Spin Transition in a 2DES at $\nu = 1/2$

The transition from partial to complete electron spin polarization as a function of density in a 2DES at $\nu = 1/2$ has been probed using a resistively-detected NMR (RDNMR) technique. Both the nuclear spin lattice relaxation time $T_1$ of $^{71}$Ga and the response in resistance to a change in the nuclear spin polarization appear to reflect this transition. At low densities, where the electron spin polarization is partial, the $T_1$ time is relatively short, due to the presence of both electron spin states at the Fermi level. In this regime $T_1$ is density independent and has a Korringa-like temperature dependence. Above a critical density $T_1$ increases and the RDNMR signal eventually vanishes, consistent with a transition to complete electron spin polarization. In the transition region we observe a non- Korringa $T_1$ temperature dependence and an unexpected enhancement of the RDNMR signal.