Microwave magnonics at millikelvin temperatures

Abstract

This thesis reports on three recent experimental studies of microwave magnons in yttrium iron garnet (YIG) systems at millikelvin temperatures. We begin with an introduction to the emerging field of quantum magnonics and its underpinning motivations. Basic theory of dipolar spin-wave or magnon dynamics in various sample geometries is presented. This introduction is followed by a brief description of the specificities of our experimental setup and properties of YIG --- the magnetic material used in our studies. The first experiment involves a hybrid system combining a YIG sphere and a niobium-based superconducting planar resonator. The device is measured at millikelvin temperatures and signals of strong magnon-photon coupling are observed when the excitation energy is at the level of single photons. The superconducting resonator is shown to maintain a good quality factor even under sizeable in-plane magnetic field that is required to support the excitation of magnons in YIG. The second experiment demonstrates the operation of a magnonic crystal based on an etched YIG film at millikelvin temperatures. A magnonic bandgap is successfully observed both under continuous- and pulsed- microwave excitation. High magnon damping is observed in the device at low temperature. The third experiment involves the measurement of magnon damping in YIG films. Comparisons between results from different samples and at different temperatures provide insight into the role of the substrate and two-level fluctuators at low temperature. A brief review of the known damping processes in bulk YIG from room temperature down to millikelvin temperature is also presented. The final chapter summarises results in this thesis and suggests possible future research directions.</p