NMR determination of 2D electron spin polarization at ν=1/2

Using a standard NMR spin-echo technique we determined the spin polarization P of two-dimensional electrons, confined to GaAs quantum wells, from the hyperfine shift of Ga nuclei located in the wells. Concentrating on the temperature (0.05 less than or similar to T less than or similar to 10 K) and magnetic field (7 less than or similar to B less than or similar to 17 T) dependencies of P at Landau level filling factory nu = 1/2, we find that the results are described well by a simple model of noninteracting composite fermions, although some inconsistencies remain when the two-dimensional electron system is tilted in the magnetic field.