Nuclear cooperative phenomena in copper at nanokelvin temperatures

We have studied nuclear cooperative phenomena in metallic copper by demagnetizing highly polarized spins to low fields where spin‐spin interactions dominate. In zero external field, the critical temperature T_c of the antiferromagnetic phase was 60±10 nK, and during the first order transition the entropy changed from 0.49Rln4 to 0.65Rln4. The critical field B_c=0.25±0.01 mT. Evidence for another transition at 0.09 mT was also seen. The entropy of nuclear spins in zero external field is presented as a function of temperature; our S vs. T diagram, however, is provisional especially in the ordered phase.