Ultrafast thermal switches are essential for thermal management in next-generation solid-state technologies, including high-speed chipsets, thermal logic circuits, and on-chip lasers. However, conventional thermal switches fail to satisfy the stringent requirements for picosecond-scale response times, whereas existing laser-triggered thermal switches that rely on conformational rearrangements suffer from inherent limitations in reliability and operational lifetime. To overcome these challenges, we propose a laser-driven ultrafast thermal switching in antiferromagnetic monolayer MnSe by using real-time time-dependent density functional theory and the Boltzmann transport equation. This work reports the timescale of complete laser-induced demagnetization. Specifically, a laser pulse with a fluence of $5.1\mathrm{mJ}/{\mathrm{cm}}^2$ triggers an ultrafast demagnetization between the two Mn sublattices in monolayer MnSe via the interplay of optical intersite spin transfer and phonons, resulting in a transient nonmagnetic state within approximately 80 fs. The transient magnetic transition from antiferromagnetism to nonmagnetism drastically suppresses thermal conductivity from 8.33 to 0.64 W/(m K), corresponding to a 13-fold reduction. This modulation primarily arises from demagnetization-enhanced phonon scattering, which is dominated by four-phonon recombination and redistribution processes involving the in-plane acoustic phonon branches. These findings establish two-dimensional magnetic materials as a viable platform for ultrafast thermal switching.

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• Thermal conductivity
• Ultrafast demagnetization
• Monolayer films
• Density functional calculations