Enhancing spin–orbit torque in tri-layer devices with an ultra-thin light metal spacer

The discovery of spin current generation through an additional ferromagnetic layer has unlocked new possibilities for spin–orbit torque (SOT) devices, particularly in tri-layer SOT configurations. This breakthrough facilitates field-free switching (FFS) with perpendicular magnetic anisotropy (PMA) under the influence of z-polarization. Despite this progress, a significant challenge persists in lowering the switching current density (JSW) in SOT devices, which remains roughly an order of magnitude higher than in spin-transfer torque devices. We incorporated an ultra-thin (1 nm) copper (Cu) spacer into a tri-layer SOT structure where the orange peel effect likely arose. This ultra-thin Cu layer, combined with the optimized Pt-based SO layer, resulted in approximately a 35% reduction in JSW compared to the scenario without Cu insertion. The damping-like torque efficiency was analyzed concerning the FFS performance at optimized Cu spacer thickness to support the finding of JSW reduction. Using magneto-optic Kerr effect measurements and micromagnetic simulations, we identified domain nucleation as the primary mechanism driving FFS, confirming the role of z-polarization in reducing JSW effectively. This study proposes a design that simultaneously addresses three key challenges of PMA, JSW, and FFS in current SOT systems.