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°K. The experimental data indicate that $T_1$ is at a minimum between 14 and 63°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 $\rho$ of the paramagnetic impurity. The analysis gives $\rho \propto H^{-\frac{7}{4}}{T}^{-1\mathrm{}}$ at helium and hydrogen temperatures for $300 \mathrm{G}<H<\mathrm{}3\mathrm{} \mathrm{kG}$. The impurity concentration is calculated to be approximately one part in ${10}^6$ and $\rho$ is calculated to be ≈${10}^{-6\mathrm{}}$ sec at 14°K, assuming the impurity to be iron.