Influence of Grain Size Dispersion on the Magnetic Properties of Nanogranular BaTiO3-CoFe2O4 Thin Films

Abstract

Thin film nanogranular composites of cobalt ferrite (CoFe2O4) dispersed in a barium titanate (BaTiO3) matrix were deposited by laser ablation with different cobalt ferrite concentrations (x). Their structural and magnetic properties were characterized. The films were polycrystalline and composed by a mixture of tetragonal-BaTiO3 and CoFe2O4 with the cubic spinel structure. A slight (111) barium titanate phase orientation and (311) CoFe2O4 phase orientation were observed. The lattice parameter of the CoFe2O4 was always smaller than the bulk value indicating that the cobalt ferrite was under compressive stress. From atomic force microscopy a broad distribution of grain sizes was observed in the nanocomposites, with a significant amount of smaller grains (<40nm) from the CoFe2O4 phase. The magnetic measurements show an increase of the magnetic moment from the low concentration region where the magnetic grains are more isolated and their magnetic interaction is small, towards the bulk value for higher CoFe2O4 content in the films. A corresponding decrease of coercive field with increasing cobalt ferrite concentration was also observed, due to the higher inter-particle magnetic interaction (and reduced stress) of the agglomerated grains.