Microthermoreflectance Characterization of the Band-Structure Transformations Observed During the Magnetic-Ordering...

2D layered ferromagnetic (FM) materials hold significant promise for various applications owing to their hysteretic behavior and the ultrathin magnetic-ordering control of spin electrons below the Curie temperature (T_C). The spin-polarized order in the FM zone (e.g., T ≤ T_C = 225 K for Fe₃GeTe₂), paramagnetic (PM) zone (e.g., T > T_C), and low-temperature anisotropic (AI) zone (e.g., T < 75 K < T_C for Fe₃GeTe₂) are typically characterized by observing changes in the magnetization and magnetoresistance under the application of an external magnetic (electric) field. However, observing the near-band-edge transitions across the FM, PM, and AI regions of a layered magnetic metal is challenging utilizing optical spectroscopy due to its high surface carrier density for shielding incident lights. In this study, three interband transitions, designated as A1, A2, and A3, are measured and identified through temperature-dependent microthermoreflectance (μTR) measurements of multilayered Fe₃GeTe₂ metal across a temperature range of 20-300 K, enabling the distinction of the FM, PM, and AI zones using optical methods. The A1 transition (≈1.73 eV) is detected across the entire temperature range of 20-300 K, whereas the A2 transition (≈1.95 eV) appears only in the AI zone below 75 K, and the A3 transition (≈2.23 eV) is observed only in the FM phase below T_C (≈225 K). Density functional theory calculations suggest that all transitions (A1-A3) originate from the down-spin-polarized band of Fe 3d electrons. Moreover, temperature-dependent X-ray diffraction, Raman, magnetization, and magneto-resistivity measurements are performed to confirm the T_C and AI temperatures observed for multilayered FGT by μTR.