Unconventional ferromagnetic and spin-glass states of the reentrant spin glass Fe0.7Al0.3

Spin excitations of single crystal Fe0.7Al0.3 were investigated over a wide range in energy and reciprocal space with inelastic neutron scattering. In the ferromagnetic phase, propagating spin wave modes become paramagnon-like diffusive modes beyond a critical wave vector q0, indicating substantial disorder in the long-range ordered state. In the spin glass phase, spin dynamics is strongly q-dependent, suggesting remnant short-range spin correlations. Quantitative model for S(energy,q) in the ``ferromagnetic'' phase is determined.Comment: 4 pages, 5 figure

Depletion and voids formation in the substrate during high temperature oxidation of Ni-Cr alloys

A numerical model to treat the kinetics of vacancy annihilation at the metal/oxide interface but also in the bulk metal has been implemented. This was done using EKINOX, which is a mesoscopic scale 1D-code that simulates oxide growth kinetics with explicit calculation of vacancy fluxes. Calculations were performed for high temperature Ni-Cr alloys oxidation forming a single chromia scale. The kinetic parameters used to describe the diffusion in the alloy were directly derived from an atomistic model. Our results showed that the Cr depletion profile can be strongly affected by the cold work state of the alloy. In fact, the oversaturation of vacancies is directly linked to the efficiency of the sinks which is proportional to the density of dislocations. The resulting vacancy profile highlights a supersaturation of vacancy within the metal. Based on the classical nucleation theory, the possibility and the rate of void formation are discussed

Etude du phénomène de corrosion d'un acier bas carbone étame 25Pb/75Sn dans un environnement pollue par HCl

International audienceL'incineration des ordures menageres, la combustion du charbon ou encore certaines activites industrielles comme le nettoyage peuvent conduire a la formation de produits corrosifs tels que le chlorure d'hydrogene ou l'acide chlorhydrique car au contact de l'eau, le chlorure d'hydrogene se solubilise et cree instantanement un electrolyte acide, charge en chlorures. L'agressivite de ces agents conduit a des phenomenes de corrosion dans les aciers qui peuvent etre particulierement forts. Afin de limiter les consequences, l'acier peut subir un traitement de surface tel que l'etamage Pb/Sn. Si dans la plupart des applications, ce revetement est mis en place pour ses proprietes de lubrification, peu d'entre elles l'utilisent comme barriere contre la corrosion, et ceci en raison du manque de donnees dans la litterature. Afin de comprendre ces phenomenes de corrosion, nous nous sommes interesses a la corrosion d'un acier bas carbone recouvert d'une couche d'etamage 75Sn/25Pb expose a un flux de gaz humide pollue par HCL. L'etude montre la particularite du mecanisme de corrosion de la couche d'etamage qui se fait en plusieurs etapes au cours desquelles la couche evolue

Superficial segregation in nanoparticles from facets to infinite surfaces

International audienceWe compare the superficial segregations of the Cu-Ag system for a nanoparticle and for surfaces that are structurally equivalent to each of its facet. Based on a lattice-gas model and within a mean-field formalism, we derive segregation isotherms at various temperatures in the canonical ensemble, i.e., for a given overall solute concentration, and in the semi-grand canonical ensemble, i.e., for a given bulk solute concentration. If both processes are very similar for high temperatures, they differ substancially at lower temperatures. Due to the finite-size effect and the indirect coupling between facets and edges, the relative position of the phase transitions of the facets and the corresponding surfaces are inversed when displayed as function of the solute bulk concentration. Moreover, we show that working in the semi-grand canonical ensemble is a much more efficient way to study this phenomenon, although nanoparticles are canonical- objects in essence

Superficial segregation, wetting, and dynamical equilibrium in bimetallic clusters: A Monte Carlo study

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X-ray spectrometer for anomalous scattering experiments : tests of a graphite mosaïc and a germanium monocrystal as analyzers

Anomalous X-ray scattering provides a useful way for improving the contrast between different atoms present in multi-element samples. To take full advantage of this technique, the detector has to have a good energy resolution in the diffracted beam to separate the elastic scattering signal from the inelastic one due to fluorescence or resonant Raman scattering (RRS). Solid state detectors are usually used, but the energy resolution is limited to about 150 eV which does not allow the separation RRS from elastic scattering. Ice and Sparks [ l ] described a mosaic crystal spectrometer which resolves this problem. We have adapted their setup on our beam line D23A at LURE-DCI but we have encountered a loss of resolution when the Bragg planes of the sample are inclined. The shape of the diffracted beam incoming on the position-sensitive detector depends strongly on the inclination of the diffracting planes ; this explains the loss of resolution which is increased in our case by the large horizontal incoming beam size. Alternatively, we recommend the use of a perfect crystal as an analyzer coupled with a 0d detector which gives a very good resolution (≈10eV) without modification of resolution when Bragg planes of the sample are inclined in X. Furthermore this kind of spectrometer allows a high counting rate so that it is possible to measure close to Bragg peaks (clustering studies, for example) and even Bragg peaks. The loss of efficiency is not dramatic considering the high brightness of third generation synchrotron radiation and is more than compensated by the improved signal to noise ratio

Corrosion influence on the evaporation of sessile droplet

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MAGNETIC FIELD DEPENDENCE OF STATIC CORRELATIONS AND SPIN DYNAMICS OF REENTRANT SPIN GLASSES STUDIED BY NEUTRON SCATTERING

We report small angle (SANS) and inelastic neutron scattering in zero and applied field for a-FeMn, NiMn and AuFe at composition where both ferromagnetic and frustration characters occur. We discuss the field evolution of the transverse correlations which arise below Tc. A study of the field dependence of the spin wave anomalies in a-Fe-Mn is reported

Interfacial structure and giant magnetoresistance in Fe/Cr superlattices

A correlation between interfacial structure and giant magnetoresistance (GMR) has been established for Fe/Cr superlattices [l-8]. However, previous studies were qualitative in that the interfacial structural disorder was expressed in terms of growth temperature, sputtering pressure or annealing temperature. In order to understand more quantitatively the effects of interfacial structure on GMR, we have undertaken [9] parallel measurements of x-ray diffraction (XRD) and magnetoresistance for two Fe/Cr samples subjected to a series of anneals at successively higher temperatures, both samples having initial GMR > 50 %. Such anneals are known to increase the thickness of the Fe/Cr interface through atomic diffusion, a structural change which can be characterized quantitatively by XRD. The advantages of high intensity and anomalous dispersion provided by synchrotron radiation has greatly contributed to the quality of our x-ray data, for which the use of simulation programs has allowed a robust and precise extraction of several structural parameters for the two samples, including the Fe/Cr interfacial thicknesses