  Spin-orbit torque (SOT) induced by spin Hall and interfacial effects in heavy
metal(HM)/ferromagnetic(FM) bilayers has recently been employed to switch the
magnetization direction using in-plane current injection. In this paper, using
the Keldysh Green's function approach and first principles electronic structure
calculations we determine the Field-Like (FL) and Damping-Like (DL) components
of the SOT for the HM/Co (HM = Pt, Pd) bilayers. Our approach yields the
angular dependence of both the FL- and DL-SOT on the magnetization direction
without assuming a priori their angular form. Decomposition of the SOT into the
Fermi sea and Fermi surface contributions reveals that the SOT is dominated by
the latter. Due to the large lattice mismatch between the Co and the HM we have
also determined the effect of tensile biaxial strain on both the FL- and DL-SOT
components. The calculated dependence of FL- and DL-SOT on the HM thickness is
overall in good agreement with experiment. The dependence of the SOT with the
position of the Fermi level suggests that the DL-SOT dominated by the Spin Hall
effect of the bulk HM.
