Spin-lattice relaxation of fluorine-19 in calcium fluoride at low temperatures

Abstract

A study of nuclear relaxation due to paramagnetic impurities has been made, using the system of F19 nuclei in Ca F 2. T1 was measured at helium, hydrogen, and nitrogen temperatures. The field dependence of T1 at helium temperatures consists of three parts: at fields less than a few hundred gauss, the dependence of T1 on the field is parabolic; between a few hundred gauss and about 2.5 Kg, the dependence is linear; and at higher fields, it is 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. At temperatures above 14°K, T1 has only a weak orientation dependence. The experimental data indicate that T1 is minimum between 14°K and 63°K. An analysis of the experimental results with existing theories permits a determination of the field and temperature dependence of the relaxation time rho of the spin of the paramagnetic impurity. The analysis gives rho ∼ H-2T-1 for nitrogen and hydrogen temperatures. In the helium range, rho ∼ H-2 T-1 for Ho ≲ 2.5 Kg and rho ∼ H-1T-2 for Ho ≳ 2.5 Kg. rho is calculated to be ∼10-6 sec. at 14°K and the impurity concentration is calculated to be approximately one part in 106, assuming the impurity is iron.