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Study of dielectric and magnetic properties of PbZr0.52Ti0.48O3–Mn0.3Co0.6Zn0.4Fe1.7O4 composite
ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 322 (2010) 1020–1025
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Study of dielectric and magnetic properties of PbZr0.52Ti0.48O3– Mn0.3Co0.6Zn0.4Fe1.7O4 composite Arti Gupta, Ratnamala Chatterjee n Advanced Ceramic Laboratory, Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India
a r t i c l e in f o
a b s t r a c t
Article history: Received 23 September 2009 Available online 16 December 2009
Results of detailed structural, dielectric, magnetic and magnetoelectric studies of (x)PbZr0.52Ti0.48O3– (1 x)Mn0.3Co0.6Zn0.4Fe1.7O4 composites where x=65, 70, 75 and 80 are shown in this work. Manganese substituted cobalt ferrites are known to exhibit large strain derivative (dx/dH) and on the other hand substitution of Zn in pure cobalt ferrite is known to enhance its permeability m and permittivity e. The choice of ferrite as Mn, Zn simultaneously substituted cobalt ferrite (MCZFO) is made keeping in view that for good magnetoelectric (ME) voltage coefficient the magnetostrictive constituent phase of the composite should have large strain derivative (dx/dH) along with large permittivity and permeability. It is shown here that although the dielectric transition temperature changes significantly with change in the mole ratio of the two component phases, magnetic transition temperature (much less compared to the bulk cobalt ferrite) is relatively non-responsive to the changing molar ratio of the two component phases. In the vicinity of the magnetic transition temperature we observed an anomaly in tan d vs. T plots, which indicates a possible magnetoelectric coupling in the samples. Magnetoelectric voltage coefficient (aE) has been measured using static magnetoelectric method. Highest magnetoelectric voltage coefficient (aE = 0.312 mV/cmOe) is obtained for sample 80:20 at HDC = 1000 Oe. & 2010 Published by Elsevier B.V.
Keywords: Structrural Magnetic Dielectric Magnetoelectric Permeability and corecivity
1. Introduction Multiferroic materials are those which combine at least two or more ferroic properties like ferroelasticity/ferroelectricity/ferromagnetism in the same temperature range. Magnetoelectric (ME) effect, i.e., induction of ferroelectric polarization by magnetic field and magnetization by electric field can be achieved in multiferroic materials that show coexistence of ferroelectricity and ferromagnetism. The ME effect was first observed in the antiferromagnetic Cr2O3 [1]. The magnetoelectric (ME) activity in multiferroics results either from the direct coupling between the two order parameters in single phase systems, or indirectly via strain mediated coupling in composites. Use of biphase composites has been more successful in the device applications as they have better ME output as compared to single-phase materials. Another difficulty with the single phase multiferroics is that usually their Neel or Curie temperature was observed to be far below room temperature [2–4]. In 1972 Suchtelene et al. [5] introduced the concept of magnetoelectric composite by combining the magnetostrictive and piezoelectric materials, in which the magnetoeletric effect appears as a product property of the composite. In these materials, as the magnetic field is applied, magnetostrictive
n
Corresponding author. Tel.: + 91 11 2659 1354; fax: + 91 11 2658 1114. E-mail address: [email protected] (R. Chatterjee).
0304-8853/$ - see front matter & 2010 Published by Elsevier B.V. doi:10.1016/j.jmmm.2009.12.007
particles change their shape due to magnetostriction. The strain thus produced passes to piezoelectric phase through interfacial boundary and generates stress over the piezoelectric phase that results into the generation of induced voltage. Magnetoelectric voltage coefficient of composite is defined [6] as
aE ¼ dE=dH ¼ ðdx=dHÞmagnetostrictive :ðdE=dxÞpiezoelectric ¼ ðdx=dHÞmagnetostrictive :ðg33 :C33 Þpiezoelectric where g33 is the piezoelectric voltage coefficient and C33 is the stiffness coefficient of the piezoelectric phase. Thus, (dx/dH) is a useful parameter in deciding the magnetoelectric properties. Hence a suitable combination of piezoelectric and magnetostrictive materials may give rise to appreciable magnetoelectric effect [7–9]. It is theoretically studied and shown that magnetoelectric voltage coefficient depends on the material properties like permittivity, permeability and elastic modulus of the magnetostrictive phase [10]. Cobalt ferrite has a large l but it has the disadvantage of large magnetic anisotropy and coercivity. Substitution of Zn in pure cobalt ferrite is observed to decrease the magnetic anisotropy in the composition that leads to high permeability value in them. This has been shown to increase the magnetoelectric coupling in PZT–CoxZn(1 x)Fe2O4 laminated composite [11]. Substitution of zinc in cobalt ferrite is known to increase the resistivity and
ARTICLE IN PRESS A. Gupta, R. Chatterjee / Journal of Magnetism and Magnetic Materials 322 (2010) 1020–1025
decrease the Curie temperature [12,13]. Increase in resistivity of ferrite is an important factor for the improvement of ME coupling in case of particulate magnetoelectric composite [9]. There are many reports [14–20] in literature which show that the manganese (Mn) substituted cobalt ferrite exhibit large slope of strain x(H) or its field derivative (dx/dH) and low magnetic transition temperature as compared to pure cobalt ferrite. Hence simultaneous substitution of Mn and Zn in cobalt ferrite with iron deficiency i.e., Mn0.3Co0.6Zn0.4Fe1.7O4 (MCZFO) can make a good candidate as a magnetostrictive component.
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In this work we have studied the system (x)PbZr0.52Ti0.48O3– (1 x)Mn0.3Co0.6Zn0.4Fe1.7O4 for x= 65, 70, 75 and 80. All the compositions lie in the ferroelectric rich region and the ferroelectric component PbZr0.52Ti0.48O3 (PZT) is at the morphotropic phase boundary (MPB). Results of detailed structural, dielectric, magnetic and magnetoelectric studies are shown in this work. Highest ME coefficient (aE = 0.312 mV/cmOe at 1000 Oe) is observed for 80:20 composite.
2. Experimental details
Fig. 1. X-ray diffraction patterns for (a) 80:20, (b) 75:25, (c) 70:30 and (d) 65:35.
The individual phase Mn0.3Co0.6Zn0.4Fe1.7O4 (MCZFO) has been prepared using the conventional solid state reaction method taking MnO2, Co3O4, ZnO and Fe2O3 as a raw materials. All the raw materials were first weighed in the stoichiometric proportion and then mixed in acetone medium for 5–6 h. The mixed powder was then calcined at 1000 1C for 6 h. The calcined powder was again mixed in the acetone medium for nearly 2 h. PbZr0.52Ti0.48O3 (PZT) was also prepared by the conventional ceramic method. All the raw materials PbO, ZrO2 and TiO2 were weighed in stoichiometric ratio and then mixed in the acetone medium for 4–5 h. The mixed powder was then calcined at 950 1C for 2 h. The individual phase formations of PZT and MCZFO were confirmed by the X-ray diffraction (not shown). The individual components PZT:MCZFO were taken in the mole ratio 65:35, 70:30, 75:25 and 80:20, respectively. They were mixed individually in the acetone medium for several hours and finally 2 wt% of polyvinyl alcohol (PVA) was added to these mixed powders and pressed in hydraulic press in the shape of disk with diameter of 11 mm and thickness of 1 mm. These disks were finally sintered in temperature range (1130–1150 1C) for 3 h.
Fig. 2. Scanning electron micrographs for (a) 65:35, (b) 70:30, (c) 75:25 and (d) 80:20.
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The structural characterizations were done using the X-ray diffractometer (CuKa radiation; Phillips X’pert PRO) and scanning electron microscopy using ZEISS EVO-50 system. The dielectric properties were measured using HP4192A LF impedence analyzer. Magnetic properties of the composites were measured using the superconducting quantum interference device (SQUID) (MPMS XL-7 of Quantum Design). The static magnetoelectric voltage coefficient (aE) was measured using the indigenous laboratory set up; prior to ME coupling measurement the samples were electrically and magnetically poled. For electric poling, samples were first heated at 100 1C in the presence of external electric field of 5 kV/cm, kept at this field for 30 min and then cooled to
room temperature in the presence of field. The electrically poled samples were then magnetically poled at 5 kOe for 30 min.
3. Results and discussion The X-ray diffraction pattern establishes the presence of both the phases in all the compositions. XRD pattern for all four compositions are shown in Fig. 1. Two well-defined sets (individual phases: the (i) tetragonal phase of PZT and the (ii) cubic spinel phase of MCZFO) of diffraction patterns observed are the characteristics of composite formation. No X-ray reflection
Fig. 3. Variation of dielectric constant (e) and dielectric loss (tan d) with temperature (T) for 65:35, 70:30, 75:25 and 80:20.
ARTICLE IN PRESS A. Gupta, R. Chatterjee / Journal of Magnetism and Magnetic Materials 322 (2010) 1020–1025
other than the individual phases was found which suggests the absence of any chemical reaction between the two phases during the sintering process. Intensity of the X-ray reflection corresponding to magnetostrictive phase (MCZFO) can be clearly seen to decrease with its content in the composite. A systematic variation of intensities of the two phases is observed to indicate the presence of scattering components from both the participating phases. Scanning electron micrographs for all four compositions are shown in Fig. 2. The images are taken for the fractured portions of the samples. Presence of two types of phases can be clearly seen from these images, with the smaller grains of PZT matrix and large grains of dispersed MCZFO phase. The variation of dielectric constants e of all the composite samples with temperature (330 oTo773 K) at fixed frequencies (1 kHz rfr1 MHz) and their dielectric loss (tan d) are shown in Fig. 3. The dielectric constants were observed to decrease with increasing frequencies. At all frequencies e vs. T graphs show a peak related to transition. This dielectric transition temperature for the composition 80:20 ( 673 K) is close to the ferroelectric transition temperature (TcFE ) for pure PZT. For composites TcFE is observed to decreases with the increase in the mole percentage of MCZFO. Similar feature has been seen for the PZT–NiFe2O4 [4] and PZT–CoFe2O4 [21] composites. It is also clear from these figures that the dielectric constant decreases with the addition of MCZFO due to the low dielectric constant value of ferrite. The high dielectric constant measured at low frequencies might be attributed to the Maxwell–Wagner type interfacial polarization mechanism that plays a crucial role in these types of heterogeneous composites. The interfaces between the ferroelectric and ferromagnetic phases which have significantly different conductivities cause an additional polarization that is termed as the interfacial polarization, which boosts the dielectric constant [22]. This fact is also supported by the variation of the dielectric constants (e) with frequency (f) at room temperature for all four compositions, shown in Figs. 4(a) and (b). Interfacial polarization responds very slowly to the external field, hence, it dominates in the low frequency region and has no significant contribution in the high frequency region (105–106 Hz). The magnetization (M) vs. temperature (T) measurements at field of 100 Oe for all four compositions are shown in Fig. 5. All the compositions show a ferromagnetic to paramagnetic transition around T 430 K. It is important to note that the magnetic transition temperature is nearly same for all compositions whereas the dielectric transition temperatures show a significant change with changing of ferroelectric component’s mole percentage. Also as expected, the magnetic transition temperature of composites ( 430 K) is much less compared to transition temperature of pure cobalt ferrite ( 800 K). This is expected as the substitution of both the manganese and zinc into pure cobalt ferrite reduce the Curie temperature [12, 17, 23]. For 65:35 sample, which consists of the highest mole fraction of ferrite, a built-up in magnetization with temperature up to the transition temperature can be noted. Possibly with increase in temperature i.e., with added thermal energy, the magnetic domains in the composite can easily rotate in the direction of field and as a result the magnetization increases. As the magnetic Curie temperature approaches, this process is taken over by the ferromagnetic to paramagnetic transition and correspondingly at 430 K a decrease in magnetization is observed. To confirm that this is purely a ferromagnetic to paramagnetic transition, the magnetization (M) vs. field (H) loop at room temperature T =300 K ( oTc 430 K), at T=430 K and at T= 530 K ( 4Tc 430 K) were performed for 65:35. At T=300 K and 430 K we get purely ferromagnetic loops while at T= 530 K a typical paramagnet behavior was observed (see Fig. 6). The inverse susceptibility
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Fig. 4. (a) and (b) Variation of dielectric constant (e) and dielectric loss (tan d) vs. frequency (f) for 65:35, 70:30, 75:25 and 80:20.
(1/w) vs. temperature (T) plot for 65:35 (see top inset of Fig. 6) gives the estimated Curie temperature for this composite as 471 K confirming the ferromagnetic behavior below magnetic transition temperature. The magnetization vs. field measurements (not shown here) have been done for all samples. Small corecivity (Hc) ( 48–55 Oe) obtained for these samples indicate their soft magnetic behavior (see bottom inset of Fig. 6). The saturation magnetization (MS) and remnant magnetization (MR) values for these samples were observed to increase with increasing MCZFO content. MS and MR values for sample 65:35 are 19.490 and 2.078 emu/g whereas for 80:20 sample, these are 10.228 and 0.8 emu/g, respectively. Magnetoelectric voltage coefficients (aE) vs. magnetic field (H) were also measured for these samples. Samples with large mole percentage of ferrites (65:35 and 70:30) did not show any detectable ME effect. Comparatively larger molar ratio of ferrite particles in these samples make conducting chains and it was difficult to pole these samples. Figs. 7(a) and (b) show ME voltage coefficient (aE) vs. magnetic field (H) for 75:25 and 80:20 samples,
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Fig. 5. Magnetization (M) and temperature (T) plots for 65:35, 70:30, 75:25 and 80:20 at H =100 Oe.
Fig. 7. (a) and (b) Magnetoelectric voltage coefficient (aE) vs. field (H) plots for 75:25 and 80:20.
Fig. 6. Magnetization (M) vs. field (H) for 65:35 at T=300 K, T= 430 K and T= 530 K. Top inset shows inverse susceptibility (1/w) vs. temperature (T) plot for 65:35 with y = 471 K and bottom inset indicates the low corecivity value for 65:35.
measured by static method. The magnetoelectric voltage coefficient first increases with the increase in magnetic field, attains a peak value and then decreases when the magnetostriction of the sample starts saturating. The maximum value of magnetoelectric voltage coefficient (aE) for 75:25 is 0.1 mV/cmOe at HDC =1200 Oe and for 80:20 it is 0.312 mV/cmOe at HDC = 1000 Oe. Since these samples are magnetically soft, pseudo piezomagnetic coupling in these samples are expected to be larger, which should lead to much large dynamic ME effect. However, the results shown here are for static measurements; dynamic ME coefficient measurements of these samples are in progress. Finally, we would like to bring to the notice that in the tan d vs. T plots for all four compositions, a peak or hump near the magnetic transition temperature was observed (shown in Fig. 8 at 100 kHz). Magnetoelectric coupling implies that at the magnetic
Fig. 8. Expanded view of dielectric loss (tan d) vs. temperature (T) plots for all samples at 100 kHz.
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transition temperature there should be a corresponding change in dielectric properties of the composites. This anomalous behavior of dielectric loss curve (although the e vs. T plot does not show any anomaly around this temperature) near magnetic transition temperature of the composites, in our opinion indicates that although for sample 65:35 and 70:30 we could not directly measure aE, one can infer ME coupling in these samples from their tan d vs. T plots. 4. Conclusions In conclusion, we have studied the structural, dielectric, magnetic and magnetoelectric properties of (x)PZT– (1 x)Mn0.3Co0.6Zn0.4Fe1.7O4 composite. Although dielectric transition temperature changes significantly with change in the mole ratio of the two component phases, magnetic transition temperature (much less compared to the bulk cobalt ferrite) is observed to be relatively non-responsive for the changing molar ratio of the two component phases. In the vicinity of the magnetic transition temperature we observed an anomaly in the tan d vs. T plot, which indicates a possible magnetoelectric coupling in the sample. Highest magnetoelectric voltage coefficient (aE =0.312 mV/cmOe) is obtained for sample 80:20 at HDC =1000 Oe. Acknowledgement Authors would like to acknowledge the DST funded SQUID facility at I.I.T Delhi for the magnetic measurements. One of the authors (A.G) would like to thank CSIR for providing the fellowship.
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References [1] D.N. Astrov, Soviet Physics JETP 11 (1960) 708. [2] I. Kornev, M. Bichurin, J.P. Rivera, S. Gentil, H. Schmid, A.G.M. Jansen, P. Wyder, Physical Review B 62 (2000) 12247. [3] M. Mahesh Kumar, A. Srinivas, G.S. Kumar, S.V. SuryanarayanaJournal of Physics: Condensed Matter 11 (1999) 8131. [4] J. Ryu, A.V. Carazo, K. Uchino, H.E. Kim, Journal of Electroceramics 7 (2001) 17. [5] J.V. Suchtelene, Phillips Research Report 27 (1972) 28. [6] A.S. Zubkov, Elektrichestvo 10 (1972) 77. [7] J.V.D. Boomgard, D.R. Terrell, R.A.J. Born, Journal of Material Science 9 (1974) (1705). [8] J.V.D. Boomgard, A.M.J.G.V. Run, J.V. Suchtelen, Ferroelectrics 14 (1976) 727. [9] J.V.D. Boomgard, R.A.J. Born, Journal of Material Science 13 (1978) 1538. [10] W. Yong-ping, Z. Zheng, Q. Jin-hao, Transactions of Non ferrous Metals Society of China 16 (2006) s20. [11] G. Srinivasan, R. Hayes, C.P. Devreugd, V.M. Laltensin, N. Paddubnaya, Applied Physics A 80 (2005) 891. [12] A.V.R. Reddy, G.R. Mohan, D. Ravinder, B.S. Boyanov, Journal of Material Science 34 (1999) 3169. [13] G.A. Gaballa, A.L. Elshora, M.A. Henaish, M.A. El-AttarPhase Transition: Multifunctional Journal 46 (1994) 67. [14] O. Caltun, I. Dumitru, M. Feder, N. Lupu, H. Chiriac, Journal of Magnetism and Magnetic Materials 320 (2008) e869. [15] O. Caltun, G.S.N. Rao, K.H. Rao, B.P. Rao, I. Dumitru, C.O. Kim, C.G. Kim, Journal of Magnetism and Magnetic Materials 316 (2007) e618. [16] O. Caltun, G.S.N. Rao, K.H. Rao, B.P. Rao, I. Dumitru, C.G. Kim, Journal of Magnetism and Magnetic Materials 320 (2008) e869. [17] O.F. Caltun, G.S.N. Rao, K.H. Rao, B. Parvatheeswara Rao, C.G. Kim, C.O. Kim, I. Dumitru, N. Lupu, H. Chiriac, Sensors Letters 5 (2007) 1. [18] J.A. Paulsen, A.P. Ring, C.C.H. Loa, J.E. Snyder, D.C. Jiles, Journal of Applied Physics 97 (2005) 044502. [19] S.D. Bhame, P.A. Joy, Journal of Physics D: Applied Physics 40 (2007) 3263. [20] C.H.L. Chester, IEEE Transactions on Magnetics 43 (2007) 2367. [21] S.G. Lu, Z.K. Xu, Y.P. Wang, S.S. Guo, H.D. Chen, T.L. Li, S.W. Or, Journal of Electroceramics Special Issue Sp.Iss.SI 21 (2008) 398. [22] L. Mitoseriu, V. Buscaglia, Phase Transitions 79 (2006) 1095. [23] R. Arulmurugan, B. Jeyadevan, G. Vaidyanathan, S. Sendhilnathan, Journal of Magnetism and Magnetic Material 288 (2005) 470.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Study of dielectric and magnetic properties of PbZr0.52Ti0.48O3– Mn0.3Co0.6Zn0.4Fe1.7O4 composite Arti Gupta, Ratnamala Chatterjee n Advanced Ceramic Laboratory, Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India
a r t i c l e in f o
a b s t r a c t
Article history: Received 23 September 2009 Available online 16 December 2009
Results of detailed structural, dielectric, magnetic and magnetoelectric studies of (x)PbZr0.52Ti0.48O3– (1 x)Mn0.3Co0.6Zn0.4Fe1.7O4 composites where x=65, 70, 75 and 80 are shown in this work. Manganese substituted cobalt ferrites are known to exhibit large strain derivative (dx/dH) and on the other hand substitution of Zn in pure cobalt ferrite is known to enhance its permeability m and permittivity e. The choice of ferrite as Mn, Zn simultaneously substituted cobalt ferrite (MCZFO) is made keeping in view that for good magnetoelectric (ME) voltage coefficient the magnetostrictive constituent phase of the composite should have large strain derivative (dx/dH) along with large permittivity and permeability. It is shown here that although the dielectric transition temperature changes significantly with change in the mole ratio of the two component phases, magnetic transition temperature (much less compared to the bulk cobalt ferrite) is relatively non-responsive to the changing molar ratio of the two component phases. In the vicinity of the magnetic transition temperature we observed an anomaly in tan d vs. T plots, which indicates a possible magnetoelectric coupling in the samples. Magnetoelectric voltage coefficient (aE) has been measured using static magnetoelectric method. Highest magnetoelectric voltage coefficient (aE = 0.312 mV/cmOe) is obtained for sample 80:20 at HDC = 1000 Oe. & 2010 Published by Elsevier B.V.
Keywords: Structrural Magnetic Dielectric Magnetoelectric Permeability and corecivity
1. Introduction Multiferroic materials are those which combine at least two or more ferroic properties like ferroelasticity/ferroelectricity/ferromagnetism in the same temperature range. Magnetoelectric (ME) effect, i.e., induction of ferroelectric polarization by magnetic field and magnetization by electric field can be achieved in multiferroic materials that show coexistence of ferroelectricity and ferromagnetism. The ME effect was first observed in the antiferromagnetic Cr2O3 [1]. The magnetoelectric (ME) activity in multiferroics results either from the direct coupling between the two order parameters in single phase systems, or indirectly via strain mediated coupling in composites. Use of biphase composites has been more successful in the device applications as they have better ME output as compared to single-phase materials. Another difficulty with the single phase multiferroics is that usually their Neel or Curie temperature was observed to be far below room temperature [2–4]. In 1972 Suchtelene et al. [5] introduced the concept of magnetoelectric composite by combining the magnetostrictive and piezoelectric materials, in which the magnetoeletric effect appears as a product property of the composite. In these materials, as the magnetic field is applied, magnetostrictive
n
Corresponding author. Tel.: + 91 11 2659 1354; fax: + 91 11 2658 1114. E-mail address: [email protected] (R. Chatterjee).
0304-8853/$ - see front matter & 2010 Published by Elsevier B.V. doi:10.1016/j.jmmm.2009.12.007
particles change their shape due to magnetostriction. The strain thus produced passes to piezoelectric phase through interfacial boundary and generates stress over the piezoelectric phase that results into the generation of induced voltage. Magnetoelectric voltage coefficient of composite is defined [6] as
aE ¼ dE=dH ¼ ðdx=dHÞmagnetostrictive :ðdE=dxÞpiezoelectric ¼ ðdx=dHÞmagnetostrictive :ðg33 :C33 Þpiezoelectric where g33 is the piezoelectric voltage coefficient and C33 is the stiffness coefficient of the piezoelectric phase. Thus, (dx/dH) is a useful parameter in deciding the magnetoelectric properties. Hence a suitable combination of piezoelectric and magnetostrictive materials may give rise to appreciable magnetoelectric effect [7–9]. It is theoretically studied and shown that magnetoelectric voltage coefficient depends on the material properties like permittivity, permeability and elastic modulus of the magnetostrictive phase [10]. Cobalt ferrite has a large l but it has the disadvantage of large magnetic anisotropy and coercivity. Substitution of Zn in pure cobalt ferrite is observed to decrease the magnetic anisotropy in the composition that leads to high permeability value in them. This has been shown to increase the magnetoelectric coupling in PZT–CoxZn(1 x)Fe2O4 laminated composite [11]. Substitution of zinc in cobalt ferrite is known to increase the resistivity and
ARTICLE IN PRESS A. Gupta, R. Chatterjee / Journal of Magnetism and Magnetic Materials 322 (2010) 1020–1025
decrease the Curie temperature [12,13]. Increase in resistivity of ferrite is an important factor for the improvement of ME coupling in case of particulate magnetoelectric composite [9]. There are many reports [14–20] in literature which show that the manganese (Mn) substituted cobalt ferrite exhibit large slope of strain x(H) or its field derivative (dx/dH) and low magnetic transition temperature as compared to pure cobalt ferrite. Hence simultaneous substitution of Mn and Zn in cobalt ferrite with iron deficiency i.e., Mn0.3Co0.6Zn0.4Fe1.7O4 (MCZFO) can make a good candidate as a magnetostrictive component.
1021
In this work we have studied the system (x)PbZr0.52Ti0.48O3– (1 x)Mn0.3Co0.6Zn0.4Fe1.7O4 for x= 65, 70, 75 and 80. All the compositions lie in the ferroelectric rich region and the ferroelectric component PbZr0.52Ti0.48O3 (PZT) is at the morphotropic phase boundary (MPB). Results of detailed structural, dielectric, magnetic and magnetoelectric studies are shown in this work. Highest ME coefficient (aE = 0.312 mV/cmOe at 1000 Oe) is observed for 80:20 composite.
2. Experimental details
Fig. 1. X-ray diffraction patterns for (a) 80:20, (b) 75:25, (c) 70:30 and (d) 65:35.
The individual phase Mn0.3Co0.6Zn0.4Fe1.7O4 (MCZFO) has been prepared using the conventional solid state reaction method taking MnO2, Co3O4, ZnO and Fe2O3 as a raw materials. All the raw materials were first weighed in the stoichiometric proportion and then mixed in acetone medium for 5–6 h. The mixed powder was then calcined at 1000 1C for 6 h. The calcined powder was again mixed in the acetone medium for nearly 2 h. PbZr0.52Ti0.48O3 (PZT) was also prepared by the conventional ceramic method. All the raw materials PbO, ZrO2 and TiO2 were weighed in stoichiometric ratio and then mixed in the acetone medium for 4–5 h. The mixed powder was then calcined at 950 1C for 2 h. The individual phase formations of PZT and MCZFO were confirmed by the X-ray diffraction (not shown). The individual components PZT:MCZFO were taken in the mole ratio 65:35, 70:30, 75:25 and 80:20, respectively. They were mixed individually in the acetone medium for several hours and finally 2 wt% of polyvinyl alcohol (PVA) was added to these mixed powders and pressed in hydraulic press in the shape of disk with diameter of 11 mm and thickness of 1 mm. These disks were finally sintered in temperature range (1130–1150 1C) for 3 h.
Fig. 2. Scanning electron micrographs for (a) 65:35, (b) 70:30, (c) 75:25 and (d) 80:20.
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The structural characterizations were done using the X-ray diffractometer (CuKa radiation; Phillips X’pert PRO) and scanning electron microscopy using ZEISS EVO-50 system. The dielectric properties were measured using HP4192A LF impedence analyzer. Magnetic properties of the composites were measured using the superconducting quantum interference device (SQUID) (MPMS XL-7 of Quantum Design). The static magnetoelectric voltage coefficient (aE) was measured using the indigenous laboratory set up; prior to ME coupling measurement the samples were electrically and magnetically poled. For electric poling, samples were first heated at 100 1C in the presence of external electric field of 5 kV/cm, kept at this field for 30 min and then cooled to
room temperature in the presence of field. The electrically poled samples were then magnetically poled at 5 kOe for 30 min.
3. Results and discussion The X-ray diffraction pattern establishes the presence of both the phases in all the compositions. XRD pattern for all four compositions are shown in Fig. 1. Two well-defined sets (individual phases: the (i) tetragonal phase of PZT and the (ii) cubic spinel phase of MCZFO) of diffraction patterns observed are the characteristics of composite formation. No X-ray reflection
Fig. 3. Variation of dielectric constant (e) and dielectric loss (tan d) with temperature (T) for 65:35, 70:30, 75:25 and 80:20.
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other than the individual phases was found which suggests the absence of any chemical reaction between the two phases during the sintering process. Intensity of the X-ray reflection corresponding to magnetostrictive phase (MCZFO) can be clearly seen to decrease with its content in the composite. A systematic variation of intensities of the two phases is observed to indicate the presence of scattering components from both the participating phases. Scanning electron micrographs for all four compositions are shown in Fig. 2. The images are taken for the fractured portions of the samples. Presence of two types of phases can be clearly seen from these images, with the smaller grains of PZT matrix and large grains of dispersed MCZFO phase. The variation of dielectric constants e of all the composite samples with temperature (330 oTo773 K) at fixed frequencies (1 kHz rfr1 MHz) and their dielectric loss (tan d) are shown in Fig. 3. The dielectric constants were observed to decrease with increasing frequencies. At all frequencies e vs. T graphs show a peak related to transition. This dielectric transition temperature for the composition 80:20 ( 673 K) is close to the ferroelectric transition temperature (TcFE ) for pure PZT. For composites TcFE is observed to decreases with the increase in the mole percentage of MCZFO. Similar feature has been seen for the PZT–NiFe2O4 [4] and PZT–CoFe2O4 [21] composites. It is also clear from these figures that the dielectric constant decreases with the addition of MCZFO due to the low dielectric constant value of ferrite. The high dielectric constant measured at low frequencies might be attributed to the Maxwell–Wagner type interfacial polarization mechanism that plays a crucial role in these types of heterogeneous composites. The interfaces between the ferroelectric and ferromagnetic phases which have significantly different conductivities cause an additional polarization that is termed as the interfacial polarization, which boosts the dielectric constant [22]. This fact is also supported by the variation of the dielectric constants (e) with frequency (f) at room temperature for all four compositions, shown in Figs. 4(a) and (b). Interfacial polarization responds very slowly to the external field, hence, it dominates in the low frequency region and has no significant contribution in the high frequency region (105–106 Hz). The magnetization (M) vs. temperature (T) measurements at field of 100 Oe for all four compositions are shown in Fig. 5. All the compositions show a ferromagnetic to paramagnetic transition around T 430 K. It is important to note that the magnetic transition temperature is nearly same for all compositions whereas the dielectric transition temperatures show a significant change with changing of ferroelectric component’s mole percentage. Also as expected, the magnetic transition temperature of composites ( 430 K) is much less compared to transition temperature of pure cobalt ferrite ( 800 K). This is expected as the substitution of both the manganese and zinc into pure cobalt ferrite reduce the Curie temperature [12, 17, 23]. For 65:35 sample, which consists of the highest mole fraction of ferrite, a built-up in magnetization with temperature up to the transition temperature can be noted. Possibly with increase in temperature i.e., with added thermal energy, the magnetic domains in the composite can easily rotate in the direction of field and as a result the magnetization increases. As the magnetic Curie temperature approaches, this process is taken over by the ferromagnetic to paramagnetic transition and correspondingly at 430 K a decrease in magnetization is observed. To confirm that this is purely a ferromagnetic to paramagnetic transition, the magnetization (M) vs. field (H) loop at room temperature T =300 K ( oTc 430 K), at T=430 K and at T= 530 K ( 4Tc 430 K) were performed for 65:35. At T=300 K and 430 K we get purely ferromagnetic loops while at T= 530 K a typical paramagnet behavior was observed (see Fig. 6). The inverse susceptibility
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Fig. 4. (a) and (b) Variation of dielectric constant (e) and dielectric loss (tan d) vs. frequency (f) for 65:35, 70:30, 75:25 and 80:20.
(1/w) vs. temperature (T) plot for 65:35 (see top inset of Fig. 6) gives the estimated Curie temperature for this composite as 471 K confirming the ferromagnetic behavior below magnetic transition temperature. The magnetization vs. field measurements (not shown here) have been done for all samples. Small corecivity (Hc) ( 48–55 Oe) obtained for these samples indicate their soft magnetic behavior (see bottom inset of Fig. 6). The saturation magnetization (MS) and remnant magnetization (MR) values for these samples were observed to increase with increasing MCZFO content. MS and MR values for sample 65:35 are 19.490 and 2.078 emu/g whereas for 80:20 sample, these are 10.228 and 0.8 emu/g, respectively. Magnetoelectric voltage coefficients (aE) vs. magnetic field (H) were also measured for these samples. Samples with large mole percentage of ferrites (65:35 and 70:30) did not show any detectable ME effect. Comparatively larger molar ratio of ferrite particles in these samples make conducting chains and it was difficult to pole these samples. Figs. 7(a) and (b) show ME voltage coefficient (aE) vs. magnetic field (H) for 75:25 and 80:20 samples,
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Fig. 5. Magnetization (M) and temperature (T) plots for 65:35, 70:30, 75:25 and 80:20 at H =100 Oe.
Fig. 7. (a) and (b) Magnetoelectric voltage coefficient (aE) vs. field (H) plots for 75:25 and 80:20.
Fig. 6. Magnetization (M) vs. field (H) for 65:35 at T=300 K, T= 430 K and T= 530 K. Top inset shows inverse susceptibility (1/w) vs. temperature (T) plot for 65:35 with y = 471 K and bottom inset indicates the low corecivity value for 65:35.
measured by static method. The magnetoelectric voltage coefficient first increases with the increase in magnetic field, attains a peak value and then decreases when the magnetostriction of the sample starts saturating. The maximum value of magnetoelectric voltage coefficient (aE) for 75:25 is 0.1 mV/cmOe at HDC =1200 Oe and for 80:20 it is 0.312 mV/cmOe at HDC = 1000 Oe. Since these samples are magnetically soft, pseudo piezomagnetic coupling in these samples are expected to be larger, which should lead to much large dynamic ME effect. However, the results shown here are for static measurements; dynamic ME coefficient measurements of these samples are in progress. Finally, we would like to bring to the notice that in the tan d vs. T plots for all four compositions, a peak or hump near the magnetic transition temperature was observed (shown in Fig. 8 at 100 kHz). Magnetoelectric coupling implies that at the magnetic
Fig. 8. Expanded view of dielectric loss (tan d) vs. temperature (T) plots for all samples at 100 kHz.
ARTICLE IN PRESS A. Gupta, R. Chatterjee / Journal of Magnetism and Magnetic Materials 322 (2010) 1020–1025
transition temperature there should be a corresponding change in dielectric properties of the composites. This anomalous behavior of dielectric loss curve (although the e vs. T plot does not show any anomaly around this temperature) near magnetic transition temperature of the composites, in our opinion indicates that although for sample 65:35 and 70:30 we could not directly measure aE, one can infer ME coupling in these samples from their tan d vs. T plots. 4. Conclusions In conclusion, we have studied the structural, dielectric, magnetic and magnetoelectric properties of (x)PZT– (1 x)Mn0.3Co0.6Zn0.4Fe1.7O4 composite. Although dielectric transition temperature changes significantly with change in the mole ratio of the two component phases, magnetic transition temperature (much less compared to the bulk cobalt ferrite) is observed to be relatively non-responsive for the changing molar ratio of the two component phases. In the vicinity of the magnetic transition temperature we observed an anomaly in the tan d vs. T plot, which indicates a possible magnetoelectric coupling in the sample. Highest magnetoelectric voltage coefficient (aE =0.312 mV/cmOe) is obtained for sample 80:20 at HDC =1000 Oe. Acknowledgement Authors would like to acknowledge the DST funded SQUID facility at I.I.T Delhi for the magnetic measurements. One of the authors (A.G) would like to thank CSIR for providing the fellowship.
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