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Synthesis, structure and properties of CoPt Nanoparticles

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Chemically ordered bimetallic nanoparticles 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. 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), we are presently studying CoPt nanoparticles synthesized by Pulsed Laser Deposition. This original approach enables precise control over the size, the composition and the atomic structure of bimetallic nanoparticles, which is indispensable to understand their physical properties. [1 - 3]

We have recently investigated the effects of size and morphology on the order-disorder phase transition temperature of CoPt nanoparticles (Tc). We have developed a suite of advanced transmission electron microscopy methods to characterize the size, the shape and the atomic structure of individually analyzed sub-10 nm nanoparticles : (i) The STEM / NBD technique allows the simultaneous observation of single particles image and diffraction and give the opportunity to a correlate the size and the structure of a large number of particles. [4] (ii) We have also demonstrated that focal series experiments and electron tomography open up new ways to characterize the three dimensional shape of very small clusters. [5]

Our investigations are an important step forward in determining the origin of the size effect in magnetic alloys nanoparticles. Indeed, we have experimentally demonstrated that for 2.4 - 3 nm particles, Tc is 325°C-175°C lower than the bulk material transition temperature, consistent with our Monte Carlo simulations. [6] This quantitative information on the size-dependent depression of Tc is also 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 using electron tomography, we have established that 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 Tc . [6] This work emphasizes the necessity of taking into account the 3D morphology of nano-objects to understand and control their structural properties.

The fabrication of self-organized sub-10nm L10 ordered CoPt nanoparticles with perpendicular magnetic anisotropy would be another step forward for magnetic storage applications. We are presently working on new fabrication methods to meet the technical requirements of the future ultra-high density recording system. In addition the new aberration-corrected microscope will soon enable quantitative analysis of the nanoparticle structure such as the measurement of the order parameter or the study of possible segregation effect in very small particles.

Collaborators :

ANR Project : ETNAA.

  • {{}} The Microstructural Investigation Laboratory (ONERA - CNRS) : Y. Le Bouar & A. Loiseau
  • {{}} The Interdisciplinary Center of Nanosciences at Marseille (CINaM - CNRS) : C. Mottet
  • {{}} The Japan Electron Optic Laboratory (JEOL) : T. Oikawa
  • {{}} IPCMS Strasbourg (CNRS - Louis Pasteur University) : V. Pierron Bohnes

Me-ANS’s publications on this subject :






Article highlighted in nature materials 8, 924.