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# Detecting the phonon spin in magnon–phonon conversion experiments

## Abstract

Recent advances in the emerging field of magnon spintronics have stimulated renewed interest in phenomena involving the interaction between spin waves, the collective excitations of spins in magnetic materials that quantize as magnons, and the elastic waves that arise from excitations in the crystal lattice, which quantize as phonons. In magnetic insulators, owing to the magnetostrictive properties of materials, spin waves can become strongly coupled to elastic waves, forming magnetoelastic waves—a hybridized magnon–phonon excitation. While several aspects of this interaction have been subject to recent scrutiny, it remains unclear whether or not phonons can carry spin. Here we report experiments on a film of the ferrimagnetic insulator yttrium iron garnet under a non-uniform magnetic field demonstrating the conversion of coherent magnons generated by a microwave field into phonons that have spin. While it is well established that photons in circularly polarized light carry a spin, the spin of phonons has had little attention in the literature. By means of wavevector-resolved Brillouin light-scattering measurements, we show that the magnon–phonon conversion occurs with constant energy and varying linear momentum, and that the light scattered by the phonons is circularly polarized, thus demonstrating that the phonons have spin.

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Calculations were performed by S.M.R. and J.H. Supervision of the work was carried out by S.M.R., who wrote the manuscript with contributions from all authors. Correspondence to S. M. Rezende. ## Ethics declarations ### Competing interests The authors declare no competing interests. ## Additional information Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. ## Supplementary information ### Supplementary Information Supplementary notes, Supplementary figures 1–5 ## Rights and permissions Reprints and Permissions ## About this article • #### Received • #### Accepted • #### Published • #### Issue Date • #### DOI https://doi.org/10.1038/s41567-018-0079-y ## Further reading • ### Magnon Chaos in$PT\$-Symmetric Cavity Magnomechanics

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