All-Epitaxial Fe_5-xGeTe₂/Graphene and Fe_5-xGeTe₂/WSe₂van der Waals Heterostructures With Above Room Temperature Ferromagnetism

Van der Waals (vdW) heterostructures combining 2-D ferromagnets and other nonmagnetic layered materials, such as graphene and WSe2, are highly promising for the realization of novel spintronic devices with integrated magnetic, electronic, and optical functionalities. Among different 2-D ferromagnets, Fe5-xGeTe2 (x~sim ~0 ) shows a high potential due to its high Curie temperature (TC). Furthermore, compared to commonly used top-down flake stacking strategies, large-scale, all-epitaxial vdW heterostructures are compatible with modern technologies and thus crucial for practical applications. In this contribution, we report on scalable epitaxial growth of Fe5-xGeTe2 via molecular beam epitaxy (MBE) on single-crystalline graphene (on SiC) and WSe2 (on Al2O3) templates. Structural characterizations show the good crystalline quality of the epitaxial Fe5-xGeTe2 films on graphene and WSe2. Importantly, magnetotransport investigations indicate a ferromagnetic order above room temperature for both Fe5-xGeTe2/graphene with perpendicular magnetic anisotropy (PMA) and Fe5-xGeTe2/WSe2 (with low-temperature PMA) heterostructures. Moreover, an unconventional Hall effect (UHE) has been observed in both Fe5-xGeTe2/graphene and Fe5-xGeTe2/WSe2 heterostructures, indicating the formation of topological spin structures. These results represent an important advance regarding bottom-up synthesis of vdW heterostructures beyond conventional exfoliation-based methods, which is crucial for the development of future spintronic applications based on 2-D magnetic materials.