"Optical spin mixing" and kinetics of nonlinear response in four-photon spectroscopy of thin ferromagnetic films

A semiclassical phenomenological model is developed which takes into account basic processes proceeding in experiments on picosecond nonlinear spectroscopy of thin ferromagnetic films. The results of numerical simulations of the evolution of the domain structure of Ni films upon spatially uniform biharmonic pumping (BP) and upon spatially nonuniform excitation in the case of degenerate four-photon spectroscopy (DFPS) are presented. It is shown that the destruction kinetics of the film magnetisation is much slower in the case of DFPS than upon BP. This is explained by the efficient suppression of 'optical spin mixing' (the domain-structure destruction due to the spatial migration of s electrons) by potential barriers formed upon spatially nonuniform excitation of stable domain walls.