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Pulsed nuclear magnetic resonance in metal single crystals McLachlan, Leslie Allan

Abstract

Spin-lattice relaxation times have been measured in metal single crystals with a pulsed nuclear magnetic resonance apparatus at both room and liquid nitrogen- temperatures. The values obtained for aluminum and vanadium agreed well with the values given in the literature for powdered samples. The niobium value was slightly lower than the most reliable powder value, possibly because of impurities. Measurements were made on isotopically pure tin to see if any anisotropy could be detected in the spin-lattice relaxation time. No anisotropy could be detected, but the crystal orientation used was so unfavourable that an anisotropy of less than about 50% could not be detected. The spin-spin relaxation time was measured in the isotopically pure tin for five different magnetic field orientations. These showed that exchange narrowing occurred. With a suitable choice of operating conditions, the apparatus measured the equivalent of the absorption mode in steady state nuclear magnetic resonance as a function of magnetic field orientation. This was combined with the spin-spin measurements to give the complete orientation dependence of the latter. These measurements gave a value of (2.1±0.3)Kc/s. for the pseudo-exchange constant in tin. The pseudo-dipolar second moment was found to be twice the dipolar second moment. Spin echoes were observed in the isotopically pure tin and were used to measure the spin-spin relaxation time. These gave values which were much shorter than those measured by free induction decays. The reason for this was not determined.

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