  Ultrafast laser excitation of ferromagnetic metals gives rise to correlated,
highly non-equilibrium dynamics of electrons, spins and lattice, which are,
however, poorly described by the widely-used three-temperature model (3TM).
Here, we develop a fully ab-initio parameterized out-of-equilibrium theory
based on a quantum kinetic approach--termed (N+2) temperature model--that
describes magnon occupation dynamics due to electron-magnon scattering. We
apply this model to perform quantitative simulations on the ultrafast,
laser-induced generation of magnons in iron and demonstrate that on these
timescales the magnon distribution is non-thermal: predominantly high-energy
magnons are created, while the magnon occupation close to the center of the
Brillouin zone even decreases, due to a repopulation towards higher energy
states via a so-far-overlooked scattering term. We demonstrate that the simple
relation between magnetization and temperature computed at equilibrium does not
hold in the ultrafast regime and that the 3TM greatly overestimates the
demagnetization. The ensuing Gilbert damping becomes strongly magnon wavevector
dependent and requires a description beyond the conventional
Landau-Lifshitz-Gilbert spin dynamics. Our ab-initio-parameterized calculations
show that ultrafast generation of non-thermal magnons provides a sizable
demagnetization within 200fs in excellent comparison with experimentally
observed laser-induced demagnetizations. Our investigation emphasizes the
importance of non-thermal magnon excitations for the ultrafast demagnetization
process.
