NUCLEAR ANTIFERROMAGNETISM IN RHODIUM METAL AT POSITIVE AND NEGATIVE NANOKELVIN TEMPERATURES

We have measured the nuclear susceptibility of polycrystalline rhodium foils down to 280 pK and up to -750 pK. In spite of these record-low and -high spin temperatures, the ordered states were not reached. At T > 0, the measured susceptibility follows the antiferromagnetic Curie- Weiss law with theta = -(1.8 +/- 0.3) nK. At T < 0, however, ferromagn etic behavior at high entropy crosses over to antiferromagnetic tenden cy around \T\ < 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.