Degradation of Fe–Pt thin film permanent magnets by irradiation with 4He and H ions

Fe55Pt45 thin film permanent magnets having a thickness of 120 nm are irradiated with 180 keV 4He and 90 keV H ions. In-plane and out-of-plane coercivities of the magnets are decreased to half by irradiation with the 4He ions at a fluence of 3.2 × 1015 ions/cm2 or with the H ions at a fluence of 5.7 × 1016 ions/cm2. In-plane magnetizations without a magnetic field are decreased by 10%, but out-of-plane magnetizations without a magnetic field are decreased by 90% by irradiation with the 4He ions at a fluence of 1 × 1016 ions/cm2 or with the H ions at a fluence of 3 × 1017 ions/cm2. Because their magnetizations do not recover to their original levels prior to the irradiation by re-magnetization in a magnetic field of 40 kOe, it is proved that the degradation is caused by disorder of crystallographic structure. Atomic densities of the Fe and the Pt atoms displaced from their lattice sites are calculated as a function of depth by a TRIM simulation program.