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 approximate 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.