Spin polarization of two-dimensional electrons in GaAs quantum wells around Landau level filling ν=1 from NMR measurements of gallium nuclei

A nuclear magnetic resonance (NMR) study has been carried out on a GaAs/Al_0.3Ga_0.7As multiple-quantum-well sample, containing two-dimensional (2D) electron layers, in the quantum Hall regime. The spin polarization P of 2D electrons is determined from the hyperfine shift of ^69,71Ga nuclei, measured by standard pulsed NMR. We have used the tilted-magnetic field technique to investigate the effect of an increasing Zeeman energy on P around Landau level filling factor ν=1. Mostly on the basis of measurements performed at B=17 T and T=1.5 K, we conjecture that in the high-B and low-T limit, P(ν) follows the predictions of the noninteracting-electron model. The low-temperature P(ν=1) is found to be far below the expected full polarization, and the observed nuclear spin-lattice relaxation rate is not reduced, in contrast to the observations by optically pumped NMR. The two techniques obtain different results only close to ν=1, and the reason why remains unclear; simple explanation in terms of the nonuniform electron density does not seem satisfactory.