Ultrafast nonequilibrium spin dynamics in a ferromagnetic thin film

An ultrafast time-resolved magneto-optical pump probe is applied to study spin-relaxation processes in a ferromagnetic ${\mathrm{CoPt}}_3$ alloy film. Following spin-selective photoexcitation with circularly polarized femtosecond pulses, transient Kerr ellipticity tracks the evolution of the nonequilibrium spin system. We isolate two distinct processes: a subpicosecond component due to the relaxation of coherently spin polarized electrons, and a slower component (∼10 psec) associated with the evolution of the thermalized spin distribution and the photoinduced transient magnetization.