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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Fig. 1: Schematics of the sample structure and apparatus used in the time-resolved measurements of the time delay of spin wavepackets in a YIG film.
Fig. 2: Illustration of the magnon–phonon conversion process in a YIG film under a non-uniform magnetic field.
Fig. 3: Schematics of the BLS set-up for wavevector-resolved detection of magnetoelastic waves.
Fig. 4: Wavenumber-resolved BLS demonstration of the magnon–phonon conversion in a YIG film under a non-uniform magnetic field.
Fig. 5: Wavenumber-resolved BLS measurements of the polarization of the light scattered by magnetoelastic waves in a YIG film under a non-uniform magnetic field.

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Acknowledgements

This research was supported by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Financiadora de Estudos e Projetos (FINEP) and Fundação de Amparo à Ciência e Tecnologia do Estado de Pernambuco (FACEPE).

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All experimental arrangements and measurements were performed by J.H. and D.S.M. with consultation from A.A. and S.M.R. A.A. prepared the YIG films using liquid phase epitaxy. 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.

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Correspondence to S. M. Rezende.

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Supplementary notes, Supplementary figures 1–5

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Holanda, J., Maior, D.S., Azevedo, A. et al. Detecting the phonon spin in magnon–phonon conversion experiments. Nature Phys 14, 500–506 (2018). https://doi.org/10.1038/s41567-018-0079-y

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