Capping Layer (CL) Induced Antidamping in CL/Py/β-W System (CL: Al, β-Ta, Cu, β-W)

For achieving ultrafast switching speed and minimizing dissipation losses, the spin-based data storage device requires a control on effective damping (α_eff) of nanomagnetic bits. Incorporation of interfacial antidamping spin orbit torque (SOT) in spintronic devices therefore has high prospects for enhancing their performance efficiency. Clear evidence of such an interfacial antidamping is found in Al capped Py(15 nm)/β-W(t_W)/Si (Py = Ni₈₁Fe₁₉ and t_W = thickness of β-W), which is in contrast to the increase of α_eff (i.e., damping) usually associated with spin pumping as seen in Py(15 nm)/β-W(t_W)/Si system. Because of spin pumping, the interfacial spin mixing conductance (g^↑↓) at Py/β-W interface and spin diffusion length (λ_SD) of β-W are found to be 1.63(±0.02) × 10¹⁸ m^-2 (1.44(±0.02) × 10¹⁸ m^-2) and 1.42(±0.19) nm (1.00(±0.10) nm) for Py(15 nm)/β-W(t_W)/Si (β-W(t_W)/Py(15 nm)/Si) bilayer systems. Other different nonmagnetic capping layers (CL), namely, β-W(2 nm), Cu(2 nm), and β-Ta(2,3,4 nm) were also grown over the same Py(15 nm)/β-W(t_W). However, antidamping is seen only in β-Ta(2,3 nm)/Py(15 nm)/β-W(t_W)/Si. This decrease in α_eff is attributed to the interfacial Rashba like SOT generated by nonequilibrium spin accumulation subsequent to the spin pumping. Contrary to this, when interlayer positions of Py(15 nm) and β-W(t_W) is interchanged irrespective of the fixed top nonmagnetic layer, an increase of α_eff is observed, which is ascribed to spin pumping from Py to β-W layer.