  Using the first-principles non-equilibrium Green's function technique with
supercell disorder averaging, we study the influence of interfacial intermixing
on the spin-orbit torque in Co$\mid$Pt bilayers. Intermixing is modeled by
inserting one or more monolayers of a disordered CoPt alloy between Co and Pt.
Dampinglike torque is moderately enhanced by interfacial intermixing, while the
fieldlike torque, which is small for abrupt interfaces, is strongly enhanced
and becomes comparable to the dampinglike torque. The enhancement of the
fieldlike torque is attributed to the interface between Co and the intermixed
region. The planar Hall-like torque increases with intermixing but remains
relatively small. The behavior of the torques is similar for bilayers with
(111) and (001)-oriented interfaces. Strong dependence of the fieldlike torque
on intermixing could provide a way to tune the fieldlike-to-dampinglike torque
ratio by interface engineering.
