In-plane spin excitation of skyrmion bags

Abstract

Skyrmion bags are spin structures with arbitrary topological charges, which consist of a large skyrmion and multiple small skyrmions. In this work, by applying in-plane ac magnetic fields, we investigate the spin-wave modes of skyrmion bags, which behave differently from the clockwise (CW) and counterclockwise (CCW) rotation modes of skyrmions because of their complex spin topological structure. The in-plane excitation power spectral density shows that there are four resonance frequencies for each skyrmion bag. By further calculating the spin dynamics of a skyrmion bag at each resonant frequency, the four spin-wave modes appear as the composition modes of outer skyrmion-inner skyrmions, i.e., a CCW-CW mode, two CW-breathing modes with different resonance strength, and an inner CCW mode. Our results provide an understanding of the in-plane spin excitation of skyrmion bags, which may contribute to the characterization and detection of skyrmion bags, as well as the applications in logic devices.