  We report the observation of a unidirectional magnetoresistance (UMR) that
originates from the nonequilibrium orbital momentum induced by an electric
current in a naturally oxidized Cu/Co bilayer. The orbital-UMR scales with the
torque efficiency due to the orbital Rashba-Edelstein effect upon changing the
Co thickness and temperature, reflecting their common origin. We attribute the
UMR to orbital-dependent electron scattering and orbital-to-spin conversion in
the ferromagnetic layer. In contrast to the spin-current induced UMR, the
magnon contribution to the orbital-UMR is absent in thin Co layers, which we
ascribe to the lack of coupling between low energy magnons and orbital current.
The magnon contribution to the UMR emerges in Co layers thicker than about 5
nm, which is comparable to the orbital-to-spin conversion length. Our results
provide insight into orbital-to-spin momentum transfer processes relevant for
the optimization of spintronic devices based on light metals and orbital
transport.
