Strain-Induced Enhancement of Thermal Conductivity of Monolayer NiI2 via Suppressed Spin-Phonon Coupling.

Strain engineering provides a versatile route for continuously tuning thermal conductivity. In this work, using spin-polarized density functional theory in conjunction with the phonon Boltzmann transport equation, we reveal that a small biaxial tensile strain can significantly increase the thermal conductivity of the planar 2D ferromagnetic NiI2 through suppression of spin-phonon coupling, identifying a distinct magnetic mechanism for strain-controlled thermal transport. At 21 K, only 0.4% biaxial tensile strain enhances the thermal conductivity from 0.61 to 3.71 W/(mK). This improvement originates from a substantial increase in phonon relaxation time driven by symmetric lattice expansion, along with a marked suppression of spin-phonon coupling. Temperature-resolved scattering calculations show that, within the investigated range of 5-21 K, the temperature dependence of three-phonon (3ph) scattering is more pronounced than that of the four-phonon (4ph) contribution, while 3ph scattering remains dominant in absolute magnitude. These results establish spin-phonon coupling as an additional degree of freedom for regulating heat transport in planar 2D magnets.