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We report and quantify a large orbital-Hall torque generated by Nb and Ru, which we identify from the strong dependence of torques on the ferromagnets. This is manifested as strong enhancement in the dampinglike torques measured in Nb (or Ru)/Ni bilayers as compared to Nb (or Ru)/FeCoB bilayers including the sign reversal in the case of Nb/(Ni or FeCoB) samples. We experimentally observe a striking enhancement of the measured torques with the increase of ferromagnetic Ni thickness which is markedly different from the regular spin-transfer torque; this could be a unique signature of long-range action of the orbital-Hall torques.
Bose et al. (2023) studied this question.