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Nanocrystalline Fe-based spinels with composition Fe 3-x Ti x O 4 are synthesized using soft chemistry. Two steps are involved: precipitation in an aqueous solution followed by thermal annealing under a reducing mixture of N 2 /H 2 /H 2 O gases. Fe-segregation is found inside stoichiometric particles when the powders are studied ex situ; they exhibit a strong surface iron enrichment. This heterogeneity is related to kinetic effects linked to the difference of mobility between Fe 2+ and Ti 4+ cations during the partial oxidation of cations occurring ex situ. Stresses in the grains induced by oxidation govern the oxidation kinetics and lead to an abrupt compositional variation inside each particle. These heterogeneities in stoichiometric powders have been investigated by a combination of averaging and local techniques: XRD and Mössbauer spectrometry for an average analysis of powders, XPS for an analysis of the surface of the grains, and HRTEM for a local analysis of single grains.

A detailed understanding of the formation of magnetic vortices in closely spaced ferromagnetic nanoparticles is important for the design of ultra-high-density magnetic devices. Here, we use electron holography and micromagnetic simulations to characterize three-dimensional magnetic vortices in chains of FeNi nanoparticles. We show that the diameters of the vortex cores depend sensitively on their orientation with respect to the chain axis and that vortex formation can be controlled by the presence of smaller particles in the chains.

Ductile metals and alloys undergo plastic yielding at room temperature, during which they exhibit work-hardening and the generation of surface instabilities that lead to necking and failure. We show that pure nanocrystalline copper behaves differently, displaying near-perfect elastoplastic behavior characterized by Newtonian flow and the absence of both work-hardening and neck formation. We observed this behavior in tensile tests on fully dense large-scale bulk nanocrystalline samples. The experimental results further our understanding of the unique mechanical properties of nanocrystalline materials and also provide a basis for commercial technologies for the plastic (and superplastic) formation of such materials.

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Mechanical properties Mechanical properties D 3000Near-Perfect Elastoplasticity in Pure Nanocrystalline Copper. -Fully dense bulk nanocrystalline Cu samples are prepared by powder metallurgy processing followed by cold isostatic pressing and annealing under H2 at low temperature. Ultimate densification is achieved by differential extrusion. Tensile tests show that these Cu samples display near-perfect elastoplastic behavior characterized by Newtonian flow and the absence of both work-hardening and neck formation. It is expected that these properties will not necessarily be limited to Cu and other metals and alloys may also show such properties. With grain refinement, near-perfect elastoplasticity opens new perspectives for commercial shaping and machining of nanocrystalline materials at room temperature. -(CHAMPION*, Y.; LANGLOIS, C.; GUERIN-MAILLY, S.; LANGLOIS, P.; BONNENTIEN, J.-L.; HYTCH, M. J.; Science (Washington, D. C.) 300 (2003) 5617, 310-311; Cent. Etud. Chim. Metall., CNRS, F-94407 Vitry-sur-Seine, Fr.; Eng.) -W. Pewestorf 29-011