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, MncZn_1-cF₂ and KMncZn_1-cF₃, with c close to the percolation concentration cₚ. The results for c<cₚ are consistent with the multicritical point picture for the percolation point with the temperature scale determined by the inverse correlation length of the appropriate linear chain. This approach gives a very satisfactory account of the spin-space crossover from Heisenberg-like to Ising-like behavior observed in tetragonal MncZn_1-cF₂. The exponent νT describing the temperature dependence of the inverse correlation length is found to be 0.85 ±{} 0.10 for this system and, with rather less certainty, we infer νT=0.95±0.05 for the cubic KMncZn_1-cF₃. The results for c>cₚ 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 KMncZn_1-cF₃ 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.