Paramagnetic Relaxation of Some Rare-Earth Ions in Diamagnetic Crystals

The pulsed-saturation method at 8.9 kMc/sec was used to measure the spin-lattice relaxation rate ${T_1}^{-1\mathrm{}}$ for ${\mathrm{Eu}}^{2+}$, ${\mathrm{Ho}}^{2+}$, and ${\mathrm{Tm}}^{2+}$ in Ca${\mathrm{F}}_2$, for ${\mathrm{Yb}}^{3+}$ in yttrium gallium garnet, yttrium aluminum garnet, and lutetium gallium garnet; for ${\mathrm{Nd}}^{3+}$ in yttrium gallium garnet and yttrium aluminum garnet; and for ${\mathrm{Sm}}^{3+}$ in lanthanum ethyl sulphate at low temperatures. The experimental data in most cases are in satisfactory agreement with the theoretical predictions based on the combined Van Vleck-Orbach theory. Besides the expected conventional processes, a process ${T_1}^{-1\mathrm{}}\propto T^5$ was observed for ${\mathrm{Eu}}^{2+}$ in Ca${\mathrm{F}}_2$ in the temperature range 15°K<30°K. Furthermore, a pronounced anisotropy in $T_1$ (a factor ∼6) for ${\mathrm{Sm}}^{3+}$ in lanthanum ethyl sulphate was found both experimentally and theoretically.