Electronic and Nuclear Magnetic Relaxation in Crystals with Fluorite Structure ContainingEu2+orMn2+

The spin-lattice relaxation time $T_1$ was measured for ${\mathrm{Eu}}^{2+}$ in Ba${\mathrm{F}}_2$ and for ${\mathrm{Mn}}^{2+}$ in Ba${\mathrm{F}}_2$ and Sr${\mathrm{F}}_2$ at about 9 GHz. The nuclear relaxation time of the ${\mathrm{F}}^{19}$ nucleus was also measured at 29 MHz at temperatures below 77°K. Each impurity ion shows a one-phonon relaxation with $T_1\propto T^{-1\mathrm{}}$ in the liquid-helium region, and a Raman relaxation with $T_1\propto T^{-5\mathrm{}}$ at higher temperatures. In the Raman region, the matrix element of the dynamic crystalline field for ${\mathrm{Eu}}^{2+}$ in Ba${\mathrm{F}}_2$ is smaller than for ${\mathrm{Eu}}^{2+}$ in Ca${\mathrm{F}}_2$, in accordance with simple crystal-field theory. The opposite effect is observed for the case of the smaller ${\mathrm{Mn}}^{2+}$ ion; the matrix element decreases as the host lattice size decreases. A minimum in the $T_1$ vs $T$ curve for the ${\mathrm{F}}^{19}$ nucleus occurs near 30°K for Ba${\mathrm{F}}_2$ containing ${\mathrm{Mn}}^{2+}$ and near 50°K for Sr${\mathrm{F}}_2$ containing ${\mathrm{Mn}}^{2+}$, in approximate agreement with theory. For temperatures below the minima, the relaxation rate is attributed predominantly to the effect of ${\mathrm{Mn}}^{2+}$ pairs or clusters and to iron, which was found by spectroscopic analysis.