Magnetic skyrmions have attracted attention because of their emergent electromagnetic properties. In particular, non-equilibrium-state skyrmions, which are protected by topology and hence can exist over a wider temperature–magnetic-field region, show promise for possible practical applications, but their dynamics remain elusive. Here, we report the observation of a magnetic-field-induced dynamical transition from the metastable hexagonal-lattice skyrmion crystal (SkX) at a zero bias-field to an amorphous state via the densely vacancy-populated SkX. With decreasing field, on the other hand, the aggregate transforms from ‘random particles’ to ‘microcrystals’ of skyrmions in a non-equilibrium state, in analogy to colloidal crystallization, and subsequently undergoes a topologically distinct phase separation between the SkX and helical/conical domains accompanied by topological defects. These observations directly demonstrate the aggregation and collapse dynamics of metastable skyrmions and may provide a route to other nontrivial topological phenomena such as the zero-magnetic-field topological Hall effect.
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Supplementary information, Supplementary Figures 1–9, Supplementary References 1–2
In situ Lorentz TEM movie shows the recrystallization of skyrmion aggregates
Micromagnetic simulations for metastable skyrmions in a micromagnet (1,024 nm × 1,024 nm × 128 nm in size) with decreasing the bias field