Our celebration of Dr. Caleb Hughes's successful doctoral dissertation defense, with his supervisor, Dr. Josh Long.
Dissertation Title: Rare Earth Iron Garnets as Probes of Exotic Spin-Dependent Forces
Abstract: Rare-earth iron garnets constitute a model system for Néel ferrimagnetism. These materials exhibit temperature-dependent orbital cancellation of the magnetism associated with the electron spins, leaving a dense ensemble of polarized electrons which are of interest as a source for exotic spin-dependent fifth-force searches. We describe the preparation and characterization of these samples as well as two sets of experiments in which they have been used.
The first is an experimental search with sensitivity to fifteen exotic spin-dependent potentials in the sub-millimeter range, using planar resonant test masses with operational frequencies near 1 kHz. The test masses have been augmented with a polarized dysprosium iron garnet (Dy3Fe5O12). Cancellation of the magnet moment, and hence the magnetic backgrounds, is maximal (ideally perfect) at the compensation temperature near 225 K. A complete apparatus, designed to make optimal use of the ferrimagnet, has been tested. The apparatus has been surrounded with a radiative shield containing a liquid nitrogen reservoir for cooling. To assess the remnant magnetization during cooldown and locate the compensation temperature, the apparatus contains a set of coils to drive the detector with a resonant magnetic gradient. At 223.7 K, the magnetic driving force is reduced by at least a factor of 240 relative to its room temperature value. We describe this operation and discuss the projected sensitivity to exotic potentials.
The second set of experiments are two novel investigations of a ferrimagnetic terbium iron garnet (Tb3Fe5O12) sample: one using neutron spin echo spectroscopy, the other using neutron radiography. In contrast to measurements of external fields, neutrons can probe the internal state of the ferrimagnet and determine if the cancellation of moments occurs at the unit cell level or is an emergent property as a result of compensation among inhomogeneously-distributed domains. The use of polarized neutrons in both of these techniques enabled a search for exotic interactions between the polarized electrons in the sample and incident neutrons. We describe the apparatus used as well as preliminary results.
