Thickness-dependent Transport and Magnetic Properties of All-epitaxial Fe₅GeTe₂/WSe₂ Van der Waals Heterostructures

The discovery of two-dimensional (2D) magnetic materials has opened a new avenue for the design of future spintronic devices. Furthermore, building van der Waals (vdW) heterostructures by combining 2D magnetic and non-magnetic materials holds great promise for tuning their magnetic and transport properties via proximity induced interfacial interactions. For their integration into established device fabrication schemes, it is crucial to develop a bottom-up scalable synthesis beyond the exfoliation-based methods commonly used for state-of-the-art vdW heterostructures. Among other 2D materials, the ferromagnetic metal Fe-Ge-Te compounds and the non-magnetic semiconductor WSe₂ have been considered as excellent material candidates due to their high Curie temperature and large spin-orbit coupling, respectively. Furthermore, the investigation and tuning of their magnetic and transport properties is of great importance for the design of spintronic devices. Here, we report the thickness dependent magnetic and transport properties of all-epitaxial Fe_5-xGeTe₂ (FGT, where x = 0.2) /WSe₂ vdW heterostructures.
The FGT films with thicknesses of 5, 12 and 17 nm were grown by molecular beam epitaxy (MBE) at a substrate temperature around 300 °C on the monolayer-bilayer thick WSe₂, which was also epitaxially grown on Al₂O₃(0001) substrate. Structural characterizations performed by different methods, including synchrotron-based grazing-incidence X-ray diffraction, confirm the high-quality formation of all-vdW FGT/WSe₂ heterostructures with their well-defined epitaxial relationship. Superconducting quantum interference device (SQUID) magnetic characterizations reveal a remarkable perpendicular magnetic anisotropy (PMA) with ferromagnetic order persisting up to room temperature, which is further confirmed by anomalous Hall effect (AHE) measurements. In the thinner FGT sample, a significantly higher coercivity and slower temperature-dependent decay of the AHE resistivity indicate an enhanced PMA. The transport measurements with in-plane magnetic field indicate a notable presence of higher order PMA and large spin-orbit coupling with high anisotropy magnetoresistance (AMR). In addition, the occurrence of an asymmetric longitudinal magnetoresistance in the 12-nm-thick FGT sample reveals a multi-step magnetization switching, which can be attributed to a thickness-dependent competition between vdW heterointerface interaction and FGT bulk properties. Further transport anomalies can be well explained by considering the possible formation of skyrmions accompanying with the topological Hall effect. In summary, our work realizes the all-epitaxial growth of high-quality FGT/WSe₂ vdW heterostructures with a high tunability of their magnetic and transport properties by adjusting the FGT thickness, which provides an ideal platform for the development of 2D spintronic devices.