Geometrically frustrated magnets provide an ideal platform for exploring the interplay between lattice geometry and spin degrees of freedom. Here, we investigate the interactions between lattice and spin via thermal-transport measurements on the triangular sawtooth-lattice olivine magnet ${\mathrm{Fe}}_2{\mathrm{SiSe}}_4$, which exhibits successive magnetic transitions at $T_1=110\mathrm{K}$ (antiferromagnetic) and $T_2=50\mathrm{K}$ (ferrimagnetic). Although phonons dominate the thermal conductivity, its temperature dependence displays a pronounced double-peak structure arising from spin–phonon coupling. In the intermediate temperature range between $T_1$ and $T_2$, resonant scattering of phonons by magnetic excitations around 5 meV produces a broad maximum around 60 K. Below $T_2$, the resonant spin–phonon scattering is strongly suppressed, leading to a rapid increase in thermal conductivity upon cooling and a pronounced low-temperature peak near 11 K, characteristic of heat transport governed by conventional phonon scattering mechanisms. Notably, this low-temperature peak is enhanced by a factor of $\sim 5$ compared to the broad maximum at higher temperatures. These results demonstrate the strong sensitivity of thermal transport to spin–lattice interactions and highlight spin-phonon scattering as an effective mechanism for tailoring thermal conductivity in geometrically frustrated magnets.

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• Frustrated magnetism
• Magnetism
• Thermal conductivity
• Thermal expansion
• Thermal transport
• Antiferromagnets