Joe Orenstein - Spin Coulomb drag results because the motion of spin through...

Spin Coulomb drag results because the motion of spin through the semiconductor is sensitive to collisions between electrons, whereas the transport of charge is not. When electrons bump into one another, the mutual repulsion of their negative charges creates a drag on their spin current, which is a relative motion between individual electrons, but not on their charge current, which is the collective transport of all the electrons in motion.