Skyrmion jellyfish in driven chiral magnets

Abstract

Chiral magnets can host topological particles known as skyrmions which carry an exactly quantised topological charge Q = −1. In the presence of an oscillating magnetic field B1(t), a single skyrmion embedded in a ferromagnetic background will start to move with constant velocity vtrans. The mechanism behind this motion is similar to the one used by a jellyfish when it swims through water. We show that the skyrmion’s motion is a universal phenomenon, arising in any magnetic system with translational modes. By projecting the equation of motion onto the skyrmion’s translational modes and going to quadratic order in B1(t), we obtain an analytical expression for vtrans as a function of the system’s linear response. The linear response and consequently vtrans are influenced by the skyrmion’s internal modes and scattering states, as well as by the ferromagnetic background’s Kittel mode. The direction and speed of vtrans can be controlled by changing the polarisation, frequency and phase of the driving field B1(t). For systems with small Gilbert damping parameter α, we identify two distinct physical mechanisms used by the skyrmion to move. At low driving frequencies, the skyrmion’s motion is driven by friction, and vtrans ∼ α, whereas at higher frequencies above the ferromagnetic gap the skyrmion moves by magnon emission, and vtrans becomes independent of α.