Modification of structure and magnetic anisotropy of epitaxial CoFe<inf>2</inf>O<inf>4</inf> films by hydrogen reduction

Heteroepitaxial CoFe2O4 (CFO) thin films with different thicknesses were deposited on MgO (001) substrates. The as-deposited CFO films show a clear switching of magnetic anisotropy with increasing film thickness. The thinner films (<100 nm) show a perpendicular magnetic anisotropy due to the out-of-plane compressive strain. The thicker films exhibit an in-plane easy axis owing to the dominating shape anisotropy effect. The magnetostriction coefficient of CFO films is estimated to be λ[001] = -188 × 10-6. Metallic CoFe2 films were obtained by annealing the as-deposited CFO films in forming gas (Ar 93% + H2 7%) at 450 °C. XRD shows that CoFe2 films are textured out-of-plane and aligned in-plane, owing to lattice matching between CoFe2 and MgO substrate. TEM results indicate that as-deposited films are continuous while the annealed films exhibit a nanopore mushroom structure. The magnetic anisotropy of CoFe2 films is dominated by the shape effect. The results demonstrate that hydrogen reduction can be effectively used to modify microstructures and physical properties of complex metal oxide materials.