  We study the dynamics of paramagnetic molecular superrotors in an external
magnetic field. Optical centrifuge is used to create dense ensembles of oxygen
molecules in ultra-high rotational states. It is shown for the first time, that
the gas of rotating molecules becomes optically birefringent in the presence of
magnetic field. The discovered effect of "magneto-rotational birefringence"
indicates preferential alignment of molecular axes along the field direction.
We provide an intuitive qualitative model, in which the influence of the
applied magnetic field on the molecular orientation is mediated by the
spin-rotation coupling. This model is supported by the direct imaging of the
distribution of molecular axes, the demonstration of the magnetic reversal of
the rotational Raman signal, and by numerical calculations.
