Interfacial engineering of spin currents is crucial for developing spintronic and memory devices. Here, we demonstrate the nonreciprocal transport of spin currents in Pt/Ti and Ti/Pt bilayers adjacent to a Py ferromagnetic layer using spin pumping and spin-torque ferromagnetic resonance (ST-FMR) techniques. Systematic thickness-dependent measurements reveal that the magnetic damping, which reflects the efficiency of spin-current injection, depends strongly on the stacking order. In Py/Pt/Ti structures, the magnetic damping increases with Pt thickness and saturates, following the conventional spin-diffusion model. In contrast, Py/Ti/Pt structures exhibit a rapid decline and saturate at thicker Ti thicknesses. The reduction of damping in the Py/Ti/Pt structures upon adding the Pt layer indicates strong orbital pumping and spin backflow at the Ti/Pt interface. Furthermore, ST-FMR measurements show opposite signs and distinct magnitudes of the spin-orbit torque efficiency for the two stacking configurations. These results confirm that the Pt layer dominates spin-current generation via the spin Hall effect, while the Ti layer contributes through the orbital Hall effect. Our results establish a clear asymmetry in spin-current propagation across Pt/Ti interfaces and provide a quantitative framework for understanding interfacial spin transport in multilayer spintronic systems.

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• Non-reciprocal propagation
• Spin current
• Spin pumping
• Spin-orbit torque
• Diodes
• Magnetic multilayers
• Ferromagnetic resonance
• Landau-Lifschitz-Gilbert equation