Characterization of the Dynamics of FeRh using Time-Domain Thermoreflectance (TDTR)*

FeRh exhibits an antiferromagnetic(AF)/ferromagnetic (FM) phase transition. This change in magnetic order is accompanied by an increase in lattice parameter. Using Time-Domain Thermoreflectance (TDTR), a pump probe technique used to determine the thermoreflectance (TR) of samples, we studied the dynamics of FeRh. For these measurements TDTR was conducted at various temperatures within the transition region. At each base temperature, we used small temperature excursions to determine the dynamics near each temperature. We found that for time delays shorter than the thermally-driven transition mechanism, the TR of the AF and FM phases have opposing signs. Using this feature, we observed a pronounced response within the AF phase for time delays less than 1 ps. These results suggest that shortly following excitation, the electrons, magnons and phonons exhibit significant non-equilibrium behavior, while equilibrium is achieved on the picosecond timescale. These findings also suggest that the dynamics of the FeRh sample occur on both the subpicosecond timecale and the picosecond timescale.