Enhanced spin orbit torque in Gd/FeCo multilayers via layer thickness gradient

The dependence of Spin-Orbit-Torque (SOT) and SOT magnetization switching in Gd/FeCo multilayers with layer thickness gradient t Grad sandwiched between Ta and Pt layers was investigated. The damping-like SOT effective field H DL changed linearly with respect to t Grad , while the field-like SOT effective field H FL exhibited much smaller values than H DL in all samples. The magnitude of SOT τ DL,FL and SOT efficiencies χ were calculated using the magnetization of Gd/FeCo multilayers, where the magnetization was estimated by the result of harmonic Hall measurements and perpendicular anisotropy of the Gd/FeCo multilayer. τ DL and χ increased only along a specific directional composition gradient, with χ showing three times increase compared to the non-gradient sample. The critical current density for SOT magnetization switching was slightly lower in the sample with higher χ .