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We present results of x-ray absorption spectroscopy and x-ray magnetic circular dichroism investigations at the L-3,L-2 edges of Fe in FePt nanoparticles. Plasma treated, oxide-free FePt nanoparticles with a high degree of L1(0) ordering and a mean diameter of about 6 nm were produced by gas phase condensation followed by in situ flight annealing. The presence of the L1(0) phase is observed by high resolution transmission electron microscopy. Compared to chemically disordered A1 FePt particles of similar size and composition, we find a fourfold enhanced orbital magnetic moment mu(L)=0.19 (+/- 0.04)mu(B), a nearly unchanged effective spin magnetic moment mu(eff)(S)=2.21 (+/- 0.24)mu(B) and a 2% increase in the white-line ratio of the L-3 and L-2 Fe edges. Nevertheless, the room temperature coercive field amounts to mu H-0(C)=38(+/- 7) mT only. These results are discussed in relation to possible effects of chemical order and surface anisotropy.
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... The phase diagram of the Fe-Pt system also reveals several types of magnetic orders such as ferromagnetism in Fe3Pt, FePt and FePt3 and antiferromagnetism in FePt3 [109,110]. In the alloys, the magnetic moments of Fe in the FePt alloy ranges from 2.8 to 3.5 µB/atom [111][112][113][114]. The different chemical composition and fabricating temperature of the alloys lead to different magnetic states and thus there is dependence of TC or TN on composition of Fe-Pt system. ...
Thesis
Ce document est le fruit d’un travail de thèse réalisé au sein de l’institut IRCER à Limoges au cours des trois dernières années. Il présente toutes les étapes qui ont conduit à la synthèse de nano-composites ferromagnétiques composé d’une matrice ferromagnétique dure et de nanoparticules magnétiques douces. Plus généralement, ce travail de thèse s’insère dans un projet collaboratif ANR-SHAMAN sur les composites magnétiques réunissant trois laboratoires (l’Institut Néel de Grenoble, l’Institut Lumière Matière de Lyon et l’Institut de Recherche sur les Céramiques de Limoges) et une société civile (l’European Synchrotron Radiation Facility de Grenoble). Aussi, la plupart des caractérisations magnétiques présentés dans ce document ont été réalisées à/et en collaboration avec l’Institut Néel de Grenoble. Des films minces de NdFeB et de FePt ont été développés par dépôt par ablation laser (PLD) sur différents substrats (Si/SiO2, Al2O3, MgO). Le contrôle du procédé à partir d’une cible unique ainsi que la maitrise de la structure et de la microstructure des matériaux ont conduit à l’obtention de propriétés magnétiques tout à fait remarquables. Des films (150 nm) composés de grains de Nd2Fe14B1 découplés grâce à une phase riche en Néodyme présentent un couple, rémanence/coercivité, de valeurs proches des meilleurs aimants macroscopiques du marché μoHc ~1.3 T, μoMr ~1.1 T et une courbe typique d’aimantation carré sans phase secondaire. Profitant d’un processus de démouillage induit par recuit rapide, une collection de grains isolés de FePt (15 nm) réalisés par PLD présente aussi de très bonnes caractéristiques magnétiques, μoHc ~4.4 T, μoMr ~1.3 T, des phases secondaires persistantes sont toutefois à déplorer. Parallèlement à ces développements, un générateur de nanoparticules entièrement réalisé à l’institut IRCER et associé à l’enceinte principale permet la synthèse de nanoparticules ferromagnétique de Co et Fe65Co35. Les particules métalliques dont la taille varie de 2 à 5 nm de diamètres, en fonction des paramètres appliqués au générateur, sont cristallisées et magnétiquement douces. Des nano-composites, d’architectures définis, composés de grains de FePt et de nanoparticules de Fe65Co35, à 25% volumique en proportion, ont montrées une augmentation importante (+24%) de la rémanence par rapport à un film mince de FePt conventionnel, tout en préservant intact les propriétés de coercivité. La difficulté réside dans la préservation des propriétés des composés magnétiques durs et doux malgré l’application de température élevées ~750°C et des phénomènes de diffusion associés. Ces améliorations constituent une preuve expérimentale validant la théorie sur l’augmentation des propriétés magnétiques des composites basés sur une interaction de matériaux magnétiquement durs et doux. Ces travaux de thèse se situent dans la perspective d’une maitrise des architectures à l’échelle micro/nanométrique de matériaux modèles afin d’en améliorer les propriétés magnétiques.
... In order to see the interfacial interactions at top interface of Fe, the experimental data of 21 nm thick Fe film taken in situ after deposition on Si substrate was normalized with respect to SXAS data of sample S Fe . One may note that Fe spectrum consists of 2 peaks, one at 708 eV, and other at 721 eV which corresponds to L 3 and L 2 edges of Fe respectively [59,60]. An additional intensity contribution around 709.6 eV for SXAS spectra from S Fe at various thicknesses is mainly because of interfacial Fe atoms in the vicinity of Ta. ...
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... In fact, the experimental data agree well with the range of moments reported in previous works on L1 0 FePt and L1 0 FePt-C. 3,7,13,14 Overall, the spin moment is within experimental uncertainty unaffected by the carbon concentration affirming the absence of chemical disorder or a deadlayer surrounding each grain. The slightly increased orbital moment, virtually the same for any investigated carbon concentration, indicates an enhanced vertical ordering of the Pt/Fe layer system compared with pure L1 0 FePt, which does not further improve with the carbon concentration. ...
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