Ferromagnetic resonance method for determining the magnetic surface anisotropy of amorphous films

The theory of a ferromagnetic resonance (FMR) method for determining the magnetic surface anisotropy of amorphous films is presented. This method enables one to deduce the surface anisotropy constant $K_s$ of an amorphous material from the dependence of the magnetic resonance field on the film thickness. The analysis includes spin-wave modes and surface-induced modes, perpendicular and parallel FMR configurations, and thin as well as ultrathin films. No approximations are made other than the linearization of the equation of motion and the assumption that skin-depth effects and electromagnetic propagation effects are negligible. Good agreement is found between the theory of the method and experimental FMR data on ultrathin films of amorphous Fe-B alloys. The reliability of the $K_s$ values deduced by means of the theory from experimental FMR and superconducting quantum-interference device data is briefly discussed.