Abstract

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 temperature T_N = 560 pK. Above T_c = -1.9 nK, ferromagnetic order has been found, which is a consequence of the fact that the spin system tries to maximize its Helmholtz free energy. The ferromagnetic ordering is observed as a saturation of the susceptibility to -1, which is caused by the formation of plate-shaped domains. Except for the re-entrant form of the antiferromagnetic phase diagram at T > 0, the overall agreement between experimental and theoretical results is good.