Room Temperature Ferromagnetism in Metal-Rich, Large-Area Fe_3+xGeTe₂ Films Synthesized by van der Waals Epitaxy on Graphene

Ferromagnetic two-dimensional materials such as Fe₃GeTe₂ have attracted great interest due to their potential applications in ultra-compact spintronic devices. Recently, we have demonstrated the synthesis of large-area Fe₃GeTe₂/graphene heterostructures with high structural and interface quality by employing molecular beam epitaxy (MBE). In order to increase the Curie temperature from about 220 K towards room temperature, one promising approach reported in the literature is the synthesis of metal-rich Fe_3+<i>x</i>GeTe₂ (FGT) films. However, in these studies, micrometer-sized flakes exfoliated from bulk crystals have been studied. In this work, we report on electrical and magnetic properties of large-area FGT films with different chemical compositions.<br/>The van der Waals epitaxy of FGT films with 10 to 20 nm thickness was accomplished via MBE at a substrate temperature around 300 °C. Epitaxial graphene on 4H-SiC (0001) was used as a substrate. Morphological and structural characterization performed with different methods including synchrotron-based, grazing-incidence X-ray diffraction confirmed the formation of continuous FGT/graphene heterostructure films with stable interfaces and good crystalline quality.<br/>From temperature-dependent resistivity measurements, metallic transport behavior with positive temperature coefficient has been revealed for all FGT films with various chemical compositions. The magnetic characteristics of the films was investigated by the analysis of magnetotransport measurements, for which a large-area van der Pauw geometry was used. At low temperatures, the transverse resistance measured during subsequent downward and upward sweeps of an out-of-plane external magnetic field exhibits a clear square-shape hysteresis loop for all samples. This observation provides evidence for the occurrence of the anomalous Hall effect (AHE) and, consequently, the ferromagnetic order in the FGT films with a strong perpendicular anisotropy. As expected, the AHE of each individual film vanishes with increasing temperature. At the same time, the coercive field (<i>H</i>_C) extracted from the hysteresis loops decreases with increasing temperature, which can be explained by a uniform, long-range ferromagnetic order with uniaxial anisotropy described by a single magnetic domain. Most importantly, the Curie temperature of the FGT films extracted from the temperature dependence of <i>H</i>_C is found to increase with increasing Fe content. In fact, the FGT films with the highest Fe content remains ferromagnetic above room temperature. Information on the dominant scattering mechanism responsible for the AHE is gained from the relationship between the AHE conductivity (<i>σ_xy</i>) and the longitudinal conductivity (σ<i>_xx</i>). At low temperatures, σ<i>_xy</i> does not depend on σ<i>_xx</i>, which indicates that the AHE is dominated by an intrinsic mechanism, whereas extrinsic mechanisms are found to become more important at elevated temperatures. However, from the viewpoint of spintronic applications, the metallic transport behavior, the robust perpendicular magnetic anisotropy, and the room temperature ferromagnetism make metal-rich, large-area FGT films grown by MBE extremely promising for device fabrication.