  We investigate ultrafast demagnetization due to electron-phonon interaction
in a model band-ferromagnet. We show that the microscopic mechanism behind the
spin dynamics due to electron-phonon interaction is the interplay of scattering
and the precession around momentum-dependent effective internal spin-orbit
magnetic fields. The resulting magnetization dynamics can only be mimicked by
spin-flip transitions if the spin precession around the internal fields is
sufficiently fast (compared to the scattering time) so that it averages out the
transverse spin components.
