Spin‐Phonon Scattering‐Induced Low Thermal Conductivity in a van der Waals Layered Ferromagnet Cr 2 Si 2 Te 6

Layered van der Waals (vdW) magnets are prominent playgrounds for developing magnetoelectric, magneto‐optic, and spintronic devices. In spintronics, particularly in spincaloritronic applications, low thermal conductivity (κ) is highly desired. Herein, by combining thermal transport measurements with density functional theory calculations, this study demonstrates low κ down to 1 W m −1 K −1 in a typical vdW ferromagnet Cr 2 Si 2 Te 6 . In the paramagnetic state, development of magnetic fluctuations way above T c = 33 K strongly reduces κ via spin‐phonon scattering, leading to low κ ≈ 1 W m −1 K −1 over a wide temperature range, in comparable to that of amorphous silica. In the magnetically ordered state, emergence of resonant magnon‐phonon scattering limits κ below ≈2 W m −1 K −1 , which will be three times larger if magnetic scatterings are absent. Application of magnetic fields strongly suppresses the spin‐phonon scattering, giving rise to large enhancements of κ. This study's calculations well capture these complex behaviors of κ by taking the temperature‐ and magnetic‐field‐dependent spin‐phonon scattering into account. Realization of low κ, which is easily tunable by magnetic fields in Cr 2 Si 2 Te 6 , may further promote spincaloritronic applications of vdW magnets. This study's theoretical approach may also provide a generic understanding of spin‐phonon scattering, which appears to play important roles in various systems.