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Accueil du site > Productions scientifiques > Séminaires à PHENIX > 2012 > Séminaire 24.01.2012 - 11h00

Séminaire 24.01.2012 - 11h00

par Benjamin Rotenberg - 10 janvier 2012

Jérôme Depeyrot, de l’Université de Brasilia, présentera un séminaire le 24 janvier 2012 à 11h00 dans la bibliothèque du laboratoire PECSA (7e étage, bâtiment F, porte 754) intitulé :

Local structure of spinel-type nanocrystals of ferrofluids : non equilibrium cation distributions

Résumé

Magnetic nanocolloids constitute a very attractive and promising class of nanomaterials as they may be confined, displaced, deformed and controlled by an external magnetic field. These unique and striking features make them suited for a quite large number of applications, from engineering to biomedicals. We are currently working on the synthesis of magnetic nanoparticles based on composites of metal oxides (ferrites) and on their dispersion in acidic and neutral media or in more complex systems like clays or liquid crystals. A core-shell strategy has been developed to protect the stoichiometric ferrite nanocore by a maghemite shell [1]. In this work, we explore the local structure of magnetic nanocrystals based on Zn-, Cu-, Ni and Mn-ferrite core. Measurements of neutron diffraction realized at Laboratoire Léon Brillouin (LLB-Saclay/France), of X- ray absorption spectroscopy (XAS) and X-ray Diffraction performed at the Brazilian Synchrotron Light Laboratory (LNLS) are undertaken to investigate the cations distribution, their valence state and coordination. The combined analysis suggest in all samples, a non equilibrium cation distribution among interstitial sites of the spinel-type nanocrystals structure. As an example, this cation inversion is responsible for the observed enhancement of the magnetic response of Zn-ferrite nanoparticles [2]. In Mn- ferrite nanoparticles, Rietveld refinement of neutron diffraction patterns shows that more than 50 % of Mn ions are located at octahedral sites. Moreover, the oxidation state of the manganese ions is 3+, a result different from bulk ferrite. Then, the presence of a shoulder before the main absorption peak may be attributed to Mn3+ ions in octahedral environment and the expected Jahn-Teller distortions.

We acknowledge the Brazilian agencies CNPq, CAPES and FAP/DF.

[1] J. A. Gomes, M. H. Sousa, F. A. Tourinho, R. Aquino, G. J. da Silva, J. Depeyrot, E. Dubois, R. Perzynski, , J. Phys. Chem C 112, 6220 (2008).

[2] J. A. Gomes, G. M. Azevedo, J. Depeyrot, J. Mestnik-Filho, G. J. da Silva, F. A. Tourinho, R. Perzynski, J. Magn. Magn. Mater. 323, 1203 (2011).

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