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 less than or equal to x less than or equal to 0.4, is investigated in the limit of vanishing external field. 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, Fisher's (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 T-N(x), but also in relation to the "local" phase transition temperatures T-N' throughout the Griffiths regime, T-N(x)less than or equal to T-N'less than or equal to T-N(0), a satisfactory quantitative description of chi vs T is deduced for any x above the percolation threshold.