Role of interfacial orbital hybridization in spin-orbit torque generation in Pt-based heterostructures

Abstract

The spin-orbit torque (SOT) has been used to efficiently manipulate the magnetization in the ferromagnet (FM)/heavy metal (HM) heterostructures for non-volatile storage applications. Engineering of the interface in FM/HM has attracted great attention due to its crucial role in the spin transfer and generation. However, the underlying mechanism for the interfacial contribution to SOT generation is still a subject of debate. Interfacial transparency has been widely used to illustrate the interfacial contribution to the SOT efficiency, for example, the Pt/Co bilayer has a larger interfacial transparency and thus a larger SOT efficiency than Pt/Py bilayer despite the same spin Hall effect of Pt. However, this explanation has been questioned since the Pt/Co interface also has a large spin memory loss, which can lead to a substantial drop of the SOT efficiency [1]. The orbital hybridization is another phenomenon at the FM/HM interface, which is particularly prominent for Pt/Co. Theoretical studies have predicted a strong effect of hybridization on the SOT generated at the Pt/Co interface [2,3], which has not been proved experimentally. In this work, we perform the STFMR on the epitaxially grown Pt/Ni81Fe19 (as known as permalloy, Py) and Pt/Co/Py heterostructures with (111) crystalline orientation to investigate both the strength and components of their SOT. We demonstrate that inserting an ultra-thin Co layer between epitaxial Pt and Py substantially enhances both the in-plane (IP) and out-of-plane (OP) damping-like SOTs, both of which can be attributed to the strong interfacial hybridization between Co and Pt.