By determining the lattice parameters as a function of temperature of the hexagonal van der Waals ferromagnet ${\mathrm{Fe}}_{2.92\left(1\right)}{\mathrm{Ge}}_{1.02\left(3\right)}{\mathrm{Te}}_2$ we obtain the temperature dependence of the spontaneous in-plane magnetostriction in the ferromagnetic and the linear thermal expansion coefficients in the paramagnetic state. The spontaneous magnetostriction is clearly seen in the temperature dependence of the in-plane lattice parameter $a\left(T\right)$, but less well pronounced perpendicular to the planes along $c$. Below $T_{\mathrm{C}}$ the spontaneous magnetostriction follows a power law $M{\left(T\right)}^{3/2}$ of the magnetization and leads to an expansion of the hexagonal layers. Extrapolating to $T\to$ 0 K we obtain a spontaneous in-plane saturation magnetostriction of ${\lambda }_{\mathrm{sp},a}(T\to 0)\approx -220\times {10}^{-6}$. In the paramagnetic state the linear thermal expansion coefficients amount to $13.9\left(1\right)\times {10}^{-6}{\mathrm{K}}^{-1}$ and to $23.2\left(2\right)\times {10}^{-6}{\mathrm{K}}^{-1}$ for the in-plane and out-of-plane direction, respectively, indicating a linear volume thermal expansion coefficient of $50.8\left(4\right)\times {10}^{-6}{\mathrm{K}}^{-1}$ which we use to estimate the volume thermal expansion contribution to the heat capacity determined at constant pressure. A Sommerfeld-type linear term in the low-temperature heat capacities can be quantitatively ascribed to two-dimensional (2D) ferromagnetic magnon excitations.

• Received 23 May 2023
• Revised 3 October 2023
• Accepted 22 January 2024

DOI:https://doi.org/10.1103/PhysRevMaterials.8.024002

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Published by the American Physical Society