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LuFe2O4 exhibits a unique mixed-valence superstructure arising from charge-ordering (CO) within the lattice. The unit cell consists of a characteristic stacking of bi-layers, in which Fe is configured in a triangular network. This results in charge and spin frustration in the material, with a resulting plenitude of degenerate magnetic-electronic ground states. Furthermore the Fe is coordinated with oxygen in a trigonal bi-pyramidal polyhedron, a rather unusual co-ordination geometry for inorganic compounds. This mixed-valence characteristic within the bi-layers has been claimed to give rise to a dipole moment, i.e., ferroelectric effects arising from the CO. However such claims of electronically driven CO are under dispute . Recent work has also demonstrated remarkable oxygen storage capacities in this compound . Previous studies have indeed shown that the Fe2+/Fe3+ ratio and magnetic ordering temperature TN are influenced by the oxygen stoichiometry [3-4]. We will report our investigations of the effect of oxygen stoichiometry on the CO and magnetic-electronic properties of LuFe2O4-δ, from a comparison of stoichiometric and oxygen deficient samples. Such samples of LuFe2O4-δ of varying oxygen stoichiometry, δ, have been synthesised by solid state reaction as a polycrystalline powder. These have been characterised by x-ray diffraction, 57Fe Mössbauer-effect spectroscopy (MES), SQUID magnetometry and TGA chemical analysis. Using different masses for the overall starting mixture has a radical effect on the purity of the as-synthesised LuFe2O4-δ sample. Magnetisation measurements show that TN is confined to 245-250 K for the synthesised samples, albeit with quite significant differences in the magnetisation-temperature envelopes. Variable cryogenic temperature MES measurements are used to compare the effect of oxygen content on both the CO and magnetic hyperfine structure.
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Prof. Giovanni Hearne (firstname.lastname@example.org)
UJ Dept. Physics