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(1 − x)PbZr0.8Ti0.2O3 composites
Materials Research Bulletin 40 (2005) 2064–2072 www.elsevier.com/locate/matresbu
Dielectric and magnetoelectric properties of (x)Ni0.8Co0.1Cu0.1Fe2O4/(1 x)PbZr0.8Ti0.2O3 composites S.R. Kulkarni, C.M. Kanamadi, B.K. Chougule * Composite Materials Laboratory, Department of Physics, Shivaji University, Kolhapur 416004, Maharashtra, India Received 10 March 2005; received in revised form 21 May 2005; accepted 8 July 2005 Available online 2 August 2005
Abstract Ceramic composites of Ni0.8Co0.1Cu0.1Fe2O4 and lead–zirconate–titanate (PZT) were prepared using conventional solid state reaction method. The presence of constituent phases in composites was confirmed by X-ray diffraction (XRD). The variation of dielectric constant with frequency (100 Hz–1 MHz) and temperature has been studied. The variation of loss tangent (tan d) with temperature (at frequency 1 kHz) has also been studied. The magnetoelectric (ME) output was measured as a function of dc magnetic field. The maximum value of ME output (625 mV/cm) was observed for 25% ferrite + 75% ferroelectric phase. The maximum ME response can be explained in terms of the content of ferrite, permittivity of dielectric material and the intensity of magnetic field. The ME response of these composites was observed to be linear within low dc magnetic field. These composites may form the basis for the development of magnetic sensors and transducers for use in solid state microelectronics and microwave devices. # 2005 Elsevier Ltd. All rights reserved. Keywords: A. Composites; D. Piezoelectricity
1. Introduction The magnetoelectric effect is a product property exhibiting a complex behaviour. The magnetoelectric materials become magnetized when placed in an external electric field and electrically polarized when placed in a magnetic field [1]. Such a property is not shown by their constituent phases [2]. It occurs due * Corresponding author. Tel.: +91 231 2690571x5230; fax: +91 231 2691533. E-mail address: [email protected] (B.K. Chougule). 0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2005.07.014
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to interaction between the magnetic and electric dipoles [3,4]. The ME materials are used as magnetic sensors for dc and ac magnetic field measurements, transducers and actuators [5]. The effect can also be used in various applications such as microwave field and current measurement [6,7], integral optics and fiber communication technology [8]. The ME materials can be classified into two groups, single phase and composites. The single phase materials exhibit ME effect due to local interaction between the ordered magnetic and ferroelectric sublattices [1]. The simultaneous coexistence of electric and magnetic dipoles activates the coupling between the spontaneous polarization and spontaneous magnetization. The ME effect in composite materials is observed as a consequence of the concept of product property introduced by Van Suchetelene [2]. A suitable combination of piezoelectric and piezomagnetic phases gives rise to this property. The ME effect obtained in composite ceramics is reported to be larger than that of single phase ME materials [9]. To obtain better ME effect, the magnitude of magnetostriction coefficient of ferrite phase and the magnitude of piezoelectric coefficient of piezoelectric phase must be high. Also the resistivity of both phases should be high in order to avoid the leakage of accumulated charges through the magnetostrictive phase. Ni ferrites are resistive and magnetostrictive under low magnetic field bias [10]. Hence, we have chosen it as a ferrite phase. PZT is one of the ideal piezoelectric material having high piezoelectric constant, high dielectric permittivity and superior coupling factor [11]. The objective of this paper is to investigate the effect of dielectric behaviour on the ME response of these composites.
2. Experimental techniques 2.1. Preparation of ME composites The samples were prepared by standard ceramic method which has many advantages over the unidirectional solidification method [12]. The piezomagnetic ferrite phase was prepared by solid state reaction using NiO, CoO, CuO and Fe2O3 in molar proportions as starting materials. Similarly, the ferroelectric phase was prepared by using PbO, ZrO2 and TiO2 in molar proportions. The constituent phases were presintered at 900 8C for 10 h separately. After presintering, the constituent phases were ground to fine powder. The composites were prepared with compositions (x)Ni0.8Co0.1Cu0.1Fe2O4/ (1 x)PbZr0.8Ti0.2O3 where x = 0.15, 0.25, 0.35 and 0.45. These composites were again ground for 3 h so as to mix them thoroughly. The powder was then pressed into pellets having diameter of 1.5 cm and thickness 2–3 mm. The pelletized samples were sintered at 1000 8C for 12 h. 2.2. Characterization The samples were characterized by X-ray diffractometer (Philips Model PW 1710) using Cu Ka ˚ ). The XRD patterns show the presence of constituent phases. The patterns do not radiation (l = 1.5418 A indicate any chemical reaction between the components during sintering. 2.3. Measurement of dielectric constant and tan d The dielectric measurements were carried out as a function of frequency in the range of 100 Hz– 1 MHz at room temperature and temperature at frequencies of 1 kHz, 10 kHz, 100 kHz and 1 MHz
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using LCR meter bridge (HP 4284A). The dielectric constant e0 was calculated using the following formula: e0 ¼ Cp t=e0 pr 2
(1)
where Cp is the capacitance of the pellet in F, t the thickness of the pellet in m, r the radius of circular cross-section of the pellet in m, and e0 is the permittivity of free space. The variation of e0 with temperature for the samples was studied at a fixed frequency of 1 kHz. 2.4. Measurement of ME output The ME output of the samples was measured by static method. In this method, ME voltage is measured by poling the samples electrically and magnetically [13]. During electrical poling, the samples were slowly heated to 450 K. The samples were then cooled to room temperature in an electric field of 3 kV/ cm. The space charges were built up in ferroelectric phase. After electric poling, samples were subjected to the dc magnetic field of 5 kOe by holding them in the electromagnet at room temperature. The stray charges developed during poling were removed by grounding the plates of the holder. The static ME voltage (in mV/cm) was measured using Keithley’s electrometer (Model 2000) as a function of magnetic field.
3. Results and discussion The representative XRD pattern of a composite containing 25% ferrite and 75% ferroelectric phase is shown in Fig. 1. It shows the presence of both ferrite and ferroelectric phases. It can be confirmed that the ferrite phase has cubic spinel structure and ferroelectric phase has tetragonal perovskite structure. The ˚ while the c/a ratio (tetragonality) of lattice parameter for ferrite phase varies between 8.20 and 8.34 A
Fig. 1. XRD pattern of composite with x = 0.25.
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Table 1 Data on structural and dielectric properties of (x)Ni0.8Co0.1Cu0.1Fe2O4/(1 x)PbZr0.8Ti0.2O3 composites x (mol%)
Lattice parameter, ˚ ) (ferrite phase) a (A
c/a ratio (ferroelectric phase)
tan dRT (1 kHz)
rRT (V m)
e0RT (1 kHz)
Tc (K)
e0max (1 kHz)
ME output (mV/cm)
0.00 0.15 0.25 0.35 0.45 1.00
– 8.231 8.203 8.337 8.337 8.346
1.056 1.022 1.001 1.015 1.005 –
0.039 0.111 0.077 0.097 0.146 0.290
3759 3054 3511 3066 2893 2303
394 214 287 312 184 98
603 613 623 633 643 483
2009 3192 3065 2320 3174 262
– 625 513 431 345 –
perovskite phase is nearly equal to 1.02 (Table 1). The intensity of ferrite peaks increases with its content but the intensity of ferroelectric phase is not changed. The variation of dielectric constant e0 with frequency for the composites is shown in Fig. 2. The figure reveals the dielectric dispersion in all composites. The value of e0 slowly decreases at lower frequencies and remains constant at higher frequencies. The large value of e0 has been attributed to the effect of heterogeneity of the samples [14,15]. The dielectric dispersion is due to interfacial polarization in agreement with Koop’s phenomenological theory [15]. At higher frequencies only the electronic polarizability contributes to the polarization and hence the dielectric constant attains a constant value [16]. The fall of dielectric constant e0 at lower frequencies is attributed to the fact that ferroelectric regions are surrounded by nonferroelectric regions similar to the relaxor ferroelectric materials [17]. The variation of dielectric constant with temperature for the composites at 1 kHz, 10 kHz, 100 kHz and 1 MHz frequencies are shown in Figs. 3–6. From the plots, it is seen that e0 increases slowly with temperature up to Curie temperature (Tc) and then it decreases. These figures represent the diffused phase transition for all the compositions [18]. The maximum dielectric constant was observed at 1 kHz frequency. Secondly, as the percentage of ferrite in the composite increases, the transition temperature also increases (Table 1). Whereas pure PZT shows normal ferroelectric behaviour [19], the composites of PZT with Ni–Co–Cu ferrite exhibit a mixture of normal and relaxor characteristics. In this case, the
Fig. 2. Variation of dielectric constant with frequency for x(Ni0.8Co0.1Cu0.1Fe2O4) + (1 x)PbZr0.8Ti0.2O3 composites.
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Fig. 3. Variation of dielectric constant with temperature for 0.15(Ni0.8Co0.1Cu0.1Fe2O4) + 0.85PbZr0.8Ti0.2O3 composite.
Fig. 4. Variation of dielectric constant with temperature for 0.25(Ni0.8Co0.1Cu0.1Fe2O4) + 0.75PbZr0.8Ti0.2O3 composite.
Fig. 5. Variation of dielectric constant with temperature for 0.35(Ni0.8Co0.1Cu0.1Fe2O4) + 0.65PbZr0.8Ti0.2O3 composite.
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Fig. 6. Variation of dielectric constant with temperature for 0.45(Ni0.8Co0.1Cu0.1Fe2O4) + 0.55PbZr0.8Ti0.2O3 composite.
maximum in dielectric constant is shifted to higher temperature side with increasing frequency. In relaxor materials a sharp Curie transition is turned into a diffuse phase transition and the dielectric constant shows frequency dispersion. It is reported that the diffused phase transition (DPT) is caused by the chemical inhomogeneity from the cation disorder in complex perovskite. It is related to the nanoscale ordered microregions existing in the material acting as locations of spontaneous polarization above the transition temperature [20]. The present composites follow the rule of mixtures as far as the dielectric constant and resistivity values are concerned [21]. The variation of dielectric constant with temperature at 1 kHz frequency is shown in Fig. 7. The maxima in these plots are also observed to be broader and Tc values are shifted towards higher temperature side confirming the diffused phase transition. The variation of tan d with frequency is shown in Fig. 8. At lower frequencies, tan d is large and it decreases with increasing frequency. The tan d is the energy dissipation in the dielectric system which is proportional to the imaginary part of dielectric constant e00 . At higher frequencies, the losses are reduced
Fig. 7. Variation of dielectric constant with temperature for x(Ni0.8Co0.1Cu0.1Fe2O4) + (1 x)PbZr0.8Ti0.2O3 composites at 1 kHz.
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Fig. 8. Variation of loss tangent with frequency for x(Ni0.8Co0.1Cu0.1Fe2O4) + (1 x)PbZr0.8Ti0.2O3 composites.
and the dipoles contribute to the polarization. The variation of tan d with temperature is shown in Fig. 9. The plots are similar to the behaviour of e0 against temperature. There is a rapid increase of tan d near the transition temperature indicating large energy losses during phase transition. The variation of ME output with applied dc magnetic field for x = 0.15, 0.25, 0.35 and 0.45 composites is shown in Fig. 10. From these plots, it is seen that the ME output increases initially with magnetic field and then gradually decreases with further increases in field except for the composite with x = 0.45. Beyond a certain applied magnetic field, the magnetostriction gets saturated and it produces constant electric field in the piezoelectric phase [22]. The ME effect in the composites is associated with the movement of domains in the ferromagnetic phase. In ferrites, domains are spontaneously deformed in the direction of magnetization which contributes to the magnetostriction [23]. The maximum value of ME output was observed to be 625 mV/cm (ME voltage coefficient, dE/dH = 0.375 mV/cm/Oe) at a field of 0.8 kOe for the composite with x = 0.25. The decrease of ME output with the ferrite content above
Fig. 9. Variation of loss tangent (tan d) with temperature for x(Ni0.8Co0.1Cu0.1Fe2O4) + (1 x)PbZr0.8Ti0.2O3 composites at 1 kHz.
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Fig. 10. Variation of ME output with dc magnetic field for x(Ni0.8Co0.1Cu0.1Fe2O4) + (1 x)PbZr0.8Ti0.2O3 composites at room temperature.
x = 0.25 is attributed to the lower resistivity of ferrite phase which provides a leakage path for the charges developed in the piezoelectric phase during poling [10,24,25]. A Jahn–Teller ion like Cu leads to improve the lattice distortion which produces a strain in the piezomagnetic phase, thereby improving the ME effect in these composites.
4. Conclusions Structural, dielectric and magnetoelectric measurements of (x)Ni0.8Co0.1Cu0.1Fe2O4/(1 x)PZT composites reveal the following: 1. The ferrite phase has cubic spinel structure and piezoelectric phase has perovskite tetragonal structure. XRD patterns do not indicate the presence of any impurity phase. 2. The dielectric behaviour of these composites shows a mixed character of normal and DPT dispersion. It can be explained in terms of heterogeneity of the sample and the ordering of microregions of spontaneous polarization. The values of dielectric constant and resistivity of all the samples are in general high, e.g. for the composite with x = 0.25, the dielectric constant is 3065 and the resistivity 3.511 105 V cm. It is due to this the sample shows highest ME output (625 mV/cm). 3. The dielectric loss in these composites is very small at higher frequencies and it increases considerably at the transition temperature. 4. The ME effect is strongly dependant on ferrite content and resistivity of constituent phases. The ME output in these composites, which is observed to be linear in the low magnetic field, may be used in the applications such as magnetic sensors and transducers.
Acknowledgement The work was supported by financial assistance under UGC-DRS II phase of UGC, New Delhi.
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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
J. Ryu, S. Priya, K. Uchino, H.-E. Kim, J. Electroceram. 8 (2002) 107. J. Van Suchetelene, Philips Res. Rep. 27 (1972) 28. S. Lopatin, I. Lopatin, I. Lisnevskaya, Ferroelectrics 162 (1994) 63. S.V. Suryanarayana, Bull. Mater. Sci. 17 (7) (1994) 1259. J. Zhai, N. Cai, Z. Shi, Y. Lin, C.-W. Nan, J. Phys. D: Appl. Phys. 37 (2004) 823. T.H. O’Dell, Electron. Power 11 (1965) 266. L.M.P. Bracke, R.G. Van Vliet, J. Electron. 51 (1981) 255. E. Wood, A.E. Austin, Int. J. Magn. 5 (1974) 303. A.M.J.G. Van Run, D.R. Terrell, J.H. Scholing, J. Mater. Sci. 9 (1974) 1710. S.L. Kadam, C.M. Kanamadi, K.K. Patankar, B.K. Chougule, Mater. Lett. 59 (2005) 215. J. Ryu, A. Vazquez Carazo, K. Uchino, H.-E. Kim, Jpn. J. Appl. Phys. 40 (2001) 4948. J. Ryu, A.V. Carazo, K. Uchino, H.-E. Kim, J. Electroceram. 7 (2001) 17. J. Van Den Boomgaard, A.M.J.G. Van Run, J. Van Suchetelene, Ferroelectrics 14 (1976) 727. M.B. Kothale, K.K. Patankar, S.L. Kadam, V.L. Mathe, A.V. Rao, B.K. Chougule, Mater. Chem. Phys. 77 (2002) 691. Vishvanathan, V.R.K. Murthy, Ferrites (1990) 219. H.V. Keer, Principles Solid State (2000) 302. S. Upadhyay, Devendrakumar, Omprakash, Bull. Mater. Sci. 19 (1996) 513. K.K. Patankar, V.L. Mathe, A.N. Patil, S.D. Lotake, Y.D. Kolekar, P.B. Joshi, J. Electroceram. 6 (2) (2001) 115. R. Yimnirun, S. Ananta, P. Laoratakul Songklanakarin, J. Sci. Technol. 26 (4) (2004) 529. L.E. Cross, Ferroelectrics 76 (1987) 241. H. Gui, X. Zhang, Appl. Phys. Lett. 69 (16) (1996) 2353. W.D. Kingery, Introduction to Ceramics, John Wiley and Sons, Inc., London, 1988, p. 720. C.W. Nan, M. Li, J.H. Huang, Phys. Rev. B63 (2001) 144415. G. Srinivasan, E.T. Rasmusen, Appl. Phys. A (2003) 1. A. Hanumaiah, T. Bhimasankaram, S.V. Suryanarayana, G. Kumar, Bull. Mater. Sci. 17 (1994) 405.
Dielectric and magnetoelectric properties of (x)Ni0.8Co0.1Cu0.1Fe2O4/(1 x)PbZr0.8Ti0.2O3 composites S.R. Kulkarni, C.M. Kanamadi, B.K. Chougule * Composite Materials Laboratory, Department of Physics, Shivaji University, Kolhapur 416004, Maharashtra, India Received 10 March 2005; received in revised form 21 May 2005; accepted 8 July 2005 Available online 2 August 2005
Abstract Ceramic composites of Ni0.8Co0.1Cu0.1Fe2O4 and lead–zirconate–titanate (PZT) were prepared using conventional solid state reaction method. The presence of constituent phases in composites was confirmed by X-ray diffraction (XRD). The variation of dielectric constant with frequency (100 Hz–1 MHz) and temperature has been studied. The variation of loss tangent (tan d) with temperature (at frequency 1 kHz) has also been studied. The magnetoelectric (ME) output was measured as a function of dc magnetic field. The maximum value of ME output (625 mV/cm) was observed for 25% ferrite + 75% ferroelectric phase. The maximum ME response can be explained in terms of the content of ferrite, permittivity of dielectric material and the intensity of magnetic field. The ME response of these composites was observed to be linear within low dc magnetic field. These composites may form the basis for the development of magnetic sensors and transducers for use in solid state microelectronics and microwave devices. # 2005 Elsevier Ltd. All rights reserved. Keywords: A. Composites; D. Piezoelectricity
1. Introduction The magnetoelectric effect is a product property exhibiting a complex behaviour. The magnetoelectric materials become magnetized when placed in an external electric field and electrically polarized when placed in a magnetic field [1]. Such a property is not shown by their constituent phases [2]. It occurs due * Corresponding author. Tel.: +91 231 2690571x5230; fax: +91 231 2691533. E-mail address: [email protected] (B.K. Chougule). 0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2005.07.014
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to interaction between the magnetic and electric dipoles [3,4]. The ME materials are used as magnetic sensors for dc and ac magnetic field measurements, transducers and actuators [5]. The effect can also be used in various applications such as microwave field and current measurement [6,7], integral optics and fiber communication technology [8]. The ME materials can be classified into two groups, single phase and composites. The single phase materials exhibit ME effect due to local interaction between the ordered magnetic and ferroelectric sublattices [1]. The simultaneous coexistence of electric and magnetic dipoles activates the coupling between the spontaneous polarization and spontaneous magnetization. The ME effect in composite materials is observed as a consequence of the concept of product property introduced by Van Suchetelene [2]. A suitable combination of piezoelectric and piezomagnetic phases gives rise to this property. The ME effect obtained in composite ceramics is reported to be larger than that of single phase ME materials [9]. To obtain better ME effect, the magnitude of magnetostriction coefficient of ferrite phase and the magnitude of piezoelectric coefficient of piezoelectric phase must be high. Also the resistivity of both phases should be high in order to avoid the leakage of accumulated charges through the magnetostrictive phase. Ni ferrites are resistive and magnetostrictive under low magnetic field bias [10]. Hence, we have chosen it as a ferrite phase. PZT is one of the ideal piezoelectric material having high piezoelectric constant, high dielectric permittivity and superior coupling factor [11]. The objective of this paper is to investigate the effect of dielectric behaviour on the ME response of these composites.
2. Experimental techniques 2.1. Preparation of ME composites The samples were prepared by standard ceramic method which has many advantages over the unidirectional solidification method [12]. The piezomagnetic ferrite phase was prepared by solid state reaction using NiO, CoO, CuO and Fe2O3 in molar proportions as starting materials. Similarly, the ferroelectric phase was prepared by using PbO, ZrO2 and TiO2 in molar proportions. The constituent phases were presintered at 900 8C for 10 h separately. After presintering, the constituent phases were ground to fine powder. The composites were prepared with compositions (x)Ni0.8Co0.1Cu0.1Fe2O4/ (1 x)PbZr0.8Ti0.2O3 where x = 0.15, 0.25, 0.35 and 0.45. These composites were again ground for 3 h so as to mix them thoroughly. The powder was then pressed into pellets having diameter of 1.5 cm and thickness 2–3 mm. The pelletized samples were sintered at 1000 8C for 12 h. 2.2. Characterization The samples were characterized by X-ray diffractometer (Philips Model PW 1710) using Cu Ka ˚ ). The XRD patterns show the presence of constituent phases. The patterns do not radiation (l = 1.5418 A indicate any chemical reaction between the components during sintering. 2.3. Measurement of dielectric constant and tan d The dielectric measurements were carried out as a function of frequency in the range of 100 Hz– 1 MHz at room temperature and temperature at frequencies of 1 kHz, 10 kHz, 100 kHz and 1 MHz
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using LCR meter bridge (HP 4284A). The dielectric constant e0 was calculated using the following formula: e0 ¼ Cp t=e0 pr 2
(1)
where Cp is the capacitance of the pellet in F, t the thickness of the pellet in m, r the radius of circular cross-section of the pellet in m, and e0 is the permittivity of free space. The variation of e0 with temperature for the samples was studied at a fixed frequency of 1 kHz. 2.4. Measurement of ME output The ME output of the samples was measured by static method. In this method, ME voltage is measured by poling the samples electrically and magnetically [13]. During electrical poling, the samples were slowly heated to 450 K. The samples were then cooled to room temperature in an electric field of 3 kV/ cm. The space charges were built up in ferroelectric phase. After electric poling, samples were subjected to the dc magnetic field of 5 kOe by holding them in the electromagnet at room temperature. The stray charges developed during poling were removed by grounding the plates of the holder. The static ME voltage (in mV/cm) was measured using Keithley’s electrometer (Model 2000) as a function of magnetic field.
3. Results and discussion The representative XRD pattern of a composite containing 25% ferrite and 75% ferroelectric phase is shown in Fig. 1. It shows the presence of both ferrite and ferroelectric phases. It can be confirmed that the ferrite phase has cubic spinel structure and ferroelectric phase has tetragonal perovskite structure. The ˚ while the c/a ratio (tetragonality) of lattice parameter for ferrite phase varies between 8.20 and 8.34 A
Fig. 1. XRD pattern of composite with x = 0.25.
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Table 1 Data on structural and dielectric properties of (x)Ni0.8Co0.1Cu0.1Fe2O4/(1 x)PbZr0.8Ti0.2O3 composites x (mol%)
Lattice parameter, ˚ ) (ferrite phase) a (A
c/a ratio (ferroelectric phase)
tan dRT (1 kHz)
rRT (V m)
e0RT (1 kHz)
Tc (K)
e0max (1 kHz)
ME output (mV/cm)
0.00 0.15 0.25 0.35 0.45 1.00
– 8.231 8.203 8.337 8.337 8.346
1.056 1.022 1.001 1.015 1.005 –
0.039 0.111 0.077 0.097 0.146 0.290
3759 3054 3511 3066 2893 2303
394 214 287 312 184 98
603 613 623 633 643 483
2009 3192 3065 2320 3174 262
– 625 513 431 345 –
perovskite phase is nearly equal to 1.02 (Table 1). The intensity of ferrite peaks increases with its content but the intensity of ferroelectric phase is not changed. The variation of dielectric constant e0 with frequency for the composites is shown in Fig. 2. The figure reveals the dielectric dispersion in all composites. The value of e0 slowly decreases at lower frequencies and remains constant at higher frequencies. The large value of e0 has been attributed to the effect of heterogeneity of the samples [14,15]. The dielectric dispersion is due to interfacial polarization in agreement with Koop’s phenomenological theory [15]. At higher frequencies only the electronic polarizability contributes to the polarization and hence the dielectric constant attains a constant value [16]. The fall of dielectric constant e0 at lower frequencies is attributed to the fact that ferroelectric regions are surrounded by nonferroelectric regions similar to the relaxor ferroelectric materials [17]. The variation of dielectric constant with temperature for the composites at 1 kHz, 10 kHz, 100 kHz and 1 MHz frequencies are shown in Figs. 3–6. From the plots, it is seen that e0 increases slowly with temperature up to Curie temperature (Tc) and then it decreases. These figures represent the diffused phase transition for all the compositions [18]. The maximum dielectric constant was observed at 1 kHz frequency. Secondly, as the percentage of ferrite in the composite increases, the transition temperature also increases (Table 1). Whereas pure PZT shows normal ferroelectric behaviour [19], the composites of PZT with Ni–Co–Cu ferrite exhibit a mixture of normal and relaxor characteristics. In this case, the
Fig. 2. Variation of dielectric constant with frequency for x(Ni0.8Co0.1Cu0.1Fe2O4) + (1 x)PbZr0.8Ti0.2O3 composites.
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Fig. 3. Variation of dielectric constant with temperature for 0.15(Ni0.8Co0.1Cu0.1Fe2O4) + 0.85PbZr0.8Ti0.2O3 composite.
Fig. 4. Variation of dielectric constant with temperature for 0.25(Ni0.8Co0.1Cu0.1Fe2O4) + 0.75PbZr0.8Ti0.2O3 composite.
Fig. 5. Variation of dielectric constant with temperature for 0.35(Ni0.8Co0.1Cu0.1Fe2O4) + 0.65PbZr0.8Ti0.2O3 composite.
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Fig. 6. Variation of dielectric constant with temperature for 0.45(Ni0.8Co0.1Cu0.1Fe2O4) + 0.55PbZr0.8Ti0.2O3 composite.
maximum in dielectric constant is shifted to higher temperature side with increasing frequency. In relaxor materials a sharp Curie transition is turned into a diffuse phase transition and the dielectric constant shows frequency dispersion. It is reported that the diffused phase transition (DPT) is caused by the chemical inhomogeneity from the cation disorder in complex perovskite. It is related to the nanoscale ordered microregions existing in the material acting as locations of spontaneous polarization above the transition temperature [20]. The present composites follow the rule of mixtures as far as the dielectric constant and resistivity values are concerned [21]. The variation of dielectric constant with temperature at 1 kHz frequency is shown in Fig. 7. The maxima in these plots are also observed to be broader and Tc values are shifted towards higher temperature side confirming the diffused phase transition. The variation of tan d with frequency is shown in Fig. 8. At lower frequencies, tan d is large and it decreases with increasing frequency. The tan d is the energy dissipation in the dielectric system which is proportional to the imaginary part of dielectric constant e00 . At higher frequencies, the losses are reduced
Fig. 7. Variation of dielectric constant with temperature for x(Ni0.8Co0.1Cu0.1Fe2O4) + (1 x)PbZr0.8Ti0.2O3 composites at 1 kHz.
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Fig. 8. Variation of loss tangent with frequency for x(Ni0.8Co0.1Cu0.1Fe2O4) + (1 x)PbZr0.8Ti0.2O3 composites.
and the dipoles contribute to the polarization. The variation of tan d with temperature is shown in Fig. 9. The plots are similar to the behaviour of e0 against temperature. There is a rapid increase of tan d near the transition temperature indicating large energy losses during phase transition. The variation of ME output with applied dc magnetic field for x = 0.15, 0.25, 0.35 and 0.45 composites is shown in Fig. 10. From these plots, it is seen that the ME output increases initially with magnetic field and then gradually decreases with further increases in field except for the composite with x = 0.45. Beyond a certain applied magnetic field, the magnetostriction gets saturated and it produces constant electric field in the piezoelectric phase [22]. The ME effect in the composites is associated with the movement of domains in the ferromagnetic phase. In ferrites, domains are spontaneously deformed in the direction of magnetization which contributes to the magnetostriction [23]. The maximum value of ME output was observed to be 625 mV/cm (ME voltage coefficient, dE/dH = 0.375 mV/cm/Oe) at a field of 0.8 kOe for the composite with x = 0.25. The decrease of ME output with the ferrite content above
Fig. 9. Variation of loss tangent (tan d) with temperature for x(Ni0.8Co0.1Cu0.1Fe2O4) + (1 x)PbZr0.8Ti0.2O3 composites at 1 kHz.
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Fig. 10. Variation of ME output with dc magnetic field for x(Ni0.8Co0.1Cu0.1Fe2O4) + (1 x)PbZr0.8Ti0.2O3 composites at room temperature.
x = 0.25 is attributed to the lower resistivity of ferrite phase which provides a leakage path for the charges developed in the piezoelectric phase during poling [10,24,25]. A Jahn–Teller ion like Cu leads to improve the lattice distortion which produces a strain in the piezomagnetic phase, thereby improving the ME effect in these composites.
4. Conclusions Structural, dielectric and magnetoelectric measurements of (x)Ni0.8Co0.1Cu0.1Fe2O4/(1 x)PZT composites reveal the following: 1. The ferrite phase has cubic spinel structure and piezoelectric phase has perovskite tetragonal structure. XRD patterns do not indicate the presence of any impurity phase. 2. The dielectric behaviour of these composites shows a mixed character of normal and DPT dispersion. It can be explained in terms of heterogeneity of the sample and the ordering of microregions of spontaneous polarization. The values of dielectric constant and resistivity of all the samples are in general high, e.g. for the composite with x = 0.25, the dielectric constant is 3065 and the resistivity 3.511 105 V cm. It is due to this the sample shows highest ME output (625 mV/cm). 3. The dielectric loss in these composites is very small at higher frequencies and it increases considerably at the transition temperature. 4. The ME effect is strongly dependant on ferrite content and resistivity of constituent phases. The ME output in these composites, which is observed to be linear in the low magnetic field, may be used in the applications such as magnetic sensors and transducers.
Acknowledgement The work was supported by financial assistance under UGC-DRS II phase of UGC, New Delhi.
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