Structural and magnetic transformation in Fe1-xCoxOy films grown by molecular beam epitaxy

Fe_1-xCo_xO_y(0 ≤ x ≤ 1,1 ≤ y ≤ 4/3) mixture oxide films are grown, with epitaxial quality, on MgO(001) substrates by molecular beam epitaxy. The X-ray diffraction result along [001] plane is similar to Fe₃O₄ for the films with x < 0.5 (Fe rich) and is similar to CoO for the films with x>0.5 (Co rich). An abrupt change, over 2%, on the lattice spacing is observed around Fe/Co ∼1(x = 0.5) suggesting a phase transformation from spinel to rocksalt. A fast suppression of the saturation magnetization is found markedly different from the bulk materials as the Co ratio increases. Crystalline anisotropy and anomalous magnetic properties are affected substantially by Co ions. These unique magnetic behaviors of Fe-Co-O films are believed to be related to cation distribution in spinel structure originated by the low-temperature process of thin-film fabrication.