  We investigate exciton-polariton condensation under magnetic field in a
single high-quality semiconductor micropillar cavity. We observe successive
polariton condensation of each spin component for two distinct threshold
powers. Pronounced and non-monotonous variations of both the Zeeman splitting
and the circular polarization of the emission are measured across these two
condensation thresholds. This unexpected behavior deeply deviates from the
so-called spin Meissner effect predicted for a fully thermalized system. Our
measurements can be understood in a kinetic approach taking into account
spin-anisotropic interactions within the entire system: the polariton
condensate and the cloud of uncondensed excitons.
