Spin fluctuations in random magnetic-nonmagnetic two-dimensional antiferromagnets. II. Heisenberg percolation

In this paper we report an extensive study using neutron scattering techniques of the spin fluctuations in the two-dimensional diluted near-Heisenberg antiferromagnet Rb2Mn/sub c/Mg/sub 1-c/F4. The concentrations studied are c=0.54, c=0.57, and c=0.60; the site-percolation concentration for the nearest-neighbor square lattice is c/sub p/=0.593 so that these experiments span the percolation threshold. The point c=c/sub p/, T=0 represents the percolation multicritical point which terminates the line of second-order transition of the infinite network. We give a detailed description of the theory of the magnetic behavior around the percolation point; as the temperaturelike scaling field we suggest μ (T) =kappa1(T) when kappa1(T) is the inverse correlation length for the associated one-dimensional chain; for the static structure factor we propose the formula S ( vertical-barc/sub p/-cvertical-bar,μ,Q) proportional kappa/sup eta/(kappa2+Q2)-1, where kappa=kappa ( vertical-barc/sub p/-cvertical-bar,0)+kappa (0,μ); that is, we assume that the geometrical and thermal inverse correlation lengths are simply additive. The c=0.60 sample is found to have a smeared second-order phase transition at about 8 K to a state with two-dimensional long-range order but only weak correlations in the third direction. The spin fluctuations in the precritical region are essentially identical to those in the concentrated systems, thus demonstrating that the phase transition in the c=0.60 infinite network is little affected by the proximity to the percolation threshold