  We find that the temperature dependence of the Electron Paramagnetic
Resonance (EPR) linewidth observed in
Bi$_{0.5}$Sr$_{0.5}$Mn$_{0.9}$Cr$_{0.1}$O$_3$ (BSMCO), as well as in certain
other 3-dimensional manganites undergoing antiferromagnetic transitions is
satisfactorily described by the Berezinskii-Kosterlitz-Thouless (BKT) model. We
understand this unexpected result in terms of an effective 2-dimensional XY
easy plane anisotropy induced by the magnetic field applied in the EPR
experiment. This conclusion is supported by the field dependence of the BKT
correlations observed in the quasi two-dimensional antiferromagnetic compound
BaNi$_{2}$V$_{2}$O$_{8}$.
