Tunable spin wave frequency gap in anisotropy-graded FePt films obtained by ion irradiation

The effect of graded anisotropy on static and dynamic magnetic properties of Ar+-irradiated FePt films has been investigated. A gradual variation of magnetic anisotropy with film thickness was obtained by Ar+ irradiation. The irradiation incidence angle influences the anisotropy profile: on decreasing α, a decreasing thickness of the hard L10 phase and an increasing thickness of the soft A1 phase were obtained. Accordingly, the zero-field spin-wave frequency gap was found to decrease. In the sample with the highest soft-phase thickness the spin-wave frequency gap takes a substantial value (ν0 ≈ 6 GHz), which could be reproduced assuming the presence of a nonzero “rotatable” anisotropy. The hypothesis is supported by both magnetometry and magnetic force microscopy data.