  The magnetization manipulation by spin-orbit torques (SOTs) in
nonmagnetic-metal (NM)/ferromagnet (FM) heterostructures has provided great
opportunities for spin devices. Besides the conventional spin Hall effect (SHE)
in heavy metals with strong spin-orbit coupling, the orbital currents have been
proposed to be another promising approach to generate strong SOTs. Here, we
systematically study the SOTs efficiency and its dependence on the FM thickness
and different NM/FM interfaces in two prototypical Pt/Py and Ta/Py systems by
inserting an ultrathin magnetic layer (0.4 nm thick ML = Co, Fe, Gd, and Ni).
The dampinglike (DL) torque efficiency $\xi_{DL}$ is significantly enhanced by
inserting ultrathin Co, Fe, and Ni layers and is noticeably suppressed for the
Gd insertion. Moreover, the Ni insertion results in a sign change of the
field-like (FL) torque in Pt/Py and substantially reduces $\xi_{DL}$ in Ta/Py.
These results are likely related to the additional spin currents generated by
combining the orbital Hall effect (OHE) in the NM and orbital-to-spin
conversion in the ML insertion layer and/or their interfaces, especially for
the Ni insertion. Our results demonstrate that inserting ultrathin ML can
effectively manipulate the strength and sign of the SOTs, which would be
helpful for spintronics applications.
