Non-uniform along thickness spin excitations in magnetic vortex-state nanodots

Abstract

We summarize our experimental findings in the arrays of Ni80Fe20 circular nanodots with diameter 300 nm and thickness 20 nm ≤ L ≤ 100 nm, probed by broadband ferromagnetic resonance spectroscopy in the absence of external magnetic field. Spin excitation modes related to the vortex core gyrotropic dynamics were observed in the gigahertz frequency range. Micromagnetic simulations revealed that they are flexure oscillations of the vortex core string with n = 0, 1, 2 nodes along the dot thickness. It was found that for L > 70 nm the intensity of more complicated n = 1 vortex gyrotropic mode is unexpectedly higher than the one of the lowest n = 0 gyrotropic mode. This behavior was clarified on the basis of the inhomogeneous vortex mode phase profiles extracted from micromagnetic simulations and calculated analytically. Precise measurements of the dependence of resonance frequency of the vortex n = 0 mode on the dot thickness demonstrated a clear maximum around L = 70 nm, that was theoretically explained by introducing a vortex mass, which is a result of the vortex distortion due to interaction with spin waves having azimuthal indices m = ±1. Finally, several azimuthal spin-wave modes having curled structure at the dot top and bottom faces were found in the spectrum of the dots with thicknesses L ≥ 40 nm.