We experimentally demonstrate the nonreciprocal generation of spin current ($J_s$) in a surface-oxidized copper film. The efficiency of conversion is at least 320 times larger than the inverse conversion. This nonreciprocity is due to a novel type of $J_s$ generation, which relies on the transfer of angular momentum from the velocity field of free electrons. A gradient in the electrical mobility in the film produces vorticity in the in-plane drift velocity of the free electrons. The inverse process can hardly occur when $J_s$ is collinear with the gradient in the electrical mobility.

• Received 8 August 2018

DOI:https://doi.org/10.1103/PhysRevLett.122.217701