  We show that the formation of a magnon condensate in thin ferromagnetic films
can be explained within the framework of a classical stochastic non-Markovian
Landau-Lifshitz-Gilbert equation where the properties of the random magnetic
field and the dissipation are determined by the underlying phonon dynamics. We
have numerically solved this equation for a tangentially magnetized
yttrium-iron garnet film in the presence of a parallel parametric pumping
field. We obtain a complete description of all stages of the nonequilibrium
time evolution of the magnon gas which is in excellent agreement with
experiments. Our calculation proves that the experimentally observed
condensation of magnons in yttrium-iron garnet at room temperature is a purely
classical phenomenon which should be called Rayleigh-Jeans rather than
Bose-Einstein condensation.

Mesoscale and Nanoscale Physics