Spin-current-mediated rapid magnon localization and coalescence after ultrafast optical pumping of ferrimagnetic alloys

We find evidence of rapid magnetic order recovery via nonlinear magnon processes after ultrafast demagnetization of amorphous GdFeCo thin films with perpendicular magnetic anisotropy. The spatial evolution of the magnetization is measured by time-resolved resonant X-ray scattering and modelled by atomistic and multiscale micromagnetic simulations. We identify both localization and coalescence processes. During localization a paramagnetic state evolves into a collection of localized textures while coalescence describes their growth, break-up, and merging. The characteristic length scale during coalescence is found to grow according to a power law in both experiments and simulations. Our results shed light into the physical mechanisms that are important for the picosecond recovery of magnetic order.