Phase Formation, Microstructure, and Magnetocapacitance Behaviour Of Ex-situ Combustion Derived Magnetodielectric Composites

Abstract

Magnetodielectric (MD) compounds belong to multifunctional materials which can show both magnetic and dielectric along with MD properties including magnetocapacitance (MC) and magnetoresistance (MR). MD properties depend on the interface of both dielectric and magnetic phases of the composite. In this work, a novel combustion derived ex-s tu synthesis was adopted to develop MD composites. Here, one of the phases in the calcined powder form was dispersed in precursor of other phase followed by combustion. In this approach, by interchanging the dispersed (varying particle size) and precursor can alter the microstructure during sintering, and it can tune MD properties of the composites. Based on this concept, the prime objective of this research work is to develop ferrite- BT and BT-ferrite composites via ex-situ combustion in which the gel-combustion/solid-state derived calcined powders of dispersed phase (i.e., 30 wt % ferrite / 70 wt % BT in ferrite-BT / BT-ferrite composite, respectively) were introduced into the precursor to tune the microstructure (in terms of the distribution of BT, CF and in-situ phases) and explore the dielectric, magnetic and frequency/field dependent MC and MR of these composites. The calcined (800 °C and 1200 °C) powders of (GCF/SCF)800/1200 (i.e., gel-combustion/solid-state derived CF) and (GBT SBT)800/1200 (i.e., gel-combustion/solid-state derived BT) have been used as a dispersed phase to develop ferrite-BT and BT-ferrite composites, respectively. These composite powders were calcined at 800 °C/4h, pelletized and sintered at 1000 °C, 1100 °C, 1200 °C and 1250 °C. Different characterization techniques including XRD, FESEM, Raman, dielectric, magnetic, and MD have been performed on all composites. The results are analysed and compared among each other considering the source and size of the dispersed phase, as well as sintering temperature of the composite. The percentage of BT and HBT found inversely proportional to each other, and a similar relation was observed between CF and BHF phases. Moreover, the presence of in-situ phases (BHF and HBT) was higher in GCF/SCF/GBT/SBT-800-based composites. Highest HBT and BHF phases were observed in ferrite-BT composites. The spherical or polyhedral BT/CF and plate-like HBT/BHF phases were found indistinguishable and evenly distributed in the GCF/SCF/GBT/SBT-800-based composites sintered at lower temperatures. Higher permittivity was observed at lower frequencies due to the space-charge polarization caused by electrical inhomogeneity in the composites. The permittivity of both ferrite-BT and BT-ferrite composites was found dependent on the type of dispersed phase and sintering temperature. The permittivity at 1 MHz in all the composites (except [SCF800]→[ferrite-BT]-1250) was found directly proportional to the relative density and the percentage of the dielectric phase. The magnetization of all the composites was partially saturated due to the presence of non-magnetic BT and/or plate-like HBT phases. The MC% response of the composites was found mostly field-dependent and analogous to the MR% response, indicating the influence of space-charge polarization (M-W polarization). The highest positive magnetocapacitance percentage of ~ 45% was observed in the [GCF1200]→[ferrite-BT]-1200 composite, which contains higher plate-like HBT phase. Furthermore, the magnetocapacitance behaviour of the composites was correlated with the dM/dH behaviour, field and frequency-dependent magnetoresistance, and Cole-Cole plots. The results of all composites are analysed with respect to the type/interchanging dispersed phase, in-situ phases, microstructure, sintering temperature, density, resistivity, MC and MR and also compared with the relevant literature. The developed MD composite may have a potential application in the field of magnetoresistive sensors, actuators, transducers, spintronic devices, and memory devices.