Skyrmion dynamics under uniaxial anisotropy

Abstract

Broadband microwave spectroscopy has been performed on single-crystalline ${\mathrm{GaV}}_{4}{\mathrm{S}}_{8}$, which exhibits a complex magnetic phase diagram including cycloidal and N\'eel-type skyrmion lattice phases below 13 K. At small magnetic fields two collective modes with large frequency separation are found that reflect the cycloid state subject to a large uniaxial anisotropy. In the skyrmion lattice phase three modes are resolved. Analyzing the spin-precessional motion microscopically, we explain the low frequency of the breathing mode in these N\'eel skyrmions by the hard-axis orientation of relevant spins.