Evidence of orbital Hall current induced correlation in second harmonic response of longitudinal and transverse voltage in light metal-ferromagnet bilayers

Abstract: We investigate the effect of orbital current arising from orbital Hall effect in thin films of Nb and Ti in ohmic contact with ferromagnetic Ni in the second harmonic longitudinal and transverse voltages in response to an a.c. current applied to the bilayer structures. Our experiments were analogous to those on Heavy Metal-Ferromagnet bilayers and we extract the Orbital Hall Torque efficiency and unidirectional magnetoresistance (UMR). Through second-harmonic measurements, we investigate orbital Hall torque and UMR in bilayer devices composed of ferromagnetic materials (FM), such as Ni and NiFe, paired with light metals (LM), such as Ti and Nb. Our results demonstrate that LM/Ni bilayers exhibit enhanced damping-like torque and unidirectional magnetoresistance (UMR) compared to LM/NiFe bilayers. This enhancement suggests that angular momentum is generated via the orbital Hall effect within the light metal, where it undergoes orbital-to-spin conversion within the Ni ferromagnet, ultimately transferring to the magnetization of the ferromagnetic layer. Torque and UMR are also absent in single-layer devices, highlighting the necessity of the bilayer structure for orbital current generation.