Abstract
There are three different magnetic structures in the α-Mn2O3–Fe2O3 system. Magnetic transitions have been determined by measurement of susceptibility vs temperature and by Mössbauer spectroscopy. X-ray powder data on α-Mn2O3 at 6.5°K show no additional lines or symmetry change, implying that the transition at 25°K involves a shift in the symmetry center of the structure, a diffusionless but probably first-order transition; a change in space group is not required. Published neutron diffraction powder data1 on α-Mn2O3 and our own observations lead to the hypothesis that the magnetic space group of both orthorhombic phases is Pcab and of the cubic phase, Ipa3. In the latter, the directions of the ordered spins of cations in the 8b sites are absolutely fixed by symmetry, while those in the 24d sites are constrained to lie in planes perpendicular to the twofold axes. There are no symmetry restrictions on the spins in Pcab, but it is probable that all three magnetic structures are closely related. The subject matter of this abstract is included in a paper entitled ``Magnetic and Crystallographic Transitions in the α-Mn2O3–Fe2O3 System,'' which has been published.2
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