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Développement de matériaux nanocomposites ferro-magnétiques par ablation laser. Application à la synthèse de nanoparticules de Co et FeCo magnétiques douces noyées dans des matrices magnétiques dures de NdFeB et FePt

Abstract : The results of the thesis were carried out at the IRCER institute in Limoges, France over the past three years. It presents all the steps to synthesize ferromagnetic nanocomposites composed of hard magnetic matrix and soft magnetic nanoparticles (NPs). This work is a part of a joint project SHAMAN ANR-16-CE09-0019 on magnetic nanocomposites bringing collaboration between three laboratories Néel Institute in Grenoble, Lumière Matière Institute in Lyon and Institute of Research for Ceramics in Limoges, and the European Synchrotron Radiation Facility in Grenoble. All of the magnetic characterizations presented in this work were conducted at Néel Institute. Thin films of NdFeB and FePt have been successfully developed by Pulsed Laser Deposition (PLD) on various substrates (Si/SiO2, Al2O3, MgO) using single alloy targets. The control of the structure and microstructure of materials have led to achieve quite remarkable magnetic properties. 150 nm-thin films composed of Nd2Fe14B1 grains decoupled by a secondary Nd-rich paramagnetic phase present values of magnetic coercivity and remanence close to the best macroscopic magnets on the market, e. g., μoHc ~1.3 T, μoMr ~1.1 T and a square loop shape with single magnetic phase behavior. Taking advantage of dewetting process induced by rapid thermal annealing process, 15 nm-FePt granular films possess also very good magnetic characteristics up to μoHc ~4.4 T, μoMr ~1.3 T, but with remaining secondary phase. Moreover, a homemade nanoparticle generator entirely developed at IRCER and associated with the main PLD chamber allowed to fabricate ferromagnetic Co and Fe65Co35 nanoparticles. The diameters of NPs are well controlled in the range 2-5 nm, depending on the parameters in the nucleation cavity. All NPs are crystallized and magnetically soft. Soft-hard magnetic nanocomposites with defined architectures and composed of FePt grains with 25% volume fraction of Fe65Co35 NPs, exhibit a significant increase of + 24% in remanence compared to that of conventional FePt thin film, while the coercivity remains constant at around 2 T. The challenge is about preserving simultaneously the properties of hard and soft magnetic phases, with the required high temperatures of 750°C and the associated diffusion phenomena. These results constitute an experimental evidence validating the theory about the enhancement of the magnetic properties in the composites, allowing exchange-coupling interaction between hard and soft phases. This work opens the way to micro/nanoscale architectures of model materials in order to augment their magnetic properties.
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Tuan Nguyen Van. Développement de matériaux nanocomposites ferro-magnétiques par ablation laser. Application à la synthèse de nanoparticules de Co et FeCo magnétiques douces noyées dans des matrices magnétiques dures de NdFeB et FePt. Matériaux. Université de Limoges, 2021. Français. ⟨NNT : 2021LIMO0085⟩. ⟨tel-03661802⟩

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