Preparation of composite nanoparticles based on cobalt ferrite

Recently, materials composed of both soft and hard magnetic phases have been attracting a lot of attention. This paper describes the preparation of composite nanoparticles by the reduction and oxidation of cobalt ferrite (CoFe₂O₄) nanoparticles. Neither the composite nanoparticles of hard ferrites and soft magnetic materials nor the hydrogen reduction of ferrite nanoparti-cles has been reported. Therefore, investigation of the relationship between their microstructure and magnetic properties is needed. Our study revealed the cobalt ferrite powders prepared by the coprecipitation method showed the highest coercivity H_c = 750 kA·m^-1 in the case of ternary cobalt ferrite. After the cobalt ferrite powders were reduced by heat treatment at 450°C for 15 min in hydrogen, the reduced powders were shown to consist of a Fe-Co (bcc) phase and XRD analysis of the lattice parameter suggests the composition was around Fe₆₇Co₃₃. After oxidization of the reduced powder by heat treatment at 300°C for 15 min in air, the powders consisted of a Fe-Co (bcc) phase and a spinel-type phase. TEM analysis revealed that the oxidized particles had a core-shell structure and the shell was shown to have a spinel-type phase with some amorphous structures. Therefore the magnetic properties of the core-shell powders were shown to exhibit low coercivity H _C = 120A kA·m^-1 and low saturation magnetization σ_s=80.2 Am²·kg^-1.