Observation of orbit-orbit torques: Highly efficient torques on orbital moments induced by orbital currents

We study the current-induced torques in bilayers composed of a light 3d metal, chromium, and a rare-earth ferromagnet with finite orbital moments, terbium, utilizing second-harmonic Hall-response measurements. The damping-like torque efficiency [Formula: see text] of chromium is found to be positive and reaches a giant value [Formula: see text]3.66 in this system, in sharp contrast to the negative and subtle [Formula: see text] in general Cr/ferromagnet heterostructures with quenched orbital moments. We suggest that the orbital currents generated by the orbital Hall effect in Cr can be injected into Tb with negligible loss at the interface and then efficiently interact with the orbital moments. We term such an exotic effect as the orbit-orbit torque (OOT). Our work implies that orbital currents could be harnessed to manipulate the orbital magnetization of materials, which would advance the development of orbitronics.