  Magnons are one of the carriers of angular momentum that are involved in the
ultrafast magnetization dynamics in ferromagnets, but their contribution to the
electronic dynamics and their interplay with other scattering process that
occur during ultrafast demagnetization has not yet been studied in the
framework of a microscopic dynamical model. The present paper presents such an
investigation of electronic scattering dynamics in itinerant ferromagnets at
the level of Boltzmann scattering integrals for the magnon distributions and
spin-dependent electron distributions. In addition to electron-magnon
scattering, we include spin-conserving and effective Elliott-Yafet like
spin-flip electron-electron scattering processes and the influence of phonons.
In our model system, the creation or annihilation of magnons leads to
transitions between two spin-split electronic bands with energy and momentum
conservation. Due to the presence of spin-orbit coupling, Coulomb scattering
transitions between these bands are also possible, and we describe them on an
equal footing in terms of Boltzmann scattering integrals. For an instantaneous
carrier excitation process we analyze the influence of both interaction
processes on the magnon and spin-dependent electron dynamics, and show that
their interplay gives rise to an efficient creation of magnons at higher
energies and wave vectors accompanied by only a small increase of the
electronic spin polarization. These results present a microscopic dynamical
scenario that shows how non-equilibrium magnons may decisively influence the
magnetic response of a ferromagnet on ultrafast timescales.
