Observation of a Magnetic Phase Transition in the Nuclear Spin System of Metallic Copper at Nanokelvin Temperatures

Abstract

A phase transition was observed in the nuclear spin system of metallic copper in external fields less than 0.22 ± 0.02 mT and at entropies in the range of $\mathrm{}(0.4-0.6)R\ln 4$; the transition temperature was estimated to be of the order of 100 nK. Indications of a second anomaly at 0.08 ± 0.01 mT were also seen. The transition is antiferromagnetic and of first order. It is accompanied with large overcooling of the paramagnetic phase, which explains the discrepancies in the previous results.