Spin–lattice coupling and anisotropic magneto-transport in Fe-ordered Fe5−xGeTe2

Two-dimensional van der Waals (vdW) magnetic materials have attracted extensive attention for the development of next-generation spintronic devices. The vdW magnet Fe5−xGeTe2 exhibits reversible spin structure and distinct electronic band structures tuned by thermal process with the Curie temperature (TC) above room temperature. By using a quench process, Fe5−xGeTe2 shows Fe(1) sites ordered in the crystal structure and nontrivial flatbands in electronic structures. However, detailed experimental investigations of physical properties in the Fe-ordered phase remain limited. In this work, we synthesize the Fe-ordered Fe5−xGeTe2 crystals and systematically investigate the physical properties by the combination of multiple measurements. The enhanced electronic specific heat coefficient (γ = 70.43 mJ mol−1 K−2) and Kadowaki–Woods ratio (A/γ2≈ 2.6 μΩ cm mol2 K2 J−2) provide evidence for the existence of flatbands and strong electron correlations. Furthermore, strong spin–lattice coupling is observed, where magnetic phase transitions coincide with lattice vibrations and Raman spectral shifts. In addition, anisotropic magneto-transport properties reveal the coupling between Fe(1) magnetic moments and charge carriers in the ordered phase. Our results provide further experimental evidence of the flatbands in the Fe-ordered phase as well as offer a strategy for designing tunable spintronic devices by exploiting the synergy between spin and lattice dynamics in vdW heterostructures.