All-optical Detection of Spin Hall Angle in W/CoFeB/SiO2 Heterostructures by Varying Tungsten Layer Thickness

TL;DR

This study demonstrates an all-optical method to measure the spin Hall angle in W/CoFeB/SiO2 heterostructures, revealing a non-monotonic dependence on tungsten layer thickness and highlighting the influence of structural phase transitions.

Contribution

It introduces an optical technique for unambiguous measurement of spin Hall angle and explores its variation with tungsten thickness in heterostructures, linking it to structural and electronic effects.

Findings

Maximum SHA of about 0.4 at 3 nm W thickness

Sharp reduction of SHA at 6 nm W thickness

Correlation of SHA variation with W phase transition

Abstract

The development of advanced spintronics devices hinges on the efficient generation and utilization of pure spin current. In materials with large spin-orbit coupling, the spin Hall effect may convert charge current to pure spin current and a large conversion efficiency, which is quantified by spin Hall angle (SHA), is desirable for the realization of miniaturized and energy efficient spintronic devices. Here, we report a giant SHA in beta-tungsten (\b{eta}-W) thin films in Sub/W(t)/Co20Fe60B20(3 nm)/SiO2(2 nm) heterostructures with variable W thickness. We employed an all-optical time-resolved magneto-optical Kerr effect microscope for an unambiguous determination of SHA using the principle of modulation of Gilbert damping of the adjacent ferromagnetic layer by the spin-orbit torque from the W layer. A non-monotonic variation of SHA with W layer thickness (t) is observed with a maximum of about 0.4 at about t = 3 nm, followed by a sudden reduction to a very low value at t = 6 nm. This variation of SHA with W-thickness correlates well with the thickness dependent structural phase transition and resistivity variation of W above the spin diffusion length of W, while below this length the interfacial electronic effect at W/CoFeB influences the estimation of SHA.