Exchange relaxation as a mechanism of the ultrafast reorientation of spins in a two-sublattice ferrimagnet

An exact solution for the evolution of the spins of sublattices in a two-sublattice ferrimagnet has been obtained in the exchange approximation. The nonlinear regimes of the spin dynamics including the longitudinal evolution of the spin magnitudes of the sublattices and the spin precession have been found taking into account exchange relaxation. In particular, these regimes describe the switching of the directions of spins observed in GdFeCo alloy under the action of a femtosecond laser pulse.