  We study the nonequilibrium spin current through a quantum dot consisting of
two localized spin-1/2 coupled to two ferromagnetic insulators. The influence
of an intra-dot magnetic field and exchange coupling, different dot-reservoir
coupling configurations, and the influence of magnon chemical potential
differences vs. magnetic field gradients onto the spin current are examined. We
discuss various spin switching mechanisms and find that, in contrast to
electronic transport, the spin current is very sensitive to the specific
coupling configuration and band edges. In particular, we identify 1- and
2-magnon transport processes which can lead to resonances and antiresonances
for the spin current.
