Matt Dwyer Final Defense

In this study, I report the measurement of the temperature dependent macro­ scopic phonon angular momentum using a fiber-optic-interferometer and mechanical oscillator system. An oscillating magnetic field is applied to an insulating ferromag­ net attached to a 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 observation of the changing phonon an­ gular momentum. A force change on the order of 100 nN was detected between 77 K and 300 K for a 3 mm³ MgZn ferrite sample, in fair agreement with theoretical predictions¹. 

 Our oscillator, with resonances at 1.3, 2.1, 9.4, and 12.4 kHz has a thermal noise limit on the order of 10-¹⁴ N/ JHz, allowing the possibility of high­ accuracy detection. Competing effects were minimized; for example, induced eddy current momentum can overwhelm the phonon effect for metallic ferromagnets, and careful temperature-dependent force calibrations were required.