Investigations of nuclear magnetism in silver down to picokelvin temperatures. II

We have studied magnetic ordering in the frustrated antiferromagnetic spin-1/2 system of the natural isotopic mixture of silver nuclei. The dynamic susceptibility of polycrystalline foils was measured down to about 0.5 nK (S>0.4R ln 2) using SQUID-NMR techniques. Saturation and a decrease of the static susceptibility were observed at the lowest temperatures, as well as clear changes in the NMR line shapes, which are associated with spontaneous, first-order antiferromagnetic alignment of silver nuclei. Our results are consistent with the presence of a single 1 - k state producing two antiferromagnetic resonance modes. We obtain T_c=560±60 pK for the transition temperature at B=0, and B_c=100 μT for the critical field extrapolated to T=0. The critical entropy was found to increase from 0.52 R ln 2 in zero field to 0.64 R ln 2 at B=30 μT. Combined with the Clausius-Clapeyron equation, this suggests that the isentropes are non-monotonous as a function of field in the low-temperature paramagnetic region. Above T_c, the Curie-Weiss behavior was observed for the static susceptibility both at T>0 and T