NEUTRON EXPERIMENTS ON ANTIFERROMAGNETIC NUCLEAR-ORDER IN SILVER AT PICOKELVIN TEMPERATURES

Spontaneous long-range nuclear antiferromagnetic order in the spin-1/2 system of silver nuclei was observed by neutron diffraction on a sing le crystal of Ag-109. A structure with the ordering vector k = (2 pi/a ) (0,0,1) developed when the antiferromagnetic state was entered by ad iabatic demagnetization along the [001] axis. No domains of the other two symmetry-equivalent k vectors, (2 pi/a) (0,1,0) and (2 pi/a) (1,0, 0), formed during warmup in zero field. Nuclear susceptibility and ent ropy were found by neutron transmission. In B = 0, the critical entrop y of ordering was S-c = (0.54 +/- 0.03)R ln2.