Reversible to Irreversible Transitions for ac Driven Skyrmions on Periodic Substrates

TL;DR

This study uses atomistic simulations to explore how magnetic skyrmions behave under ac driving on periodic substrates, revealing reversible, translating, and chaotic irreversible regimes with distinct dynamical features.

Contribution

It uncovers the dynamical phase transitions of skyrmions under ac drive, including the identification of reversible, translating, and chaotic irreversible states in various lattice arrangements.

Findings

Reversible motion with zero diffusion at low ac amplitudes.

Translating regime with skyrmions moving by one lattice constant per cycle.

Chaotic irreversible motion with long-time diffusion at higher ac amplitudes.

Abstract

Using atomistic simulations, we investigate the dynamical behavior of magnetic skyrmions in dimer and trimer molecular crystal arrangements, as well as bipartite lattices at 3/2 and 5/2 fillings, under ac driving over a square array of anisotropy defects. For low ac amplitudes, at all fillings we find reversible motion where the skyrmions return to their original positions at the end of each ac drive cycle and the diffusion is zero. We also identify two distinct irreversible regimes. The first is a translating regime in which the skyrmions form channels of flow in opposing directions and translate by one substrate lattice constant per ac drive cycle. The translating state appears in the dimer and trimer states, and produces pronounced peaks in the diffusivity in the direction perpendicular to the external drive. For larger ac amplitudes, we find chaotic irreversible motion in which the skyrmions can randomly exchange places with each other over time, producing long-time diffusive behavior both parallel and perpendicular to the ac driving direction.