Skyrmion lattice creep at ultra-low current densities

TL;DR

This paper investigates the creep motion of skyrmion lattices in bulk materials at ultra-low current densities, revealing a new thermally-activated creep regime that impacts potential skyrmion-based memory devices.

Contribution

It demonstrates that skyrmion lattice depinning occurs at only 4% of the critical current density, using resonant ultrasound spectroscopy to explore this ultra-low current regime.

Findings

Depinning at 4% of critical current density

Observation of thermally-activated creep in skyrmion lattices

Agreement with Anderson-Kim creep theory

Abstract

Magnetic skyrmions are well-suited for encoding information because they are nano-sized, topologically stable, and only require ultra-low critical current densities $j_c$ to depin from the underlying atomic lattice. Above $j_c$ skyrmions exhibit well-controlled motion, making them prime candidates for race-track memories. In thin films thermally-activated creep motion of isolated skyrmions was observed below $j_c$ as predicted by theory. Uncontrolled skyrmion motion is detrimental for race-track memories and is not fully understood. Notably, the creep of skyrmion lattices in bulk materials remains to be explored. Here we show using resonant ultrasound spectroscopy--a probe highly sensitive to the coupling between skyrmion and atomic lattices--that in the prototypical skyrmion lattice material MnSi depinning occurs at $j_c^*$ that is only 4 percent of $j_c$. Our experiments are in excellent agreement with Anderson-Kim theory for creep and allow us to reveal a new dynamic regime at ultra-low current densities characterized by thermally-activated skyrmion-lattice-creep with important consequences for applications.