Ultrafast dynamics in ferrimagnetic materials with a quantum Monte Carlo atomistic model

We investigate the ultrafast dynamics of the atomic angular momentum in ferrimagnets irradiated by laser pulses. Our study is based on a quantum atomistic approach and the particle dynamics is performed using a Monte Carlo technique. We focus on microscopic mechanisms that lead to the dissipation of the total angular momentum in a rare earth–transition metal (RE-TM) alloy in which the two sublattices have opposite spin orientation. We describe the coherent transfer of atomic angular momentum between the spin and the orbital momentum. The orbital momentum quenching induced by the lattice field and the Elliott-Yafet collision mechanism are also included. The simulations show that the observed ultrafast magnetization quenching may be explained at a microscopical level by the combined effects of the coherent spin transfer between the RE and the TM sublattices along with the quenching of the localized orbital angular momentum induced by the lattice field.

Figure 1

Ultrafast evolution of the spin (blue curve) and orbital angular momentum (red curve) after laser excitation for cobalt (left panel) and terbium (right panel). The dashed curves are obtained by imposing the conservation of total angular momentum of the system. The inset in the right panel depicts the zoom of the results for small times.

Figure 2

Upper panel: Total angular momentum of the alloy. Lower panel: Ultrafast evolution of the total angular momentum (spin plus orbital momentum) for cobalt (red curve) and terbium (blue curve). The continuous curves refer to the full solution and the dashed curves refer to the case with conservation of the total angular momentum.