Revealing spin-phonon interaction in ferrimagnetic insulators by ultrafast lattice excitation

In spintronic materials, control and transport of magnetic order require a fundamental understanding of the way spins interact with the surrounding crystal lattice. However, direct measurement and analysis even of basic collective processes such as spin-phonon equilibration have remained challenging. Here, we reveal the flow of energy and angular momentum in the model insulating ferrimagnet yttrium iron garnet, following resonant lattice excitation. Remarkably, on a time scale as fast as 1 ps, spins and phonons reach quasi-equilibrium in terms of energy through phonon-induced modulation of the exchange interaction. This mechanism leads to identical emagnetization of the ferrimagnet's two spin sublattices. The resulting spin pressure is released by angular-momentum equilibration on a much slower, 100 ns time scale. Our results indicate that the spin Seebeck effect and efficient spin control by phonons can be extended to antiferromagnets and into the terahertz frequency range.