We report a detailed high-field/high-frequency ferromagnetic resonance (HF-FMR) study of low-energy magnon excitations in the van der Waals ferromagnet ${\mathrm{Fe}}_{2.92\left(1\right)}{\mathrm{Ge}}_{1.02\left(3\right)}{\mathrm{Te}}_2$. At $2\mathrm{K}$, the field dependence of the magnon branches is well described by a semiclassical domain-based model, from which we extract key microscopic parameters including the anisotropy gap $\mathrm{\Delta }=170\left(4\right)\mathrm{GHz}$, the anisotropy field $B_{\mathrm{A}}=5.85\left(8\right)\mathrm{T}$, and the effective $g$-factor $g_{\mathrm{ab}}=g_{\mathrm{c}}=2.07\left(4\right)$. Furthermore the uniaxial anisotropy constant is determined to be $K=10.50\left(23\right)\times {10}^6\text{erg}/{\mathrm{cm}}^3$. Anisotropic short-range magnetic order persists above $T_{\mathrm{C}}$ up to approximately $270\mathrm{K}$, as evidenced by a finite anisotropy gap and anisotropic shifts in the ferromagnetic resonance (FMR) resonance fields. Both results clearly show the presence of anisotropic local magnetic fields well above $T_{\mathrm{C}}$. Our findings underscore the crucial role of magnetocrystalline anisotropy in driving long-range magnetic order in ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$.

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• Spin waves
• 2-dimensional systems
• Ferromagnets
• Electron spin resonance
• Ferromagnetic resonance