Cobalt nanoparticles encapsulated in carbon shells have been synthesized by catalytic chemical vapor deposition (CCVD) in high yield by reducing with a H 2 /CH 4 gas mixture a Mg 0.9 Co 0.1 O solid solution impregnated MgO (SSI-MgO) catalyst. The carbon-encapsulated Co nanoparticles have a narrow distribution of diameters within the range 5-15 nm. They are made of (fcc)-Co as shown by XRD and are very stable to air oxidation; the magnetic properties have been investigated using a SQUID magnetometer and confirm that Co is present in the metallic state.

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Cubic antiferromagnetic nanoparticles of KMnF3 (see Figure) have been prepared and their synthesis as well as their properties are described in this communication. Small, ordered domains of about 350 nm size were obtained. The narrow size distribution provided by the reverse micelle technique allows the particles to self‐assemble into a well‐ordered array. Work towards bigger ordered areas is currently in progress.

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The iron manganites FexMn( 3 _X)0 4 synthesis by soft chemistry method have been studied. The main difficulty is to obtain single phase spinel with high Mn content (0.4< x < 1.3). Oxalate precursor powders of these materials with controlled shape and nanoscopic size have been prepared. The precursors are then heat treated with a H2/H 2 O/N 2 gas mixture at low temperature. The resulting stoechiometric spinels are metastable phases with high specific surface area and are highly reactive toward oxygen. Therefore, these oxide can be oxidized in air at low temperature in order to produce mixed valence defect manganites 0 4+8 with a good reproducibility on the oxygen content. Although, some problems persist for the higher Mn contents, as the oxygen partial pressure for the reduction must be controlled precisely in order to produce the stoechiometric spinel at low temperature. The development of a low temperature reduction system, with oxygen partial pressure controlled by oxygen electrochemical pumping, is in progress.

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