  We propose and analyze a theoretical model of ultrafast light-induced
magnetization dynamics in systems of localized spins that are coupled to
carriers' spins by sp-d exchange interaction. A prominent example of a class of
materials falling into this category are ferromagnetic (III,Mn)V
semiconductors, in which ultrafast demagnetization has been recently observed.
In the proposed model light excitation heats up the population of carriers,
taking it out of equilibrium with the localized spins. This triggers the
process of energy and angular momentum exchange between the two spin systems,
which lasts for the duration of the energy relaxation of the carriers. We
derive the Master equation for the density matrix of a localized spin
interacting with the hot carriers and couple it with a phenomenological
treatment of the carrier dynamics. We develop a general theory within the sp-d
model and we apply it to the ferromagnetic semiconductors, taking into account
the valence band structure of these materials. We show that the fast spin
relaxation of the carriers can sustain the flow of polarization between the
localized and itinerant spins leading to significant demagnetization of the
localized spin system, observed in (III,Mn)V materials.
