Femtosecond electron and spin dynamics probed by nonlinear optics

A theoretical calculation is performed for the ultrafast spin dynamics in n ickel using an exact diagonalization method. The present theory mainly focu ses on the situation where the intrinsic charge and spin dynamics is probed by the nonlinear (magneto-)optical responses on the femtosecond time scale , i.e. optical second harmonic generation (SHG) and the nonlinear magneto-o ptical Ken effect (NOLIMOKE). It is found that the ultrafast charge and spi n dynamics are observable on the time scale of 10 fs. The charge dynamics p roceeds ahead of the spin dynamics, which indicates the existence of a spin memory time. The fast decay results from the loss of coherence in the init ial excited state. Both the material specific and experimental parameters a ffect the dynamics. We find that the increase of exchange interaction accel erates mainly the spin dynamics rather than the charge dynamics. A reductio n of the hopping integrals, such as present at interfaces, slows down the s pin dynamics significantly. Furthermore, it is found that a spectrally broa d excitation yields the intrinsic speed limit of the charge (SHG) and spin dynamics (NOLIMOKE) while a narrower width prolongs the dynamics. This magn etic interface dynamics should then become accessible to state-of-the-art t ime-resolved nonlinear-optical experiments.