Double-spin-flip resonance of rhodium nuclei at positive and negative spin temperatures

Sensitive SQUID-NMR measurements were used to study the mutual interactions in the highly polarized nuclear-spin system of rhodium metal. The dipolar coupling gives rise to a weak double-spin-flip resonance. The observed frequency shifts allow deducing separately the dipolarlike contribution and the isotropic exchange term. For the first time, such measurements were extended to negative absolute temperatures as well. We find an effective dipolar moment 0.10 mu(N) of which about 15% is attributed to a conduction electron mediated pseudodipolar interaction. The isotropic exchange is described by R = -0.9 +/- 0.1.