Large Negative Uniaxial Magnetic Anisotropy in Epitaxially Strained Nickel Ferrite Films

We investigated the magnetic properties of NiFe2O4(NFO) epitaxial films grown on MgAl2O4(MAO) substrates by the reactive radio frequency magnetron sputtering method. The films were found to be coherently distorted to at least 61 nm thickness because of the lattice mismatch between MAO and NFO. The NFO(001) films exhibited large negative uniaxial anisotropy that can be quantitatively explained by the magneto-elastic theory despite the lattice distortion being as much as 3%. We also discovered magnetic anomalies in both the saturation magnetization and anisotropy of the thinnest film, which may because of the reconstruction of the electronic structures at NFO interfaces.