Measurement of Phonon Angular Momentum via the Einstein-deHaas Effect, Fiber-Optic Interferometry, and a High-Q Oscillator*

We report the design and use of a fiber-optic-interferometer

system to measure the predicted¹ macroscopic phonon angular

momentum. An oscillating magnetic field is applied to an

insulating ferromagnet attached to our single-crystal high-Q

double torsional oscillator. By the Einstein-de Haas effect,

oscillator displacement measurements between low

temperatures and those closer to the Debye temperature allow

extraction of the changing phonon angular momentum. A force

change of 5 x 10^-8 N was detected between 77 K and 300 K for a

1 mm³ MgZn ferrite sample. Our oscillator, with a resonance at

1.3 kHz, has a thermal noise limit on the order of 10^-14 N/√Hz,

allowing the possibility of high-accuracy detection. Competing

effects are being minimized; for example, induced eddy current

momentum can overwhelm the phonon effect for metallic

ferromagnets, and careful temperature-dependent studies are

required for force calibrations.