Crystalline phase dependent spin current efficiency in sputtered Ta thin films

We report on the optical detection of the spin Hall effect (SHE) as a function of the crystalline structure of sputtered Ta thin films using a magneto-optical Kerr system. The growth rate of Ta films is found to influence the crystalline phase of Ta films. At a lower growth rate, G(R) the pure alpha-phase of Ta is formed, which changes to the pure beta-phase for G(R) >= 1.44 angstrom/s. For an intermediate growth rate, 0.62 angstrom/s <= G(R) < 1.44A angstrom/s, an admixture of alpha and beta phases is formed. We optically detect spin accumulation due to the spin Hall effect in Ta films by applying a square wave current and using Fourier analysis in a magneto-optical Kerr effect setup that uses spatially modulated incident light. We show that there exists a threshold current density (Jth) above which spin current can be detected via the optical technique. Jth, which is a measure of spin current efficiency, is found to be the lowest in the mixed phase of Ta and is strongly correlated with the crystalline phase of Ta films. Published by AIP Publishing.