RARE-EARTH PEROVSKITES, SYNCHROTRON ELECTRON DENSITIES AND MAGNETIC PROPERTIES

Victor A. Streltsov, Crystallography Centre, University of Western Australia, Nedlands 6907, Australia

The relationship of the electron density to the magnetic properties of some rare earth (RE) perovskite-orthoferrites, REFeO₃, was studied by imaging the deformation electron density (Dr) with synchrotron radiation.

Strong magnetic effects are due primarily to electron spin density. Exceptional precision is required to measure small spin-dependent cross-sections directly. Spin density and charge density are related by the effect of un-compensated spins on the electron probability density. Exchange correlation between electrons occurs when atoms overlap. Their "fermion" character prevents two electrons from occupying the same quantum state. If a two-electron spin wave function component is symmetric, the corresponding anti-symmetric space component reduces electron density in the region of maximum overlap.

Exchange Hamiltonians are invariant to arbitrary simultaneous rotation of all spins, causing degeneracy additional to that for spatial symmetry. Ordering of magnetic states with the same exchange energy for a crystal generates an exchange multiplet. The effects of the higher symmetry of the exchange Hamiltonian may influence a deformation density image strongly.

High precision synchrotron X-ray imaging can be applied to such systems, sometimes with unexpected consequences. Metal-metal interactions, though subordinate to nearest-neighbour cation-anion interactions in terms of energy, can affect electron density strongly. Results for a-Al₂O₃, a-Fe₂O₃ and the C-type RE oxides suggest that cation-anion effects on the electron density could become larger if the RE atom site symmetry were lowered. The REFeO₃ compounds, have distorted perovskite structures with low (C_s) symmetry of the RE atom sites, suitable for testing this hypothesis. The conclusions, based on high precision diffraction imaging for YFeO₃, SmFeO₃, GdFeO₃ and DyFeO₃ with synchrotron radiation, will be described.