Structural Phase Engineering of ( plus )-W for a Large Spin Hall Angle and Spin Diffusion Length

Spin-orbit coupling (SOC) plays a crucial role in spin-to-charge interconversion in ferromagnet (FM)-heavy metal (HM) heterostructures. The SOC of a heavy metal strongly depends on its crystalline phase. Here, we report a systematic study of the effect of growth rate (D-r) on SOC-related parameters such as the spin-mixing conductance (g(down arrow up arrow)), spin diffusion length (lambda(sd)), spin transparency, and spin Hall angle (theta(SH)) in (alpha + beta)-W. X-ray diffraction (XRD) results for W sputtered at different D-r values show that the adatom energy is critical in determining the structural phase. The thickness dependence of the XRD results shows that all films sputtered at 0.06 nm/s nucleate as (alpha + beta)-W on the Si substrate. Four-probe resistivity measurements for (alpha + beta)-W (10) show that the resistivity is linearly proportional to D-r with a negative slope. A ferromagnetic resonance (FMR)-based spin pumping study shows that the line width (Delta H) is enhanced due to spin current transport across the Py/(alpha + beta)-W interface. The g(down arrow up arrow) shows a maximum of 10.1 x 10(18) m(-2) and a minimum of 4.2 x 10(18) m(-2) at D-r of 0.08 and 0.06 nm/s, respectively. The extracted spin diffusion length for (alpha + beta)-W is 0.98 +/- 0.01 nm. The calculated spin transparency for (alpha + beta)-W is 0.74. The inverse spin Hall effect (ISHE) is demonstrated for Py(20)/(alpha + beta)-W(10) bilayer heterostructures. From ISHE measurements, the calculated spin Hall angle for (alpha + beta)-W is found to be theta(SH) = -0.24 +/- 0.08. It is observed that D-r strongly influences the spin diffusion length, spin transparency, and spin Hall angle. Our systematic study may open new possibilities for tuning spin transparency and spin Hall angle for developing low-power memory and logic devices.