Measurement of the Nuclear Spin Diffusion Coefficient in CaF2

Abstract

Measurements are reported of the spin-lattice relaxation time $T_1$ at 80°K and at room temperature for ${\mathrm{F}}^{19}$ spins in Ca${\mathrm{F}}_2$ doped with one ${\mathrm{U}}^{3+}$ ion per 9×${10}^3$ fluorine atoms. $T_1$ was found to be strongly dependent on the orientation of magnetic field with respect to the crystalline axes. Observation at 80°K of the recovery of nuclear magnetization for very short times after saturation indicated a $t^{\frac{1}{2}}$ behavior, as predicted by Blumberg for the diffusion-limited regime of relaxation via paramagnetic impurities. The magnetization which starts out proportional to $t^{\frac{1}{2}}$ consists of spins near a paramagnetic impurity, i.e., those relaxed by direct contact with the paramagnetic ions rather than by diffusion of Zeeman energy via the nuclear dipoledipole interaction. The latter measurements, coupled with the $T_1$ measurements, yield values for the effective spin-diffusion coefficient of 0.77×${10}^{-12\mathrm{}}$ ${\mathrm{cm}}^2$/sec when the dc magnetic field is in the [111] direction, and 4.2×${10}^{-12\mathrm{}}$ ${\mathrm{cm}}^2$/sec when it is in the [100] direction. These results are in disagreement with present theories, which predict little variation in the effective diffusion coefficient with orientation of the applied field.