Spin-Lattice Relaxation ofF19in CaF2at Low Temperatures

Abstract

A study of nuclear relaxation caused by paramagnetic impurities has been made, using the system of ${\mathrm{F}}^{19}$ nuclei in Ca${\mathrm{F}}_{2}$. The nuclear spin-lattice relaxation time ${T}_{1}$ was measured at helium, hydrogen, and nitrogen temperatures. The dependence of ${T}_{1}$ on magnetic field at helium temperatures consists of two parts: at fields between a few hundred gauss and about 3 kG, the dependence is linear; at higher fields it also appears to be linear, but has a steeper slope. Moreover, there is a strong orientation dependence in the helium range for a single crystal, and a size effect appears at higher fields. ${T}_{1}$ has only a weak orientation dependence at temperatures above 14\ifmmode^\circ\else\textdegree\fi{}K. The experimental data indicate that ${T}_{1}$ is at a minimum between 14 and 63\ifmmode^\circ\else\textdegree\fi{}K. Employing existing theories, an analysis of the experimental results permits a determination of the field and temperature dependence of the spin-lattice relaxation time $\ensuremath{\rho}$ of the paramagnetic impurity. The analysis gives $\ensuremath{\rho}\ensuremath{\propto}{H}^{\ensuremath{-}\frac{7}{4}}{T}^{\ensuremath{-}1}$ at helium and hydrogen temperatures for $300 \mathrm{G}<H<3 \mathrm{kG}$. The impurity concentration is calculated to be approximately one part in ${10}^{6}$ and $\ensuremath{\rho}$ is calculated to be \ensuremath{\approx}${10}^{\ensuremath{-}6}$ sec at 14\ifmmode^\circ\else\textdegree\fi{}K, assuming the impurity to be iron.