Evidence for thermal mechanisms in laser-induced femtosecond spin dynamics

Recent pump-probe experiments using powerful femtosecond lasers and x-ray magnetic circular dichroism have opened a debate on the origin of the magnetization modification on the femtosecond time scale. We show a quantitative agreement between femtosecond optical pump-probe experiments and thermal micromagnetic modeling in nickel, which reveals a predominant thermal demagnetization mechanism. Magnetic fluctuations are introduced in the system as spin-flip processes due to scattering mechanisms in the electron system. In our model the Landau-Lifshitz-Bloch equation for a macrospin (containing the statistically averaged magnetic fluctuations) is coupled to the electronic temperature of the two-temperature model whose parameters are extracted from the measured reflectivity. We show that the demagnetization and the magnetization recovery time slow down as the laser pump fluence is increased and identify the longitudinal relaxation as a key factor for the observed behavior.