Topological defect-mediated skyrmion annihilation in three dimensions

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. Skyrmions are topologically non-trivial, vortex-like magnetic structures the dynamics of which have been mostly studied in 2D systems, but they are also able to exist as 3D tube-like structures. Here, the authors report a combination of experimental and computational results investigating the annihilation dynamics of 3D skyrmion structures in order to better understand how to stabilise topological structures in other bulk magnetic systems.