Modulation of spin torque from spin transport through two nearby domain walls

The motion of domain walls due to the spin torque generated by coherent coherent carrier transport [1] is of considerable interest for the development of spintronic devices [2]. We model two $\pi$ N\'eel walls [3] separated by a variable distance, and calculate transport characteristics and spin torque through the system [4]. We find that for large separations, the domain walls show the resonant transmission behavior of a spin-dependent double barrier; for small separations, the transmission spectrum resembles that of a $2\pi$ wall. We also find that the spin torque across the system initially increases as the separation between the walls increases from zero, then decreases slightly before reaching a saturation value that is larger than both the spin torque of a $2\pi$ wall and that of two individual $\pi$ walls. This work is supported by an ARO MURI.\\[4pt] [1] M. Yamanouchi, D. Chiba, F. Matsukura, and H. Ohno, Nature 428, 539 (2004).\\[0pt] [2] S. Parkin,M . Hayashi, L. Thomas, Science 320, 190 (2008)\\[0pt] [3] G. Vignale and M. Flatt\'e, Phys. Rev. Lett. 89 (2002).\\[0pt] [4] D. Ralph and M. Stiles, J.M.M.M. 320, 1190 (2008).