The Orbital Angular Momentum of Azimuthal Spin-Waves

Abstract: In the context of a growing interdisciplinary interest in the angular momentum of wave fields, the spin-wave case has yet to be fully explored, with the extensively studied notion of spin transport being only part of the broader picture. Here we report experimental evidence for magnon orbital angular momentum, demonstrating that the mode exhibits rotation rather than remaining stationary. This conclusion is drawn from observations of the lifted degeneracy of waves with counter-rotating wave fronts. This requires an unambiguous formulation of spin and orbital angular momenta for spin waves, which we provide in full generality based on a systematic application of quantum field theory techniques. The results unequivocally establish magnetic dipole-dipole interactions as a magnetic-field controllable spin-orbit interaction for magnons. Our findings open a new research direction, leveraging the spectroscopic readability of angular momentum for azimuthal spin waves and other related systems.