Determination Of Spin Hall Angle And Spin Diffusion Length In Beta-Phase-Dominated Tantalum

The dc spin-to-charge conversions of tantalum (Ta) in Ta/Co40Fe40B20 bilayer structures are investigated utilizing spin pumping and inverse spin Hall effects (ISHE). From Ta thickness (t(Ta))-dependent resistivity and x-ray diffraction measurements, we found that Ta films, below 30 nm in thickness, are beta-phase dominated. The damping enhancement shows a fast increase with t(Ta) when t(Ta) < 1 nm and reaches a saturation value at similar to 1.5 nm. The ISHE induced charge voltages have opposite signs for Ta and Pt. From t(Ta)-dependent spin pumping produced ISHE voltage and precession angle measurements, the normalized spin-charge conversion signal is found to increase with t(Ta) and saturate at similar to 15 nm. Our findings can be understood with a recently developed theory [Phys. Rev. Lett. 114, 126602 (2015)], which includes spin backflow and a spin loss at the interface. With a fitted spin loss factor of 0.02 +/- 0.02, we extract the spin Hall angle and spin diffusion length of high resistivity Ta to be theta(SH) = -0.0062 +/- 0.001 and lambda(sd) = 5.1 +/- 0.6 nm, respectively.