Crossover from pure Ising to random exchange dominated behavior of the two-dimensional antiferromagnet Rb2Co1-xMgxF4

The temperature dependence of the uniform susceptibility, chi, of diluted two-dimensional Ising antiferromagnets Rb2Co1-xMgxF4,0<=x<=0.4, is investigated in the limit of vanishing external field. Novel data for x = 0.15 are compared with those obtained for x = 0 and 0.4 by Breed et al. (1969) and Ikeda (1983), respectively. Whereas in the pure case, x = 0, Fishers (1962) "energetic contribution" dominates, Aharony et al.s (1979,1986) "random contribution" becomes increasingly important with increasing x. Taking into account both terms not only with respect to the global ordering temperature, TN(x), but also in relation to the "local" phase transition temperatures TN throughout the Griffiths regime,TN(x)<=TN<=TN(0), a satisfactory quantitative description of chi vs. T is deduced for any x above the percolation threshold.