  In the exchange approximation, an exact solution is obtained for the
sublattice magnetizations evolution in a two-sublattice ferrimagnet. Nonlinear
regimes of spin dynamics are found that include both the longitudinal and
precessional evolution of the sublattice magnetizations, with the account taken
of the exchange relaxation. In particular, those regimes describe the spin
switching observed in the GdFeCo alloy under the influence of a femtosecond
laser pulse.
