Enhancement of spin current generation in epitaxial α-Ta/CoFeB bilayer

We report the enhancement of the spin Hall effect in a (111)-oriented epitaxial alpha-Ta/CoFeB bilayer system, which was previously believed to show the small spin Hall angle (SHA). The SHA of the epitaxial alpha-Ta/CoFeB structure estimated by spin Hall magnetoresistance measurement is 0.15, a value that is 1.5 times larger than the conventional amorphous-Ta/CoFeB structure. Effective SHA estimated by harmonic Hall measurement qualitatively supports the SMR result, whereas the strength of the field-like field of epitaxial alpha-Ta samples is similar to that of amorphous Ta samples. According to the resistivity dependence on the SHA, the origin of the enhanced SHA in epitaxial alpha-Ta is due to the extrinsic contribution to the SHA.