For the development of room temperature spin-filtering devices with a ferromagnetic barrier, very thin ferromagnetic barrier films were required. Co-ferrite thin films are prepared by the surface plasma oxidization of a CoFe2 underlayer deposited on MgO (001) single crystal substrates. During oxidization, the substrate temperature was varied from 523 K to 673 K. The plasma oxidized CoFe2 films had a spinel structure, 4-fold symmetry in the plane, and an epitaxial relationship with the CoFe2 underlayer. The lattice parameter of Co-ferrite thin film was 0.828 nm, which is 1.2% less than that of bulk. The interface between the CoFe2 and its oxide was relatively smooth. When the substrate temperature during plasma oxidization was lower than 623 K, magnetization of CoFe2 and Co-ferrite rotated independently. XPS and MOKE measurements identified that the hard and soft phases as Co-ferrite and CoFe2, respectively. Measurements of the magnetic properties showed that the magnetization of the Co-ferrite thin films was about 1100 emu/cm3, which is 2.6 times more than that of bulk (420 emu/cm3). This phenomenon may be explained by the exchange of Co and Fe cation distribution in our Co-ferrite films.
