Functional properties of randomly mixed and layered BaTiO3 - CoFe2O4 ceramic composites close to the percolation limit

Abstract

Di-phase ferroelectric-ferrite composites with the same composition close to the percolation limit (0.66BaTiO3-0.33CoFe2O4) and with different phase arrangements (randomly mixed phases and tri-layer structures) have been consolidated by using spark plasma sintering and their properties were comparatively analyzed. The intrinsic effective dielectric constant has been estimated by using finite element method and a value almost ten times higher was predicted for the randomly mixed composite, as result of different field distributions inside the ferrite and ferroelectric phases. At room temperature, experimental permittivity contains an intrinsic behavior overlapped onto a very strong extrinsic component determined by the interfaces and by local charge inhomogeneity. The extrinsic dielectric behavior has been discussed in terms of thermally activated relaxation mechanisms developed in both types of investigated structures, as revealed by the temperature-dependence impedance spectroscopy. The magnetic properties are derived as “sum property” from the ferrimagnetic character of CoFe2O4 and show the lowest magnetization for the randomly mixed composites and a weak magnetic anisotropy when the magnetic field was applied in-plane or out-of-plane in the layered structure. Due to the different levels of interface doping specific to the different types of interfaces, the magnetic Curie temperature of the two composites ranges from 720 K in the randomly mixed structure to 746 K for the layered one. The ferroelectric P(E) loops and a modest ME coefficient of 14 mVOe−1cm−1 at 100 Hz were determined only for the layered structure, due to the high leakage and to the small grain size (below 200 nm) in the ferroelectric BaTiO3 component.