Thermal spin dynamics of yttrium iron garnet

Yttrium Iron Garnet is the prototypical material used to study pure spin currents. It is a complex material with 20 magnetic atoms in the unit cell. Almost all theories and experimental analysis approximates this complicated material to a simple ferromagnet with a single spin wave mode. We use the method of atomistic spin dynamics to study the temperature evolution of the full 20 mode exchange spin wave spectrum. Our results show a strong frequency dependence of the modes in quantitative agreement with neutron scattering experiments. We find this causes in a reduction in the net spin pumping due to the thermal occupation of optical modes with the opposite chirality to the FMR mode.