Anisotropy, Transport, and Spin-Phonon Interplay in Insulating Magnets

Insulating magnets are a diverse platform for uniquely quantum behavior and the realization of novel cooperative spin states with exotic magnetic and topological excitations. In addition to fundamental interest, such materials have many potential technological applications, from spintronic sensing devices to the next generation of solid-state memory storage and even crucial roles in quantum information processing. The bulk magnetic and thermal properties of these materials are a product of the complex interplay of many interacting spin, orbit, and lattice degrees of freedom on an atomic scale, thus experimental probes that can dissect these microscopic details and recognize new phenomena are highly desirable. This thesis provides an overview of magnetism, thermal transport theory and experimentation, and high-field resonant torsion magnetometry techniques for a wide range of unconventional quantum magnets. We focus on two particular insulating magnets, CrCl3 and CsYbSe2, each of which exemplifies distinct mechanisms of spin-phonon interaction that are manifest in the anisotropic, field-dependent behavior of their respective thermal conductivity tensors. Our work develops new tools to advance the understanding of spin-phonon interactions, and the identification of truly exotic spin states of matter in these, and many other insulating systems.</p

Generic magnetic field dependence of thermal conductivity in magnetic insulators via hybridization of acoustic phonons and spin-flip excitations

Magnetic insulators provide excellent playgrounds to realize a range of exciting spin models, some of which predict exotic spin ground states, and thermal transport properties have been taking center stage in probing the spin excitations. Despite the fact that acoustic phonons make the major contribution to heat conduction in a crystalline system, their interplay with magnetic excitations is often viewed as peripheral to the physics of interest, for instance as an inconvenient source of scattering or decoherence. Here, we present a comprehensive study on the longitudinal magneto-thermal transport in a paramagnetic effective spin-1/2 magnetic insulator CsYbSe$_2$. We introduce a minimal model requiring only Zeeman splitting and magnetoelastic coupling, and use it to argue that hybridized excitations -- formed from acoustic phonons and localized spin-flip-excitations across the Zeeman gap of the crystal electric field ground doublet -- are responsible for a striking non-monotonic field dependence of longitudinal thermal conductivity. Beyond highlighting a starring role for phonons, our results raise the prospect of universal magneto-thermal transport phenomena in magnetic insulators that originate from simple features shared across many systems.Comment: 8 pages, 4 figure