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

TL;DR

This study investigates the ultrafast spin-phonon energy exchange in ferrimagnetic insulators, revealing two distinct equilibration stages that influence magnetic order and could impact spin manipulation technologies.

Contribution

It provides the first direct measurement and analysis of two-stage spin-lattice equilibration in ferrimagnets using ultrafast lattice excitation.

Findings

Identified 1-ps and 100-ns spin-lattice equilibration stages.

Demonstrated phonon-induced modulation of exchange interaction causes rapid demagnetization.

Revealed potential for extending spin control to terahertz frequencies.

Abstract

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.