Abstract
The magnetic‐field dependence of the order‐disorder transition temperature Tc in RbMnF3 and KNiF3 was measured in fields H up to 180 kOe. In both of these very nearly isotropic cubic antiferromagnets, Tc(H) increases with increasing H at low H, as predicted by Fisher, Nelson and Kosterlitz. The values for the crossover exponent φ obtained from the data for two RBMnF3 samples are 1.278±0.026 and 1.274±0.045, compared to the theoretical value 1.250±0.015.