Reversible Vector Ratchets for Skyrmion Systems

TL;DR

This paper introduces the concept of vector ratchets in skyrmion systems driven by ac forces, demonstrating their ability to produce controllable, reversible motion in any direction due to the Magnus effect, with potential applications in spintronics.

Contribution

It presents the first demonstration of vector ratchets in skyrmion systems, highlighting their reversible and multi-directional motion capabilities driven by the Magnus term.

Findings

Vector ratchets enable 360° control of skyrmion motion.

Reversible vector ratchets can be achieved by varying ac amplitude or skyrmion density.

Collective effects are necessary for ratchet reversal; isolated skyrmions do not exhibit reversals.

Abstract

We show that ac driven skyrmions interacting with an asymmetric substrate provide a realization of a new class of ratchet system which we call a vector ratchet that arises due to the effect of the Magnus term on the skyrmion dynamics. In a vector ratchet, the dc motion induced by the ac drive can be described as a vector that can be rotated clockwise or counterclockwise relative to the substrate asymmetry direction. Up to a full $360^{\circ}$ rotation is possible for varied ac amplitudes or skyrmion densities. In contrast to overdamped systems, in which ratchet motion is always parallel to the substrate asymmetry direction, vector ratchets allow the ratchet motion to be in any direction relative to the substrate asymmetry. It is also possible to obtain a reversal in the direction of rotation of the vector ratchet, permitting the creation of a reversible vector ratchet. We examine vector ratchets for ac drives applied parallel or perpendicular to the substrate asymmetry direction, and show that reverse ratchet motion can be produced by collective effects. No reversals occur for an isolated skyrmion on an asymmetric substrate. Since a vector ratchet can produce motion in any direction, it could represent a new method for controlling skyrmion motion for spintronic applications.