Two-dimensional Behavior of the Sublattice Magnetization in Three Dimensional Ising Antiferromagnets

A three-dimensional layered Ising-Antiferromagnet with a ferromagnetic intra-layer coupling to z neighbors, zJ > 0, and an antiferromagnetic interlayer coupling to z neighbors, zJ ′ < 0, is investigated by Monte Carlo simulations on a hexagonal lattice. The physical nature of the anomalous temperature bahavior of the sublattice magnetizations, which is found for certain values of r = zJ/zJ ′ and z in magnetic fields is explained in terms of successive phase transitions. They take place on the ferromagnetic 2-dimensional spin-down sublattice at T ≈ T 2d c , smeared by a finite stabilizing molecular field, and on both antiferromagnetically coupled sublattices at T 3d c > T 2d c .