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 ${\mathrm{Ar}}⁺$-irradiated FePt films has been investigated by static magnetometry, magnetic force microscopy, and Brillouin light scattering from thermally excited spin waves. A gradual variation of magnetic anisotropy with film thickness was obtained by ${\mathrm{Ar}}⁺$ irradiation. The irradiation incidence angle influences the anisotropy profile: on decreasing $\ensuremath{\alpha}$, a decreasing thickness of the hard $\mathrm{L}{1}₀$ 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 (${\ensuremath{\nu}}₀\ensuremath{\approx}6$ GHz), which could be reproduced assuming the presence of a nonzero ``rotatable'' anisotropy (i.e., any direction in the film plane can be established as the easy axis by the application of a saturating magnetic field along this direction). The hypothesis is supported by both magnetometry and magnetic force microscopy data.