Azimuthal Spin Wave Excitations in Magnetic Nanodots over the Soliton Background: Vortex, Bloch and N\'eel-like skyrmions

TL;DR

This paper investigates azimuthal spin-wave excitations in magnetic nanodots with vortex and skyrmion states, revealing how topological differences and Dzyaloshinskii-Moriya interaction influence the spectrum, degeneracy, and mode coupling.

Contribution

It provides a continuous mapping of spin-wave spectra across different topological states and uncovers the effects of Berry phase, DMI, and geometry on spin-wave behavior in nanodots.

Findings

SW frequency degeneracy is lifted by topological state changes.

DMI induces nonreciprocal channeling and large frequency splitting.

Coupling of magnetization dynamics to uniform fields is enhanced in non-circular dots.

Abstract

We study azimuthal spin-wave (SW) excitations in a circular ferromagnetic nanodot in different inhomogeneous, topologically non-trivial magnetization states, specifically, vortex, Bloch-type skyrmion and N\'eel-type skyrmion states. Continuous mapping of the SW spectrum between these states is realized with gradual change of the out-of-plane magnetic anisotropy and Dzyaloshinskii-Moriya exchange interaction (DMI). Our study shows lifting of the SW frequencies degeneracy and change in systematics of the frequency levels. The change is induced by a geometrical Berry phase, that is present for the dot-edge localized SWs in a vortex state and vanishes in skyrmion states. Furthermore, channeling of the azimuthal SWs localized at the skyrmion edge is present and induces large frequency splitting. This is attributed to DMI induced nonreciprocity, while coupling of the breathing and gyrotropic modes is related to soliton motion. Finally, an efficient coupling of the dynamic magnetization in the skyrmion state to uniform magnetic field in nanodots with non-circular symmetry is shown.