Surface pinning and triggered unwinding of skyrmions in a cubic chiral magnet

TL;DR

This study demonstrates how surface pinning influences skyrmion stability in a cubic chiral magnet and shows that magnetic tips can locally trigger skyrmion unwinding by overcoming topological protection.

Contribution

It reveals the role of surface pinning and magnetic tip stray fields in controlling skyrmion stability and unwinding in a bulk chiral magnet.

Findings

Surface pinning sites preferentially host skyrmions.

Magnetic tips can locally unwind skyrmions when aligned with the external field.

Skyrmion unwinding is controllable via magnetic field and tip magnetization.

Abstract

In the cubic chiral magnet Fe$_{1-x}$Co$_{x}$Si a metastable state comprising of topologically nontrivial spin whirls, so-called skyrmions, may be preserved down to low temperatures by means of field cooling the sample. This metastable skyrmion state is energetically separated from the topologically trivial ground state by a considerable potential barrier, a phenomenon also referred to as topological protection. Using magnetic force microscopy on the surface of a bulk crystal, we show that certain positions are preferentially and reproducibly decorated with metastable skyrmions, indicating that surface pinning plays a crucial role. Increasing the magnetic field allows an increasing number of skyrmions to overcome the potential barrier, and hence to transform into the ground state. Most notably, we find that the unwinding of individual skyrmions may be triggered by the magnetic tip itself, however, only when its magnetization is aligned parallel to the external field. This implies that the stray field of the tip is key for locally overcoming the topological protection. Both the control of the position of topologically nontrivial states as well as their creation and annihilation on demand pose important challenges in the context of potential skyrmionic applications.