Probing the energy barriers in nonuniform magnetization states of circular dots by broadband ferromagnetic resonance

Abstract

The time evolution of the ferromagnetic resonance output signal in the arrays of permalloy circular dots of submicron sizes was measured near the critical fields of the vortex nucleation and annihilation. Surprisingly short times of the transition from the quasiuniform to the vortex magnetization state (several milliseconds) were detected. The observed effects are explained by overcoming the field dependent energy barriers in the process of vortex core nucleation. The energy barrier values found from the time dependences of the ferromagnetic resonance peak intensities were compared with the ones calculated within the rigid vortex model. The rigid vortex model overestimates the nucleation barriers and a more adequate magnetization reversal model is needed. There is a strong dependence of the stable, metastable energy minima and energy barriers on the magnetic field and dot geometrical parameters.