Electrons carry both charge and spin as they flow in a material. For many applications, the spins are artificially polarized by magnetic fields or are naturally polarized in ferromagnets such as iron and nickel. The recent history of spintronics is rich with examples of heterostructures carefully engineered to send a current of spin-polarized electrons from a ferromagnet to metals, semiconductors, or superconductors (see the article by Jagadeesh Moodera, Guo-Xing Miao, and Tiffany Santos in Physics Today, April 2010, page 46).
In fact, though, a spin current may appear wherever there is a charge current—no magnetism required. The spin polarization arises from the relativistic coupling of the electrons’ spin and orbital angular momenta. As electrons scatter from atomic impurities, they feel an effective magnetic field that alters their paths: Spin-up electrons bend in one direction, spin-down electrons in the other, both transverse to the original direction of the charge...
