Compression of gold sub-micron crystallites: Method and experiments

Understanding and characterizing the mechanical response of individual nanostructure is of great importance for both fundamental prospects and device reliability. Higher flow stress with decreasing sample size is observed together with jerky flow. Compression of pristine submicron gold crystallites yield at very large stress in a stochastic manner, followed by large displacement bursts reaching up to 50% of the initial height [1,2]. In this work, by collecting a large set of measurements, we investigate the small and large strain behavior of crystallites loaded in compression. Large arrays of [111] oriented gold crystallites are prepared by solid state dewetting of initial cylinders of different volumes on sapphire substrates. Dedicated flat punch compression in-situ a FEG-SEM (figure 1a) has been carried out in load controlled mode [3]. Microstructure of defects is investigated using synchrotron radiation by nanoscale 3D imaging (Bragg Coherent X-ray Diffraction Imaging) [4] and Atomic Force Microscopy observations. The analysis of the plastic instability and its amount of deformation is carried out taking into account the inertial effect of the instrument, using a 1D dynamic model and Finite Element Method calculations. Simulations are made with different estimates of the shape of each individual crystallite, from an ideal cylinder of equivalent volume to the one based on SEM or AFM observations. We show that prior to the displacement burst, plastic events take place and that the sudden displacement does not necessarily relates to the onset of dislocation nucleation (figure 1b). Moreover, using the collection of measurements, we show that a unique stress-strain response can be obtained which can be used as a lower bound estimate of the mechanical response in compression of the crystallites. Please click Additional Files below to see the full abstract

Antiferromagnetic order and the structural order-disorder transition in the Cd6Ho quasicrystal approximant

International audienceIt has generally been accepted that the orientational ordering of the Cd4 tetrahedron within the Cd6 R quasicrystal approximants is kinetically inhibited for R = Ho, Er, Tm and Lu by steric constraints. Our high-resolution X-ray scattering measurements of the Cd6Ho quasicrystal approximant, however, reveal an abrupt (first-order) transition to a monoclinic structure below T S = 178 K for samples that have 'aged' at room temperature for approximately one year, reopening this question. Using X-ray resonant magnetic scattering at the Ho L 3-edge we have elucidated the nature of the antiferromagnetic ordering below T N = 8.5 K in Cd6Ho. The magnetic Bragg peaks are found at the charge forbidden H + K + L = 2n + 1 positions, referenced to the high-temperature body-centred cubic structure. In general terms, this corresponds to antiferromagnetic arrangements of the Ho moments on adjacent clusters in the unit cell as previously found for Cd6Tb

Strain inhomogeneity in copper islands probed by coherent X-ray diffraction

International audienceThe strain field of individual epitaxial sub-micrometric copper islands is studied using coherent X-ray diffraction and finite element modelling. The strain inhomogeneity in each island is so large that the characteristic features of the island shape tend to disappear in the diffraction pattern, which is dominated by strain effects. The model confirms the tensile strain imposed to the island by the thermal mismatch occurring during the preparation of the samples. An evaluation of the residual strain is obtained by qualitatively fitting the diffraction data

ETUDE DE NANOSTRUCTURES<br />MAGNÉTIQUES PAR DIFFRACTION<br />RÉSONANTE ET COHÉRENTE DES<br />RAYONS X MOUS

In this thesis, I present a study of magnetic nanostructures with soft x-ray resonant scattering.The samples are stacks of epitaxial thin layers made of FePd alloys and Co/Pt multilayers depositedon patterned Silicon substrates. In the first part, the samples are presented and charcterizedby means of conventional techniques and neutron measurements. Additionally, a micromagneticmodelization is described. In the second part, soft x-ray scattering from the FePd layers ismeasured at the L3-edge of Iron and modelized, in order to obtain information on the periodicmagnetic configuration of the samples. In the third part, I used a soft x-ray coherent beam inorder to characterize in detail the magnetic configuration of model samples. For this, an experimentalset-up is developed and a methodology is described to count photons on a CCD camera.A Monte-Carlo algorithm is proposed and discussed, aiming to reconstruct the exact magneticconfiguration of a nanoline grating with perpendicular magnetization.Dans cette thèse, je présente une étude de nanostructures magnétiques par la diffusion résonantedes rayons X mous. Les échantillons étudiés sont des empilements de couches mincesépitaxiées d'alliages de FePd et des multicouches de Co/Pt déposées sur des substrats de Siliciumnanostructurés. Dans une première partie, les échantillons sont présentés et caractérisés par destechniques conventionnelles de laboratoire, ainsi que par des mesures de neutrons. En outre, unemodélisation micromagnétique est décrite. Dans une deuxième partie, la diffusion des rayons Xmous par les couches de FePd est mesurée au seuil L3 du Fer et modélisée afin de tirer des informationssur la configuration magnétique périodique des échantillons. Dans une troisième partie,j'utilise un faisceau cohérent de rayons X mous afin de caractériser en détail la configurationmagnétique d'échantillons modèles. A cette fin, un dispositif instrumental est développé et uneméthodologie est décrite pour le comptage des photons sur une camera CCD. Un algorithme deMonte-Carlo est proposé et discuté en vue de reconstruire la configuration magnétique exacted'un réseau de nanolignes à aimantation perpendiculaire

ETUDE DE NANOSTRUCTURES<br />MAGNÉTIQUES PAR DIFFRACTION<br />RÉSONANTE ET COHÉRENTE DES<br />RAYONS X MOUS

In this thesis, I present a study of magnetic nanostructures with soft x-ray resonant scattering.The samples are stacks of epitaxial thin layers made of FePd alloys and Co/Pt multilayers depositedon patterned Silicon substrates. In the first part, the samples are presented and charcterizedby means of conventional techniques and neutron measurements. Additionally, a micromagneticmodelization is described. In the second part, soft x-ray scattering from the FePd layers ismeasured at the L3-edge of Iron and modelized, in order to obtain information on the periodicmagnetic configuration of the samples. In the third part, I used a soft x-ray coherent beam inorder to characterize in detail the magnetic configuration of model samples. For this, an experimentalset-up is developed and a methodology is described to count photons on a CCD camera.A Monte-Carlo algorithm is proposed and discussed, aiming to reconstruct the exact magneticconfiguration of a nanoline grating with perpendicular magnetization.Dans cette thèse, je présente une étude de nanostructures magnétiques par la diffusion résonantedes rayons X mous. Les échantillons étudiés sont des empilements de couches mincesépitaxiées d'alliages de FePd et des multicouches de Co/Pt déposées sur des substrats de Siliciumnanostructurés. Dans une première partie, les échantillons sont présentés et caractérisés par destechniques conventionnelles de laboratoire, ainsi que par des mesures de neutrons. En outre, unemodélisation micromagnétique est décrite. Dans une deuxième partie, la diffusion des rayons Xmous par les couches de FePd est mesurée au seuil L3 du Fer et modélisée afin de tirer des informationssur la configuration magnétique périodique des échantillons. Dans une troisième partie,j'utilise un faisceau cohérent de rayons X mous afin de caractériser en détail la configurationmagnétique d'échantillons modèles. A cette fin, un dispositif instrumental est développé et uneméthodologie est décrite pour le comptage des photons sur une camera CCD. Un algorithme deMonte-Carlo est proposé et discuté en vue de reconstruire la configuration magnétique exacted'un réseau de nanolignes à aimantation perpendiculaire

Etude de nanostructures magnétiques par diffraction résonante et cohérente des rayons X mous

Les rayons X mous (~400-1500 eV) sont un outil de choix pour l'étude des nanostructures ferromagnétiques. Leur longueur d'onde (1-3 nm) est tout à fait adaptée à l'échelle caractéristique de ces systèmes : les couches minces ferromagnétiques étudiées pour le stockage magnétique sont épaisses de quelques dizaines de nanomètres, et présentent des domaines magnétiques de quelques dizaines de nanomètres également. De plus, les métaux de transition, principaux éléments magnétiques de ces matériaux, ont leur seuil de résonance L3 dans cette gamme d'énergie. Cette résonance exalte la sensibilité des rayons X au moment magnétique local. Dans cette thèse, nous démontrons que cette sensibilité peut être utlisée pour étudier la configuration des moments magnétiques dans le volume. Nous étudions le cas particulier de couches minces de FePd. En filtrant le faisceau de rayonnement synchrotron avec un diaphragme microscopique, on sélectionne un faisceau très cohérent. Sa diffraction, mesurée avec une caméra CCD utilisée comme compteur de photons, est une mesure haute résolution du module de la transformée de Fourier de l'objet éclairé. Nous présentons ici la diffraction magnétique cohérente d'une microstructure de FePd et d'un réseau périodique de nanolignes parallèles à aimantation perpendiculaire. La possibilité de reconstruire l'image exacte de la configuration magnétique des nanolignes est discutée.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

Ordering fluctuation dynamics in AuAgZn 2

International audienceThe second-order ordering transition of the AuAgZn2 alloy has been studied by coherent x-ray scattering. Within a few degrees above the critical temperature Tc, equilibrium critical fluctuations are observed together with some pre-transitional local ordering connected to sample defects. The speckles observed correspond to heterodyne interference between local ordering and fluctuations and show a mixed static and dynamical behaviour in a narrow domain of a few tenths of degree above Tc. The dynamical behaviour is shown to correspond to the critical slowing down of the fluctuations in the vicinity of the transition (model " A " of Hohenberg and Halperin 1). A rough comparison can be carried out with the classical diffusion models. Some improvements of the method are discussed

Charge superstructure in single-layered La2-xCaxCoO4±δ (0.4≤x≤0.7) studied by resonant x-ray scattering

Resumen del trabajo presentado a la 17th International Conference on X-Ray Absorption Fine Structure (XAFS), celebrada en Kraków (Polonia) del 22 al 27 de julio de 2018.Hole-doped La2-xCaxCoO4±δ single-layered perovskites undergoes structural phase transitions with decreasing temperature due to the rich interplay between charge, spin, orbital and lattice degrees of freedom. These structural transitions are coupled to an electronic localization due to the nominal fractional valence of the Co atoms resulting in charge superstructure. In this work, we propose a model for the Co charge superstructure in the singlelayered La2-xCaxCoO4±δ (0.4≤x≤0.7) perovskites based on the photon energy, azimuth angle and polarization dependence of the scattered intensity of selected reflections measured in a resonant x-ray scattering experiment. Room temperature resonant x-ray experiments at the Co K-edge we carried out at the BM02-D2AM beam line at the ESRF. Single crystals were cut with the surface perpendicular to the [1, 1, 0]t direction for La2-xCaxCoO4±δ (0.4, 0.5 and 0.6) whereas La1.3Ca0.7CoO3.9 was cut along the [0, 0, 1]t direction. An empirical analysis of the resonant data, once corrected for selfabsorption, was carried out by the experimental determination of the resonant atomic scattering factor tensor. Theoretical simulations of the resonant reflections were also carried out by the FDMNES code to estimate the magnitude of the local structural distortions associated with the charge disproportionation. Tetragonal superlattice (h/2, h/2, l)-type reflections were detected for all the samples independently from the Ca content in agreement with a checkerboard ordering of two different Co sites. The corresponding average charge disproportionation results in about 0.5±0.1 e-. No forbidden (h/4, h/4, l)-type reflections were observed at room temperature indicating the lack of any local anisotropy ordering in contrast with the behavior found in the single-layered manganites. Finally, we found the pattern of small distortions (~ 0.05 Å) of the alternating expanded and compressed CoO6 octahedra compatible with the resonant x-ray scattering results. The symmetry of this displacement pattern is consistent with the A2mm (or Ammm) orthorhombic structure of the ordered phase and the structural transition is accounted for by the condensation of two soft modes ─X1 +(B2u) and X1 +(A1) ─ acting on the oxygen atoms. Thus, the single-layered RE1.5A0.5TMO4 (RE=rare earth, A=Ca, Sr and TM=Mn, Ni, Co) perovskites show a common checkerboard ordering of two non-equivalent TM atoms associated to two distinct valence states. However, compared to La1.5Sr0.5MnO4, where the maximum charge disproportionation is about 0.15 e-, the charge disproportionation (or the compression/expansion of the oxygen octahedra) in the single-layered cobalt oxides is relatively high (~0.5 e-). This is understood by the fact that charge superstructures (driven by concomitant structural transitions) appears at much higher temperatures (well above room temperature) in the layered cobalt oxides than in the layered manganites (TCO~230 K).Peer Reviewe