  Efficient electron-refrigeration based on a
normal-metal/spin-filter/superconductor junction is proposed and demonstrated
theoretically. The spin-filtering effect leads to values of the cooling power
much higher than in conventional
normal-metal/nonmagnetic-insulator/superconductor coolers and allows for an
efficient extraction of heat from the normal metal. We demonstrate that highly
efficient cooling can be realized in both ballistic and diffusive multi-channel
junctions in which the reduction of the electron temperature from 300 mK to
around 50 mK can be achieved. Our results indicate the practical usefulness of
spin-filters for efficiently cooling detectors, sensors, and quantum devices.
