LAUSR

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 (\ensuremath{\beta}-W) thin films in $\mathrm{Sub}\text{/}\mathrm{W}(t)\text{/}\mathrm{C}{\mathrm{o}}_{20}\mathrm{F}{\mathrm{e}}_{60}{\mathrm{B}}_{20}(3\phantom{\rule{0.16em}{0ex}}\mathrm{nm})\text{/}\mathrm{Si}{\mathrm{O}}_{2}(2\phantom{\rule{0.16em}{0ex}}\mathrm{nm})$ 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 nonmonotonic variation of SHA with W layer thickness ($t$) is observed with a maximum of about 0.4 at about $t=3\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$, followed by a sudden reduction to a very low value at $t=6\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$. 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.