the coercivity decreases with a further increase in temperature to 600 °C. If the increase in coercivity depends only on the crystallite size then it would not decrease on a further increase in temperature above 600 °C which results in additional nanocrystallite growth. There may be magnetization pinning defects incorporated into the nanocrystallites, due to lattice mismatch between the intergrowing nanocrystallites, after the onset of significant crystallite growth at intensity of the peaks increased. All the peaks can be identified; hence it is clear that there is no intermediate
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Fig. 4. Variation of e and tand of xNiFe2O4–(1x)BiFeO3 (x = 0.1, 0.2, 0.3 and 0.4) nanocomposites annealed at 600 °C with frequency at room temperature.
phase or interphase formed in the composite. Annealing at higher temperature would reduce these defects and result in the decrease in Hc observed. Magnetic properties of the spinel – perovskite xNiFe2O4–(1x)BiFeO3 with x = 0.1, 0.2, 0.3 and 0.4, annealed at
600 °C is shown in Fig. 6(b). All the samples exhibited strong magnetic characteristics indicating the presence of ordered magnetic structure in the nanocomposites. The hysteresis is small,which shows the nanocomposites are still soft magnetic materials, similar to NiFe2O4. The plots of saturation magnetization (Ms) and coercivity (Hc) of the nanocomposites as a function NiFe2O4 concentration is shown in the inset of Fig. 6 (b). The saturation magnetization (Ms) increases while the coercivity (Hc) decreases with increasing nickel ferrite content. The increase in saturation magnetization with increasing NiFe2O4 content provides an indication that the spontaneous magnetization of the nanocomposites originates from unbalanced antiparallel spins of the ferrimagnetic NiFe2O4. The decrease in the coercivity with increasing NiFe2O4 content means that magnetization can be realized with increasing NiFe2O4 content so that the interaction of the magnetic poles on the magnetic powders becomes energetic. To investigate the coupling of electric and magnetic polarizations between NiFe2O4 and BiFeO3, the dielectric constant at frequency 1 kHz is measured as a function of the applied magnetic field and is plotted in Fig. 7(a). For multiferroic composite materials under an external magnetic field, the magnetostriction in the magnetic phase produces stresses that are transferred in the ferroelectric phase, resulting in an electric polarization via the ME effect [18,19]. As a result, the dielectric behavior is modified. The change in dielectric constant (e), e(H) – e(0)/e(0), where e(H) and e(0) denotes the dielectric constants at applied magnetic field H and zero field, is a measure of the ME response. The value of ME
Fig. 5. Dielectric constant and dielectric loss tangent for xNiFe2O4–(1x)BiFeO3 (x = 0.1, 0.2, 0.3 and 0.4) nanocomposites as a function of temperature.
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ME response of NiFe2O4–BiFeO3 nanocomposites is stronger than Tb doped BFO [18], Nb doped BiFeO3 [20], La and Nb codoped BiFeO3 [21]. In a ferroelectromagnet, the thermodynamic potential U can be written in the form
b 2
U ¼ U0 þ aP2 þ P4 PE þ a0 P2 þ
b0 4 M MH þ cP2 M2 2
where P and M are the order parameters for the polarization and the magnetization, respectively, and a, b, a0 , b0 , and c are coupling coefficients. The term representing magnetoelectric coupling, P2M2, is always allowed regardless of symmetry. Kimura et al. used this free energy expression to calculate the effect of magnetic ordering on the dielectric susceptibility and shown that the effect of magnetic ordering on the dielectric constant will be proportional to De cM2 [22]. The sign of De depends on the sign of the constant magnetoelectric interaction c, and can be either positive or negative. The variation of De/e(0) as a function of the square of magnetization (M2) is shown in Fig. 7(b) for 0.3NiFe2O4–0.7BiFeO3 nanocomposite annealed at 600 °C. Inset of Fig. 7(b) shows the variation of M2 with the applied magnetic field. It can be seen that De/e(0) changes linearly with M2. Thus De/e(0) versus M2 can also be expressed by De cM2 for 0.3NiFe2O4–0.7BiFeO3 nanocomposite. Linear fitting with this formula give the value of c 4.38 104 (emu/g)2 for 0.3NiFe2O4–0.7BiFeO3 nanocomposite. The values of magnetoelectric coupling coefficient c are 3.87 103, 5.98 104 and 2.85 104 (emu/g)2 for the xNiFe2O4–(1x)BiFeO3 with x = 0.1, 0.2 and 0.4 respectively. 4. Conclusions
Fig. 6. (a) M–H curves of 0.3NiFe2O4–0.7BiFeO3 annealed at various temperatures. Inset shows plots of coercivity (Hc) and squareness (Mr/Ms) of 0.3NiFe2O4–0.7BiFeO3 nanocomposites as a function of the annealing temperature. (b) M–H curves of xNiFe2O4–(1x)BiFeO3 (x = 0.1, 0.2, 0.3 and 0.4) annealed at 600 °C. Inset shows variation of saturation magnetization (Ms) and coercivity (Hc) of for xNiFe2O4– (1x)BiFeO3 (x = 0.1, 0.2, 0.3 and 0.4) nanocomposites as a function of the NiFe2O4 content.
response at 8 k Oe is 2.8%, 3%, 3.6% and 4.5% for the xNiFe2O4– (1x)BiFeO3 nanocomposites with x = 0.1, 0.2, 0.3 and 0.4. The
The spinel – perovskite structured nanocomposites of xNiFe2O4–(1x)BiFeO3 nanocomposites with x = 0.1, 0.2, 0.3 and 0.4 have been synthesized by chemical solution method at various temperatures ranging from 500 to 700 °C. Nanocrystalline xNiFe2O4– (1x)BiFeO3 composites show the phase formation at 600 °C annealing temperature. TEM observation showed that the average particles size is around 30–70 nm. The variation of the dielectric constant and the dielectric loss with frequency showed dispersion in the low frequency range. An anomaly in dielectric constant with temperature suggests the magnetoelectric nature of the naocomposites. The effects of annealing temperature and the NiFe2O4 content in the composites on the magnetic properties of the spinel – perovskite system have been studied in detail. The saturation
Fig. 7. (a) Variation of dielectric constant with the applied magnetic field of xNiFe2O4–(1x)BiFeO3 (x = 0.1, 0.2, 0.3 and 0.4) nanocomposites annealed at 600 °C. (b) Variation of magnetocapacitance with (magnetization)2 for 0.3NiFe2O4–0.7BiFeO3 nanocomposite annealed at 600 °C. Inset shows variation of M2 with magnetic field.
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magnetization, squareness and coercivity values have been found to vary with the composition and annealing temperature due to the crystallite growth. The saturation magnetization increases with the increase of NiFe2O4 content in the nanocomposites. Strong dependence of dielectric constant on magnetic field of the synthesized nanocomposites proves the magnetoelecgtric coupling in the nanocomposite system and magnetoelectric coupling coefficient (c) is approximated by De cM2. Acknowledgments Hemant Singh would like to acknowledge University Grants Commission, New Delhi for approving Teacher fellowship under faculty improvement programme. References [1] W. Eerenstein, N.D. Mathur, J.F. Scott, Nat. (London) 442 (2006) 759–765. [2] M. Fiebig, J. Phys. D: Appl. Phys. 38 (2005) R123–152. [3] M. Zeng, J.G. Wan, Y. Wang, H. Yu, J.M. Liu, X.P. Jiang, C.W. Nan, J. Appl. Phys. 95 (2004) 8069–8073.
[4] G.T. Rado, V.J. Folen, Phys. Rev. Lett. 7 (1961) 310–311. [5] J.V. Suchethelene, Phillips Res. Rep. 27 (1972) 28. [6] C.W. Nan, M.I. Bichurin, S. Dong, D. Viehland, G. Srinivasan, J. Appl. Phys. 103 (2008) 031101. [7] Z. Cheng, X. Wang, C.V. Kannan, K. Ozawa, H. Kimura, T. Nishida, S. Zhang, T.R. Shrout, Appl. Phys. Lett. 88 (2006) 132909. [8] P. Uniyal, K.L. Yadav, J. Phys.: Condens. Matter 21 (2009) 405901. [9] P. Uniyal, K.L. Yadav, J. Phys.: Condens. Matter 21 (2009) 012205. [10] S.K. Singh, H. Ishiwara, K. Sato, K. Maruyama, J. Appl. Phys. 102 (2007) 094109. [11] Y.H. Lee, J.M. Wu, C.H. Lai, Appl. Phys. Lett. 88 (2006) 042903. [12] X.M. Liu, S.Y. Fu, C.J. Huang, Mater. Sci. Eng. B 121 (2005) 255–260. [13] P. Uniyal, K.L. Yadav, J. Alloy Comp. 492 (2010) 406–410. [14] M. Kumar, K.L. Yadav, Mater. Lett. 61 (2007) 2089–2092. [15] N.A. Spaldin, Magnetic Materials Fundamentals and Applications, second ed., Cambridge University Press, Cambridge UK, 2011. [16] C.G. Koops, Phys. Rev. 83 (1951) 121–124. [17] R.K. Mishra, D.K. Pradhan, R.N.P. Choudhary, A. Banerjee, J. Phys.: Condens. Matter 20 (2008) 045218. [18] V.R. Palkar, D.C. Kundaliya, S.K. Malik, S. Bhattacharya, Phys. Rev. B 69 (2004) 212102. [19] S.N. Babu, J.H. Hsu, Y.S. Chen, J.G. Lin, J. Appl. Phys. 109 (2011) 07D904. [20] Y.K. Jun, W.T. Moon, C.M. Chang, H.S. Kim, H.S. Ryu, J.W. Kim, K.H. Kim, S.H. Hong, Solid State Commun. 135 (2006) 133–137. [21] H. Singh, K.L. Yadav, J. Phys.: Condens Matter. 23 (2011) 385901. [22] T. Kimura, S. Kawamoto, I. Yamada, M. Azuma, M. Takano, Y. Tokura, Phys. Rev. B 67 (2003) 180401.