Linear optical anisotropy and domain structures at two successive antiferromagnetic phase transitions in α-MnS |
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Authors: | W Kleemann FJ Schäfer H van der Heide |
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Institution: | Fachbereich 6 (Physik), Universität GH Paderborn, Postfach 1621, D 4790 Paderborn, Fed. Rep. Germany;Laboratory of Inorganic Chemistry, Material Science Center of the University, Nyenborg 16, NL 9747 AG Groningen, The Netherlands |
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Abstract: | Two successive antiferromagnetic phase transitions of α-MnS are confirmed to occur at TC1 = 152.7 K and TC2 = 129.5 K, respectively, by microscopic observations of T domain formation and rearrangement, linear birefringence (LB), optical absorption and linear dichroism (LD) measurements. Stress experiments and X-ray data reveal a rhombohedral lattice contraction below TC1, which switches into a trigonal elongation below TC2. The second-order transition at TC1 (β = 0.40) is proposed to yield the familiar fcc type-2 single axis spin order of NiO and MnO, whereas a multispin axis structure seems to be achieved via a first-order transition below TC2. Model calculations of the exchange striction essentially agree with the observed lattice distortions, whereas neutron diffraction data are less conclusive with respect to the proposed spin structure transformation. The interdependence of LB and LD is shown via dispersion relations and a Kramers-Kronig analysis involving the d-d transitions of Mn2+ in the visible region. |
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