Hall voltage against out-of-plane field from minus 400 to plus 400 millitesla on a dark ground: a blue hysteresis loop recorded after a plus 2.5 tesla preset is shifted towards negative field, and a red loop recorded after a minus 2.5 tesla preset is shifted towards positive field, arrows marking each shift away from the dashed zero-field line.

Research · spintronics

Exchange bias between magnets pointing at right angles, and the domain structure behind it

Two magnets pointing at right angles that pin each other anyway, and a pinning direction set by a field pulse rather than by cooling.

The short version

Exchange bias, one magnet pinning another across an interface, is normally described between two magnets that share an axis, and setting its direction normally means cooling the device through an ordering transition in a field. This work paired two magnets whose preferred directions are perpendicular, where that picture says neither can pin the other.

CrSBr is an antiferromagnet with in-plane spins; Fe3GeTe2 (FGT) is a ferromagnet with out-of-plane spins. They couple regardless.

Four panels. Top left: an optical micrograph of the device with coloured outlines labelling the FGT, CrSBr and h-BN flakes over blue and gold contacts. Top right: two Hall-voltage hysteresis loops against out-of-plane field between minus 400 and 400 millitesla, a blue loop shifted to negative field after a plus 2.5 tesla preset and a red loop shifted to positive field after a minus 2.5 tesla preset. Bottom left: a greyscale cross-sectional electron micrograph with a 20 nm scale bar showing a platinum capping layer over a CrSBr layer over an FGT layer. Bottom right: a colour-coded magnetic induction map of the same cross-section with a 100 nm scale bar, the CrSBr, FGT and SiO2 layers marked by dashed yellow lines; within the FGT band a row of concentric closed contour loops marks circulating flux-closure domains. Opposite presets shift the loop in opposite directions, and the closed contours inside the FGT are the flux-closure domains that carry the coupling. Adapted from Small 21, e06284 (2025), © Wiley-VCH GmbH.

How

  • Set the pinning with a ±2.5 T out-of-plane preset at the measurement temperature. Hall-voltage hysteresis loops show a bias of about ±47 mT at 10 K whose sign follows the preset, surviving up to the 132 K Néel temperature of CrSBr.
  • Imaged the coupling by cross-sectional off-axis electron holography: CrSBr drives stripe-like flux-closure domains in FGT, with Bloch walls through the centre and Néel-type closure caps at the surfaces.
  • Reproduced the asymmetric reversal by domain nucleation with a five-state model built from interfacial exchange, the two competing anisotropies and dipolar fields.
  • Mapped where it stops working: a strong training effect, and no out-of-plane bias in the 9–12 nm FGT regime.

Sole first author. Published in Small 21, e06284 (2025), 10.1002/smll.202506284. The analysis code is public on GitHub as EB-in-FGT_CrSBr-vdW-heterostructure.