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Etude de l'anisotropie d'échange dans des agrégats de cobalt nanométriques

Abstract : This work deals with the study of exchange anisotropy in nanometric cobalt clusters (1-6 nm) prepared by magnetron sputtering and condensation in a gas aggregation cluster source.High resolution transmission electron microscopy and X-rays diffraction measurements show that clusters are icosahedra. This structure confirmed by calculations on the basis of atomistic model is non homogeneously strained. The magnetic anisotropy of these nanoparticles, calculated within the framework of Néel's pair model, oscillates as a function of the filling sequence of the icosahedra.
Exchange bias is first studied on an assembly of cobalt clusters for different particle density, coupled with an AF material (CoO or NiO) in thin films or shell geometries. Maximum effects are obtained when the AF layer is continuous, whether by direct contact between oxidized clusters or via an AF thin film.
Finally, we measured for the first time exchange bias on individual cobalt clusters using the micro-SQUID technique. Cobalt clusters, oxidized or not, are deposited on a CoO thin film and then covered with a superconducting niobium layer in which the micro-SQUID loops are etched. These measurements show that exchange bias is present even without field cooling the F/AF system. They also give evidence that each cluster has its own exchange anisotropy with direction and sense that are fixed by the local anisotropy of the AF.
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Contributor : Céline Portemont <>
Submitted on : Monday, December 18, 2006 - 3:53:15 PM
Last modification on : Wednesday, November 4, 2020 - 2:46:20 PM
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Céline Portemont. Etude de l'anisotropie d'échange dans des agrégats de cobalt nanométriques. Matière Condensée [cond-mat]. Université Joseph-Fourier - Grenoble I, 2006. Français. ⟨tel-00120880⟩

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