Skyrmion-Skyrmionium Phase Separation and Laning Transitions via Spin-Orbit Torque Currents

TL;DR

This study uses atomistic simulations to reveal how mixtures of skyrmions and skyrmioniums exhibit phase separation, laning transitions, and complex motion behaviors under spin-orbit torque currents, with implications for magnetic material control.

Contribution

The paper demonstrates for the first time that skyrmion-skyrmionium mixtures exhibit laning and phase separation, revealing new dynamic phases driven by spin-orbit torque currents.

Findings

Identified three phases: jammed, laned, and skyrmion-only states.

Observed tilted laning structures due to skyrmion Hall effects.

Mapped velocity and Hall response across different phases.

Abstract

Many driven binary systems can exhibit laning transitions when the two species have different mobilities, such as colloidal particles with opposite charges in electric fields. Another example is pedestrian or active matter systems, where particles moving in opposite directions form a phase-separated state that enhances the overall mobility. In this work, we use atomistic simulations to demonstrate that mixtures of skyrmions and skyrmioniums also exhibit pattern formation and laning transitions. Skyrmions move more slowly and at a finite Hall angle compared to skyrmioniums, which move faster and without a Hall effect. At low drives, the system forms a partially jammed phase where the skyrmionium is dragged by the surrounding skyrmions, resulting in a finite angle of motion for the skyrmionium. At higher drives, the system transitions into a laned state, but unlike colloidal systems, the lanes in the skyrmion skyrmionium mixture are tilted relative to the driving direction due to the intrinsic skyrmion Hall angle. In the laned state, the skyrmionium angle of motion is reversed when it aligns with the tilted lane structure. At even higher drives, the skyrmioniums collapse into skyrmions. Below a critical skyrmion density, both textures can move independently with few collisions, but above this density, the laning state disappears entirely, and the system transitions to a skyrmion-only state. We map out the velocity and Hall responses of the different textures and identify three distinct phases: partially jammed, laned, and skyrmion-only moving crystal states. We compare our results to recent observations of tilted laning phases in pedestrian flows, where chiral symmetry breaking in the particle interactions leads to similar behavior.