Complex temperature-dependent thermal conductivity in the sawtooth chain magnet Fe$_2$SiSe$_4$

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 Fe$_2$⁢SiSe$_4$, which exhibits successive magnetic transitions at 𝑇$_1$=110K (antiferromagnetic) and 𝑇$_2$=50K (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 ∼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.