We investigate the temperature-dependent magnetic phase evolution in the van der Waals layered magnet ${\mathrm{Fe}}_{4.5}{\mathrm{GeTe}}_2$ using static magnetometry and broadband ferromagnetic resonance (FMR) spectroscopy. Below the Curie temperature ($T_{\mathrm{C}}\approx 270\mathrm{K}$), ${\mathrm{Fe}}_{4.5}{\mathrm{GeTe}}_2$ exhibits a sophisticated sequence of magnetic transitions. Static magnetization measurements identify anomalies at ∼180 and ∼100 K, indicating a departure from a simple collinear ferromagnetic ground state. Temperature-dependent FMR spectra reveal a crossover from a dominant single resonance to two dynamically distinct in-plane branches below approximately 180 K. The prominent high-field branch exhibits Kittel-like frequency-field behavior and is consistent with the approximately uniform precession of the field-polarized net magnetization, whereas the additional low-field branch may be associated with canted or spatially nonuniform magnetic components. Below approximately 100 K, the spectra become strongly broadened and multimode, indicating an increasingly nonuniform magnetic response without allowing a unique microscopic mode assignment. The temperature-driven anisotropy crossover is quantitatively tracked through the evolution of the magnetocrystalline anisotropy field, effective magnetization, and $g$-factor anisotropy. The anisotropic $g$-factor and its deviation from the free-electron value are consistent with an appreciable orbital and spin-orbit-coupling contribution associated with the reconstructed magnetic response. Our findings establish $\mathrm{F}{\mathrm{e}}_{4+\delta }\mathrm{GeT}{\mathrm{e}}_2$ as a platform for exploring anisotropy-controlled spin dynamics and nontrivial magnetic phases in high-$T_{\mathrm{C}}$ two-dimensional magnets.

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• Magnetic anisotropy
• Magnetic interactions
• Magnetometry
• Spin reorientation transition
• van der Waals systems
• Ferromagnetic resonance
• Magnetization measurements