Skyrmion dynamics under uniaxial anisotropy

Broadband microwave spectroscopy has been performed on single-crystalline GaV4S8, which exhibits a complex magnetic phase diagram including cycloidal and Neel-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 Neel skyrmions by the hard-axis orientation of relevant spins.