Influence of ion implantation parameters on the perpendicular magnetic anisotropy of Fe-N thin films with stripe domains

Abstract

Nitrogen-martensite thin films are known to present a perpendicular magnetic anisotropy (PMA) depending on the nitrogen content. Additionally, weak magnetic stripe domains have been studied in Fe-N samples made by ion implantation. In this work, ion implantation proves to be a good technique to make nitrogen-martensite thin films presenting both tunable PMA and stripe domains. We report on the changes in magnetic and structural properties of nitrogen-implanted iron thin films, resulting from various implantation conditions. Fluences from 1.8 × 1016 N2+/cm2 to 3.5 × 1016 N2+/cm2 at 26 keV and 5.3 × 1016 N2+/cm2 at 40 keV were used to implant iron thin films epitaxially grown on ZnSe/GaAs(001). X-ray diffraction measurements disclosed the presence of body-centered tetragonal nitrogen-martensite whose c-axis is perpendicular to the thin film plane and the c-parameter increases with fluence. Vibrating sample magnetometer measurements revealed that nitrogen implantation induced strong changes in magnetic properties such as an increasing PMA with fluence. Therefore, this PMA may originate from the magnetocrystalline anisotropy of nitrogen-martensite and stress-induced anisotropy. Magnetic stripe domains are notably observed by magnetic force microscopy for the highest fluences. Furthermore, ferromagnetic resonance measurements lead to the magnetic anisotropy constants calculation. The results show a significant increase of the PMA, related to the presence of nitrogen-martensite whose c-parameter is close to that of α′-Fe8N, which reaches a maximum value of 4.9 × 106 erg/cm3. In addition, an interesting comparison is done between the anisotropy constants of Fe-N and Fe-Ga samples. Fe-Ga thin films are also well known to present PMA and stripe domains.