The coupling of electron and nuclear spins in n-GaAs changes significantly as the donor concentration n increases through the insulator-metal critical concentration n C ϳ 1.2ϫ 10 16 cm −3 . The present measurements of the 71 Ga relaxation rates W made as a function of magnetic field ͑1-13 T͒ and temperature ͑1.5-300 K͒ for semi-insulating GaAs and for three doped n-GaAs samples with donor concentrations n = 5.9ϫ 10 15 , 7ϫ 10 16 , and 2 ϫ 10 18 cm −3 , show marked changes in the relaxation behavior with n. Korringa-like relaxation is found in both metallic samples for T Ͻ 30 K, while for T Ͼ 30 K phonon-induced nuclear quadrupolar relaxation is dominant. The relaxation rate measurements permit determination of the electron probability density at 71 Ga sites. A small Knight shift of −3.3 ppm was measured on the most metallic ͑2 ϫ 10 18 cm −3 ͒ sample using magic-angle spinning at room temperature. For the n = 5.9ϫ 10 15 cm −3 sample, a nuclear relaxation model involving the Fermi contact hyperfine interaction, rapid spin diffusion, and exchange coupled local moments is proposed. While the relaxation rate behavior with temperature for the weakly metallic sample, n =7 ϫ 10 16 cm −3 , is similar to that found for the just-insulating sample, the magnetic field dependence is quite different. For the 5.9ϫ 10 15 cm −3 sample, increasing the magnetic field leads to a decrease in the relaxation rate, while for the 7 ϫ 10 16 cm −3 sample this results in an increase in the relaxation rate ascribed to an increase in the density of states at the Fermi level as the Landau level degeneracy is increased.

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Absorption line shape and spin-lattice relaxation-time measurements have been made on ' C nuclei (1.1% abundant) in a type-IIb single-crystal semiconducting diamond. The line shape measurements are consistent with theoretical predictions for a dilute spin system. Over the temperature range 295-375 K, the spin-lattice relaxation time is long (3-4 h) and shows a temperature dependence which may be explained by means of a model in which spin diffusion plays a role, and the paramagnetic acceptor centers relax via an Orbach-type process involving excited states near the valence band.

Poly(vinyl acetate) (PVAc) has been studied by proton magnetic relaxation methods. The polymer exhibits a typical glass transition in that the nmr relaxation times behave in much the same way as observed for other substances. Pressure dependence results yield an activation volume of about 160 cm3/mol in the glass-transition region. Ester methyl reorientations, evident in dielectric studies, have almost no effect on the nmr parameters. Methyl group rotations dominate relaxation at low temperatures, and quantum mechanical tunneling is probably involved. The methyl rotational barriers are