Observation of nonlocal orbital transport and sign reversal of dampinglike torque in Nb/Ni and Ta/Ni bilayers

Abstract

We study the efficiencies of dampinglike and fieldlike torques in Nb/Ni and Ta/Ni bilayers and compare them with $\mathrm{Nb}/{\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$(Py) and Ta/Py as a function of the thickness of a ferromagnet. In both these systems, we observe that the sign of the dampinglike torque is negative for Py whereas it is positive for nickel. The negative sign of the dampinglike torque in Nb/Py and Ta/Py is consistent with the negative sign of the spin Hall effect in niobium and tantalum. The sign reversal of the dampinglike torque in Nb/Ni and Ta/Ni suggests a strong generation of the orbital current from niobium and tantalum, which is converted into the spin current inside nickel and produces the orbital torque. We also find that both the dampinglike and fieldlike efficiencies change signs when nickel is replaced by Py.