Chiral skyrmion auto-oscillations in a ferromagnet under spin-transfer torque

A skyrmion can be stabilized in a nanodisk geometry in a ferromagnetic material with Dzyaloshinskii-Moriya (DM) interaction. We apply spin torque uniform in space and time and observe numerically that the skyrmion is set in steady rotational motion around a point off the nanodisk center. We give a theoretical description of the emerging auto-oscillation dynamics based on the coupling of the rotational motion to the breathing mode of the skyrmion and to the associated oscillations of the in-plane magnetization. The analysis shows that the achievement of auto-oscillations in this simple setup is due to the chiral symmetry breaking. Thus, we argue that the system is turned into a spin-torque oscillator due to the chiral DM interaction. We also show injection locking of this skyrmion oscillator demonstrating feasibility of application by using synchronization of multiple skyrmion auto-oscillators.