  We investigate a heat- to charge-current converter consisting of a
single-level quantum dot coupled to two ferromagnetic metals and one
ferromagnetic insulator held at different temperatures. We demonstrate that
this nano engine can act as an optimal heat to spin-polarized charge current
converter in an antiparallel geometry, while it acts as a heat to pure spin
current converter in the parallel case. We discuss the maximal output power of
the device and its efficiency.
