Nuclear spin-lattice relaxation time in copper below 10 −2° K

Abstract

THE nuclear spin-lattice relaxation in metals is governed primarily by the interaction of the nuclear spins with the conduction electrons. The corresponding relaxation time has been calculated by Korringa, ~ who gave an expression for r which is inversely proportional to the conduction electron temperature and to the square of the Knight shift. The theory has been modified subsequently by Pines, 2 Redfield, 3 and Hebel and Slichter. 4 Experimental v, ork on copper at low temperatures has been done by Anderson and Redfieid s in the liquid helium temperature range and by Kurti 6 and others in the adiabatic demagnetization temperature range down to 1-2 x 10 -2° K. The first authors used resonance techniques for the determination of the relaxation time, whereas the method of the Oxford group was based on a nuclear cooling of the copper spin system. Also in the investigations reported here a nuclear cooling method was used. The nuclear spin system was precooled by rotational demagnetization of CMN (cerium magnesium nitrate) which is highly anisotropic and which has only small interactions, so that temperatures down to a few millidegrees can easily be attained. The nuclear stage consisted of a bundle of copper wires in thermal contact with the salt. Copper was used as a nuclear sample because of its large nuclear susceptibility. Both salt and copper were located in the pole gap of an electromagnet, which could be rotated for precooling the salt and removed after the nuclear demagnetization. The spin-lattice relaxation time was determined by recording the decay of the nuclear susceptibility of the copper after the final demagnetization. The susceptibility was measured by means of an electronic mutual inductance bridge, the output of which was detected and recorded. A ballistic method was used for the determination of the susceptibility of the salt. The corresponding temperature was calibrated in the usual way by comparison with the vapour pressure of the liquid helium between 1 ° K and 4.2 ° K. The nuclear spin temperature was not calibrated; it was calculated using the theoretical value of the nuclear spin susceptibility and the sensitivity of the mutual inductance measurement. The salt tempera-