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 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 −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.