<p> Neutron diffraction techniques have been used to determine the chemical and magnetic order in a single crystal of the Heusler alloy Ni2Mn0.8v0.2sn. This material orders in the Heusler L21 structure and is ferromagnetic. Nuclear Bragg scattering intensity ratios have been measured at 298 K and compared with nuclear structure factor calculations based on a model of the crystal structure. This comparison is used to determine chemical disorder. Magnetic Bragg scattering intensity ratios have been measured at 117 K. These ratios, along with bulk magnetization measurements, are used to determine the spatial distribution of the magnetic moment. The crystal is found to have the L21 structure with possibly 5% or so Ni-Sn disorder. Nearly all of the magnetic moment, which is 3.19 ± .03 μ8/mol, is found to exist on the Mn-V sites. The rest is present on the Sn atoms. The values 3.74 ± .10 μ8/Mn atom and .21 ± .08 μ8/Sn atom result from assuming the V atoms carry no moment. </p> / Thesis / Master of Science (MSc)

Cette étude a pour objectif de comprendre comment les effets de température, de cinétiques de croissance, ou les effets d’environnement peuvent influencer la structure (cristalline ou non cristalline) et la configuration chimique (mélange/ ségrégation/ séparation de phase) de nanoparticules supportées d’Ag et de CoAg. Pour cela, des nanoparticules de CoAg de différentes tailles et compositions ont été préparées par condensation sous ultravide selon différents modes de croissance (co-dépôt ou dépôts séquentiels des deux métaux). Afin d’accéder à l’ensemble des caractéristiques des nanoparticules, des techniques complémentaires ont été couplées: la diffusion de rayons X aux petits et aux grands angles en incidence rasante et les techniques de microscopie en mode d’imagerie haute résolution ou filtrée en énergie. L’analyse préliminaire de particules d’Ag a montré l’existence de structures cristallines et non cristallines (icosaédriques) pour les petites (2-2.5nm) et grandes tailles (6-8nm). Une dominance de la structure décaédrique a été montrée entre ces deux extrêmes. Cette dernière disparaît complètement lors de l’élaboration en température. Pour le système bimétallique Co-Ag, à température ambiante et indépendamment de la taille, de la composition et du mode d’élaboration, les nanoparticules présentent une ségrégation avec une configuration de type coeur d’argent entouré d’une coquille plus ou moins continue à base de Co métallique et d’oxyde de Co. Lorsque les échantillons sont soumis à un traitement thermique, une transition s’opère conduisant à une ségrégation plus importante de type Janus. / This study aims to understand how the temperature, the kinetic growth conditions or the environment can influence the structure (crystalline or non-crystalline) and the chemical order (mixing/ segregation/phase separation) of Ag and CoAg supported nanoparticles. Different samples of CoAg nanoparticles with different sizes and compositions were prepared by condensation under ultrahigh vacuum with different growth modes (co-deposition or sequential deposition of the two metals) and different thermal treatements. To access all the characteristics of the nanoparticles, complementary techniques were coupled: the X ray scattering at small and wide angles under grazing incident X Ray beam and the electron microscopy techniques: (high resolution and energy filtered modes). Preliminary analysis of Ag particles prepared at room temperature showed the existence of crystalline and non-crystalline (icosahedral) structures for small (2-2.5nm) and large sizes (6-8nm). A dominant feature of the decahedral structure was shown between these two extremes. This structure disappears completely when increasing elabration temperature. For the bimetallic Co-Ag system, at room temperature and independently of the size, composition and growth mode, the nanoparticles present a segregated configuration with a silver core surrounded by a more or less continuous shell, based on metallic Co or cobalt oxyde. After heating the samples, a transition takes place, leading to a more important segregation such as Janus one.

Many physical properties, for example structural or magnetic, of a material are directly dependent on elemental composition. Tailoring of properties through highly accurate composition control is possible in thin films. This work exemplifies such tailoring. A short review is given of the current status for research in the area of permanent magnets, focusing on rare earth element free alternatives, where FeNi in the L10 phase is a possible candidate. Epitaxial FeNi L10 thin films were successfully synthesized by magnetron sputtering deposition of monoatomic layers of Fe and Ni on HF-etched Si(001) substrates with Cu or Cu100-xNix/Cu buffers. The in-plane lattice parameter aCuNi of the Cu100-xNix buffer layer was tuned by the Ni content. Through matching of aFeNi to aCuNi, the strain state (c/a)FeNi was controlled, where c is the out-of-plane lattice parameter. The 001 reflection indicative of chemical order, as measured by resonant x-ray diffraction, was in most cases split in two peaks due to a composition modulation of Fe and Ni. This chemical disorder contributed to that the uniaxial magnetocrystalline anisotropy energy, KU≈0.35 MJ/m3, was smaller than predicted. In later experiments the composition modulation could partly be compensated for. Remaining discrepancies with respect to predicted KU values were attributed to additional disorder induced by surface roughness of the buffer layer. The interface sharpness between Fe and Ni was explored by producing epitaxial symmetric multilayers with individual layer thicknesses n = 4-48 monolayers (ML). For n ≤ 8 ML the films had pure fcc structure, with antiferromagnetic Fe layers. For n ≥ 8 ML the Fe layers relaxed to bcc structure. A combinatorial sputter chamber, which has the capability to deposit samples with composition and thickness gradients, was assembled. A model for simulation of composition and thickness across large substrates, for the conditions in this chamber, is presented. The model is verified by comparison to experimental data. Some challenges inherent in combinatorial sputtering are discussed, and two experimental studies employing the technique are presented as examples. These investigated magnetic and structural properties of Tb-Co films, with 7-95 at.% Tb, and of amorphous and crystalline ternary gradient Co-Fe-Zr films, respectively.