Thickness Dependence of Interface‐Generated Spin Currents in Ferromagnet/Ti/CoFeB Trilayers

Abstract Interface‐generated spin currents in ferromagnet (FM)/nonmagnet (NM) structures provide both in‐plane and out‐of‐plane spin–orbit torques (SOTs), enabling the field‐free switching of perpendicular magnetization of the other FM layer in magnetic trilayers. In this study, the NM thickness dependence of interface‐generated spin currents and associated SOTs in FM/Ti/CoFeB trilayers is investigated. In such magnetic trilayers, it is known that the in‐plane SOT results from the spin–orbit filtering of the interface‐generated spin current, while the out‐of‐plane SOT is due to the spin–orbit precession. These results show that the polarity of current‐induced magnetization switching under an in‐plane magnetic field reverses with increasing Ti thickness. This indicates that the sign of the in‐plane SOT depends on the current distribution between the bottom FM and Ti layers. On the other hand, field‐free switching occurs only for a Ti thickness of up to ≈4 nm, and the same polarity is retained, demonstrating that out‐of‐plane SOT is governed by the charge current flowing near the interface. These results suggest that field‐free switching efficiency can be enhanced by engineering the relative conductance of the FM/NM bilayers to constructively combine in‐plane and out‐of‐plane SOTs caused by interface‐generated spin currents.