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Pulsed nuclear magnetic resonance in metal single crystals Apps, Michael John

Abstract

The study of pulsed n. m. r. in single crystal metallic samples, initiated by McLachlan, has been extended to liquid helium temperatures with special emphasis on Sn¹¹⁹ . Contrary to McLachlan's belief it was found that cooling to 4. 2°K and lower afforded significant improvements to the signal to noise ratio and in many cases the n. m. r. signals (including spin echoes in Sn¹¹⁹ ) could readily be seen on an oscilloscope without the use of a signal averager. The theory of magnetic resonance in metallic samples was studied in some detail with particular emphasis on the experimental situation where matters are complicated by the high conductivity which modulates both the amplitude and the phase of the exciting r.f. magnetic field as it penetrates into the sample. It is shown theoretically that several assumptions must be made to show that the conventional methods of pulsed n. m. r. used to measure T₂ (by either spin echo or free induction decay) and T₁ yield true meaningful results. In particular it is found that the spin lattice relaxation time T₁ is obtained by the conventional two pulse sequence only when the magnetic field is exactly on resonance; this was observed to be the case experimentally as well. In sharp distinction to McLachlan's findings for Sn¹¹⁹, the spin-spin relaxation time T₂ obtained by FID methods (175 ± 18μsec) was much smaller than that obtained by spin echo techniques (390 ± 48μsec) in the present research on Sn¹¹⁹. The spin-lattice relaxation time was also measured at liquid helium temperatures and yielded a value of 56 ± 4 millisec deg for T₁, in excellent agreement with Dickson although twice as large as McLachlan’s value.

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