Above-room-temperature Ferromagnetism in Wafer-scale Two-dimensional Fe_3+xGeTe₂ Films

Recently, 2D ferromagnetic materials have been discovered with tunable magnetism and nodal-line features. Controlling 2D magnetism in exfoliated nanoflakes via electric-field enables the boosted Curie temperature (T_C). One of the most intriguing challenges, however, is the realization of high Tc materials that are tunable, robust and compatible with the commercial-level manufacturability. Here, we report the above-room-temperature ferromagnetic order in wafer-scale 2D Fe_3+xGeTe₂ films through a non-equilibrium growth process in molecular beam epitaxy. The perpendicular magnetic anisotropy in Fe_3+xGeTe₂ is found to persist up to 320 K, significantly higher than the stoichiometric bulk counterpart (Fe₃GeTe_2, T_C~220 K). By controlling the atomic ratio, we found a largely-modulated Tc that depends on carrier density. Corroborated with DFT calculations, we demonstrated that the higher atomic ratio further stabilizes the FM ground state by yielding a larger energy difference of E_AFM-E_FM, therefore enabling the enhanced ferromagnetic order in this system. Our results show an effective approach, i.e., the element doping, to produce a robust ferromagnetic order beyond room temperature in wafer-scale Fe_3+xGeTe₂ films, which may render practical applications for 2D spintronic devices.