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