Linear optical anisotropy and domain structures at two successive antiferromagnetic phase transitions in α-MnS

Abstract

Two successive antiferromagnetic phase transitions of α-MnS are confirmed to occur at T C1 = 152.7 K and T C2 = 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 T C1, which switches into a trigonal elongation below T C2. The second-order transition at T C1 (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 T C2. 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 Mn 2+ in the visible region.