A near-monochrome collage of seven laboratory photographs cut into diagonal panels: a cryostat and vacuum chamber trailing cabling and steel flanges; an optical table with posts, a glowing lamp and a circuit board; a bright ring of light seen through a dark chamber viewport; a chamber interior with a finned copper disc, translation stages and stepper motors; a rack of long cryostat probe inserts with stacked baffles; a vacuum chamber wrapped in aluminium foil with pneumatic valves; and a chamber with a brilliant glowing plume beneath a circular electrode.

Surface oxidation as the dominant source of exchange bias

The short version

Fe3GeTe2, one of the magnetic crystals in this stack, grows a thin rust layer within half an hour of air exposure. Rather than treating that layer as damage, we measured what it does, and found it takes over the magnetic coupling between the two crystals and can reverse its direction. It is a reminder that at these thicknesses an unintended interface layer is not a defect at the edge of the device, it is the device.

Fe3GeTe2 (FGT) oxidises within half an hour of ambient exposure. In this study we let it, then measured what the resulting native oxide does to the magnetic coupling across an FGT/CrPS4 stack.

The oxide is itself magnetically ordered. Cross-sectional electron microscopy with elemental spectroscopy (STEM-EELS) and X-ray photoelectron spectroscopy resolve it into two sublayers, a thick ferrimagnetic Fe3O4 film over a thin antiferromagnetic FeO one, and each pins the ferromagnet across its own temperature range. The exchange bias is non-monotonic in temperature, with minima near 20 K and 70 K, and survives to a blocking temperature of about 140 K. A pristine, hBN-capped control device shows bias only below 36 K, at −12.5 mT. A ±1 T preset field applied at fixed temperature then sets the sign of the bias, with no field cooling required.

For device integration this makes surface oxidation a process-control parameter: in this stack the oxide sets both the magnitude and the sign of the coupling. The layer-by-layer assignment rests on a three-region spin model rather than on direct depth-resolved magnetometry, and the whole effect is cryogenic.

Three panels. Left: an optical micrograph of the device, a red and green flake stack sitting across eight yellow gold contacts on a dark blue substrate, with arrows labelling the FGT/O-FGT and CrPS4 regions. Centre: a greyscale cross-sectional electron micrograph with a 5 nm scale bar, showing three stacked bands labelled thick O-FGT, thin O-FGT and FGT. Right: a schematic plot of exchange bias against temperature divided into three shaded regions, with the blocking temperatures of CrPS4, FeO and Fe3O4 marked in sequence.

The device, the two-sublayer oxide seen in cross-section, and the three temperature regimes it produces: each magnetic phase in the stack stops contributing at its own blocking temperature, which is what makes the bias non-monotonic.

Shared first authorship with Aravind Puthirath Balan. Published in ACS Nano 18, 8383–8391 (2024), 10.1021/acsnano.3c13034. Analysis code: EB-in-FGT-OFGT-CrPS4-vdW-heterostructure.