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To gain control over magnetic order on ultrafast time scales, a fundamentalunderstanding of the way electron spins interact with the surrounding crystallattice is required. However, measurement and analysis even of basic collectiveprocesses such as spin-phonon equilibration have remained challenging. Here, wedirectly probe the flow of energy and angular momentum in the model insulatingferrimagnet yttrium iron garnet. Following ultrafast resonant latticeexcitation, we observe that magnetic order reduces on distinct time scales of 1ps and 100 ns. Temperature-dependent measurements, a spin-coupling analysis andsimulations show that the two dynamics directly reflect two stages ofspin-lattice equilibration. On the 1-ps scale, spins and phonons reachquasi-equilibrium in terms of energy through phonon-induced modulation of theexchange interaction. This mechanism leads to identical demagnetization of theferrimagnet's two spin-sublattices and a novel ferrimagnetic state of increasedtemperature yet unchanged total magnetization. Finally, on the much slower,100-ns scale, the excess of spin angular momentum is released to the crystallattice, resulting in full equilibrium. Our findings are relevant for allinsulating ferrimagnets and indicate that spin manipulation by phonons,including the spin Seebeck effect, can be extended to antiferromagnets and intothe terahertz frequency range.

Dissecting spin-phonon equilibration in ferrimagnetic insulators by ultrafast lattice excitation