Driving force of ultrafast magnetization dynamics

Irradiating a ferromagnetic material with an ultrashort laser pulse leads to demagnetization on the femtosecond timescale. We implement Elliott–Yafet-type spin-flip scattering, mediated by electron–electron and electron–phonon collisions, in the framework of a spin-resolved Boltzmann equation. Considering three mutually coupled reservoirs, (i) spin-up electrons, (ii) spin-down electrons and (iii) phonons, we trace non-equilibrium electron distributions during and after laser excitation. We identified the driving force for ultrafast magnetization dynamics as the equilibration of temperatures and chemical potentials between electronic subsystems. This principle can be used to easily predict the maximum quenching of magnetization upon ultrashort laser irradiation in any material, as we show for the case of 3d-ferromagnetic nickel.