Generation, transmission, and conversion of orbital torque by an antiferromagnetic insulator.

Electrical control of magnetization in nanoscale devices can be significantly improved through the efficient generation of orbital currents and their conversion into spin currents. In nonmagnetic/ferromagnetic bilayers, this conversion produces a torque on the magnetization, enabling magnetization switching and dynamic manipulation. While previous studies focus on metallic ferromagnets, we demonstrate a large orbital torque and enhanced orbital-to-spin conversion by an antiferromagnetic insulating CoO layer. Measurements in CuOx/CoO/Co trilayers show that inserting CoO reverses the torque's sign and triples its magnitude compared to CuOx/Co. This behaviour stems from the inverted oxygen gradient at the CuOx/CoO interface and CoO's high orbital multiplicity, which favours the transmission of orbital momenta and efficient orbital-to-spin conversion. At low temperatures, the onset of antiferromagnetic order induces a further many-fold increase of the torque, which we attribute to the efficient excitation and propagation of spin-orbit excitons induced by magnetic coupling. Comparative measurements of CuOx/NiO/Co and CuOx/MnO/Co trilayers show that the torque efficiency scales with the orbital momentum of the Co2+, Ni2+, and Mn2+ ions in the antiferromagnet. These results reveal that antiferromagnetic insulators like CoO provide highly effective orbital-to-spin transduction, combining orbital torque and exchange bias functionalities to improve the performance of spintronic devices.