Irreversible transformation of ferromagnetic ordered stripe domains in single-shot IR pump - resonant X-ray scattering probe experiments

The evolution of a magnetic domain structure upon excitation by an intense, femtosecond Infra-Red (IR) laser pulse has been investigated using single-shot based time-resolved resonant X-ray scattering at the X-ray Free Electron laser LCLS. A well-ordered stripe domain pattern as present in a thin CoPd alloy film has been used as prototype magnetic domain structure for this study. The fluence of the IR laser pump pulse was sufficient to lead to an almost complete quenching of the magnetization within the ultrafast demagnetization process taking place within the first few hundreds of femtoseconds following the IR laser pump pulse excitation. On longer time scales this excitation gave rise to subsequent irreversible transformations of the magnetic domain structure. Under our specific experimental conditions, it took about 2 nanoseconds before the magnetization started to recover. After about 5 nanoseconds the previously ordered stripe domain structure had evolved into a disordered labyrinth domain structure. Surprisingly, we observe after about 7 nanoseconds the occurrence of a partially ordered stripe domain structure reoriented into a novel direction. It is this domain structure in which the sample's magnetization stabilizes as revealed by scattering patterns recorded long after the initial pump-probe cycle. Using micro-magnetic simulations we can explain this observation based on changes of the magnetic anisotropy going along with heat dissipation in the film.Comment: 16 pages, 6 figure

Laser-induced ultrafast demagnetization in the presence of a nanoscale magnetic domain network

International audienceFemtosecond magnetization phenomena have been challenging our understanding for over a decade. Most experiments have relied on infrared femtosecond lasers, limiting the spatial resolution to a few micrometres. With the advent of femtosecond X-ray sources, nanometric resolution can now be reached, which matches key length scales in femtomagnetism such as the travelling length of excited 'hot' electrons on a femtosecond timescale. Here we study laser-induced ultrafast demagnetization in [Co/Pd]30 multilayer films, which, for the first time, achieves a spatial resolution better than 100 nm by using femtosecond soft X-ray pulses. This allows us to follow the femtosecond demagnetization process in a magnetic system consisting of alternating nanometric domains of opposite magnetization. No modification of the magnetic structure is observed, but, in comparison with uniformly magnetized systems of similar composition, we find a significantly faster demagnetization time. We argue that this may be caused by direct transfer of spin angular momentum between neighbouring domains

Revising the structure of a new eicosanoid from human platelets to 8,9-11,12-diepoxy-13-hydroxy-eicosadienoic acid

Eicosanoids are critical mediators of fever, pain, and inflammation generated by immune and tissue cells. We recently described a new bioactive eicosanoid generated by cyclooxygenase-1 (COX-1) turnover during platelet activation that can stimulate human neutrophil integrin expression. On the basis of mass spectrometry (MS/MS and MS3), stable isotope labeling, and GC-MS analysis, we previously proposed a structure of 8-hydroxy-9,11-dioxolane eicosatetraenoic acid (DXA3). Here, we achieved enzymatic synthesis and 1H NMR characterization of this compound with results in conflict with the previously proposed structural assignment. Accordingly, by using LC-MS, we screened autoxidation reactions of 11-hydroperoxy-eicosatetraenoic acid (11-HpETE) and thereby identified a candidate sharing the precise reverse-phase chromatographic and MS characteristics of the platelet product. We optimized these methods to increase yield, allowing full structural analysis by 1H NMR. The revised assignment is presented here as 8,9–11,12-diepoxy-13-hydroxyeicosadienoic acid, abbreviated to 8,9–11,12-DiEp-13-HEDE or DiEpHEDE, substituted for the previous name DXA3. We found that in platelets, the lipid likely forms via dioxolane ring opening with rearrangement to the diepoxy moieties followed by oxygen insertion at C13. We present its enzymatic biosynthetic pathway and MS/MS fragmentation pattern and, using the synthetic compound, demonstrate that it has bioactivity. For the platelet lipid, we estimate 16 isomers based on our current knowledge (and four isomers for the synthetic lipid). Determining the exact isomeric structure of the platelet lipid remains to be undertaken

Photon shot-noise limited transient absorption soft X-ray spectroscopy at the European XFEL

Femtosecond transient soft X-ray Absorption Spectroscopy (XAS) is a very promising technique that can be employed at X-ray Free Electron Lasers (FELs) to investigate out-of-equilibrium dynamics for material and energy research. Here we present a dedicated setup for soft X-rays available at the Spectroscopy & Coherent Scattering (SCS) instrument at the European X-ray Free Electron Laser (EuXFEL). It consists of a beam-splitting off-axis zone plate (BOZ) used in transmission to create three copies of the incoming beam, which are used to measure the transmitted intensity through the excited and unexcited sample, as well as to monitor the incoming intensity. Since these three intensity signals are detected shot-by-shot and simultaneously, this setup allows normalized shot-by-shot analysis of the transmission. For photon detection, the DSSC imaging detector, which is capable of recording up to 800 images at 4.5 MHz frame rate during the FEL burst, is employed and allows approaching the photon shot-noise limit. We review the setup and its capabilities, as well as the online and offline analysis tools provided to users.Comment: 11 figure

Croissance et structure electronique de couches ultra-minces de cobalt deposees sur la face (100) du platine

SIGLEINIST T 73493 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Couplage magnétique de films ultra-minces NiO antiferromagnétiques sur des films ferrimagnétiques de Fe3O4(001)

Nous avons étudié les corrélations entre les propriétés structurales, chimiques et les propriétés magnétiques de couches ultra-minces de NiO déposées sur un substrat de Fe3O4 (001) afin de décrire le couplage magnétique. Dans une première partie, le travail a été consacré à la caractérisation structurale de NiO/Fe3O4 (001). Les observations par microscopie électronique en transmission à haute résolution ont révélé la présence d une structure cristalline contrainte à l interface NiO/Fe3O4 (001). La seconde partie du travail concerne les propriéte s physiques des couches obtenues. Les données quantitatives, obtenues par dichroïsme circulaire magnétique des rayons X ont montré une forte réduction des moments magnétiques de spin et orbitaux, accentuée pour les plus fortes températures de dépôt de NiO. Nous avons attribué cette diminution à la formation d un nouveau composé NiFe2O4 à l interface. Les résultats X-PEEM ont montré un couplage parallèle entre les couches NiO et Fe3O4 (001).We have studied the correlations between structural, chemical and magnetic properties in ultrathin NiO films deposited on Fe3O4 (001) in order to describe the magnetic coupling. In the first part, we present the structural characteristics of NiO/Fe3O4 (001). In particular, the High-Resolution Transmission Electron Microscopy observations have revealed a strained crystalline structure at the NiO/Fe3O4 (001) interface. In a second part, we have studied the magnetic properties of NiO/Fe3O4 (001) bilayers. The quantitative data, obtained by X-ray Magnetic Circular Dichroïsm, show strong reductions in the magnetic spin and orbital moments proportional with the growth temperature of NiO. The reduced magnetic moments can be explained by the formation of a spinel compound NiFe2O4 at the interface. The last part of the work is devoted to the X-PEEM results. The magnetic contrast images obtained show perfect and parallel coupling between the spins in the NiO and Fe3O4 layers.STRASBOURG-Sc. et Techniques (674822102) / SudocSudocFranceF

Structure and morphology of NiO / Cu(111) and NiO / FeNi / Cu(111) ultra-thin layers and nanostructures

Les premiers stades de la croissance du NiO/Cu(111) ont été caractérisés du point de vue chimique, morphologique et structurale à une échelle microscopique. Pour cela, nous avons utilisé différentes méthodes d'élaboration et combiné différentes techniques de laboratoire in situ ainsi que le rayonnement synchrotron. Les bicouches à couplage d'échange NiO/FeNi/Cu(111) ont aussi été étudiées. L'interface Ni/Cu(111) a été étudié en mettant en évidence les liens entre la morphologie, la structure et les propriétés magnétiques. Les valeurs réduites du moment de spin sont reliées à l'hybridation 3d à l'interface Ni-Cu et au recouvrement de Cu. Dans toute la gamme des épaisseurs étudiées l'anisotropie du moment orbital est dans le plan, déterminant l'orientation de l'axe de facile aimantation. Cette axe peut être reliée à la tétragonalisation du réseau cristallin du Ni. Des couches ultraminces de NiO ont été obtenues par évaporation à partir de pépites de NiO à température ambiante ainsi qu'à 250ʿC. Les couches minces peuvent être décrite par une bicouche NiO/Ni/Cu(111). Le GISAXS permettent de confirmer les observations STM et de mettre en évidence l'auto-organisation des îlots de NiO. Due à la faible efficacité d'oxydation, les couches minces de NiO déposées à partir du Ni métallique sous oxygène, présentent quelques différences par rapport aux couches déposées à partir des pépites. Le mécanisme de formation des îlots proposé est basé sur un processus de nucléation/agrégation de clusters. Les résultats structurales (LEED et GIXD) peuvent être attribués soit à la formation d'une phase hexagonale a-Ni2O3, soit à une distorsion structurale de la maille NiO(111)( )R30ʿ.Le système à couplage d'échange NiO/FeNi/Cu(111) a été élaboré en utilisant les deux méthodes d'élaboration du NiO. L'interface NiO/FeNi est très abrupte, présentant une texturation suivant les axes cristallographiques. Les analyses structurales montrent une bonne épitaxie du NiO sur les films d'alliage FeNi.The very first stages of the growth of NiO/Cu(111) interface, were characterized on a microscopic scale including chemical, morphological, and structural aspects. Different elaboration procedures were used and we combined in-situ laboratory and synchrotron radiation techniques. Complete exchange coupled NiO/FeNi/Cu(111) bilayer system have also been investigated.The metallic Ni/Cu(111) interface has been studied, evidencing the close relationship between morphology, structure and magnetic properties. The reduced spin magnetic moments were correlated with the Ni-Cu 3d hybridization and with the Cu capping. The in-plane orbital moment anisotropy, related with the Ni tetragonalization, confirms that the easy axis of magnetization is in the plane for all measured thin nickel films.Ultra-thin NiO films, obtained through MBE evaporation of NiO nuggets, were grown at room temperature and at 250ʿC. The resulting system can globally be described in terms of a spontaneous NiO/Ni/Cu(111) layering. Confirming the STM real-space observations, GISAXS experiments and calculations allowed evidencing the self-organized nature of the NiO islands obtained at room temperature. Due to the poor oxidizing efficiency of the molecular oxygen, the NiO films deposited from metallic Ni at 250ʿC, exhibit several differences with respect to those obtained by NiO nuggets evaporation at room temperature. A cluster nucleation/aggregation mechanism was proposed, based on the STM observations. Formation of a-Ni2O3 hexagonal phase, or structural distortion of the NiO(111)( )R30ʿ structure could both explain the LEED and GIXD results.Exchange coupled NiO/FeNi/Cu(111) bilayer was elaborated using both, NiO nuggets and metallic Ni in oxygen partial pressure evaporation. Sharp NiO/FeNi interfaces were obtained, with textured NiO. Structural analysis of the NiO films deposited on FeNi/Cu(111) films evidenced twinned NiO(111), in much better epitaxy than the oxides deposited on the Cu(111) substrate.STRASBOURG-Sc. et Techniques (674822102) / SudocSudocFranceF

MORPHOLOGIE, STRUCTURE ET MAGNETISME DES COUCHES ULTRAMINCES D'ALLIAGES FE XNI 1 X DEPOSEES SUR DES SURFACES VICINALES DU CU(111)

NOUS AVONS ETUDIE LES PROPRIETES MAGNETIQUES ET STRUCTURALES DE FILMS ULTRAMINCES (2D) ET DE NANO-FILS (1D) D'ALLIAGES MAGNETIQUES DEPOSES SUR DES SURFACES A MARCHES PERIODIQUEMENT ESPACEES. NOTRE OBJECTIF ETAIT DE MONTRER L'EXISTENCE D'UNE ANISOTROPIE MAGNETIQUE INDUITE PAR LES MARCHES DU SUBSTRAT DANS LE PLAN DES COUCHES ET DE CORRELER CETTE ANISOTROPIE A LA MORPHOLOGIE (FORMES 1D, 2D, MEMOIRE DES MARCHES) AINSI QU'A LA STRUCTURE DE L'ALLIAGE (CONTRAINTES, RELAXATION) ; LE TOUT DANS UN SOUCI DE COMPREHENSION DES MECANISMES QUI GOUVERNENT L'ANISOTROPIE MAGNETIQUE. NOUS MONTRONS PAR STM QUE L'EPAISSEUR ET LA CONCENTRATION DE FE XNI 1 X/CU(111)-VIC SONT DES FACTEURS QUI INFLUENCENT FORTEMENT LA MORPHOLOGIE. NOTRE CHOIX C'EST NEANMOINS PORTE SUR L'ALLIAGE FE 0 . 6 5NI 0 . 3 5 QUI PRESENTE L'AVANTAGE D'ADOPTER UNE STRUCTURE CFC AINSI QU'UNE CROISSANCE EN DECORATION DE MARCHES. CETTE AUTO-ORGANISATION DES ATOMES LE LONG DU BORD SUPERIEUR DES MARCHES NOUS PERMET D'OBTENIR DES STRUCTURES A 1D (FILS) POUR DES FAIBLES TAUX DE RECOUVREMENT ALORS QU'AU DELA DE LA COALESCENCE 2D, DES RUBANS D'ALLIAGE SONT OBSERVES. L'APPLICATION DES REGLES DE SOMME AUX MESURES XMCD EFFECTUEES AUX SEUILS L 2 , 3 DU FE EN FONCTION DES ANGLES POLAIRE : ET AZIMUTAL : NOUS ONT PERMIS DE METTRE EN EVIDENCE L'EXISTENCE D'UNE FORTE ANISOTROPIE DU MOMENT ORBITAL DANS LE PLAN DES FILMS. L'EFFET LE PLUS IMPORTANT EST OBSERVE POUR LES FILS 1D AINSI QUE POUR LES RUBANS D'ALLIAGE A 3 MC. CETTE ANISOTROPIE MAGNETIQUE EST CORRELEE, DANS LE CAS DES RUBANS, A LA RELAXATION ANISOTROPE DES FILMS. NOUS AVONS MONTRE PAR S-EXAFS QU'UNE FAIBLE COMPRESSION DU PARAMETRE DE MAILLE PEUT INDUIRE UNE AUGMENTATION SIGNIFICATIVE DU MOMENT ORBITAL. LES DEPOTS ULTRAMINCES (0.3 ET 0.7 MC) MONTRENT QUANT A EUX UN ETAT DE SPIN ELEVE CORRELE AU VOLUME ATOMIQUE DU FER.STRASBOURG-Sc. et Techniques (674822102) / SudocSudocFranceF

Pisum sativum Response to Nitrate as Affected by Rhizobium leguminosarum-Derived Signals

Legumes are suitable for the development of sustainable agroecosystems because of their ability to use atmospheric N2 through symbiotic nitrogen fixation (SNF). However, a basic NO3− input is necessary before SNF takes place to ensure successful seedling establishment. Since Rhizobia not only induce nodulation but also affect root branching by stimulating the development of lateral roots, and NO3− as a signal also modulates root system architecture, we investigated whether Rhizobium-derived signals interfere in nitrate signaling. Here, we bring evidence that (i) Rhizobium-altered NO3−-mediated processes in pea expressions of major players in NO3− transport, sensing, and signaling were affected, and (ii) the characteristic limitation of root foraging and branching in response to NO3− supply was abolished. The number of tertiary roots per secondary root was higher in infected compared to uninfected peas, thus indicating that the Rhizobium effect allows for favorable management of trade-offs between nodules growth for nitrogen capture and root foraging for water and other nutrient uptake in pea. The outcome of this basic research can be used to produce molecular tools for breeding pea genotypes able to develop deep-foraging and branched root systems, and more competitive architectures and molecular levels for soil NO3− absorption during seedling establishment without jeopardizing nodulation