Magnetization dynamics in dysprosium orthoferrites via the inverse Faraday effect

The ultrafast nonthermal control of magnetization has recently become feasible in canted antiferromagnets through photomagnetic instantaneous pulses [A. V. Kimel et al., Nature 435, 655 (2005)]. In this experiment, circularly polarized femtosecond laser pulses set up a strong magnetic field along the wave vector of the radiation through the inverse Faraday effect, thereby exciting nonthermally the spin dynamics of dysprosium orthoferrites. A theoretical study is performed by using a model for orthoferrites based on a general form of free energy whose parameters are extracted from experimental measurements. The magnetization dynamics is described by solving coupled sublattice Landau-Lifshitz-Gilbert equations whose damping term is associated with the scattering rate due to magnon-magnon interaction. Due to the inverse Faraday effect and the nonthermal excitation, the effect of the laser is simulated by magnetic-field Gaussian pulses with temporal width of the order of $100\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$. When the field is along the $z$ axis, a single resonance mode of the magnetization is excited. The amplitude of the magnetization and out-of-phase behavior of the oscillations for fields in the $z$ and $\ensuremath{-}z$ directions are in good agreement with the cited experiment. The analysis of the effect of the temperature shows that the magnon-magnon scattering mechanism affects the decay of the oscillations on the picosecond scale. Finally, when the field pulse is along the $x$ axis, another mode is excited, as observed in experiments. In this case, a comparison between theoretical and experimental results shows some discrepancies, the origin of which is related to the role played by anisotropies in orthoferrites.