Indirect Exchange Coupling of Nuclear Magnetic Moments by Conduction Electrons

A calculation is given of the indirect exchange ${\mathrm{I}}_i·{\mathrm{I}}_j$ type coupling of nuclear magnetic moments in a metal by means of the hyperfine interaction with the conduction electrons. The interaction appears to account qualitatively for the broad nuclear spin resonance lines observed in natural metallic silver. It is expected that the interaction may sharpen the resonances in pure isotopic specimens. The line shape of the minority isotope in a binary mixture may tend to be Gaussian, while that of the majority isotope may tend to be Lorentzian, if the indirect exchange interaction is dominant.