Collective antiskyrmion-mediated phase transition and defect-induced melting in chiral magnetic films

The topological stability of skyrmions goes beyond the energetics of materials and strongly depends on the dynamics of magnetization textures, and the combined effects of energetics, topology, and magnetization dynamics, lead to wildly rich phenomena. Based on detailed high-resolution micromagnetic simulations, we have found a new first-order field-induced phase transition in which a skyrmion lattice inverts its polarity through a metastable creation and annihilation of an antiskyrmion lattice. Importantly, we have observed that in the presence of even a single defect in the material, the phase transition becomes second-order and progresses through a gradual melting of the skyrmion lattice. Our analysis suggests that in a perfect skyrmion lattice the response to an external magnetic field is an abrupt collective phenomenon due to topological constraints, whereas a single defect disrupts the constraint and facilitates topological charge melting, consequently leading to the gradual response of the material to an external field. This provides a basis for a much wider scope of experiments on skyrmion lattices, with emphasis on the interplay between topological constraints and material defects.