Critical properties of the models of small magnetic particles of the antiferromagnet MnF2

The static critical behavior of the models of small magnetic particles of the real two-sublattice antiferromagnet MnF2 is investigated by the Monte Carlo method taking into account the interaction of the second nearest neighbors. Systems with open boundaries are considered to estimate the influence of the sizes of particles on the pattern of their critical behavior. The behavior of thermodynamic functions in the phase transition region is investigated. The data on the maxima of the temperature dependences of heat capacity and magnetic susceptibility are shown to be insufficient to unambiguously determine the effective temperature of the phase transition in the models of small magnetic particles. This requires an additional investigation of the spatial orientation of the sublattice (sublattices) magnetization vector for the models under study.