Magnetization of thin films with in-plane uniaxial anisotropy studied by microwave absorption

Abstract

The resonance spectra of a thin film, which possesses an in-plane uniaxial anisotropy field Hk, exhibit one or two resonance modes when the external dc field is applied along the hard axis, depending upon the respective values of the signal frequency and the natural resonance frequency f0=γ√Hk(Hk+4πMs)/2π. When f≳f0 the low-field side mode is no longer resolved and instead a peak is detected on the absorption spectrum. It is shown that the location and the shape of this peak are determined by the magnetization process occuring along the hard axis. The theoretical computation is based upon the hypothesis that the magnetization arises by a pure rotational mechanism, and therefore follows the Landau–Lifshitz equation of motion. The experiments were performed on CoZr–rare-earth-substituted amorphous thin films. The full experimental spectra can be fitted numerically with the measured magnetic parameters if one takes the effect of the large-angle magnetization ripples in the vicinity of H≂Hk into account by a phenomenological expression. This computation also makes it possible to determine an effective fluctuating field δHeff, the magnitude of which correlates nicely with the local anisotropy Kloc deduced from the transverse bias initial susceptibility measurements.