Tuning ferromagnetism in 2D magnet/topological insulator heterostructures across room temperature by epitaxial growth*

2D van der Waals ferromagnet is Fe₃GeTe₂ (FGT) has a relatively high Curie temperature (T_C) with strong perpendicular magnetic anisotropy (PMA). Also, observation of novel phenomena such as spin-orbit torque switching and skyrmion spin textures were reported in this material, which makes it an excellent 2D magnetic candidate to be integrated with topological insulators (TIs) for spin-orbit torque studies. Here, we study the optimum growth conditions for the FGT/Bi₂Te₃ system using molecular beam epitaxy, through which we observed different magnetic properties for the heterostructures of different growth conditions. With different growth temperature and Fe:Ge atomic ratio, we are able to controllably achieve above room temperature ferromagnetism. Cross-sectional scanning transmission electron microscopy (STEM) was used to characterize our high-quality epitaxial van der Waals film. Optical magnetic circular dichroism (MCD) enables us to observe and compare the shape of the hysteresis loops of FGT/Bi₂Te₃ samples grown with different growth conditions and determine their Curie temperature. This in turn gives us more insights into the origin of the elevated Curie temperature in FGT/Bi₂Te₃ heterostructure, leading to a more systematic study of synthesis and magnetic properties of this all-vdW 2D magnet/TI heterostructure. In addition, we looked into possibilities of integration of other materials belonging to FGT family (Fe_mGe_nTe₂) with Bi₂Te₃.