Role of transparency of platinum-ferromagnet interface in determining intrinsic magnitude of spin Hall effect

TL;DR

This paper demonstrates that the transparency of the platinum-ferromagnet interface significantly influences the measured spin Hall angle, revealing the intrinsic SHA in platinum to be higher than previously observed, with interface engineering further modulating this effect.

Contribution

It shows that interface transparency critically affects spin Hall angle measurements and reveals the intrinsic SHA in platinum to be higher than prior estimates, highlighting the importance of interface properties.

Findings

Higher spin Hall angle in Pt/cobalt (~0.11) than in Pt/permalloy (~0.05).

Intrinsic SHA in platinum estimated at 0.19 ± 0.04, higher than measured values.

Insertion of magnetic layers modulates the spin Hall angle.

Abstract

The spin Hall effect (SHE) converts charge current to pure spin currents in orthogonal directions in materials that have significant spin-orbit coupling.The efficiency of the conversion is described by the spin Hall Angle (SHA). The SHA can most readily be inferred by using the generated spin currents to excite or rotate the magnetization of ferromagnetic films or nano-elements via spin-transfer torques.Some of the largest spin torque derived spin Hall angles (ST-SHA) have been reported in platinum. Here we show, using spin torque ferromagnetic resonance (ST-FMR) measurements, that the transparency of the Pt-ferromagnet interface to the spin current plays a central role in determining the magnitude of the ST-SHA. We measure a much larger ST-SHA in Pt/cobalt (~0.11) compared to Pt/permalloy (~0.05) bilayers when the interfaces are assumed to be completely transparent. Taking into account the transparency of these interfaces, as derived from spin-mixing conductances, we find that the intrinsic SHA in platinum has a much higher value of 0.19 +- 0.04 as compared to the ST-SHA. The importance of the interface transparency is further exemplified by the insertion of atomically thin magnetic layers at the Pt/permalloy interface that we show strongly modulates the magnitude of the ST-SHA.