Spin-driven lattice thermal conductivity in magnetic chromium trihalide materials

Fundamental research into phonon-mediated behaviours is increasingly gaining attention in the transition metal trihalide systems. Insulating quantum magnet trihalides in bulk as well as in two-dimensional (2D) layers provide the opportunity to explore spin liquid behaviour, spintronics, 2D and bulk magnetism, and thermal transport. Thermal transport driven by phonons in the presence of spins has been barely explored. This work has examined the harmonic and anharmonic vibrational properties of magnetic bulk CrCl₃ and CrI₃ using density functional theory and experimental comparison. Our theoretical results strongly emphasize that van der Waals and magnetic interactions are important for the dynamical stability of such structures. The vibrational spectrum of CrCl₃ and CrI₃ are significantly different, as CrI₃ has a phonon band gap, and low energy phonons have dominant iodine contributions. Engaging phononic vibrational spectrum leads to interesting anisotropic lattice thermal conductivity (κ_l) as CrCl₃ has higher in-plane κl than CrI₃, whereas CrI₃ shows larger out-of-plane κl values than CrCl₃. We illustrate that even though the scattering strength of phonons is higher in CrCl₃ by virtue of their higher phonon group velocities, the heat transfer capability of CrCl₃ is larger than CrI₃. Our work provides a basic understanding and insights into the phononic thermal transport in the magnetic bulk transition metal trihalide materials.