Origin of spin–orbit torques and spin transport in Pt/Co/Cu/NiFe/Cu/capping multilayers

Abstract

Spin–orbit torque (SOT) enables efficient current-driven control of magnetization, offering a promising pathway toward low-power spintronic devices. However, the origin of both damping-like (DL) and field-like (FL) SOTs and associated spin transport in complex multilayers remain unclear. Here, we investigated the dependence of SOT efficiencies on NiFe thickness within Ta/Pt/Co/Cu/xNiFe/Cu/Capping multilayers (x = 1–5 nm; Capping = Pt, Al, and SiO2). By employing a spin rotation geometry, the perpendicularly magnetized Pt/Co/Cu stacks serve as a spin source introducing an unconventional spin polarization orthogonal to the Oersted field, eliminating its contribution and enabling unambiguous extraction of SOTs using planar Hall and polar magneto-optic Kerr effect measurements. To distinguish bulk and interfacial contributions, we introduce a sample-area-normalized moment m = mNiFe/S, accounting for thickness-dependent magnetization and eliminating uncertainties arising from nominal thickness scaling and magnetic dead layers. We find that DL-SOT follows nearly linear 1/m scaling, consistent with rapid spin absorption at the Cu/NiFe interface but exhibits finite βSOT when 1/m approaches zero in both Pt- and Al-capped samples, indicating additional interfacial spin-current contributions at Cu/Pt and Cu/Al interfaces. In contrast, SiO2-capped samples show negligible interfacial contributions. Furthermore, FL-SOT deviates markedly from 1/m scaling, indicating a significantly longer spin dephasing length (∼1.7 nm) and thus more extended propagation of the spin currents responsible for FL-SOT in NiFe than for DL-SOT. Comparative capping-layer studies further corroborate this behavior through interface-dependent spin transport. Our findings clarify the origin of DL and FL torques and spin-transport processes, providing guidelines for engineering interfacial spin–orbit functionalities in ultrathin metallic heterostructures.