CrSBr is a van der Waals magnetic semiconductor. Below 140 K, its layers order antiferromagnetically — neighboring layers have opposite spin orientation, canceling the net magnetization. To make it ferromagnetic, you would conventionally apply a magnetic field strong enough to force the layers into alignment, or dope the material to change the exchange coupling. Both approaches fight the material's natural tendency.
Li and colleagues (arXiv:2602.20940) insert organic molecules between the layers instead. Tetramethylammonium and tetrapropylammonium ions, intercalated into the van der Waals gaps, transform the magnetic order from antiferromagnetic to ferromagnetic while raising the transition temperature from 140 K to 190 K (TMA) and 230 K (TPA). The larger molecule produces the larger effect.
The mechanism is structural: intercalation changes the interlayer spacing and modifies the orbital overlap pathways that mediate the exchange coupling. The sign of the interlayer exchange flips from antiferromagnetic to ferromagnetic. Raman spectroscopy and density functional calculations confirm that the lattice distortion changes the superexchange pathway rather than introducing new magnetic species. The organic molecules are not magnetic — they are spacers that reshape the geometry through which the magnetic layers communicate.
The result: air-stable ferromagnetism above 200 K with hysteretic magnetoresistance exceeding 60%. No applied field needed to establish the ferromagnetic state — the intercalation does it permanently.
The general observation: when the sign of an interaction depends on geometry, a passive structural modification can reverse the interaction without adding new active components. The molecules that flip the magnetism carry no magnetic moment. They change the conversation between layers by changing the distance and angle at which the conversation occurs.