Abstract
We report studies on dielectric properties of magnetoelectric composites of La-substituted lead zirconate titanate (PLZT) and zinc-doped nickel ferrite (NZF) with compositional formula 0.95 Pb1-3x/2LaxZr0.65Ti0.35O3-0.05 Ni0.8Zn0.2Fe2O4 (, 0.01, 0.02, and 0.03). The materials were synthesized by conventional solid state reaction route. The presence of individual phases (PLZT and NZF) was confirmed by using X-ray diffraction technique. Dielectric properties were studied as a function of temperature and frequency. Significant improvement was observed in dielectric properties with addition of La.
1. Introduction
Single-phase multiferroics have small magnetoelectric (ME) voltage coefficient and temperature constraints [1–3] which restricts their technological applications. To overcome these difficulties, composites are made from ferroelectrics and ferromagnetic, and they are found to exhibit large extrinsic ME effect [4, 5]. Magnetoelectric composites have significant potential for applications in multifunctional devices like sensors for magnetic field measurements, transducers for magnetoelectric conversion, and magnetoelectric memory devices [6–8]. The most widely studied systems that have been reported correspond to NiFe2O4, CoFe2O4 ferrites with PZT and BaTiO3. PZT has been selected due to higher resistivity, lower sintering temperature, and stronger dielectric and piezoelectric properties [9] as compared to other ferroelectrics. Further, varying Zr/Ti ratio and substitution of some suitable ions improve piezoelectric and dielectric properties. A number of researchers reported that La-doped PZT ceramics (PLZT) have been found to be most suitable to provide improved properties [10, 11]. Furthermore, zinc-doped Nickel ferrite particles are very stable in the PZT matrix, do not react even at high sintering temperature [12], and have high resistivity and high piezomagnetic coefficient [13]. The focus of this work is on understanding of the effects of La content on structural and dielectric properties of PLZT-NZF composites.
2. Experimental
Studied system (PLZT-NZF) consists of two phases (ferroelectric and ferrite). Individual phases were prepared by conventional solid state reaction route. The raw materials of AR grade PbO, ZrO2, TiO2, and La2O3 for ferroelectric phase were weighed in required molar proportions and mixed. An excess of 2% of PbO was added to compensate lead loss during sintering. Mixture of the raw materials was milled in distilled water using zirconia balls. The dried powder was calcined at 800°C for 4 hrs. The reacted powder mixture was ball milled again and then recalcined at 1000°C for 4 hrs. The ferrite phase was prepared using starting materials of AR grade NiO, ZnO, and Fe2O3 in molar proportions. The powder mixture was ball-milled and then calcined at 1000°C for 4 hrs. A small amount of MnO2 (0.5% by weight) was added to increase the resistivity of the ferrite phase. After ball milling powder mixture was recalcined at 1100°C for 4 hrs. Ferroelectric-Ferrite (PLZT-NZF) composites with general formula 0.95Pb1-3x/2LaxZr0.65Ti0.35O3-0.05Ni0.80Zn0.20Fe2O4 were prepared. Small amount of diluted polyvinyl alcohol (2-3 drops) was added to the powder mixture as binder and pressed into circular discs of 1 mm thickness and 15 mm diameter using hydraulic press. The pellets were finally sintered at 1200°C for 4 hrs in a programmable furnace. Experimental density of sintered pellets was determined using Archimedes principle. Theoretical density of the samples was calculated using the following formula: where is lattice constant, is molecular weight, is Avogadro’s number, and is number of molecules per unit cell ( for ferrite phase and 1 for ferroelectric phase). For measuring dielectric properties, the sintered pellets were ground and then electroded properly using silver paste on flat surfaces by heating at 400°C for 30 min. XRD patterns were recorded for all compositions (using Philips XPERT-PRO Diffractogram with Cukα ( Å) source operated at 45 kV/40 mA). The dielectric properties were measured as a function of frequency (100 Hz–100 kHz) at room temperature and temperature (35°C–500°C) at 1, 10, and 100 kHz using an Agilent 4263B LCR meter.
3. Results and Discussion
X-ray diffraction (XRD) patterns for all the compositions were recorded and compared in Figure 1. For all the composite samples, peaks with specific indices confirm the coexistence of cubic spinel structure in ferrite phase (NZF) and rhombohedral perovskite structure in ferroelectric phase (PLZT).
Intensity of peaks corresponding to ferrite phase is very small due to small concentration of ferrite phase. Shifting in ferroelectric peaks towards higher angle is observed with increase in La ( pm) concentration which indicates the decrease in lattice constant due to substitution of less ionic size substituent at Pb ( pm) site, as shown in Table 1. Random values of lattice constant of ferrite phase were observed. This may be due to stresses induced by ferroelectric phase on ferrite phase in composites [14]. Relative density for all the samples was calculated and is given in Table 1. Comparison shows that sample with has lower relative density than undoped one, and for , there is increase in densification whereas density further decreases for . Lower densification for small concentration of dopants is already observed in many ferroelectric ceramics [15, 16]. This variation in density can be explained by vacancy concentrations which affects volume diffusion for different dopant concentrations [17]. A typical SEM micrograph for the composite with is shown in Figure 2.
Figure 3 shows the variation of dielectric constant and loss tangent with temperature for composite samples at 1, 10, and 100 kHz. The maxima in dielectric constant () were observed for all samples which correspond to ferroelectric-paraelectric transition at ferroelectric Curie temperature (). was found to decrease with increase in La content, and increases with increase in La content. This decrease in is mainly due to shrinkage in lattice volume and substitution of less ionic size dopant. Increase in room temperature dielectric constant and loss tangent with increase in dopant concentration for PZT are well reported in the literature [18]. At 1 kHz, increase in dielectric constant with increase in temperature and high values of loss tangent in paraelectric region were seen that may be due to conductivity losses which occurs due to presence of ferrite phase.
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Figure 4 shows the variation of dielectric constant and loss tangent with frequency at 30°C. The dielectric constant decreases with increase in frequency, showing dispersion in the lower frequency range for all the composite samples. Higher value of dielectric constant at low frequencies is due to the presence of all types of polarizations. Further, if we compare the low frequency dispersion for undoped composite sample () with that for doped ones, it can easily be seen that dispersion is more in case of undoped one. This may be due to more inhomogeneity [19] created because of Pb vacancies in case of undoped sample than doped ones. Values of ferroelectric Curie temperature (), room temperature dielectric constant, loss tangent and their maximum values are shown in Table 1.
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4. Conclusions
Composites synthesized by conventional solid state method were characterized by X-ray diffraction technique, and coexistence of both phases (ferroelectric and ferrite) was confirmed. There was decrease in relative density for small concentration of dopant (). Decrease in ferroelectric Curie temperature () and increase in room temperature dielectric constant with increase in La content were observed for all the composite samples. All of the aforementioned brings us to the conclusion that La-doped magnetoelectric composites have improved dielectric properties as compared to undoped ones.
Acknowledgment
One of the authors (R. Rani) would like to thank Department of Science & Technology for awarding INSPIRE fellowship to her.