Orbital currents — flows of orbital angular momentum through materials — are predicted to be orders of magnitude larger than spin currents. In theory, this should make orbital effects dominant in next-generation electronics. In practice, there has been a bottleneck: most magnets are spin-dominated. Orbital currents had to be converted to spin currents before they could interact with the magnetization, and the conversion wastes most of the advantage.
Schmitt et al. bypassed the conversion by using cobalt oxide, an antiferromagnet where the magnetic moments are dominated by orbital angular momentum rather than spin. When orbital currents from surface-oxidized copper interact with CoO's orbital magnetization, the coupling is direct: orbital to orbital, no spin intermediary. The result is a fifty-fold enhancement in orbital Hall magnetoresistance compared to conventional spin-dominated interfaces, with a sign reversal indicating a fundamentally different scattering mechanism.
The sign reversal is the diagnostic. In conventional CoO/Pt interfaces, the magnetoresistance has one sign because orbital currents are converted to spin before coupling. In the CoO/Cu* interface, the sign flips because the orbital current couples directly to the orbital magnetization — the scattering physics is different at the interface level. Same magnet, different transport channel, opposite resistance response.
The broader point is architectural. Spintronics spent decades optimizing spin-based coupling because the magnets available were spin-dominated. Orbital currents were always larger, but they couldn't be used directly. The solution wasn't to improve the conversion — it was to change the magnet. Use an orbital-dominated material, and the larger currents couple efficiently without the lossy spin intermediary.
The bottleneck was never in the current. It was in the receiver. Match the carrier to the listener, and the signal that was always there gets through.