Nuclear magnetism in silver at positive and negative absolute temperatures in the low nanokelvin range

We have investigated the susceptibility and entropy in the thermally isolated system of silver nuclei down to 0.8 nK and, at negative temperatures, up to -4.3 nK. Low-frequency SQUID-NMR techniques were employed to measure the dynamic sysceptibility. Curie-Weiss behavior was observed for the static susceptibility both at T>0 and T<0; for FTHETA we deduce -4.4±1.0 nK. Our results show directly that antiferromagnetic nuclear alignment at positive temperatures transforms into ferromagnetic orientation at T<0 in the nuclear-spin system of silver, dominated by exchange interaction.