NEUTRON-DIFFRACTION STUDIES OF NUCLEAR MAGNETIC-ORDERING IN SILVER

Nuclear antiferromagnetism in fee silver metal, already investigated b y NMR measurements, has been studied in a single crystal of Ag-109 by neutron absorption and diffraction techniques. Below the Neel temperat ure T-N, a (001) Bragg reflection with a resolution limited width demo nstrates long range order in a simple type-I AFM structure with the or dering vector k=(2 pi/a)(001) (''up-down'' structure). The entropy at the transition, S-c = 0.54R ln 2 in zero magnetic field, corresponding to a critical polarization P-c = 0. 75 and T-N= 700 +/- 80 pK. Magnet ic field B versus entropy S phase diagrams of the (001) structure have been constructed for two directions of B: [001] and [0.8 0.8 1]. The critical field extrapolated to S=0 is 100 +/- 10 mu T, and for both fi eld directions S, is highest around B = 30 mu T. The transition to the paramagnetic state is presumably of second order, The nuclear magneti zation was measured by transmission of unpolarized neutrons, and the d imensionless 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 su sceptibility at 7.9 Hz showed a kink at the transition only when the s ample was not exposed to neutrons.