Nuclear-Magnetic-Resonance Spin Echoes in Alloys

Abstract

In disordered alloys, the Knight shift and the quadrupolar coupling vary from one nuclear site to the next. These variations have a strong influence on the shape of the spin-echo envelope of a given nuclear spin species. A general calculation has been developed involving 90°-180° pulse sequences. Two types of modulation of the spin-echo envelope are predicted under certain conditions. The first is related to quadrupolar effects, the second to spin-spin interactions. Experiments on ${\mathrm{Pb}}^{207}$ (I=½) have been performed at liquid-helium temperature in various lead solid solutions. The shape of the experimental echo envelopes agrees with the theoretical predictions. In particular, there is no dependence of the modulation period on the nature of the solute or on its concentration. The value of this period ($2{\tau }_M=305\mathrm{}\pm 15$ μ sec) together with the exchange-narrowed linewidth in pure Pb metal is used to determine the indirect spin-spin-interaction coupling constants. The values of these coupling constants between first-nearest neighbors are, respectively, $\frac{J}{2\pi }=4.8\mathrm{}$ kHz for the indirect exchange and $\frac{b}{2\pi }=2.2\mathrm{}$ kHz for the pseudodipolar interaction. Similar results are analyzed for platinum. An attempt is made to interpret the results in terms of the band structure.