Increase in coercivity of Fe–Pt thin film permanent magnets by N2+ ion implantation and heat treatment in hydrogen gas

We investigated Fe–Pt thin films implanted with nitrogen ions followed by heat treatment under a vacuum at 650 °C and further treatment in hydrogen gas of 0.1 atm at 350 °C. The elemental composition of the Fe–Pt films is Fe54Pt46, and the thickness of the films is 110 nm. Energies of the implanted N2+ ions are 120 or 130 keV. In-plane and out-of-plane coercivities of the Fe–Pt films after the heat treatment at 650 °C without the N2+ ion implantation are 9.8 and 7.7 kOe, respectively. The in-plane coercivity for an implanted N dose of 3.5 × 1017 atoms/cm2 increases to 13.3 kOe after the heat treatment at 650 °C under a vacuum and increases to 15.4 kOe after the heat treatment at 350 °C in hydrogen gas. The out-of-plane coercivity for the same dose increases to 12.7 and 14.4 kOe after these heat treatments, respectively. Backscattering Spectrometry with 6.0 MeV 4He ions indicates that the implanted nitrogen atoms are not released but stay in the films and that the amount of the nitrogen atoms decreases below a detection limit after the heat treatment at 650 °C. Widths of FePt (1 1 1) peaks in X-ray diffraction patterns indicate that the size of the FePt crystallites decreases with increasing the dose of nitrogen after the heat treatment at 650 °C and that the size remains unchanged by the heat treatment at 350 °C in hydrogen gas.