Experimental demonstration of the coexistence of spin Hall and Rashba effects in β-tantalum/ferromagnet bilayers

We have measured the spin torques of beta-tantalum/Co20Fe60B20 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(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 +/- 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 +/- 0.01 and 1.92 +/- 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 beta-tantalum to be 2.5 nm.