Clogging, Diode and Collective Effects of Skyrmions in Funnel Geometries

TL;DR

This study uses a particle-based model to explore how skyrmions behave in funnel geometries under different driving conditions, revealing clogging, diode effects, and the influence of Magnus forces on skyrmion dynamics.

Contribution

It introduces a detailed analysis of skyrmion collective behavior in funnel geometries, highlighting the impact of density, drive direction, and Magnus force on clogging and diode effects.

Findings

Increased skyrmion density causes jamming and reduces velocity.

A critical density exists below which skyrmions become trapped.

Strong diode effect observed with higher depinning forces in hard direction.

Abstract

Using a particle-based model, we examine the collective dynamics of skyrmions interacting with a funnel potential under dc driving as the skyrmion density and relative strength of the Magnus and damping terms are varied. For driving in the easy direction, we find that increasing the skyrmion density reduces the average skyrmion velocity due to jamming of skyrmions near the funnel opening, while the Magnus force causes skyrmions to accumulate on one side of the funnel array. For driving in the hard direction, there is a critical skyrmion density below which the skyrmions become trapped. Above this critical value, a clogging effect appears with multiple depinning and repinning states where the skyrmions can rearrange into different clogged configurations, while at higher drives, the velocity-force curves become continuous. When skyrmions pile up near the funnel opening, the effective size of the opening is reduced and the passage of other skyrmions is blocked by the repulsive skyrmion-skyrmion interactions. We observe a strong diode effect in which the critical depinning force is higher and the velocity response is smaller for hard direction driving. As the ratio of Magnus force to dissipative term is varied, the skyrmion velocity varies in a non-linear and non-monotonic way due to the pile up of skyrmions on one side of the funnels. At high Magnus forces, the clogging effect for hard direction driving is diminished.