Proximity-tuned Magnetic and Transport Anomalies in All-epitaxial Fe5-xGeTe2/WSe2 Van der Waals Heterostructures

The gist The realization of all-epitaxial, high-quality Fe5−xGeTe2(FGT)/WSe2 heterostructures exhibiting perpendicular magnetic anisotropy (PMA) and room-temperature ferromagnetism provides a scalable platform for exploring emergent 2D physics and advancing next-generation 2D spintronic technologies.

Structural Properties

The FGT films were grown by Molecular beam epitaxy (MBE) with thicknesses of 6, 12, and 17 nm directly on single-crystalline, continuous monolayer–bilayer thick WSe2 synthesized by chemical vapor deposition (CVD) on Al2O3(0001)<ref:2512.13888#pg5>. The FGT growth process was continuously monitored via in-situ reflection-high-energy-electron-diffraction (RHEED), which confirmed the epitaxial growth of FGT films having a smooth 2D surface<ref:2512.13888#pg5>. Due to trigonal symmetry, the H and K axes form an angle of 60◦<ref:2512.13888#pg5>. Synchrotron-based grazing incidence diffraction (GID) was used to probe in-plane properties such as strain and crystallographic orientation of 2D layers<ref:2512.13888#pg5>. The GID measurements revealed a coincidence-site lattice matching between FGT and WSe2, with a resulting ratio 5aFGT ≈ 6aWSe2<ref:2512.13888#pg5>. Scanning transmission electron microscopy (STEM) was employed to investigate the structural properties of the layers<ref:2512.13888#pg5>. STEM analysis showed that the FGT film comprises not only Fe5GeTe2 layers but also with thicknesses ranging from 1 to 0.8 nm<ref:2512.13888#pg5>. The average thickness of the FGT single layers obtained from TEM was found to be 0.93 nm<ref:2512.13888#pg5>. A low growth temperature was intentionally chosen to mitigate the possible intermixing at the FGT/WSe2 interface<ref:2512.13888#pg5>.

Magnetic Properties

The magnetic properties of FGT/WSe2 vdW heterostructures were investigated using superconducting quantum interference device (SQUID) magnetometry<ref:2512.13888#pg7>. The OP hysteresis loop exhibits an unconventional shape with a kink near the zero field<ref:2512.13888#pg7>. The larger MR in the OP configuration indicates the persistence of PMA and ferromagnetic ordering up to room temperature<ref:2512.13888#pg7>. Both the OP M–T curve (black) and its derivative dM/dT (green) exhibit two clear magnetic transitions at temperatures T1 ≈ 110 K and T2 ≈ 220 K<ref:2512.13888#pg7>. The transition at T1 is associated with the Fe(1) sublattice, which remains magnetically ordered below 110 K but undergoes strong fluctuations in the intermediate temperature range<ref:2512.13888#pg7>. The ferromagnetism above T2 is primarily attributed to the long-range ferromagnetic order of Fe(3)<ref:2512.13888#pg7>.

Anomalous Hall Effect (AHE)

To further investigate the magnetic and transport behaviors, AHE measurements were performed due to their high sensitivity to magnetization in ultrathin films<ref:2512.13888#pg8>. At low temperatures, the square-shaped AHE loops originate from the FGT layer and confirm the presence of strong PMA<ref:2512.13888#pg8>. The absence of kink-like features in the AHE data indicates that the zero-coercivity component in the SQUID signal likely arises from the insulating surface oxide<ref:2512.13888#pg8>. At high temperature, the AHE loops become rounded with reduced coercivity, indicating a transition from hard to soft ferromagnetism<ref:2512.13888#pg8>. The TC of our FGT/WSe2 heterostructures lies slightly above room temperature, with negligible dependence on FGT thickness<ref:2512.13888#pg8>.

Sign Reversal of Exchange Bias Effect

An exchange bias effect is observed in our FGT/WSe2 heterostructures at low temperatures after cooling without additional field<ref:2512.13888#pg8>. The extracted exchange bias field (Hex) exhibits a sign reversal between negative and positive depending on both temperature and FGT thickness<ref:2512.13888#pg8>. This unusual exchange bias can be attributed to the interplay between two contributions: a negative Hex occurring at the O-FGT/FGT interface with an estimated TB1 ≈ 100 K, and a positive Hex emerging from the interaction between FGT sublayers with a TB = 160 K<ref:2512.13888#pg8>.

In-plane Magnetotransport

In this configuration, the magnetization vector continuously moves from the OP to IP direction as H∥ increases, settling at an angle (θ) determined by the free energy landscape<ref:2512.13888#pg10>. The analysis indicates that a higher-order PMA term (HK2) is essential to adequately capture the experimental response<ref:2512.13888#pg10>. The pronounced AMR observed in FGT can thus be primarily attributed to Fe(1), whose contribution dominates the spin-dependent scattering<ref:2512.13888#pg10>. The temperature dependence of AMR further corroborates this perspective<ref:2512.13888#pg10>.

Thickness-dependent Unconventional Hall Effect

A clear unconventional Hall effect (UHE) was observed during both forward and backward field sweeps, deviating from the conventional AHE baseline<ref:2512.13888#pg11>. The detection of UHE in transport measurements critically depends on the skyrmion density<ref:2512.13888#pg11>. This consistency further supports the correlation between the UHE and skyrmion formation in our samples<ref:2512.13888#pg11>. The non-monotonic PMA-thickness dependence can be attributed to a strong interfacial proximity effect at the FGT/WSe2 interface<ref:2512.13888#pg8>.

Proximity-induced Interfacial Contributions

The non-monotonic PMA profile, arising from the competition between bulk and interfacial contributions, is a distinctive feature of vdW heterostructures<ref:2512.13888#pg8>. The pronounced 1/tFGT term reveals a strong interface-driven enhancement of PMA<ref:2512.13888#pg8>. This interpretation is further supported by the consistently higher HC in FGT/WSe2 compared to FGT/graphene, despite similar crystalline quality<ref:2512.13888#pg8>. The non-monotonic PMA profile, arising from the competition between bulk and interfacial contributions, is a distinctive feature of vdW heterostructures<ref:2512.13888#pg8>. This tunability establishes FGT/WSe2 as a promising platform for studying proximity-driven physics<ref:2512.13888#pg8>.

Conclusion

These heterostructures establish a robust and versatile platform for both fundamental studies and nextgeneration 2D spintronic technologies, including vdW magnetoresistive devices, ultrahigh-density racetrack memories and proximity-engineered magnetic functionalities<ref:2512.13888#pg14>. The reversed thickness dependence of the UHE underscores the interplay between PMA and interfacial coupling, suggesting the possible formation of skyrmions<ref:2512.13888#pg8>. Together with the non-monotonic PMA-thickness dependence, these magnetic and transport anomalies can be attributed to a strong interfacial proximity effect at the FGT/WSe2 interface enabled by the high epitaxial quality and the strong SOC of WSe2<ref:2512.13888#pg8>. The heterostructures establish a robust and versatile platform for both fundamental studies and nextgeneration 2D spintronic technologies, including vdW magnetoresistive devices, ultrahigh-density racetrack memories and proximity-engineered magnetic functionalities<ref:2512.13888#pg14>.

Experimental Section

FGT was grown by MBE using elemental Fe, Ge, and Te evaporated from Knudsen cells<ref:2512.13888#pg15>. The FGT films were capped in-situ after their growth with a Te layer (5 nm) deposited after sample cooling to room temperature<ref:2512.

1. Improved AI systems can accurately predict unconventional Hall effect (UHE) signatures by incorporating models that account for topological spin textures and their dependence on layer thickness, as seen in Figure 6b.

2. AI-driven materials discovery tools can be trained to correlate the non-monotonic PMA–thickness dependence and the reversed thickness trend in the unconventional Hall effect (UHE) with specific interfacial contributions, such as the SOC from WSe2 underlayer, which is described as a signature of a proximity effect.

3. Advanced simulation models can be developed to predict the sign reversal of exchange bias by incorporating the competition between bulk and interfacial PMA contributions, allowing for better prediction of the observed regimes: (i) a thickness-dependent Hex below 80 K with a positive temperature coefficient, (ii) a thickness-independent positive Hex between 80 and 160 K with a negative temperature coefficient.

4. AI systems can optimize the design of next-generation spintronic devices by predicting the higher-order PMA contribution and unusually large AMR effect in transport measurements, which are linked to sublattice-specific contributions to magnetic and transport properties.

5. AI models can be used for real-time characterization guidance, such as interpreting SQUID data to distinguish between the magnetic transitions associated with different sublattices, specifically identifying the T1 temperature (110K) determined from ρmaxA agrees well with that of bulk materials[51].

Van der Waals (vdW) heterostructures combining two-dimensional (2D) ferromagnets and semiconducting transition-metal dichalcogenides (TMDCs) offer highly promising opportunities for tailoring 2D magnetism through interfacial proximity effects, enabling unique physical phenomena inaccessible in 3D systems and achieving functionalities beyond conventional spintronics. However, current fabrication of vdW heterostructures still relies heavily on the manual stacking of exfoliated 2D flakes, leading to critical challenges in scalability, interfacial quality, thickness control and device integration. This work reports on the realization of all-epitaxial, high-quality Fe5-xGeTe2(FGT)/WSe2 heterostructures exhibiting perpendicular magnetic anisotropy (PMA) and room-temperature ferromagnetism. The FGT/WSe2 system demonstrates temperature-driven magnetic transitions, higher-order PMA contributions and large anisotropic magnetoresistance, highlighting sublattice-specific contributions to magnetic and transport properties. Notably, the FGT/WSe2 heterostructures display unconventional physical phenomena, including thickness- and temperature-dependent sign reversal of exchange bias, a reversed thickness trend in the unconventional Hall effect, and a non-monotonic PMA-thickness dependence. These anomalies indicate pronounced interfacial contributions arising from proximity effects enhanced by epitaxial interface quality. Collectively, this study provides deep insights into the magnetic and transport properties of FGT/WSe2 vdW heterostructures, establishing a scalable platform for exploring emergent 2D physics and advancing next-generation 2D spintronic technologies.