  We study the spin magnetic moment of a single impurity embedded in a
finite-size non-magnetic host exhibiting a band gap. The calculations were
performed using a tight-binding model Hamiltonian. The simple criterion for the
magnetic to non-magnetic transition as given in the Anderson impurity model
breaks down in these cases. We show how the spin magnetic moment of the
impurity that normally would be quenched can be restored upon introducing a gap
at the Fermi level in the host density of states. The magnitude of the impurity
spin magnetic moment scales monotonically with the size of the band gap. This
observation even holds for a host material featuring a strongly discretized
density of states. Thus, it should be possible to tune the magnetic moment of
doped nano-particles by varying their size and thereby their band gap.
