Nuclear Magnetic Relaxation in Rare-Earth-Doped CaF2Crystals at Low Temperatures

The spin-lattice relaxation time $T_1$ of ${\mathrm{F}}^{19}$ has been measured in single Ca${\mathrm{F}}_2$ crystals containing known concentrations of three different rare-earth ions, ${\mathrm{Tb}}^{3+}$ (1.94×${10}^{19}$ ions/${\mathrm{cm}}^3$), ${\mathrm{Tm}}^{3+}$ (1.60×${10}^{19}$ ions/${\mathrm{cm}}^3$), and ${\mathrm{Sm}}^{3+}$ (1.59×${10}^{20}$ ions/${\mathrm{cm}}^3$), respectively, over a temperature range of 28 to 300°K. The measurements were made at 29.5 Mc/sec using the magnetic recovery method with the magnetic field along the [111] direction. The data indicate that the relaxation is due to "slow diffusion" of nuclear magnetization toward paramagnetic impurities. The $T_1$-versus-temperature curve exhibits a minimum at about 41°K for the Ca${\mathrm{F}}_2$ crystals doped with ${\mathrm{Tb}}^{3+}$ and ${\mathrm{Tm}}^{3+}$. These minima yield the correlation time $\tau$ at 41°K to be 5.40×${10}^{-9\mathrm{}}$ sec for both ${\mathrm{Tb}}^{3+}$ and ${\mathrm{Tm}}^{3+}$. The nuclear spin-spin diffusion coefficient $D$ has been calculated using this value of $\tau$ and the $T_1$ measured at the temperature of the minimum. The temperature dependence of $\tau$ for the three ions has been obtained by using the measured nuclear $T_1$ data and the value of $D$ obtained as indicated above.