Thermal Decay of a Single Néel Skyrmion via Helicity Slip in a Nanodisk

Dynamics of the decay of a single skyrmion confined in a ferromagnetic nanodisk in the presence of stochastic thermal noise are studied using micromagnetic simulations. By analyzing the time-varying oscillations of the in-plane and out-of-plane magnetization under the influence of the thermal field, we identified the presence of two skyrmion eigenmodes with frequencies differing by an order of magnitude– a low-frequency mode associated with gyration of skyrmion core, and a high-frequency breathing mode associated with the oscillations of the skyrmion radius. We show that the thermal fluctuations not only affects the topological charge but also the helicity of the skyrmion, changing the helicity to oscillate around ±π/4-a value which is in between that of the Néel skyrmion and a vortex. We also reveal a phenomenon of helicity slip between +π/4 and -π/4 due to the thermal fluctuations. We explain the helicity slip based on an energy barrier that arises due to a combination of thermally induced deformation of skyrmion and the finite size of the disk.