Spin dynamics inCoPt3alloy films: A magnetic phase transition in the femtosecond time scale

We have measured the magnetization dynamics of an epitaxial ${\mathrm{CoPt}}_3$ alloy film using a magneto-optical pump-probe Kerr setup with 120 fs laser pulses. At high pump fluence, the film is driven from the ferro- to the paramagnetic state with a characteristic time comparable to the pulse duration. The associated demagnetization occurs within the thermalization time of the electrons to a Fermi distribution. These results demonstrate the feasibility of ultrafast magneto-optical devices.