The spin-lattice relaxation time $T_1$ of ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$ nuclei in Si:P near the metal-insulator transition is known to have a marked magnetic field dependence. Various models have previously been proposed to account for this behavior. We present new measurements on a dilute sample with electron concentration well below the metallic range and on compensated material, Si:(P,B). We show that these data, as well as previous measurements, favor a local-moment model for the electron-spin relaxation agents rather than a Fermi-liquid model, even for samples just on the metallic side of the transition. The model, which includes effects of nuclear-spin diffusion, electron spin-spin interactions via the exchange coupling J, and frozen spin pairs, can account for the main features of the available data and connects observed temperature dependencies of $T_1$ and the spin susceptibility.

• Received 8 April 1988

DOI:https://doi.org/10.1103/PhysRevB.38.10550