  Irradiating a ferromagnetic material with an ultrashort laser pulse leads to
demagnetization on a femtosecond timescale. We implement Elliott-Yafet type
spin-flip scattering, mediated by electron-electron and electron-phonon
collisions, into 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 identify the driving force
for ultrafast magnetization dynamics as the equilibration of temperatures and
chemical potentials between the 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 example of
3d-ferromagnetic nickel.
