Experimental charge density of hematite in its magnetic low temperature and high temperature phases

Ultramicroscopy, Volume 120, September 2012, Pages 1-9
R. Theissmann, H. Fuess, K. Tsuda 

Faculty of Engineering and CeNIDE (Center for NanoIntegration Duisburg-Essen), University of Duisburg-Essen, Bismarckstr. 81, 47057 Duisburg, Germany

Institute for Materials Science, Darmstadt University of Technology, Petersenstr. 23, 64287 Darmstadt, Germany

Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, 980-8577 Sendai, Japan 

Abstract

Structural parameters of hematite ({Alpha}-Fe2O3), including the valence electron distribution, were investigated using convergent beam electron diffraction (CBED) in the canted antiferromagnetic phase at room temperature and in the collinear antiferromagnetic phase at 90 K. The refined charge density maps are interpreted as a direct result of electron–electron interaction in a correlated system. A negative deformation density was observed as a consequence of closed shell interaction. Positive deformation densities are interpreted as a shift of electron density to antibinding molecular orbitals. Following this interpretation, the collinear antiferromagnetic phase shows the characteristic of a Mott–Hubbard type insulator whereas the high temperature canted antiferromagnetic phase shows the characteristic of a charge transfer insulator. The break of the threefold symmetry in the canted antiferromagnetic phase was correlated to the presence of oxygen–oxygen bonding, which is caused by a shift of spin polarized charge density from iron 3d-orbitals to the oxygen ions. We propose a triangular magnetic coupling in the oxygen planes causing a frustrated triangular spin arrangement with all spins lying in the oxygen planes. This frustrated arrangement polarizes the super-exchange between iron ions and causes the spins located at the iron ions to orient in the same plane, perpendicular to the threefold axis.

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