NUCLEAR ANTIFERROMAGNETISM IN RHODIUM METAL AT POSITIVE AND NEGATIVE NANOKELVIN TEMPERATURES

We have measured the dynamic susceptibility of polycrystalline rhodium foils down to 280 pK and up to -750 pK. These record-low and -high nuclear spin temperatures were reached by adiabatic demagnetization using initial polarizations of 83% and -60%. At T > 0, the static susceptibility, integrated from NMR spectra, displays an antiferromagnetic Curie-Weiss law, with theta = -1.8+/-0.3 nK. At T < 0, a crossover from ferro- to antiferromagnetic tendency is found around -6 nK. We obtain J(nn)/h = -17 +/- 3 Hz and J(nnn)/h = 10 +/- 3 Hz if only nearest and next nearest neighbor interactions are assumed.