Nonlinear ferromagnetic resonance in permalloy films: A nonmonotonic power-dependent frequency shift

We present experimental data on the power-dependent shift in the ferromagnetic resonance frequency of permalloy ribbons magnetized in plane. We find the sign of the shift depends on the strength of the applied static magnetic field. For low applied fields, the power-dependent shift is upwards in frequency, while for high fields the mode softens with increasing power. We summarize two theoretical studies that are qualitatively compatible with this result. One is an analytic treatment of very thin films and the second a numerical simulation of a thin rectangular prism.