  The intensity and the giant circular polarization of edge luminescence in a
longitudinal magnetic field have been measured in nitrogen alloys GaAsN under
circularly polarized pumping. It has been found that these dependences are
shifted with respect to zero field by a value Beff. The magnitude of the
internal field Beff increases with increase in pumping intensity and reaches
saturation (~250 Gauss) at great densities of excitation. The saturation of the
Beff field with growth of pumping indicates that this is a field of nuclei,
polarized dynamically due to hyperfine interaction with optically oriented deep
paramagnetic centers, rather than a field of exchange interaction created on
the center by spin-polarized photo-excited conduction electrons. The short time
of nuclear polarization by electrons (<15 mks), measured under modulation of
circular polarization of the exciting light with high frequency, points to a
small number of nuclei undergoing hyperfine interaction with an electron
localized at a center.
