Classical description of spin wave excitation by currents in bulk ferromagnets

A modification of the Landau-Lifshitz equation accounts for the effect of the current on the magnetization in the "slow passage" limit, according to which the spins precess about the exchange field quickly in comparison to the rate of change of the orientation of the magnetization as seen in the reference frame of the conduction electrons. This model implies that the spin polarization of the current provokes an instability of the magnetization in the bulk of a ferromagnet when the current density reaches a threshold.