Direct observation of magnon-phonon coupling in yttrium iron garnet

TL;DR

This study uses inelastic neutron scattering to observe magnon-phonon interactions in yttrium iron garnet, revealing enhanced hybridized scattering intensity instead of expected gaps, challenging existing models of spin-lattice coupling.

Contribution

First direct observation of magnon-phonon coupling in YIG showing enhanced scattering, not gap formation, suggesting new understanding of magnon-polaron interactions.

Findings

Magnon-phonon coupling enhances hybridized scattering intensity.

No gap opening at dispersion intersections under high magnetic field.

Results differ from conventional spin-lattice coupling expectations.

Abstract

The magnetic insulator yttrium iron garnet (YIG) with a ferrimagnetic transition temperature of $\sim$560 K has been widely used in microwave and spintronic devices. Anomalous features in the spin Seeback effect (SSE) voltages have been observed in Pt/YIG and attributed to the magnon-phonon coupling. Here we use inelastic neutron scattering to map out low-energy spin waves and acoustic phonons of YIG at 100 K as a function of increasing magnetic field. By comparing the zero and 9.1 T data, we find that instead of splitting and opening up gaps at the spin wave and acoustic phonon dispersion intersecting points, magnon-phonon coupling in YIG enhances the hybridized scattering intensity. These results are different from expectations of conventional spin-lattice coupling, calling for new paradigms to understand the scattering process of magnon-phonon interactions and the resulting magnon-polarons.