  For a linear chain of diamond-like elements, we show that the Rashba
spin-orbit interaction (which can be tuned by a perpendicular gate voltage) and
the Aharonov-Bohm flux (due to a perpendicular magnetic field) can combine to
select only one propagating ballistic mode, for which the electronic spins are
fully polarized along a direction that can be tuned by the electric and
magnetic fields and by the electron energy. All the other modes are evanescent.
For a wide range of parameters, this chain can serve as a spin filter.
