RESUMEN
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.
RESUMEN
Recently, it has been reported that the skyrmion Hall effect can be suppressed in an elliptical skyrmion-based device. Given that defects are unavoidable in materials, it is necessary and important to investigate the dynamics of an elliptical skyrmion in a defective racetrack device. In this work, the current-driven dynamics of an elliptical skyrmion in a defective racetrack device are systematically studied using micromagnetic simulations. The system energy analysis reveals that the magnetic parameters of the circular defect play critical roles in determining the type (repulsive or attractive) and the magnitude of the force on the elliptical skyrmion. The simulated trajectories show that the primary motion modes of the elliptical skyrmion in the defective racetrack can be divided into four types, which are dependent on the values of the Dzyaloshinskii-Moriya interaction (DMI) constant Dd, the perpendicular magnetic anisotropy constant Kd, the magnitude of the driving current density J, and the size d of the defect. Further investigation of the motion-mode phases of the skyrmion reveals the synthetic effects of Dd, Kd, J, and d. Finally, the minimum depinning current density J, which linearly depends on the parameters of Dd and Kd, is obtained for a skyrmion completely pinned in the defect. Our findings give insights into the dynamics of an elliptical skyrmion in the presence of a defect with different magnetic parameters in a racetrack device and may be useful for performance enhancement of skyrmion-based racetrack memory devices.