Topological defect-mediated skyrmion annihilation in three dimensions

Max T Birch,Shinichiro Seki,David Cortés-Ortuño,Peter D Hatton,Nguyen D Khanh,Geetha Balakrishnan,Aleš Štefančič,Yoshinori Tokura

doi:10.1038/s42005-021-00675-4

Abstract

The creation and annihilation of magnetic skyrmions are mediated by three-dimensional topological defects known as Bloch points. Investigation of such dynamical processes is important both for understanding the emergence of exotic topological spin textures, and for future engineering of skyrmions in technological applications. However, while the annihilation of skyrmions has been extensively investigated in two dimensions, in three dimensions the phase transitions are considerably more complex. We report field-dependent experimental measurements of metastable skyrmion lifetimes in an archetypal chiral magnet, revealing two distinct regimes. Comparison to supporting three-dimensional geodesic nudged elastic band simulations indicates that these correspond to skyrmion annihilation into either the helical and conical states, each exhibiting a different transition mechanism. The results highlight that the lowest energy magnetic configuration of the system plays a crucial role when considering the emergence and stability of topological spin structures via defect-mediated dynamics.

Highlights

The creation and annihilation of magnetic skyrmions are mediated by three-dimensional topological defects known as Bloch points
Further calculations have demonstrated that entropic considerations, which concern the number of available paths across the annihilation energy barrier, play a more prominent role in determining the lifetime of skyrmions in twodimensional systems than the energy barrier itself[18,19,20,21]. This phenomenon has been demonstrated experimentally[18,22]. While they are commonly portrayed as twodimensional objects forming a hexagonal skyrmion lattice (SkL), in three dimensions magnetic skyrmions exist as extended tube-like objects[23,24]
This qualitatively agrees with the conclusions drawn from the experimental data in Fig. 3, where we anticipated the energy barriers varying linearly with the applied field, supporting our labelling of the two linear regimes in the lifetime data to the two distinct skyrmion tube (SkT) annihilation mechanisms

Summary

Introduction

The creation and annihilation of magnetic skyrmions are mediated by three-dimensional topological defects known as Bloch points. Investigation of such dynamical processes is important both for understanding the emergence of exotic topological spin textures, and for future engineering of skyrmions in technological applications. Similar to the melting of solids by dislocations and disclinations, magnetic skyrmions are annihilated by the proliferation and subsequent propagation of magnetic Bloch points[8] Such three-dimensional topological defects have been called hedgehog defects, or emergent magnetic monopoles[9], due to their singular nature. The chirality of these Bloch points is defined by the local topology of the spin structures (see Supplementary Fig. S1, Supplementary Note 1)

Methods

Results

Conclusion