Experimental Curves of Entropy and Susceptibility versus Temperature for Copper Nuclear Spins Down to the Ordered State

Abstract
The entropy and susceptibility of copper nuclear spins below 1 μK have been measured as a function of temperature both in the paramagnetic and in the antiferromagnetic states. The critical temperature is 63± 10 nK, about 30% of the mean-field estimate. The results are in agreement with calculations based on the spin-wave spectrum of the dipolar and the Ruderman-Kittel interactions in the fee lattice of copper.