Spin polarization of two-dimensional electrons in different fractional states and around filling factor nu=1

The spin configuration of the ground state of a two-dimensional electron system is investigated for different fractional quantum Hall effect (FQHE) states from an analysis of circular polarization of time-resolved luminescence. The method clearly distinguishes between fully spin-polarized, partially spin-polarized, and spin-unpolarized FQHE ground states. We demonstrate that FQHE states which are spin unpolarized or partially polarized at low magnetic fields become fully spin polarized at high fields. Temperature dependence of the;spin polarization reveals a nonmonotonic behavior at v = 2/3. At B>4 T and v less than or equal to 1 the electron system is found to be fully spin polarized. This result does not indicate the existence of any Skyrmionic excitations in the high-magnetic-field limit. However, at B<2 T the observed spin depolarization of the electron system at v = 2/3 and 4/3 becomes broader for lower magnetic fields, so;that full spin polarization remains only in a small vicinity around v = 1. Such a behavior could be considered as a precursor of Skyrmionic depolarization, which would dominate for smaller ratios between Zeeman and Coulomb energies.