Spin-to-charge conversion by spin pumping in sputtered polycrystalline Bi$_x$Se$_{1-x}$

TL;DR

This study investigates spin-to-charge conversion in sputtered polycrystalline Bi$_x$Se$_{1-x}$ using spin pumping, revealing an opposite sign of the spin Hall angle compared to Pt and highlighting interfacial effects impacting efficiency.

Contribution

It provides the first detailed analysis of interfacial properties and spin Hall angle sign in sputtered polycrystalline Bi$_x$Se$_{1-x}$, emphasizing the role of interdiffusion.

Findings

Bi$_x$Se$_{1-x}$ has an opposite spin Hall angle sign to Pt.

Charge current from spin-to-charge conversion is over ten times lower than Pt.

Interdiffusion at the interface affects the chemical composition and conversion efficiency.

Abstract

Topological materials are of high interest due to the promise to obtain low power and fast memory devices based on efficient spin-orbit torque switching or spin-orbit magnetic state read-out. In particular, sputtered polycrystalline Bi$_x$Se$_{1-x}$ is one of the materials with more potential for this purpose since it is relatively easy to fabricate and has been reported to have a very high spin Hall angle. We study the spin-to-charge conversion in Bi$_x$Se$_{1-x}$ using the spin pumping technique coming from the ferromagnetic resonance in a contiguous permalloy thin film. We put a special emphasis on the interfacial properties of the system. Our results show that the spin Hall angle of Bi$_x$Se$_{1-x}$ has an opposite sign to the one of Pt. The charge current arising from the spin-to-charge conversion is, in contrast, lower than Pt by more than one order of magnitude. We ascribe this to the interdiffusion of Bi$_x$Se$_{1-x}$ and permalloy and the changes in chemical composition produced by this effect, which is an intrinsic characteristic of the system and is not considered in many other studies.