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/Co sub(20)Fe sub(60)B sub(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] sub(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 + or - 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 + or - 0.01 and 1.92 + or - 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.