Thermal and magnetoelastic properties of the van der Waals ferromagnet Fe3-GeTe2: Anisotropic spontaneous magnetostriction and ferromagnetic magnon excitations

By determining the lattice parameters as a function of temperature of the hexagonal van der Waals ferromagnet Fe2.92(1)Ge1.02(3)Te2 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(T), but less well pronounced perpendicular to the planes along c. Below TC the spontaneous magnetostriction follows a power law M(T )3/2 of the magnetization and leads to an expansion of the hexagonal layers. Extrapolating to T -> 0 K we obtain a spontaneous in -plane saturation magnetostriction of lambda sp,a(T -> 0) approximate to -220 x 10-6. In the paramagnetic state the linear thermal expansion coefficients amount to 13.9(1) x 10-6 K-1 and to 23.2(2) x 10-6 K-1 for the in -plane and out-of-plane direction, respectively, indicating a linear volume thermal expansion coefficient of 50.8(4) x 10-6 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.