  We show the full control of the polarization dynamics of a propagating
exciton-polariton condensate in a planar microcavity by using a magnetic field
applied in the Voigt geometry. The change of the spin-beat frequency, the
suppression of the optical spin Hall effect and the rotation of the
polarization pattern by the magnetic field are theoretically reproduced by
accounting for the magneto-induced mixing of exciton-polariton and dark, spin
forbidden, exciton states.
