  Spintronic devices are based on heterojunctions of two materials with
different magnetic and electronic properties. Although an energy barrier is
naturally formed even at the interface of metallic heterojunctions, its impact
on spin transport has been overlooked. Here, using diffusive spin Hall
currents, we provide evidence that the inherent energy barrier governs the spin
transport even in metallic systems. We find a sizable field-like torque, much
larger than the damping-like counterpart, in
Ni$_{81}$Fe$_{19}$/Bi$_{0.1}$Sb$_{0.9}$ bilayers. This is a distinct signature
of barrier-mediated spin-orbit torques, which is consistent with our theory
that predicts a strong modification of the spin mixing conductance induced by
the energy barrier. Our results suggest that the spin mixing conductance and
the corresponding spin-orbit torques are strongly altered by minimizing the
work function difference in the heterostructure. These findings provide a new
mechanism to control spin transport and spin torque phenomena by interfacial
engineering of metallic heterostructures.
