Thermal history-dependent characteristics in van der Waals ferromagnet Fe5−xGeTe2 (x ∼ 0.16)

We investigated the thermal-history dependence of physical properties in a quenched Fe5−xGeTe2 (x ∼ 0.16) single crystal by measuring magnetization (M), electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) as a function of temperature (T). The results reveal anomalies in these physical quantities around various transition points: ferromagnetic (TC ∼ 310–300 K), helimagnetic (TH ∼ 275 K), charge ordering (TCO ∼ 165 K), spin-reorientation (T* ∼ 100–120 K), and a Fermi-liquid (FL) phase below TL ∼ 35 K. Using power-law fitting, the M(T) analysis near TC shows that Fe moments become primarily itinerant after thermal cycling. The ρ(T) results indicate inherent residual stresses in the crystal that alter with thermal cycling, influencing ferromagnetic domain formations within grain boundaries. The system exhibits a strongly correlated FL behavior at low temperatures, which ceases above TL due to spin fluctuations. In the T-range of T* ≤ T ≤ TCO, ρ(T) and S(T) analyses suggest significant electronic band structure modifications with multiband effects. The κ(T) data indicate phonon-dominated heat transport in the crystal, with a phonon behavior influenced by inherent lattice strains following initial thermal cycles, as evidenced by the decreased phonon peak height at low temperatures. In addition, there is evidence of phonon localization and electron–phonon coupling at higher temperatures.