Femtosecond thermalization of electrons and holes in transition-metal ferromagnets

We analyze ultrafast charge dynamics in transition metal ferromagnets Fe, Co and Ni by using dynamical mean-field theory and time-dependent density-functional theory (DMFT+TDDFT) . We pay special attention to the ultrafast thermalization of electron and hole subsystems after the system is excited by a few-fs laser pulse. At such a short time scales the only source of scattering is electron-electron interaction. We take into account these effects through a non-collinear TDDFT exchange-correlation kernel derived from the DMFT spin-resolved susceptibility. It is demonstrated that the charge thermalization in all systems happens at time ~10fs. This result is in agreement with available experimental data for Ni. We discuss the differences in charge relaxation in different materials, as well as the difference in the thermalization of electrons and holes. Obtained results may shed light on details of the ultrafast charge and spin dynamics in ferromagnets, including possibility of long-time metastable states and even photoinduced phase transitions.