Magnetic-field-induced spin polarization of AlAs two-dimensional electrons

Two-dimensional (2D) electrons in an in-plane magnetic field become fully spin polarized above a field ${B}_{P},$ which we can determine from the in-plane magnetoresistance. We perform such measurements in modulation-doped AlAs electron systems, and find that the field ${B}_{P}$ increases approximately linearly with 2D electron density. These results imply that the product $|{g}^{*}|{m}^{*},$ where ${g}^{*}$ is the effective g factor and ${m}^{*}$ the effective mass, is a constant essentially independent of density. While the deduced $|{g}^{*}|{m}^{*}$ is enhanced relative to its band value by a factor of $\ensuremath{\sim}4,$ we see no indication of its divergence as 2D density approaches zero. These observations are at odds with results obtained in Si, but qualitatively confirm spin-polarization studies of 2D GaAs carriers.