Synthesis and Characterizations of Magnetoelectric Materials Processed by Microwave Assisted Solid State Reaction Route for Multifunctional Device Applications

Abstract

Magnetoelectric (ME) composites, in which magnetic field can modify electrical properties and electric field can modify magnetic properties, are one of the promising as well as technologically interesting materials. Research work on bulk ME composites is based on the principle of combining highly electrostrictive material with highly magnetostrictive one in the anticipation of pronounced ME effect. ME effect in a composite is thought to take place mainly via mechanical coupling through piezoelectric and magnetostriction effects. Lack of inherent coupling in single-phase multiferroic compounds at room temperature led to the introduction of two-phase composite systems in which ME effect is regarded as the product property. Enhanced ME effect or better magneto-dielectric property between electrostrictive and magnetostrictive phases can be achieved in multiferroic composites. Hence, proper selection of electrostrictive and magnetostrictive phases is important to get better ME composite systems for multifunctional device applications. Thus, in view of the potential applications of ME materials, this work focuses on synthesizing ME composites comprising of ferrite (magnetostrictive) and ferroelectric (electrostrictive) phases by microwave-assisted solid-state reaction route. In order to synthesize bulk ME composites, morphotropic phase boundary composition (MPB) of 0.93Bi0.5Na0.5TiO3-0.07BaTiO3 (BNT-BT) and 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 (BZT-BCT) have been selected as electrostrictive phase whereas Ni0.30Cu0.08Zn0.62Fe2O4 (NCZF) and CoFe2O4(CFO) have been selected as the magnetostrictive phase. The parent systems were individually prepared and different wt. % of each of magnetostrictive and electrostrictive phases were mixed resulting in the formation of different composite systems. ME composites, synthesized via microwave assisted solid state reaction route (SSRR), are as follows:
1. (1-x)(BNT-BT)-xNCZF (where x = 0.1, 0.2, 0.3, 0.4, 0.5 wt. %)
2. (1-x)(BNT-BT)-xCFO (where x = 0.1, 0.2, 0.3, 0.4, 0.5 wt. %)
3. (1-x)(BZT-BCT)-xNCZF (where x = 0.1, 0.2, 0.3, 0.4, 0.5 wt. %)
4. (1-x)(BZT-BCT)-xCFO (where x = 0.1, 0.2, 0.3, 0.4, 0.5 wt. %)
XRD patterns of microwave calcined and sintered BNT-BT and BZT-BCT electrostrictive samples showed single perovskite phase. Experimental density of sintered BNT-BT and BZT-vii BCT samples was found to be ~94.96% and ~94.31% of their theoretical density, respectively. Average grain size (from SEM microstructure study) of microwave sintered BNT-BT and BZT-BCT samples was found to be ~0.41 μm and ~0.33 μm, respectively. Cole-Cole plots of BNT-BT and BZT-BCT samples in the 300ºC- 450ºC temperature range revealed negative temperature coefficient of resistance (NTCR) behaviour. Polarization vs. electric field (P-E) loop study of BNT-BT samples showed saturation polarization (Ps) and remnant polarization (Pr) to be ~ 23μC/cm2 and ~12μC/cm2,respectively and coercive field (Ec) was obtained ~ +/-15kV/cm. For BZT-BCT samples, Ps, Pr, Ec were obtained to be ~ 12μC/cm2, ~ 5μC/cm2 and ~ +/-8kV/cm, respectively. Leakage current of BNT-BT and BZT-BCT samples lies in 10-5-10-6 amps and in 10-7-10-8 amps range, respectively. Single phase cubic structure was obtained from XRD study of NCZF and CFO samples. Average grain size for NCZF and CFO samples were found to be ~0.30 μm and ~0.29 μm respectively. Room temperature dielectric study revealed that dielectric constant (εr) decreased with the increase in frequency whereas, εr for both the samples has been found to increase with the increase in temperature. Both the samples showed NTCR behaviour, which is evident from Cole-Cole plot study. Leakage current value of NCZF and CFO samples was obtained in 10-5-10-6 amps and in 10-4-10-5 amps range, respectively. Saturation magnetization (Ms) of NCZF and CFO samples was observed to be~ 45emu/g and ~ 60emu/g, respectively. From P-E loop study of (1-x)(BNT-BT)-xNCZF and(1-x)(BNT-BT)-xCFO composite samples it was found that with the increase of NCZF and CFO contents in value of 2Pmax and 2Pr vary and became maximum for x = 0.4. However, 2Pmax and 2Pr values of 0.6(BNT-BT)-0.4NCZF composite system were found more compared to 0.6(BNT-BT)-0.4CFO composite system. In M-H loop study, it was observed that with the increase of ferrite content (NCZF and CFO) in BNT-BT matrix, Ms and Mr values increased. Ms ~37.8 emu/g and Mr ~9.83 emu/g were obtained in 0.5(BNT-BT)-0.5CFO composite samples. P-E loop study of (1-x)(BZT-BZT)-xNCZF and(1-x)(BZT-BCT)-xCFO composite samples revealed that with the increase of NCZF as well as CFO contents in BZT-BCT matrix, value of 2Pmax and 2Pr vary and become maximum for x = 0.2. However, 2Pmax and 2Pr values were found to be maximum for 0.8(BZT-BCT)-0.2CFO composite samples.M-H loop study showed that with the increase of ferrite content (NCZF and CFO) in BZT-BCT matrix, Ms and Mr values increased for both (1-x)(BZT-BCT)-xNCZF and(1-x)(BZT-BCT)-xCFO composite system. However maximum Ms and Mr were observed in 0.5(BZT-BCT)-0.5CFO composite samples. Besides having good electrical and magnetic properties, there should be better interaction between the ferroic orders in ME composite samples. This interaction can produce other functionalities in electronic devices such as sensors, transducer, oscillators, filters, phase shifters, memory devices etc. From the analysis of present work, it can be inferred that 0.6(BNT BT)-0.4NCZF, 0.6(BNT-BT)-0.4CFO, 0.8(BZT-BCT)-0.2NCZF, 0.8(BZT-BCT)-0.2CFO composite system showed better polarization and magnetization values. Leakage current of 0.8(BNT-BT)-0.2CFO samples was found to be least. In all ME composite samples, AC conductivity increased with the increase of temperature. Grain and grain boundary contributions of synthesized composite samples were inferred from fitted Cole-Cole plot. NTCR behaviour and non-Debye type relaxation have been confirmed in all composite samples. From PFM and MFM analysis, we found evidence of M-E coupling in the composite samples. MD% (at 100 kHz) has been obtained to be ~ 2.4%, 4.7%, 3.2%, 6% for 0.6(BNT-BT)-0.4NCZF, 0.6(BNT-BT)-0.4CFO, 0.8(BZT-BCT)-0.2NCZF, 0.8(BZT-BCT)-0.2CFO composite samples, respectively. Overall, high M-D coupling has been observed in 0.8(BZT-BCT)-0.2CFO composite samples. Observance of high MD effect at low frequency (100 Hz) also suggested the presence of intrinsic ME coupling in studied composite samples.