Measurement of the local-moment-induced electric field gradient at dilute gold impurities in iron by single-passage NMR on oriented nuclei

Abstract

The effect of small quadrupole interactions on single-passage nuclear magnetic resonance of oriented nuclei is reported both in theory and experiment. In contrast to the purely magnetic (Zeeman) case, the quadrupole interaction $P$ gives rise to a sweep-direction dependence of the resonance signal. This yields the sign of $P$ and subsequent time development gives the nuclear-spin-lattice relaxation time. The magnitude of $P$ can be obtained from the signal during passage even in the presence of inhomogeneous broadening $\ensuremath{\Gamma}\ensuremath{\gg}2IP$. The technique is applied to $^{198}\mathrm{Au}$ nuclei in cubic ferromagnetic Fe and $P=0.21\ifmmode\pm\else\textpm\fi{}0.02$ MHz is obtained. The quadrupole interaction is the result of spin-orbit coupling at the spin-polarized Au ion breaking the cubic-lattice symmetry. The Korringa constant ${C}_{K}$ for relaxation of the $^{198}\mathrm{Au}$ nuclei is measured as 1.2 K sec.