Determination of spin Hall angle and spin diffusion length in β -phase-dominated tantalum

Abstract

The dc spin-to-charge conversions of tantalum (Ta) in $\mathrm{Ta}/\mathrm{C}{\mathrm{o}}_{40}\mathrm{F}{\mathrm{e}}_{40}{\mathrm{B}}_{20}$ bilayer structures are investigated utilizing spin pumping and inverse spin Hall effects (ISHE). From Ta thickness $({t}_{\mathrm{Ta}})$-dependent resistivity and x-ray diffraction measurements, we found that Ta films, below 30 nm in thickness, are $\ensuremath{\beta}$-phase dominated. The damping enhancement shows a fast increase with ${t}_{\mathrm{Ta}}$ when ${t}_{\mathrm{Ta}}l1\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ and reaches a saturation value at $\ensuremath{\sim}1.5\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$. The ISHE induced charge voltages have opposite signs for Ta and Pt. From ${t}_{\mathrm{Ta}}$-dependent spin pumping produced ISHE voltage and precession angle measurements, the normalized spin-charge conversion signal is found to increase with ${t}_{\mathrm{Ta}}$ and saturate at $\ensuremath{\sim}15\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$. 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 $0.02\ifmmode\pm\else\textpm\fi{}0.02$, we extract the spin Hall angle and spin diffusion length of high resistivity Ta to be ${\ensuremath{\theta}}_{\mathrm{SH}}=\ensuremath{-}0.0062\ifmmode\pm\else\textpm\fi{}0.001$ and ${\ensuremath{\lambda}}_{\mathrm{sd}}=5.1\ifmmode\pm\else\textpm\fi{}0.6\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$, respectively.