Magnetic critical phenomena in fcc antiferromagnets: role of strain and dimensionality

We report neutron diffraction studies of MnTe epitaxial layers and MnTe/ZnTe strained superlattices prepared on (100) GaAs substrates by molecular beam epitaxy. In these systems MnTe grows in the Zinc-Blende (ZB) phase, in contrast to bulk MnTe, found in the NiAs form. The magnetic interactions are short range and antiferromagnetic, and result in a fcc type-III antiferromagnetic ordering (AFM-III). The renormalization group theories predict a fluctuation-induced first order phase transition for such system in the absence of strain. Such transitionsis indeed reported in previous work on bulk β-MnS, and is observed in our MnTe epilayers. The epilayers seem to be virtually strain-free, with all three AFM-III domains present. This picture is complicated, however, by our discovery of a substantial magnetostriction in MnTe. This magnetostriction, observed by precise measurement of the lattice constant, seems to result in the relative population shifts between the domains. The superlattices with the MnTe layer thickness of 30 and 60 Å exhibit a single AFM-III domain that has the lowest strain-determined energy, consistent with observed small tetragonal distortion of the MnTe lattice, resulting from the ZnTe and MnTe lattice mismatch. In the presence of strain the dimensionality of the order parameter is reduced from 6 to 2, in which case the theory predicts the continuous phase transition. This is indeed observed experimentally.