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 $\left(t_{\mathrm{Ta}}\right)$-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 $t_{\mathrm{Ta}}$ when $t_{\mathrm{Ta}}<1\mathrm{nm}$ and reaches a saturation value at $\sim 1.5\mathrm{nm}$. 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 $\sim 15\mathrm{nm}$. 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 $0.02\pm 0.02$, we extract the spin Hall angle and spin diffusion length of high resistivity Ta to be ${\theta }_{\mathrm{SH}}=-0.0062\pm 0.001$ and ${\lambda }_{\mathrm{sd}}=5.1\pm 0.6\mathrm{nm}$, respectively.

• Received 16 February 2018

DOI:https://doi.org/10.1103/PhysRevMaterials.2.074406