CrSBr is a layered antiferromagnet — magnetic moments in adjacent layers point in opposite directions, canceling the net magnetization. The material's electronic structure is highly anisotropic: the Fermi surface is elongated along certain crystallographic directions and compressed along others. Electrons move easily in some directions and struggle in others. This anisotropy should show up in magnetotransport — how electrical resistance changes with applied magnetic field — but extracting a complete picture requires systematically rotating both the current direction and the field direction through all three crystal axes.
That systematic rotation reveals two distinct transport regimes (arXiv:2603.22157). When the magnetic field is perpendicular to the current and to the layers, the magnetoresistance directly probes the electronic anisotropy of the Fermi surface. The resistance changes dramatically when the current switches between crystallographic axes — not because the field changes but because different current directions sample different cross-sections of the Fermi surface. The resistance is a Fermi surface tomograph: each current orientation is a different projection angle.
When the magnetic field lies in the plane of the layers, the behavior shifts to conventional anisotropic magnetoresistance with hysteresis loops — the signature of ferromagnetic-like behavior. The antiferromagnet, under in-plane fields, shows the transport signatures of a ferromagnet. The in-plane field cants the sublattice magnetizations enough to create a net moment and the associated magnon and domain-wall scattering.
The structural insight: the same material exhibits two qualitatively different magnetotransport mechanisms depending on field orientation. Perpendicular fields reveal the electronic structure (Fermi surface geometry). In-plane fields reveal the magnetic structure (sublattice canting and domain dynamics). The resistance is sensitive to both, but they occupy different regions of the orientation space. The measurement direction doesn't just change the magnitude of the signal — it changes what the signal measures.