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

• DOI: 10.1007/bf00683320
• Publication Date: 2004-11-29T00:14:51Z
• Authors: P. J. Hakonen; S. Yin
• Keywords: Magnetism; Saturation (graph theory)

Magnetic ordering in the frustrated antiferromagnetic spin-1/2 system of the natural isotopic mixture of silver nuclei was studied. 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. The authors results are consistent with the presence of a single 1 {minus} k state producing two antiferromagnetic resonance modes. They obtain {Tc} = 560 {plus minus} pK for the transition temperature at B = 0, and B{sub c} = 100 {mu}T 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 {mu}T. 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 {Tc}, the Curie-Weiss behavior was observed for the static susceptibility both at T > 0 and T < 0; for {theta} they deduce {minus}4.9 {plus minus} 0.5more » nK. These results show directly that antiferromagnetic nuclear alignment at positive temperatures transforms into ferromagnetic orientation at T < 0 in silver, which is an exchange-dominated metal.« less