  We evidence magnetic field triggered polariton lasing in a microcavity
containing semimagnetic quantum wells. This effect is associated with a
decrease of the polariton lasing threshold power in magnetic field. The
observed magnetic field dependence of the threshold power systematically
exhibits a minimum which only weakly depends on the zero-field photon-exciton
detuning. These results are interpreted as a consequence of the polariton giant
Zeeman splitting which in magnetic field: leads to a decrease of the number of
accessible states in the lowest polariton branch by a factor of two, and
substantially changes the photon-exciton detuning.
