Spin correlations near the percolation threshold in three-dimensional antiferromagnets

In this paper we report on a study, using neutron scattering techniques, of the spin fluctuations in the random magnetic-nonmagnetic antiferromagnets, Mn/sub c/Zn/sub 1-c/F2 and KMn/sub c/Zn/sub 1-c/F3, with c close to the percolation concentration c/sub p/. The results for cc/sub p/ show two unexpected features. Firstly the diffuse scattering does not show any critical scattering associated with the onset of long-range order. We suggest this may be because only a small fraction of spins are in the backbone of the infinite cluster. Secondly in KMn/sub c/Zn/sub 1-c/F3 the long-range order decreases on cooling below 6 K. We suggest that this may result from the random magnetic dipole-dipole forces producing a type of spin-glass phase