Spin Hall effect in amorphous W-Zr and W-Hf alloys

In search for materials with large spin Hall effect, we investigated the binary alloys W-Zr and W-Hf. Thin films were prepared across the full range of stoichiometries for both alloys by magnetron co-sputtering. Due to the linear relation between the spin Hall angle and the resistivity, alloy compositions with high resistivity are expected to show a large spin Hall angle. In both stoichiometry series we found peak values of the spin Hall angle accompanied with a significant increase of the resistivity. X-ray diffraction measurements connect these observations to a transition from a bcc solid solution to an amorphous material at this concentration. In the W-Zr series, the maximum spin Hall angle of Θ = -0.31 is found at a tungsten content of 65 %, whereas in the W-Hf series the maximum spin Hall angle of Θ = -0.23 appears at a tungsten content of 70 %. For lower tungsten content, the spin Hall angle decreases in both stoichiometry series. In the W-Hf series we find a linear relation between the spin Hall conductivity and the tungsten content irrespective of the structural phase transition.