Magnon-phonon conversion experiment and phonon spin

Abstract

Recent experiment demonstrates magnon to phonon conversion in a YIG film under the application of a non-uniform magnetic field. Light scattered from phonons is observed to change its polarization state interpreted by the authors signifying phonon spin. In this note we argue that the experimental data merely shows the exchange of angular momentum $\pm \hbar$ per photon. We suggest that it has physical origin in the orbital angular momentum of phonons. The distinction between spin and orbital parts of the total angular momentum, and between phonons and photons with added emphasis on their polarizations is explained. The main conclusion of the present note is that phonon spin hypothesis is unphysical.