  Recent studies evidence the emergence of asymmetric electron transport in
layered conductors owing to the interplay between electrical conductivity,
magnetization, and the spin Hall or Rashba- Edelstein effects. Here, we
investigate the unidirectional magnetoresistance (UMR) caused by the
current-induced spin accumulation in Co/Pt and CoCr/Pt bilayers. We identify
three competing mechanisms underpinning the resistance asymmetry, namely
interface and bulk spin-dependent electron scattering and electron-magnon
scattering. Our measurements provide a consistent description of the current,
magnetic field, and temperature dependence of the UMR and show that both
positive and negative UMR can be obtained by tuning the interface and bulk
spin-dependent scattering terms relative to the magnon population.
