OBSERVATION OF NUCLEAR FERROMAGNETIC ORDERING IN SILVER AT NEGATIVE NANOKELVIN TEMPERATURES

The ferromagnetically ordered state in the nuclear spin system of silver has been reached at negative absolute temperatures by adiabatic nuclear demagnetization at entropies below 0.82 R ln2. The ordering, caused by the antiferromagnetic Ruderman-Kittel interaction, was observed below -1.9 nK as a saturation of susceptibility close to -1 and as an increase of the NMR frequencies. Comparison with recent mean-field calculations by Viertio and Oja suggests a domain configuration. The phase diagram of silver nuclei at T < 0 is outlined in the magnetic field versus entropy plane.