  A large unidirectional magnetoresistance (UMR) ratio of UMR/$R_{xx}\sim$
$0.36\%$ is found in W/CoFeB metallic bilayer heterostructures at room
temperature. Three different regimes in terms of the current dependence of UMR
ratio are identified: A spin-dependent-scattering mechanism regime at small
current densities $J \sim$ $10$$^{9}$A/m$^{2}$ (UMR ratio $\propto$ $J$), a
spin-magnon-interaction mechanism regime at intermediate $J \sim$
$10$$^{10}$A/m$^{2}$ (UMR ratio $\propto$ $J$$^{3}$), and a spin-transfer
torque (STT) regime at $J \sim$ $10$$^{11}$A/m$^{2}$ (UMR ratio independent of
$J$). We verify the direct correlation between this large UMR and the transfer
of spin angular momentum from the W layer to the CoFeB layer by both
field-dependent and current-dependent UMR characterizations. Numerical
simulations further confirm that the large STT-UMR stems from the tilting of
the magnetization affected by the spin Hall effect-induced spin-transfer
torques. An alternative approach to estimate damping-like spin-torque
efficiencies from magnetic heterostructures is also proposed.
