  The Einstein-de Haas (EdH) effect and its reciprocal the Barnett effect are
fundamental to magnetism and uniquely yield measures of the ratio of magnetic
moment to total angular momentum. These effects, small and generally difficult
to observe, are enjoying a resurgence of interest as contemporary techniques
enable new approaches to their study. The high mechanical resonance frequencies
in nanomechanical systems offer a tremendous advantage for the observation of
EdH torques in particular. At radio frequencies, the EdH effect can become
comparable to or even exceed in magnitude conventional cross-product magnetic
torques. In addition, the RF-EdH torque is expected to be phase-shifted by 90
degrees relative to cross-product torques, provided the magnetic system remains
in quasi-static equilibrium, enabling separation in quadratures when both
sources of torque are operative. Radio frequency EdH measurements are
demonstrated through the full hysteresis range of micrometer scale,
monocrystalline yttrium iron garnet (YIG) disks. Equilibrium behavior is
observed in the vortex state at low bias field. Barkhausen-like features emerge
in the in-plane EdH torque at higher fields in the vortex state, revealing
magnetic disorder too weak to be visible through the in-plane cross-product
torque. Beyond vortex annihilation, peaks arise in the EdH torque versus bias
field, and these together with their phase signatures indicate additional
utility of the Einstein-de Haas effect for the study of RF-driven spin
dynamics.
