  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 demonstrates that
these correspond to skyrmion annihilation into either the helical or 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 of topological spin structures via
defect-mediated dynamics, and their stability in future devices.
