  We report an \emph{ab inito} investigation on the ground-state atomic
configuration, electronic structures, magnetic and spin-dependent transport
properties of Mn-doped Au$_{25}$ nanoclusters protected by thiolate. It is
found that the most stable dopant sites are near surfaces, rather than the
center positions of the nanoparticles. Transport calculations show that
high-performance spin filters can be achieved by sandwiching these doped
clusters between two nonmagnetic Au electrodes. The nearly perfect spin
filtering originates from localized magnetic moments of these clusters that are
well protected by ligands from the presence of electrodes.
