Magnetic structure and collective Jahn-Teller distortions in nanostructured particles of CuFe2O4

The aim of the present work is to compare the structural, the composition and chemical state of the surface and magnetic properties of different nanosized CuFe2O4 powders exhibiting collective Jahn-Teller effect. The samples under examination consist of edged nanosized particles (needle like) with average length 1300 +/- 20 nm and diameter 300 +/- 20 nm obtained after high temperature synthesis, and superparamagnetic (at room temperature) spherical particles (d = 6 +/- 2 nm), obtained by soft chemistry techniques. The surface composition of the particles was investigated by X-ray photoelectron spectroscopy (XPS). Mossbauer spectroscopy (MoS), including at high magnetic field up to 5 T and 4.2 K, was used for characterization of cation distribution in the samples. The data yielded by the XPS and Mos analyses for spherical nanosized particles led us to the assumption for the existence of a Jahn-Teller effect gradient-from the B-sublattice on the surface to a compensation of the tetragonal distortion in the two sublattices in the core. The analysis of the contribution of the anisotropy energy in edged and spherical nanoparticles shows that it must be considered as an effective value reflecting the influence of the individual factors depending on the particle shape and surface. (c) 2006 Elsevier B.V. All fights reserved.