Transient inverse Faraday effect and ultrafast optical switching of magnetization

A model is proposed for describing the nonthermal switching of the spin magnetization M-s caused by circularly polarized femtosecond laser pulses. At the initial stage the intense pulse induces in the magnetic medium a nonequilibrium orbital momentum L-neq which in its turn creates a nonequilibrium anisotropy field H-neq potentially capable to reorient M-s. The field H-neq is characterized by a mean value < H-neq > and fluctuations delta H-neq which both can be quite large in the nonequilibrium state. Transverse fluctuations delta H-neq(perpendicular to)perpendicular to < H-neq > provide a supplementary channel for the coupling of spins with the lattice thus accelerating the spin-lattice relaxation necessary for the reorientation of M-s. In contrast, longitudinal fluctuations delta H-neq(parallel to)parallel to < H-neq > impede the reorientation of M-s because they are responsible for the quenching of atomic-orbital momenta in the equilibrium state. They lead to strong oscillations of H-neq thus drastically reducing < H-neq > down to a value comparable with the equilibrium magnetic anisotropy field. Were that the case the switching of M-s would be illusive. In order to suppress these objectionable fluctuations of delta H-neq(parallel to) we propose a mechanism resulting from the interatomic interaction V-ll of orbital momenta. We show that the parameter V-ll can be chosen in such a way that, on one hand, it cannot restore the quenched orbital momenta in the equilibrium state, and on the other hand, it can suppress the longitudinal oscillations of H-neq to a sufficiently low level delta H-neq(parallel to)/< H-neq >approximate to 0.1. The switching of M-s becomes feasible provided the lifetime tau(q) of the nonequilibrium state is longer than the spin-lattice relaxation time tau(s).