A mechanical analogy for spin currents and torques

We map the problem of spin-transfer torques in nanostructures with diffusive spin currents on a mechanical problem involving elastic strings connecting points of attachement that slide along the fixed straight rails. The obtained mechanical analogy provides a qualitative understanding of the effects of spin relaxation on spin torques, and of the phenomenon of sign-changing angular dependence [1-3] of the efficiency factor $g(\theta)$. \\[4pt] [1] A. A. Kovalev, A. Brataas, and G. E. W. Bauer, Phys. Rev. B {\bf 66}, 224424 (2002).\\[0pt] [2] Jan Manschot, A. Brataas, and G. E. W Bauer, Phys. Rev. B {\bf 69}, 092407 (2004).\\[0pt] [3] J. Barnas, A. Fert, M. Gmitra, I. Weymann, and V. K. Dugaev, Phys. Rev. B {\bf 72}, 024426 (2005).