Magnetic Anisotropy and Ferromagnetic Resonance in Nitrogen-Incorporated NiFe2O4 Thin Films

The nitrogen-incorporated nickel ferrite (N-NFO) thin films deposited by radio-frequency magnetron sputtering were found to exhibit interesting magnetic properties upon heat treatment. The in-plane (IP) and out-of-plane (OP) magnetizations of the heat-treated N-NFO thin films measured at 20 K were seen not to saturate even at higher fields, indicating a large perpendicular magnetic anisotropy (PMA). The coercive field value was found to decrease upon heat treatment, due to the weakened pinning of domain walls upon decrease in grain boundary volume. The IP magnetization exhibits a tendency to saturate at high fields, whereas the OP magnetization does not, indicative of the existence of easy magnetization direction in the plane of the film. The room temperature ferromagnetic resonance (FMR) spectra in both IP and OP configurations indicated the increase of FMR linewidth upon nitrogenation, revealing the increased distribution of magnetic anisotropy induced by nitrogen in the lattice. An increase in the resonance field value was observed for the N-NFO film grown at 100 W compared with that of NFO in the IP configuration, while a decrease in the value was observed for OP, which is attributed to an enhanced IP magnetic anisotropy. The films grown at 120 and 140 W have shown decrease in a resonance field value in the IP configuration and increase in OP, which is attributed to the increase in thickness-dependent PMA.