Electron-spin relaxation in bulk GaAs for doping densities close to the metal-to-insulator transition

We have measured the electron-spin-relaxation rate and the integrated spin noise power in n-doped GaAs for temperatures between 4 and 80 K and for doping concentrations ranging from 2.7 x 10(15) to 8.8 x 10(16) cm(-3) using spin noise spectroscopy. The temperature-dependent measurements show a clear transition from localized to free electrons for the lower doped samples and confirm mainly free electrons at all temperatures for the highest doped sample. While the sample at the metal-to-insulator transition shows the longest spin-relaxation time at low temperatures, a clear crossing of the spin-relaxation rates is observed at 70 K and the highest doped sample reveals the longest spin-relaxation time above 70 K.