Emergence of Lissajous trajectories in skyrmion oscillator

Abstract

Understanding the dynamics of current-driven skyrmion is essential for their practical applications. In this study, we apply an AC current pulse (a) in x-- direction, and (b) in both x-- and y-- directions through the free layer of a Co/Pt thin film and investigate the motion of the skyrmion. We show that the skyrmion follows the sinusoidal current pulse and behaves like a forced oscillator in the range of current amplitude 1 $\times$ 10$^{11}$ A/m$^2$ to 1 $\times$ 10$^{12}$ A/m$^2$ and frequency 5 $\times$ 10$^{8}$ Hz to 1 $\times$ 10$^{10}$ Hz. For current pulse of (A$_1$sin$\omega_1$t, A$_2$sin($\omega_2$t+$\phi$), 0), the skyrmion forms Lissajous figures in the x-y plane, same as observed in classical mechanics. The results are compared at T = 0 K and T $>$ 0 K to analyze the effect of temperature. As the skyrmion Hall angle ($\theta_{SkH}$) and stochastic thermal fluctuation ($\textbf{F}^{Th}$) are functions of temperature, the skyrmion starts deviating from its path at T = 0 K with increasing temperature and eventually generates somewhat deformed Lissajous figures from ideal.