Studies of nuclear magnetism in silver at positive and negative nanokelvin temperatures

Abstract

We have measured the entropy and susceptibility of silver nuclei down to T = 0.8 nK and T = - 4.3 nK. A ferromagnetic Curie-Weiss law was found at negative absolute spin temperatures, in contrast to the antiferromagnetic law at T > 0. For the Curie-Weiss θ we obtain −4.4 ± 1.0 nK. These results provide the first direct demonstration that the antiferromagnetic alignment at positive temperatures transforms into a ferromagnetic preference at T < 0 in a nuclear system where exchange interaction dominates.