Nuclear magnetic ordering in silver at positive and negative spin temperatures

Recent experimental results on antiferromagnetic and ferromagnetic ordering in silver at nanokelvin temperatures are reviewed. The dominant interaction between nuclear spins is the Ruderman-Kittel exchange which leads to spontaneous antiferromagnetic ordering below the record-low Néel temperatureT_n= 560 pK. AboveT_c=—1.9 nK, ferromagnetic order has beenfound, which is a consequence of the fact that the spin system tries tomaximize its Helmholtz free energy. The ferromagnetic ordering isobserved as a saturation of the susceptibility to — 1, which is caused by theformation of plate-shaped domains. Except for the re-entrant form of theantiferromagnetic phase diagram atT >0, the overall agreement betweenexperimental and theoretical results is good.