Skyrmion ratchet effect driven by a biharmonic force

TL;DR

This paper demonstrates that skyrmion ratchet motion can be induced and controlled by biharmonic magnetic fields, with potential applications in skyrmion-based devices, using micromagnetic simulations and analytical analysis.

Contribution

It introduces a novel method to induce and control skyrmion ratchet motion using biharmonic forces, distinct from previous single-frequency or pulsed drives.

Findings

Skyrmion speed exceeds 5 m/s under biharmonic drive.

Direction of skyrmion motion can be rotated over 360 degrees.

Dissipative force is key to ratchet motion emergence.

Abstract

Based on micromagnetic simulation and analysis of Thiele's equation, in this work we demonstrate that ratchet motion of skyrmion can be induced by a biharmonic in-plane magnetic field hx(t) = h1sin(mwt) + h2sin(nwt + phi), provided that integers m and n are coprime and that m + n is odd. Remarkably, the speed and direction of the ratchet motion can be readily adjusted by the field amplitude, frequency and phase, with the maximum speed being over 5 m/s and the direction rotatable over 360 degree. The origin of the skyrmion ratchet motion is analyzed by tracing the excitation spectra of the dissipation parameter D and the skyrmion position R, and it shows that the dissipative force plays a key role in the appearance of ratchet motion. Such a ratchet motion of skyrmion is distinguished from those caused by single-frequency ac drives reported in the literature, and from that driven by pulsed magnetic fields as also predicted in this work. Our results show that skyrmion ratchet effect under biharmonic forces shares some common features of those found in many soliton systems, and the facile controllability of both the skyrmion speed and direction should be useful in practice.