Science 330, 1645–1648 (2010)

For spintronic devices to manipulate electron spin, they require the ability to isolate 'up' spins from 'down'. Although this can be done by passing an electronic current through a spin filter (often built with ferromagnets), moving charges involves an energy cost. Marius Costache and Sergio Valenzuela have now experimentally demonstrated a method to generate spin polarization with no net charge transport.

The researchers — who are based at the Campus Universitat Autònoma de Barcelona and ICREA in Spain, and at Harvard University — demonstrated a spin ratchet consisting of a nanoscale superconducting island connected to two regular metal contacts, each with a different resistance. In analogy to a hand-held ratchet, an oscillatory voltage applied across the device resulted in a unidirectional flow of spins in a direction that could be set by a series of applied electrical and magnetic fields. The ratchet effect applied only to spin, and not charge.

By using ferromagnetic contacts with known spin polarization, Costache and Valenzuela measured the efficiency of their spin ratchet to be approximately 50%. And because the device operates on a single electron at a time, it may be useful for spin-based quantum computing as well as in basic physics research.