Topological Melting of the Metastable Skyrmion Lattice in the Chiral Magnet Co9Zn9Mn2

In a β‐Mn‐type chiral magnet Co9Zn9Mn2, it is demonstrated that the magnetic field‐driven collapse of a room temperature metastable topological skyrmion crystal (SkX) passes through a regime described by a partial topological charge inversion. Using Lorentz transmission electron microscopy, the magnetization distribution is observed directly as the magnetic field is swept antiparallel to the original skyrmion core magnetization, that is, negative magnetic fields. Due to the topological stability of skyrmions, a direct transition of the metastable SkX to the equilibrium helical state is avoided for increasingly negative fields. Instead, the metastable SkX gradually transforms into giant magnetic bubbles separated by 2π domain walls. Eventually these large structures give way to form a near‐homogeneously magnetized medium that unexpectedly hosts a low density of isolated skyrmions with inverted core magnetization, and thus a total topological charge of reduced size and opposite sign compared with the initial state. A similar phenomenon has been observed previously in systems hosting ordered lattices of magnetic bubbles stabilized by the dipolar interaction and called “topological melting.” With support from numerical calculations, it is argued here that the observed regime of partial topological charge inversion has its origin in the topological protection of the starting metastable skyrmion state.