NUCLEAR MAGNETIC-RELAXATION IN LIQUID-HE-3 AND HE-3-HE-4 MIXTURES

The spin-lattice relaxation time T-1 of liquid He-3 and He-3-He-4 mixt ures is determined by two parallel relaxation processes: intrinsic rel axation, which is caused by dipolar interaction between the He-3 nucle ar spins, and surface relaxation, due to interaction of the He-3 nucle ar spins with the magnetic moments at the walls of the experimental ce ll. Using a type of torque magnetometer, we have measured T-1 of liqui d He-3 containing 0.5% He-4 and He-3-He-4 mixtures with a He-3 concent ration ranging from 6 to 95%, as a function of magnetic field up to 22 T at temperatures between 40 mK and 1 K. Due to the difference in the ir magnetic-field dependences, we have been able to separate the intri nsic and surface contributions to T-1. Our measurements reveal a surfa ce relaxation mechanism for liquid He-3, with a relaxation time propor tional to the square of the magnetic field, which can be described by the classical relaxation theory of Bloembergen, Purcell, and Pound. We relate the observed classical relaxation mechanism to the dynamics of the He-3 atoms in the He-4 film at the surface. The temperature depen dence of the surface relaxation time T-s is consistent with the hypoth esis that the surface relaxation is caused by diffusive motion of He-3 atoms near the surface. This mechanism would naturally explain the pr eviously unexplained observations that T-s is inversely proportional t o the diffusion coefficient, while T-s is clearly larger than the diff usion time. We find the intrinsic relaxation time Ti, of the pure liqu id He-3 in good agreement with existing Fermi-liquid theory, and obser ve the T-in of the He-3-He-4 mixtures at 1 K to be proportional to the He-3 concentration, in agreement with theoretical predictions. [S0163 -1829(98)07025-8].