All-optical detection of the spin Hall angle in W/CoFeB/SiO2 heterostructures with varying thickness of the tungsten layer

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 (β-W) thin films in Sub/W(t)/Co20Fe60B20(3nm)/SiO2(2nm) 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 nonmonotonic variation of SHA with W layer thickness (t) is observed with a maximum of about 0.4 at about t=3nm, followed by a sudden reduction to a very low value at t=6nm. 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.