Real-time observation of topological defect dynamics mediating two-dimensional skyrmion lattice melting.

Topological defects are the key feature mediating two-dimensional phase transitions. However, both resolution and tunability have been lacking to access the dynamics of these transitions in the various two-dimensional systems explored. Skyrmions in magnetic thin films are two-dimensional, topologically non-trivial quasi-particles that provide rich dynamics as well as tunability as an essential ingredient for the control of their phase behaviour. With dynamic Kerr microscopy, we directly capture the melting of a confined two-dimensional magnetic skyrmion lattice in a Ta/CoFeB/Ta/MgO/Ta magnetic multilayer system with high resolution in real time and real space. By the applied magnetic field, we tune the skyrmion size and effective temperature on the fly to drive the two-step melting through an intermediate hexatic regime between the solid lattice and the isotropic liquid. We quantify the characteristic occurrence of topological defects mediating the transitions and reveal the dynamics of the lattice dislocations. The full real-time and real-space imaging reveals the diffusion coefficient of dislocations, which is two orders of magnitude higher than that of skyrmions.