Transient inverse Faraday effect and ultrafast optical switching of magnetization

A model is proposed for describing the nonthermal switching of the spin magnetization Mₛ caused by circularly polarized femtosecond laser pulses. At the initial stage the intense pulse induces in the magnetic medium a nonequilibrium orbital momentum Lneq which in its turn creates a nonequilibrium anisotropy field Hneq potentially capable to reorient Mₛ. The field Hneq is characterized by a mean value ⟨Hneq⟩ and fluctuations δHneq which both can be quite large in the nonequilibrium state. Transverse fluctuations δHneq^⊥⊥⟨Hneq⟩ provide a supplementary channel for the coupling of spins with the lattice thus accelerating the spin-lattice relaxation necessary for the reorientation of Mₛ. In contrast, longitudinal fluctuations δHneq^∥∥⟨Hneq⟩ impede the reorientation of Mₛ because they are responsible for the quenching of atomic-orbital momenta in the equilibrium state. They lead to strong oscillations of Hneq thus drastically reducing ⟨Hneq⟩ down to a value comparable with the equilibrium magnetic anisotropy field. Were that the case the switching of Mₛ would be illusive. In order to suppress these objectionable fluctuations of δHneq^∥ we propose a mechanism resulting from the interatomic interaction Vₗₗ of orbital momenta. We show that the parameter Vₗₗ 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 Hneq to a sufficiently low level δHneq^∥/⟨Hneq⟩≈0.1. The switching of Mₛ becomes feasible provided the lifetime τq of the nonequilibrium state is longer than the spin-lattice relaxation time τₛ.