Observation of negative orbital torque from Vanadium

Abstract: We present systematic investigations of orbital torques generated from the light metal $V$, revealing a negative orbital torque. We observe that the damping-like torque (DLT) per unit electric field depends on the choice of the ferromagnetic layer, with approximately seven times higher torque efficiency in $Ni/V$ as compared to $Fe_{60}Co_{20}B_{20}/V$. We find the sign of DLT per unit electric field from $V$ is opposite to that from $Pt$. These results collectively confirm the existence of negative orbital Hall effect (OHE) in $V$. Furthermore, the DLT per unit electric field increases with the $V$ layer thickness, maintaining the negative sign at all thicknesses. We also note that the DLT per unit electric field exceeds that of the $Pt$ reference samples at higher $V$ thicknesses. Through fitting using the drift-diffusion equation, we extract a high effective orbital Hall conductivity of $-(1.46 \pm 0.09)\,\frac{\hbar}{2e}\,\times 10^{5}\,\Omega^{-1}\,\mathrm{m}^{-1}$ and a long orbital diffusion length of $(13.7 \pm 0.9)\,\mathrm{nm}$ in $V$. .