  The spin Hall effect originating from 5d heavy transition metal thin films
such as Pt, Ta, and W is able to generate efficient spin-orbit torques that can
switch adjacent magnetic layers. This mechanism can serve as an alternative to
conventional spin-transfer torque for controlling next-generation magnetic
memories. Among all 5d transition metals, W in its resistive amorphous phase
typically shows the largest spin-orbit torque efficiency ~ 0.20-0.50. In
contrast, its conductive and crystalline $\alpha$ phase possesses a
significantly smaller efficiency ~ 0.03 and no spin-orbit torque switching has
yet been realized using $\alpha$-W thin films as the spin Hall source. In this
work, through a comprehensive study of high quality W/CoFeB/MgO and the
reversed MgO/CoFeB/W magnetic heterostructures, we show that although
amorphous-W has a greater spin-orbit torque efficiency, the spin Hall
conductivity of $\alpha$-W
($|\sigma_{\operatorname{SH}}^{\alpha\operatorname{-W}}|=3.71\times10^{5}\operatorname{\Omega}^{-1}\operatorname{m}^{-1}$)
is ~3.5 times larger than that of amorphous-W
($|\sigma_{\operatorname{SH}}^{\operatorname{amorphous-W}}|=1.05\times10^{5}\operatorname{\Omega}^{-1}\operatorname{m}^{-1}$).
Moreover, we demonstrate spin-orbit torque driven magnetization switching using
a MgO/CoFeB/$\alpha$-W heterostructure. Our findings suggest that the
conductive and high spin Hall conductivity $\alpha$-W can be a potential
candidate for future low power consumption spin-orbit torque memory
applications.
