Abnormal sign reversal of the fieldlike spin-orbit torque in Pt/Ni/Py with an ultrathin Ni spacer

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 as another promising approach to generate strong SOTs. Here, we systematically study the SOT 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 (ML), (0.4-nm-thick $\mathrm{ML}=\mathrm{Co}$, Fe, and Ni). The dampinglike (DL) torque efficiency ${\ensuremath{\xi}}_{\mathrm{DL}}$ is significantly enhanced by inserting ultrathin ML layers. Moreover, the Ni insertion results in a sign change of the fieldlike (FL) torque in Pt/Py and substantially reduces ${\ensuremath{\xi}}_{\mathrm{DL}}$ in Ta/Py. These results are likely related to the additional spin currents generated by combining the orbital Hall effect 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.