Illustration: Alan Stonebraker

Figure 1: A double spin-filter junction sketched here in (top) a low resistance configuration and (bottom) a high resistance configuration. When the magnetic layers are antiparallel, no current flows, but when the magnetic layers are parallel, only electrons with spins parallel to the magnetization direction can cross the junction. By changing the magnetic orientation of the $EuS$ layers, the spin-polarized current can be turned on and off, like a valve.