Frustration in FCC Antiferromagnets Studied by Neutron Scattering

K sub 2 PbM (NO sub 2) sub 6 M=Cu,Ni), which approximately realize quantum FCC antiferromagnets, have been studied by neutron scattering. For M=Cu (S = 1/2) short-range correlations exists at T=5K. Upon cooling the correlations continuously change symmetric until a 2nd order phase-transition at T sub (N1) = 0.7K. The magnetic order is a complicated long wavelength incommensurate state. The incommensurability varies continuously with temperature until a 1st order transition at T sub (N2) = 0.4K where q = (1/2 0.055 0).