Nuclear Magnetic Resonance in Solid Helium-3—Helium-4 Mixtures between 0.3 and 2.0 °K

Nuclear-magnetic-resonance techniques have been used to study vacancy diffusion and the exchange interaction of ${\mathrm{He}}^{3}$ in solid ${\mathrm{He}}^{3}$-${\mathrm{He}}^{4}$ mixtures between 0.3 and 2.0 \ifmmode^\circ\else\textdegree\fi{}K for molar volumes between 20 and 22 cc/mole. The concentrations studies are 32.1, 7.78, and 1.94% ${\mathrm{He}}^{3}$ in ${\mathrm{He}}^{4}$. There is good agreement between the diffusion activation energies determined from the ${T}_{2}$ measurements and those obtained by measuring the diffusion constant directly using the field-gradient technique. These activation energies are consistently lower for mixtures than for pure ${\mathrm{He}}^{3}$. The ${T}_{1}$ data in the Zeeman-exchange plateau region indicate that the exchange interaction is independent of concentration. The ${T}_{2}$ data in the exchange-narrowed region are not in agreement with the theoretical result obtained by allowing the moments of the line shape to become concentration dependent. The experimental values of ${T}_{2}$ are much lower than the predicted values. This deviation is qualitatively explained by postulating the existence of two spin species: those that strongly experience the effects of exchange and those that do not. A small fraction of the isolated spins can then dominate the ${T}_{2}$ relaxation process.

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