Spin dynamics driven by ultrafast laser-induced heating of iron garnet in high magnetic fields

Femtosecond laser excitation heats a ferrimagnetic iron garnet across the compensation temperature and decreases the magnetizations of the constituting Fe³⁺ sublattices. Here, we explore the heat-induced magnetization dynamics in the ferrimagnet at different points in the H-T phase diagram. For magnetic field strengths high enough to promote a state with non-collinear magnetizations of the sublattices, the dynamics occurs on a sub-ns timescale, governed by the effective spin–lattice interaction throughout the whole Brillouin zone of the spin excitations. When the field is low and the magnetizations are collinear, the heating alone is not sufficient to initiate the dynamics. In that case, the dynamics can only start after the magnetizations experience an initial kick, which occurs on the timescale of the spin–lattice interaction in the center of the Brillouin zone, leading to a substantial delay of the response of the spins to the thermal excitation.