Giant thermal magnetoconductivity in CrCl3 and a general model for spin-phonon scattering

Abstract

Insulating quantum magnets lie at the forefront both of fundamental research into quantum matter and of technological exploitation in the increasingly applied field of spintronics. In this context, the magnetic thermal transport is a particularly sensitive probe of the elementary spin and exotic topological excitations in unconventional magnetic materials. However, magnetic contributions to heat conduction are invariably intertwined with lattice (phonon) contributions, and thus the issue of spin-phonon coupling in determining the spin and thermal transport properties of magnetic insulators becomes more important with every emergent topological magnetic system. Here we report the observation of an anomalously strong enhancement of the thermal conductivity, occurring at all relevant temperatures, in the layered honeycomb material CrCl3 in the presence of an applied magnetic field. Away from the magnetically ordered phase at low temperatures and small fields, there is no coherent spin contribution to the thermal conductivity, and hence the effect must be caused by strong suppression of the phonon thermal conductivity due to magnetic fluctuations, which are in turn suppressed by the field. We build an empirical model for the thermal conductivity of CrCl3 within a formalism assuming an independently determined number of spin-flip processes and an efficiency of the phonon scattering events they mediate. By extracting the intrinsic phonon thermal conductivity, we obtain a quantitative description of our measured data at all fields and temperatures, and we demonstrate that the scattering efficiency is entirely independent of the applied field. In this way, we use CrCl3 as a model system to understand the interactions between spin and phonon excitations in the context of thermal transport. We anticipate that the completely general framework we introduce will have broad implications for the interpretation of transport phenomena in magnetic quantum materials.