Thermalization of spins and charges in Ni thin films

It has been shown more than ten years ago [1] that the magnetization of a ferromagnetic material can be modified and monitored using femtosecond optical pulses. This discovery has motivated many researches showing that the magnetization of materials can be controlled at the femtosecond time scale. The initial demagnetization, which occurs during the thermalization of the spins [2] (typically a few hundreds of femtoseconds) is not yet completely understood. In the present paper, we address the physical process of thermalization of the charges (time τ e ) and the spins (time τ s ), for several laser excitation and film thcknesses.