The static susceptibility of a single-crystal copper specimen has been measured at nanokelvin temperatures, parallel and perpendicular to the external magnetic field $B$. Below $B_c=0.27\mathrm{}$ mT, three different antiferromagnetically ordered regions are described at small values of entropy $S$. The $B-S$ phase diagram is presented. Metastabilities and the observed nonadiabaticity during the final stages of demagnetization are discussed.

• Received 13 June 1984

DOI:https://doi.org/10.1103/PhysRevLett.53.1692