Dynamics of orbital skyrmions in a circular nanodisk.

Magnetic skyrmions in a circular nanodisk show great potential in nano-oscillator-based applications owing to their long-term stability and electric current-dependent circulating high-frequency. However, the circulating orbits are confined at the edge or the center of the disk and the upper bond of high-frequency motion is limited due to the skyrmion Hall effect (SkHE). Indeed, skyrmions with enhanced tunability in circulating orbits and oscillation frequencies are more expected. In this work, artificial circulating orbits of skyrmions are designed in a circular nanodisk by using annular barriers induced by voltage-controlled magnetic anisotropy (VCMA) effect, and the dynamics of the orbital skyrmions are investigated by micromagnetic simulations. Our results show that, orbital skyrmions not only can circulate in one of the designed orbits separately or in both orbits simultaneously, but also can switch from one orbit to the other by appropriate electric current density (J), providing a not-previously-reported platform for innovative applications. Furthermore, the upper bond of high-frequency motion of orbital skyrmions is lifted with respect to that of the skyrmions in a standard circular nanodisk. Detailed studies of dynamics and annihilation of skyrmions reveal the correlation between the SkHE, the VCMA effect and the geometry of the designed orbits. Our results give insights into the stability and dynamics of orbital skyrmions in the nanodisk, and may be useful for the design, fabrication and application of orbital skyrmions in electronic and spintronic devices.