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

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 $\Gamma \gg 2IP$. The technique is applied to ${}_{}{}^{198}{}_{}{}^{}\mathrm{Au}$ nuclei in cubic ferromagnetic Fe and $P=0.21\mathrm{}\pm 0.02$ 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.