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

Abstract

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\ifmmode^\circ\else\textdegree\fi{}K. Each impurity ion shows a one-phonon relaxation with ${T}_{1}\ensuremath{\propto}{T}^{\ensuremath{-}1}$ in the liquid-helium region, and a Raman relaxation with ${T}_{1}\ensuremath{\propto}{T}^{\ensuremath{-}5}$ 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\ifmmode^\circ\else\textdegree\fi{}K for Ba${\mathrm{F}}_{2}$ containing ${\mathrm{Mn}}^{2+}$ and near 50\ifmmode^\circ\else\textdegree\fi{}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.