Current-induced viscoelastic topological unwinding of metastable skyrmion strings

TL;DR

This study demonstrates that pulsed currents can induce topological unwinding of metastable skyrmion strings in MnSi, revealing viscoelastic behavior and the importance of string length in current-driven dynamics.

Contribution

It uncovers the current-induced topological unwinding mechanism of skyrmion strings and highlights the role of viscoelasticity and string segmentation in their dynamics.

Findings

Unwinding occurs at high current densities (3-5×10^6 A/m^2) under pulsed currents.

Shorter pulse widths increase the critical current density for unwinding.

Numerical simulations show depinning of segmented skyrmion strings initiates unwinding.

Abstract

In the MnSi bulk chiral magnet, magnetic skyrmion strings of 17 nm in diameter appear in the form of a lattice, penetrating the sample thickness, 10-1,000 um. Although such a bundle of skyrmion strings may exhibit complex soft-matter-like dynamics when starting to move under the influence of a random pinning potential, the details remain highly elusive. Here, we show that a metastable skyrmion-string lattice is subject to topological unwinding under the application of pulsed currents of 3-5x106 A m-2 rather than being transported, as evidenced by measurements of the topological Hall effect. The critical current density above which the topological unwinding occurs is larger for a shorter pulse width, reminiscent of the viscoelastic characteristics accompanying the pinning-creep transition observed in domain-wall motion. Numerical simulations reveal that current-induced depinning of already segmented skyrmion strings initiates the topological unwinding. Thus, the skyrmion-string length is an element to consider when studying current-induced motion.