  We study theoretically extraction of spin-polarized electrons at nonmagnetic
semiconductor/ferromagnet junctions. The outflow of majority spin electrons
from the semiconductor into the ferromagnet leaves a cloud of minority spin
electrons in the semiconductor region near the junction, forming a local
spin-dipole configuration at the semiconductor/ferromagnet interface. This
minority spin cloud can limit the majority spin current through the junction
creating a pronounced spin-blockade at a critical current. We calculate the
critical spin-blockade current in both planar and cylindrical geometries and
discuss possible experimental tests of our predictions.
