Orbital Current Pumping From Ultrafast Light-driven Antiferromagnetic Insulator.

The orbital Hall effect originating from light materials with weak spin-orbit coupling, has attracted considerable interest in spintronic applications. Recent studies demonstrate that orbital currents can be generated from charge currents through the orbital Hall effect in ferromagnetic materials. However, the generation of orbital currents in antiferromagnets has so far been elusive. In this work, this is experimentally observed that the generation of orbital currents from orbital dynamics in the antiferromagnetic insulator α-FeO via terahertz (THz) emission spectroscopy, a phenomenon known as orbital pumping. A significant increase in THz signal is obtained in α-FeO/Pt/CuO heterostructure compared to that of α-FeO/Pt, with the maximum value occurring at a Pt thickness of 2 nm. The enhancement of the THz signal is attributed to the fact that magnons injected into Pt excite a coupled spin-orbital current that flows toward the Pt/CuO interface, aside from the spin-to-charge conversion in the Pt layer. The magnetoresistance contains the conventional spin-Hall magnetoresistance contributed by the Pt layer and an additional orbital contribution from the Pt/CuO interface. The Pt/CuO interface generates an orbital current and absorbs the orbital accumulation, similar to the orbital-Hall magnetoresistance. This finding provides a rich platform for orbital-to-charge conversion and opens an interdisciplinary field of antiferromagnetic orbitronics.