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Size and Shape of magnetic nanoparticles influence their phase diagram.

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Chemically ordered bimetallic nanoparticles (Fig. a) are promising candidates for magnetic storage applications. However, the use of sub-10 nm nanomagnets requires further study of possible size effects on their physical properties. The Me-ANS group from the Materials and Quantum Phenomena Laboratory (University Paris 7 - CNRS), in collaboration with the Microstructural Investigation Laboratory (ONERA - CNRS), the Interdisciplinary Center of Nanosciences at Marseille (CINaM - CNRS) and the Japan Electron Optic Laboratory (JEOL), have investigated the effects of size and morphology on the order-disorder phase transition temperature of CoPt nanoparticles (TNP). They used and developed a suite of advanced transmission electron microscopy methods to determine the size, the morphology and the atomic structure of individually analyzed nanoparticles. Their observations are confirmed theoretically with canonical Monte Carlo simulations. For 2.4 - 3 nm particles, TNP is found to be 325°C-175°C lower than the bulk material transition temperature. This quantitative information on the size-dependent depression of TNP is very important for the technological applications of small CoPt nanoparticles, because this size effect significantly restricts the annealing temperature range in which ordering can be induced. Furthermore, by analyzing three-dimensional reconstructions of nanoparticles obtained by electron tomography (Fig. b), they established that TNP is also sensitive to the shape of the nanoparticles. Only one dimension of the particles (in-plane size or thickness) smaller than 3 nm is sufficient to induce a depression of TNP (Fig. c). This work emphasizes the necessity of taking into account the 3D morphology of nano-objects to understand and control their structural properties.


Size and shape effects on the order–disorder phase transition in CoPt nanoparticles, D. Alloyeau, C. Ricolleau, C. Mottet, T. Oikawa , C. Langlois, Y. Le Bouar, N. Braidy and A. Loiseau, Nature Materials, 8, 940 - 946 (2009).


Damien Alloyeau, Research scientist,