GROWTH and DEPTH-DEPENDENCE of SATURATION MAGNETIZATION of IRON NITRIDE THIN FILMS on MgO SUBSTRATE

We used polarized neutron reflectometry (PNR) to investigate the depth-dependent saturation magnetization (4πM_s) of bi-layer structured Fe-N/Fe grown on MgO substrates, which were prepared by a facing target sputtering method followed by a subsequent annealing process to purposely study the N inter-diffusive effect. It is observed that by tuning the Fe layer thickness from 2 nm to 5 nm, the magnetic properties of the resulting product varies substantially. According to X-ray diffraction, an additional peak, indexed to Fe₁₆N₂ (004), was developed in the sample with thinner (2 nm) underlayer. Its corresponding PNR study shows a 4πM_s of up to 2.82 T towards the substrate interface, which is 14% higher than the known limit (Fe₆₅Co₃₅ ∼ 4πM_s = 2.45 T). We attribute this giant magnetization to the presence of chemically ordered Fe₁N₂. We have seen evidence that the high M_s is favorably stabilized due to the lattice misfit. The thickness range is consistent with the strained region of the films. This depth-dependent saturation magnetization analysis can shed a light to help understand the previous magnetization reports by classical magnetometry methods for the samples prepared using the same underlayer.