Magnon activation by hot electrons via nonquasiparticle states
AbstractWe consider the situation when a femtosecond laser pulse creates a hot electron state in a half-metallic ferromagnet (e.g., ferromagnetic semiconductor) on a picosecond time scale but does not act directly on the localized spin system. We show that the energy and magnetic moment transfer from hot itinerant electrons to localized spins is facilitated by so-called nonquasiparticle states, which are the scattering states of a magnon and spin-majority electron. The magnon distribution is described by a quantum kinetic equation that we derive using the Keldysh diagram technique. In a typical ferromagnetic semiconductor such as EuO, magnons remain essentially in nonequilibrium on a scale of the order of microseconds after the laser pulse.