Crystalline phase dependent spin current efficiency in sputtered Ta thin films

Abstract

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, GR the pure α-phase of Ta is formed, which changes to the pure β-phase for GR ≥ 1.44 Å/s. For an intermediate growth rate, 0.62 Å/s ≤ GR < 1.44 Å/s, an admixture of α and β 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.