Abstract
Nanoparticle samples of Fe3O4, CoFe2O4 and NiFe2O4 with mean sizes in the 5-8 nm range and excellent crystallinity have been synthesized by thermal decomposition of organic precursors. Mössbauer spectra taken at 4.2 K in applied fields from 0 to 12 T could be fitted with two sextets corresponding to the A and B Fe sites of the spinel structure. The cation distribution was deduced from the relative intensities of both sextets. The Fe oxide spectrum was more similar to γ-Fe2O3 than to Fe3O4, suggesting this sample has undergone partial or total oxidation. A small residual intensity at lines 2 and 5 reveals some spin canting at both Fe sites, however the absence of a third sextet indicates that a "core-shell" spin structure, with disordered spins near the surface, does not occur in these nanoparticles.
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