  We propose three different mechanisms for pumping spin-polarized currents in
a ballistic circuit using a time-dependent magnetic field acting on an
asymmetrically connected quantum ring at half filling. The first mechanism
works thanks to a rotating magnetic field and produces an alternating current
with a partial spin polarization. The second mechanism works by rotating the
ring in a constant field; like the former case, it produces an alternating
charge current but the spin current is d.c.; both methods do not require a
spin-orbit interaction to achieve the polarized current, but the rotating ring
could be used to measure the spin-orbit interaction in the ring using
characteristic oscillations. On the other hand, the last mechanism that we
propose depends on the spin-orbit interaction in an essential way, and requires
a time-dependent magnetic field in the plane of the ring. This arrangement can
be designed to pump a purely spin current. The absence of a charge current is
demonstrated analytically. Moreover, a simple formula for the current is
derived and compared to the numerical results.
