Neutron diffraction studies of nuclear magnetic ordering in silver

Abstract

Nuclear antiferromagnetism in fcc silver metal, already investigated by NMR measurements, has been studied in a single crystal of109Ag by neutron absorption and diffraction techniques. Below the Neel temperature TN, a (001) Bragg reflection with a resolution limited width demonstrates long range order in a simple type-I AFM structure with the ordering vectork = (2π/a)(001) (“up-down” structure). The entropy at the transition,\({\text{S}}_c = 0.54R\) ln 2 in zero magnetic field, corresponding to a critical polarization Pc= 0.75 and TN= 700 ± 80 pK. Magnetic field B versus entropy S phase diagrams of the (001) structure have been constructed for two directions ofB: [001] and \([\overline {0.8} {\text{ }}\overline {{\text{0}}{\text{.8}}} {\text{ }}1]{\text{.}}\) The critical field extrapolated to S = 0 is 100 ± 10 μT, and for both field directions Scis highest around B = 30 μT. The transition to the paramagnetic state is presumably of second order. The nuclear magnetization was measured by transmission of unpolarized neutrons, and the dimensionless static volume susceptibility in SI units was found to be 0.36 ± 0.01 in the ordered state independently of B and S. The ac susceptibility at 7.9 Hz showed a kink at the transition only when the sample was not exposed to neutrons.