We have measured the spin torques of $\beta \text{−}\mathrm{tantalum}/\mathrm{C}{\mathrm{o}}_{20}\mathrm{F}{\mathrm{e}}_{60}{\mathrm{B}}_{20}$ bilayers versus Ta thickness at room temperature using a ferromagnetic resonance (FMR) technique. A significant fieldlike spin torque originating from Ta was identified, which is constant and independent of Ta thickness. Because of this constant torque, the spin Hall coefficient ${\theta }_{\mathrm{SH}}$ needs to be calculated from the ratio of the symmetric component of the FMR signal to the slope of the antisymmetric component with Ta thickness, from which a value of $-0.11\pm 0.01$ was determined. The saturation magnetization of the CoFeB layers for samples deposited with Ta was found to be smaller than that of a single CoFeB layer, with values of $1.84\pm 0.01$ and $1.92\pm 0.01$ T, respectively. The origin of the fieldlike torque is ascribed to an interface spin-orbit coupling, or Rashba effect, due to the strength and constancy of the torque with Ta thickness. From fitting measured data to a semiclassical diffusion model that includes interface spin-orbit coupling, we have determined the spin diffusion length for β-tantalum to be 2.5 nm.

• Received 3 December 2014
• Revised 23 February 2015

DOI:https://doi.org/10.1103/PhysRevB.91.144412