Critical Properties of Pure and Random Antiferromagnets: CoF2, Co/ZnF2, KMn/ NiF3

Neutron scattering techniques have been used to study the critical properties of CoF2 and the randomly mixed systems: Co/ZnF2 and KMn/NiF3. The results for CoF2 are in excellent accord with the critical properties of the three-dimensional Ising model. In all of the random crystals studied the transition was smeared. In CoF2 the smearing corresponds to concentration fluctuations of about 1.5% while in KMn/NiF3 it corresponds to about 2.5%. In both cases macroscopic chemical inhomogeneities were observed in these systems by electron microprobe analysis of a magnitude consistent with the observed smearing. Despite and smearing, measurements were made of the critical properties of Co/ZnF2. The results are insufficiently accurate to clearly distinguish between the properties of the pure three-dimensional Ising model and of the random system.