Skyrmion Lattice Collapse and Defect-Induced Melting in Chiral Magnetic Films

Magnetic phase transitions are a test bed for exploring the physics of non-equilibrium phenomena in condensed matter, which become even more complex when topological constraints are involved. In particular, the investigation of skyrmions and skyrmion lattices offers insight into fundamental processes of topological-charge creation and annihilation upon changing the magnetic state. Nonetheless, the exact physical mechanisms behind these phase transitions remain unresolved. Here, we have systematically compared ultra-thin films with isotropic and anisotropic Dzyaloshinskii-Moriya interactions (DMI), demonstrating a nearly identical behavior in technologically relevant materials such as interfacial systems. We numerically show that in perfect systems skyrmion lattices can be inverted in a field-induced first-order phase transition. The existence of even a single defect, however, replaces the inversion with a second-order phase transition of defect-induced lattice melting. This radical change in the system's behavior from a first-order to a second-order phase transition signifies the importance of such an analysis for all realistic systems in order to correctly interpret experimental data. Our results shed light on complex topological charge annihilation mechanisms that mediate transitions between magnetic states and pave the way for an experimental realization of these phenomena.