Significant enhancement of ferromagnetism above room temperature in epitaxial 2D van der Waals ferromagnet Fe5-δGeTe2/Bi2Te3 heterostructures

Two-dimensional (2D) van der Waals (vdW) ferromagnetic metals Fe_xGeTe₂ with x = 3-5 have raised significant interest in the scientific community. Fe₅GeTe₂ shows prospects for spintronic applications since the Curie temperature T_c has been reported near or higher than 300 K. In the present work, epitaxial Fe_5-δGeTe₂ (FGT) heterostructures were grown by Molecular Beam Epitaxy (MBE) on insulating crystalline substrates. The FGT films were combined with Bi₂Te₃ topological insulator (TI) aiming to investigate the possible beneficial effect of the TI on the magnetic properties of FGT. FGT/Bi₂Te₃ films were compared to FGT capped only with AlO_x to prevent oxidation. SQUID and MOKE measurements revealed that the growth of Bi₂Te₃ TI on FGT films significantly enhances the saturation magnetization of FGT as well as the T_c well above room temperature (RT) reaching record values of 570 K. First-principles calculations predict a shift of the Fermi level and an associated enhancement of the majority spin (primarily) as well as the total density of states at the Fermi level suggesting that effective doping of FGT from Bi₂Te₃ could explain the enhancement of ferromagnetism in FGT. It is also predicted that strain induced stabilization of a high magnetic moment phase in FGT/Bi₂Te₃ could be an alternative explanation of magnetization and T_c enhancement. Ferromagnetic resonance measurements evidence an enhanced broadening in the FGT/Bi₂Te₃ heterostructure when compared to FGT. We obtain a large spin mixing conductance of g↑↓eff = 4.4 × 10²⁰ m^-2, which demonstrates the great potential of FGT/Bi₂Te₃ systems for spin-charge conversion applications at room temperature.