The structural and magnetic properties of the hexagonal four-layer form of SrMnO3 have been investigated by combining magnetization measurements, electron diffraction and high-resolution synchrotron X-ray and neutron powder diffraction. Below 350K, there is subtle structural phase transition from hexagonal symmetry (space group P63/mmc) to orthorhombic symmetry (space group C2221) where the hexagonal metric is preserved. The second-order phase transition involves a slight tilting of the corner-sharing Mn2O9 units composed of 2 face-sharing MnO6 octahedra and the associated displacement of Sr2+ cations. The phase transition is described in terms of symmetry-adapted displacement modes of the high symmetry phase. Upon further cooling, long range magnetic order with propagation vector k=(0,0,0) sets in below 300K. The antiferromagnetic structure, analyzed using representation theory, shows a considerably reduced magnetic moment indicating the crucial role played by direct exchange between Mn centers of the Mn2O9 units.
@article{arxiv.cond-mat/0609235,
title = {Structural phase transition and magnetism in hexagonal srmno},
author = {A. Daoud-Aladine and C. Martin and L. C. Chapon and M. Hervieu and K. S. Knight and M. Brunelli and P. G. Radaelli},
journal= {arXiv preprint arXiv:cond-mat/0609235},
year = {2007}
}