Nicolas Jaouen - Academia.edu (original) (raw)

Papers by Nicolas Jaouen

Noncollinear spin textures in ferromagnetic ultrathin films are attracting a renewed interest fue... more Noncollinear spin textures in ferromagnetic ultrathin films are attracting a renewed interest fuelled by the discovery of the interfacial Dzyaloshinskii-Moriya (DM) interaction. They are at the origin of complex chiral spin textures such as chiral magnetic domain walls, spin spirals, and magnetic skyrmions. We report here on the ultrafast behavior of chiral domain walls in perpendicularly magnetized asymmetric multilayers after optical pumping probed using circular dichroism in time-resolved x-ray resonant magnetic scattering (XRMS). In addition to the reduction of the magnetization, we report an ultrafast change of the magnetic chirality occurring in the first few picoseconds after optical pumping. Then the XRMS signal, related to the homochiral N\'eel domain wall periodicity is recovered faster that the domain magnetization and subsequently accompanied by a 6 GHz oscillation lasting up to nanosecond time scales. We attribute the short timescale XRMS dichroism reduction in the ...

Advanced Electronic Materials

Physical Review B

The tensile strain is a promising tool for creation and manipulation of magnetic solitonic textur... more The tensile strain is a promising tool for creation and manipulation of magnetic solitonic textures in the chiral helimagnets via tunable control of magnetic anisotropy and Dzyaloshinskii-Moriya interaction. Here, by using the in-situ resonant small-angle x-ray scattering we demonstrate that the skyrmion and chiral soliton lattices can be achieved as metastable states in FeGe lamella as distinct states or even simultaneously by combining the tensile strain and magnetic fields in various orientations with respect to the deformation. The small-angle scattering data are discussed in the frame of the analytical model which is sufficient to describe the experimental results for soliton lattice. By using the experimental results and analytical theory, unwinding of the metastable skyrmions in the perpendicular magnetic field as seen by small-angle scattering experiment was analyzed by the micromagnetic simulation.

The magnetism of Co–Rh nanoparticles is investigated experimentally and theoretically. The partic... more The magnetism of Co–Rh nanoparticles is investigated experimentally and theoretically. The particles (E2 nm) have been synthesized by decomposition of organometallic precursors in mild conditions of pressure and temperature, under hydrogen atmosphere and in the presence of a polymer matrix. The magnetic properties are determined by SQUID, Mössbauer spectroscopy, and X-ray magnetic circular dichroism (XMCD). The structural and chemical properties are characterized by wide angle X-ray scattering, transmission electronic microscopy and X-ray absorption near edge spectroscopy. All the studied Co–Rh clusters are magnetic with an average spin moment per atom m that is larger than the one of macroscopic crystals or alloys with similar concentrations. The experimental results and comparison with theory suggest that the most likely chemical arrangement is a Rh core, with a Co-rich outer shell showing significant Co–Rh mixing at the interface. Measured and calculated magnetic anisotropy energ...

The complex refractive indices of tholins, simulating aerosols in the atmosphere of Titan and the... more The complex refractive indices of tholins, simulating aerosols in the atmosphere of Titan and the primitive earth, have been measured over a wide spectral range, including the soft X-ray, vacuum-ultraviolet (VUV), and UV-Visible. The soft X-ray and VUV spectral ranges are in particular relevant to radiative transfer models of solar irradiation of primitive atmospheres (Lammer et al. 2008) and may elucidate the (anti-)greenhouse potential of photochemical aerosols.Thin films were grown using the PAMPRE capacitively coupled plasma setup (Szopa et al. 2006; Carrasco et al. 2009). Gas mixtures consisting of CH 4 /N 2 with 5:95 ratios were used to simulate Titan's atmospheric composition. For the primitive Earth, gas mixtures of N 2 /CO 2 /H 2 and N 2 /CO 2 /CH 4 were used as described in Fleury et al. (2014).State-of-the-art laboratory techniques were used to determine the refractive indices of such tholin films. These include VUV ellipsometry (performed in collaboration with the Me...

Advanced Materials

The emergence of magnetic skyrmions, topological spin textures, has aroused tremendous interest i... more The emergence of magnetic skyrmions, topological spin textures, has aroused tremendous interest in studying the rich physics related to their topology. While skyrmions promise high-density and energy-efficient magnetic memory devices for information technology, the manifestation of their nontrivial topology through single skyrmions and ordered and disordered skyrmion lattices could also give rise to many fascinating physical phenomena, such as chiral magnon and skyrmion glass states. Therefore, generating skyrmions at designated locations on a large scale, while controlling the skyrmion patterns, is the key to advancing topological magnetism. Here, a new, yet general, approach to the "printing" of skyrmions with zero-field stability in arbitrary patterns on a massive scale in exchange-biased magnetic multilayers is presented. By exploiting the fact that the antiferromagnetic order can be reconfigured by local thermal excitations, a focused electron beam with a graphic pattern generator to "print" skyrmions is used, which is referred to as skyrmion lithography. This work provides a route to design arbitrary skyrmion patterns, thereby establishing the foundation for further exploration of topological magnetism.

Science Advances

Competition between magnetic interactions in multilayers stabilizes complex, vertically twisted t... more Competition between magnetic interactions in multilayers stabilizes complex, vertically twisted topological spin textures.

Physical review letters, Jan 19, 2018

Chirality in condensed matter has recently become a topic of the utmost importance because of its... more Chirality in condensed matter has recently become a topic of the utmost importance because of its significant role in the understanding and mastering of a large variety of new fundamental physical mechanisms. Versatile experimental approaches, capable to reveal easily the exact winding of order parameters, are therefore essential. Here we report x-ray resonant magnetic scattering as a straightforward tool to reveal directly the properties of chiral magnetic systems. We show that it can straightforwardly and unambiguously determine the main characteristics of chiral magnetic distributions: i.e., its chiral nature, the quantitative winding sense (clockwise or counterclockwise), and its type, i.e., Néel [cycloidal] or Bloch [helical]. This method is model independent, does not require a priori knowledge of the magnetic parameters, and can be applied to any system with magnetic domains ranging from a few nanometers (wavelength limited) to several microns. By using prototypical multilaye...

The Journal of chemical physics, Jan 7, 2017

We present for the first time two-dimensional resonant inelastic x-ray scattering (RIXS) maps of ... more We present for the first time two-dimensional resonant inelastic x-ray scattering (RIXS) maps of multilayer and monolayer bi-isonicotinic acid adsorbed on the rutile TiO2(110) single crystal surface. This enables the elastic channel to be followed over the lowest unoccupied molecular orbitals resonantly excited at the N 1s absorption edge. The data also reveal ultra-fast intramolecular vibronic coupling, particularly during excitation into the lowest unoccupied molecular orbital-derived resonance. Both elastic scattering and the vibronic coupling loss features are expected to contain the channel in which the originally excited electron is directly involved in the core-hole decay process. This allows RIXS data for a molecule coupled to a wide bandgap semiconductor to be considered in the same way as the core-hole clock implementation of resonant photoemission spectroscopy (RPES). However, contrary to RPES measurements, we find no evidence for the depletion of the participator channel...

Journal of Synchrotron Radiation

The SOLEIL synchrotron radiation source is regularly operated in special filling modes dedicated ... more The SOLEIL synchrotron radiation source is regularly operated in special filling modes dedicated to pump–probe experiments. Among others, the low-α mode operation is characterized by shorter pulse duration and represents the natural bridge between 50 ps synchrotron pulses and femtosecond experiments. Here, the capabilities in low-α mode of the experimental set-ups developed at the TEMPO beamline to perform pump–probe experiments with soft X-rays based on photoelectron or photon detection are presented. A 282 kHz repetition-rate femtosecond laser is synchronized with the synchrotron radiation time structure to induce fast electronic and/or magnetic excitations. Detection is performed using a two-dimensional space resolution plus time resolution detector based on microchannel plates equipped with a delay line. Results of time-resolved photoelectron spectroscopy, circular dichroism and magnetic scattering experiments are reported, and their respective advantages and limitations in the ...

The Astrophysical Journal

The deuterium enrichment of organics in the interstellar medium, protoplanetary disks and meteori... more The deuterium enrichment of organics in the interstellar medium, protoplanetary disks and meteorites has been proposed to be the result of ionizing radiation. The goal of this study is to simulate and quantify the effects of soft Xrays (0.1-2 keV), an important component of stellar radiation fields illuminating protoplanetary disks, on the refractory organics present in the disks. We prepared tholins, nitrogen-rich organic analogs to solids found in several astrophysical environments, e.g. Titan's atmosphere, cometary surfaces and protoplanetary disks, via plasma deposition. Controlled irradiation experiments with soft X-rays at 0.5 and 1.3 keV were performed at the SEXTANTS beam line of the SOLEIL synchrotron, and were immediately followed by ex-situ infrared, Raman and isotopic diagnostics. Infrared spectroscopy revealed the preferential loss of singly-bonded groups (N-H, C-H and R-N≡C) and the formation of sp 3 carbon defects with signatures at ∼1250-1300 cm −1. Raman analysis revealed that, while the length of polyaromatic units is only slightly modified, the introduction of defects leads to structural amorphization. Finally, tholins were measured via secondary ion mass spectrometry (SIMS) to quantify the D, H and C elemental abundances in the irradiated versus non-irradiated areas. Isotopic analysis revealed that significant D-enrichment is induced by X-ray irradiation. Our results are compared to previous experimental studies involving the thermal degradation and electron irradiation of organics. The penetration depth of soft X-rays in µm-sized tholins leads to volume rather than surface modifications: lower energy X-rays (0.5 keV) induce a larger D-enrichment than 1.3 keV X-rays, reaching a plateau for doses larger than 5 × 10 27 eV cm −3. Synchrotron fluences fall within the expected soft X-ray fluences in protoplanetary disks, and thus provide evidence of a new non-thermal pathway to deuterium fractionation of organic matter.

Scientific reports, Jan 3, 2017

The advent of x-ray free electron lasers has extended the unique capabilities of resonant x-ray s... more The advent of x-ray free electron lasers has extended the unique capabilities of resonant x-ray spectroscopy techniques to ultrafast time scales. Here, we report on a novel experimental method that allows retrieving with a single x-ray pulse the time evolution of an ultrafast process, not only at a few discrete time delays, but continuously over an extended time window. We used a single x-ray pulse to resolve the laser-induced ultrafast demagnetisation dynamics in a thin cobalt film over a time window of about 1.6 ps with an excellent signal to noise ratio. From one representative single shot measurement we extract a spin relaxation time of (130 ± 30) fs with an average value, based on 193 single shot events of (113 ± 20) fs. These results are limited by the achieved experimental time resolution of 120 fs, and both values are in excellent agreement with previous results and theoretical modelling. More generally, this new experimental approach to ultrafast x-ray spectroscopy paves th...

Http Www Theses Fr, 2001

Cette these traite du developpement de la reflectivite magnetique resonante des RX polarises, app... more Cette these traite du developpement de la reflectivite magnetique resonante des RX polarises, appliquee a l'etude des profils d'aimantation au sein de couches minces. Cette methode combine la sensibilite au magnetisme, avec une selectivite chimique et electronique liee aux choix de l'energie des photons incidents au voisinage d'un seuil d'absorption, et la sensibilite structurale (selectivite spatiale) inherente a une technique qui sonde l'espace reciproque. Un reflectometre sous ultra- vide, dedie a des mesures dans le domaine des x -mous, a ete developpe. Les premieres experiences sont presentees pour illustrer ses potentialites. Un programme de simulation, base sur un traitement optique, permet de calculer l'intensite reflechie pour un etat de polarisation du faisceau incident et une orientation de l'aimantation quelconques. L'etude du systeme W/Fe/W, realisees dans deux configurations de polarisation et d'aimantation, a permis de le valider et de discuter les effets de rugosites et d'absorption. Ue extension a l'etude de films de nano-particules magnetiques dans une matrice est proposee. Une deuxieme partie s'attache a l'etude de la distribution de l'aimantation induite au sein de films de Ce dans des multicouches Ce/Fe. L'analyse de la dependance en energie (aux seuils L2 et M4,5 du Ce) de la reflectivite, mesuree sur les pics de Bragg generes par la periode chimique, permet de determiner le profil magnetique des moments d'origine 5d et 4f. Dans des multicouches Ce/Fe, CeLaCe/Fe et LaCeLa/Fe, ou le Ce est de type [alpha] (niveaux 4f partiellement delocalises), une composante antiferromagnetique se superpose a l'ordre magnetique induit par le Fe aux interfaces (continument decroissant). Elle semble etre une propriete intrinseque du Ce [alpha] a la limite d'une transition vers une phase [gamma]. Dans le systeme CeH2-[delta]/Fe, une relocalisation des niveaux 4f prend place, le Ce est de type [gamma] et les deux profils d'aimantation presentent une amplitude decroissante depuis l'interface avec le fer.

Physical Review A, 2016

We present a detailed experimental-theoretical analysis of O K-edge resonant 1σ-2π inelastic x-ra... more We present a detailed experimental-theoretical analysis of O K-edge resonant 1σ-2π inelastic x-ray scattering (RIXS) from carbon monoxide with unprecedented energy resolution. We employ high-level ab initio calculations to compute the potential energy curves of the states involved in the RIXS process and simulate the measured RIXS spectra using the wave-packet-propagation formalism, including Coulomb coupling in the final-state manifold. The theoretical analysis allows us to explain all the key features of the experimental spectra, including some that were not seen before. First, we clearly show the interference effect between different RIXS channels corresponding to the transition via orthogonal Πx1 and Πy1 core-excited states of CO. Second, the RIXS region of 13 eV energy loss presents a triple structure, revealed only by the high-resolution measurement. In previous studies, this region was attributed solely to a valence state. Here we show a strong Coulomb mixing of the Rydberg and valence final states, which opens the forbidden RIXS channels to the "dark" final Rydberg states and drastically changes the RIXS profile. Third, using a combination of high-resolution experiment and high-level theory, we improve the |4σ-12π1) final-state potential-energy curve by fitting its bottom part with the experiment. Also, the coupling constants between Rydberg and valence states were refined via comparison with the experiment. Our results illustrate the large potential of the RIXS technique for advanced studies of highly excited states of neutral molecules. (Less)

AIP Conference Proceedings, 2007

ABSTRACT The Ti-K edge X-ray Absorption Near Edge Structure (XANES) for CaTiO3 and PbTiO3 have be... more ABSTRACT The Ti-K edge X-ray Absorption Near Edge Structure (XANES) for CaTiO3 and PbTiO3 have been measured under high pressure in a diamond anvil cell at room temperature. Despite the huge absorption from the diamond cell and the sample high quality XANES allows us to observe that in CaTiO3 no change occurs when applying pressure, at the opposite of PbTiO3 in which the pre-edge features vary strongly. It allows studying the phase transition from ferroelectric to paraelectric phase in PbTiO3 from the local point of view. Under pressure the change in intensity of the pre-edge indicates qualitatively that the Ti atom is moving toward the centre of the oxygen octahedron along the c-axis.

IEEE Transactions on Magnetics, 2016

Time-averaged images of the magnetization within single layer spin transfer oscillators have been... more Time-averaged images of the magnetization within single layer spin transfer oscillators have been obtained using the holography with extended reference by autocorrelation linear differential operator (HERALDO) technique. Transport measurements on a Pd(5)-Cu(20)-Ni81Fe19(7)-Cu(2)-Pd(2) (in nm) stack with a 100 nm diameter nano-contact reveal the presence of vortex dynamics. Magnetic images of the device for injected current values of 24mA and-24mA suggest that a vortex has been ejected from the nano-contact and become pinned at the edge of the region that is visible through the Au mask.

Scientific Reports, 2016

The original version of this Article contained a typographical error in the spelling of the autho... more The original version of this Article contained a typographical error in the spelling of the author Bharati Tudu, which was incorrectly given as Bahrati Tudu. This has now been corrected in the PDF and HTML versions of the Article.

Scientific Reports, 2016

The unique opportunity to study and control electron-nuclear quantum dynamics in coupled potentia... more The unique opportunity to study and control electron-nuclear quantum dynamics in coupled potentials offered by the resonant inelastic X-ray scattering (RIXS) technique is utilized to unravel an anomalously strong two-electron one-photon transition from core-excited to Rydberg final states in the CO molecule. High-resolution RIXS measurements of CO in the energy region of 12-14 eV are presented and analyzed by means of quantum simulations using the wave packet propagation formalism and ab initio calculations of potential energy curves and transition dipole moments. The very good overall agreement between the experimental results and the theoretical predictions allows an in-depth interpretation of the salient spectral features in terms of Coulomb mixing of "dark" with "bright" final states leading to an effective two-electron one-photon transition. The present work illustrates that the improved spectral resolution of RIXS spectra achievable today may call for more advanced theories than what has been used in the past.

Scientific reports, Jan 6, 2016

Does the excitation of ultrafast magnetization require direct interaction between the photons of ... more Does the excitation of ultrafast magnetization require direct interaction between the photons of the optical pump pulse and the magnetic layer? Here, we demonstrate unambiguously that this is not the case. For this we have studied the magnetization dynamics of a ferromagnetic cobalt/palladium multilayer capped by an IR-opaque aluminum layer. Upon excitation with an intense femtosecond-short IR laser pulse, the film exhibits the classical ultrafast demagnetization phenomenon although only a negligible number of IR photons penetrate the aluminum layer. In comparison with an uncapped cobalt/palladium reference film, the initial demagnetization of the capped film occurs with a delayed onset and at a slower rate. Both observations are qualitatively in line with energy transport from the aluminum layer into the underlying magnetic film by the excited, hot electrons of the aluminum film. Our data thus confirm recent theoretical predictions.

Applied Physics Letters, 2015

Ring-shaped multilayered sub-micron dots have the potential for the development of non-volatile m... more Ring-shaped multilayered sub-micron dots have the potential for the development of non-volatile multi-bit devices. We show that a Co/Cu/FeNi asymmetric ring can take four distinct remanent magnetic states, each one stabilized by applying a magnetic field pulse along one of four in-plane orthogonal directions. We use element selective x-ray holography for imaging the Co magnetic configuration following a magnetic pulse. Micro-magnetic simulations support our experimental findings; they also provide an estimate of the system magnetization dynamics, setting out the conditions for further time-resolved experiments.

Noncollinear spin textures in ferromagnetic ultrathin films are attracting a renewed interest fue... more Noncollinear spin textures in ferromagnetic ultrathin films are attracting a renewed interest fuelled by the discovery of the interfacial Dzyaloshinskii-Moriya (DM) interaction. They are at the origin of complex chiral spin textures such as chiral magnetic domain walls, spin spirals, and magnetic skyrmions. We report here on the ultrafast behavior of chiral domain walls in perpendicularly magnetized asymmetric multilayers after optical pumping probed using circular dichroism in time-resolved x-ray resonant magnetic scattering (XRMS). In addition to the reduction of the magnetization, we report an ultrafast change of the magnetic chirality occurring in the first few picoseconds after optical pumping. Then the XRMS signal, related to the homochiral N\'eel domain wall periodicity is recovered faster that the domain magnetization and subsequently accompanied by a 6 GHz oscillation lasting up to nanosecond time scales. We attribute the short timescale XRMS dichroism reduction in the ...

Advanced Electronic Materials

Physical Review B

The tensile strain is a promising tool for creation and manipulation of magnetic solitonic textur... more The tensile strain is a promising tool for creation and manipulation of magnetic solitonic textures in the chiral helimagnets via tunable control of magnetic anisotropy and Dzyaloshinskii-Moriya interaction. Here, by using the in-situ resonant small-angle x-ray scattering we demonstrate that the skyrmion and chiral soliton lattices can be achieved as metastable states in FeGe lamella as distinct states or even simultaneously by combining the tensile strain and magnetic fields in various orientations with respect to the deformation. The small-angle scattering data are discussed in the frame of the analytical model which is sufficient to describe the experimental results for soliton lattice. By using the experimental results and analytical theory, unwinding of the metastable skyrmions in the perpendicular magnetic field as seen by small-angle scattering experiment was analyzed by the micromagnetic simulation.

The magnetism of Co–Rh nanoparticles is investigated experimentally and theoretically. The partic... more The magnetism of Co–Rh nanoparticles is investigated experimentally and theoretically. The particles (E2 nm) have been synthesized by decomposition of organometallic precursors in mild conditions of pressure and temperature, under hydrogen atmosphere and in the presence of a polymer matrix. The magnetic properties are determined by SQUID, Mössbauer spectroscopy, and X-ray magnetic circular dichroism (XMCD). The structural and chemical properties are characterized by wide angle X-ray scattering, transmission electronic microscopy and X-ray absorption near edge spectroscopy. All the studied Co–Rh clusters are magnetic with an average spin moment per atom m that is larger than the one of macroscopic crystals or alloys with similar concentrations. The experimental results and comparison with theory suggest that the most likely chemical arrangement is a Rh core, with a Co-rich outer shell showing significant Co–Rh mixing at the interface. Measured and calculated magnetic anisotropy energ...

The complex refractive indices of tholins, simulating aerosols in the atmosphere of Titan and the... more The complex refractive indices of tholins, simulating aerosols in the atmosphere of Titan and the primitive earth, have been measured over a wide spectral range, including the soft X-ray, vacuum-ultraviolet (VUV), and UV-Visible. The soft X-ray and VUV spectral ranges are in particular relevant to radiative transfer models of solar irradiation of primitive atmospheres (Lammer et al. 2008) and may elucidate the (anti-)greenhouse potential of photochemical aerosols.Thin films were grown using the PAMPRE capacitively coupled plasma setup (Szopa et al. 2006; Carrasco et al. 2009). Gas mixtures consisting of CH 4 /N 2 with 5:95 ratios were used to simulate Titan's atmospheric composition. For the primitive Earth, gas mixtures of N 2 /CO 2 /H 2 and N 2 /CO 2 /CH 4 were used as described in Fleury et al. (2014).State-of-the-art laboratory techniques were used to determine the refractive indices of such tholin films. These include VUV ellipsometry (performed in collaboration with the Me...

Advanced Materials

The emergence of magnetic skyrmions, topological spin textures, has aroused tremendous interest i... more The emergence of magnetic skyrmions, topological spin textures, has aroused tremendous interest in studying the rich physics related to their topology. While skyrmions promise high-density and energy-efficient magnetic memory devices for information technology, the manifestation of their nontrivial topology through single skyrmions and ordered and disordered skyrmion lattices could also give rise to many fascinating physical phenomena, such as chiral magnon and skyrmion glass states. Therefore, generating skyrmions at designated locations on a large scale, while controlling the skyrmion patterns, is the key to advancing topological magnetism. Here, a new, yet general, approach to the "printing" of skyrmions with zero-field stability in arbitrary patterns on a massive scale in exchange-biased magnetic multilayers is presented. By exploiting the fact that the antiferromagnetic order can be reconfigured by local thermal excitations, a focused electron beam with a graphic pattern generator to "print" skyrmions is used, which is referred to as skyrmion lithography. This work provides a route to design arbitrary skyrmion patterns, thereby establishing the foundation for further exploration of topological magnetism.

Science Advances

Competition between magnetic interactions in multilayers stabilizes complex, vertically twisted t... more Competition between magnetic interactions in multilayers stabilizes complex, vertically twisted topological spin textures.

Physical review letters, Jan 19, 2018

Chirality in condensed matter has recently become a topic of the utmost importance because of its... more Chirality in condensed matter has recently become a topic of the utmost importance because of its significant role in the understanding and mastering of a large variety of new fundamental physical mechanisms. Versatile experimental approaches, capable to reveal easily the exact winding of order parameters, are therefore essential. Here we report x-ray resonant magnetic scattering as a straightforward tool to reveal directly the properties of chiral magnetic systems. We show that it can straightforwardly and unambiguously determine the main characteristics of chiral magnetic distributions: i.e., its chiral nature, the quantitative winding sense (clockwise or counterclockwise), and its type, i.e., Néel [cycloidal] or Bloch [helical]. This method is model independent, does not require a priori knowledge of the magnetic parameters, and can be applied to any system with magnetic domains ranging from a few nanometers (wavelength limited) to several microns. By using prototypical multilaye...

The Journal of chemical physics, Jan 7, 2017

We present for the first time two-dimensional resonant inelastic x-ray scattering (RIXS) maps of ... more We present for the first time two-dimensional resonant inelastic x-ray scattering (RIXS) maps of multilayer and monolayer bi-isonicotinic acid adsorbed on the rutile TiO2(110) single crystal surface. This enables the elastic channel to be followed over the lowest unoccupied molecular orbitals resonantly excited at the N 1s absorption edge. The data also reveal ultra-fast intramolecular vibronic coupling, particularly during excitation into the lowest unoccupied molecular orbital-derived resonance. Both elastic scattering and the vibronic coupling loss features are expected to contain the channel in which the originally excited electron is directly involved in the core-hole decay process. This allows RIXS data for a molecule coupled to a wide bandgap semiconductor to be considered in the same way as the core-hole clock implementation of resonant photoemission spectroscopy (RPES). However, contrary to RPES measurements, we find no evidence for the depletion of the participator channel...

Journal of Synchrotron Radiation

The SOLEIL synchrotron radiation source is regularly operated in special filling modes dedicated ... more The SOLEIL synchrotron radiation source is regularly operated in special filling modes dedicated to pump–probe experiments. Among others, the low-α mode operation is characterized by shorter pulse duration and represents the natural bridge between 50 ps synchrotron pulses and femtosecond experiments. Here, the capabilities in low-α mode of the experimental set-ups developed at the TEMPO beamline to perform pump–probe experiments with soft X-rays based on photoelectron or photon detection are presented. A 282 kHz repetition-rate femtosecond laser is synchronized with the synchrotron radiation time structure to induce fast electronic and/or magnetic excitations. Detection is performed using a two-dimensional space resolution plus time resolution detector based on microchannel plates equipped with a delay line. Results of time-resolved photoelectron spectroscopy, circular dichroism and magnetic scattering experiments are reported, and their respective advantages and limitations in the ...

The Astrophysical Journal

The deuterium enrichment of organics in the interstellar medium, protoplanetary disks and meteori... more The deuterium enrichment of organics in the interstellar medium, protoplanetary disks and meteorites has been proposed to be the result of ionizing radiation. The goal of this study is to simulate and quantify the effects of soft Xrays (0.1-2 keV), an important component of stellar radiation fields illuminating protoplanetary disks, on the refractory organics present in the disks. We prepared tholins, nitrogen-rich organic analogs to solids found in several astrophysical environments, e.g. Titan's atmosphere, cometary surfaces and protoplanetary disks, via plasma deposition. Controlled irradiation experiments with soft X-rays at 0.5 and 1.3 keV were performed at the SEXTANTS beam line of the SOLEIL synchrotron, and were immediately followed by ex-situ infrared, Raman and isotopic diagnostics. Infrared spectroscopy revealed the preferential loss of singly-bonded groups (N-H, C-H and R-N≡C) and the formation of sp 3 carbon defects with signatures at ∼1250-1300 cm −1. Raman analysis revealed that, while the length of polyaromatic units is only slightly modified, the introduction of defects leads to structural amorphization. Finally, tholins were measured via secondary ion mass spectrometry (SIMS) to quantify the D, H and C elemental abundances in the irradiated versus non-irradiated areas. Isotopic analysis revealed that significant D-enrichment is induced by X-ray irradiation. Our results are compared to previous experimental studies involving the thermal degradation and electron irradiation of organics. The penetration depth of soft X-rays in µm-sized tholins leads to volume rather than surface modifications: lower energy X-rays (0.5 keV) induce a larger D-enrichment than 1.3 keV X-rays, reaching a plateau for doses larger than 5 × 10 27 eV cm −3. Synchrotron fluences fall within the expected soft X-ray fluences in protoplanetary disks, and thus provide evidence of a new non-thermal pathway to deuterium fractionation of organic matter.

Scientific reports, Jan 3, 2017

The advent of x-ray free electron lasers has extended the unique capabilities of resonant x-ray s... more The advent of x-ray free electron lasers has extended the unique capabilities of resonant x-ray spectroscopy techniques to ultrafast time scales. Here, we report on a novel experimental method that allows retrieving with a single x-ray pulse the time evolution of an ultrafast process, not only at a few discrete time delays, but continuously over an extended time window. We used a single x-ray pulse to resolve the laser-induced ultrafast demagnetisation dynamics in a thin cobalt film over a time window of about 1.6 ps with an excellent signal to noise ratio. From one representative single shot measurement we extract a spin relaxation time of (130 ± 30) fs with an average value, based on 193 single shot events of (113 ± 20) fs. These results are limited by the achieved experimental time resolution of 120 fs, and both values are in excellent agreement with previous results and theoretical modelling. More generally, this new experimental approach to ultrafast x-ray spectroscopy paves th...

Http Www Theses Fr, 2001

Cette these traite du developpement de la reflectivite magnetique resonante des RX polarises, app... more Cette these traite du developpement de la reflectivite magnetique resonante des RX polarises, appliquee a l'etude des profils d'aimantation au sein de couches minces. Cette methode combine la sensibilite au magnetisme, avec une selectivite chimique et electronique liee aux choix de l'energie des photons incidents au voisinage d'un seuil d'absorption, et la sensibilite structurale (selectivite spatiale) inherente a une technique qui sonde l'espace reciproque. Un reflectometre sous ultra- vide, dedie a des mesures dans le domaine des x -mous, a ete developpe. Les premieres experiences sont presentees pour illustrer ses potentialites. Un programme de simulation, base sur un traitement optique, permet de calculer l'intensite reflechie pour un etat de polarisation du faisceau incident et une orientation de l'aimantation quelconques. L'etude du systeme W/Fe/W, realisees dans deux configurations de polarisation et d'aimantation, a permis de le valider et de discuter les effets de rugosites et d'absorption. Ue extension a l'etude de films de nano-particules magnetiques dans une matrice est proposee. Une deuxieme partie s'attache a l'etude de la distribution de l'aimantation induite au sein de films de Ce dans des multicouches Ce/Fe. L'analyse de la dependance en energie (aux seuils L2 et M4,5 du Ce) de la reflectivite, mesuree sur les pics de Bragg generes par la periode chimique, permet de determiner le profil magnetique des moments d'origine 5d et 4f. Dans des multicouches Ce/Fe, CeLaCe/Fe et LaCeLa/Fe, ou le Ce est de type [alpha] (niveaux 4f partiellement delocalises), une composante antiferromagnetique se superpose a l'ordre magnetique induit par le Fe aux interfaces (continument decroissant). Elle semble etre une propriete intrinseque du Ce [alpha] a la limite d'une transition vers une phase [gamma]. Dans le systeme CeH2-[delta]/Fe, une relocalisation des niveaux 4f prend place, le Ce est de type [gamma] et les deux profils d'aimantation presentent une amplitude decroissante depuis l'interface avec le fer.

Physical Review A, 2016

We present a detailed experimental-theoretical analysis of O K-edge resonant 1σ-2π inelastic x-ra... more We present a detailed experimental-theoretical analysis of O K-edge resonant 1σ-2π inelastic x-ray scattering (RIXS) from carbon monoxide with unprecedented energy resolution. We employ high-level ab initio calculations to compute the potential energy curves of the states involved in the RIXS process and simulate the measured RIXS spectra using the wave-packet-propagation formalism, including Coulomb coupling in the final-state manifold. The theoretical analysis allows us to explain all the key features of the experimental spectra, including some that were not seen before. First, we clearly show the interference effect between different RIXS channels corresponding to the transition via orthogonal Πx1 and Πy1 core-excited states of CO. Second, the RIXS region of 13 eV energy loss presents a triple structure, revealed only by the high-resolution measurement. In previous studies, this region was attributed solely to a valence state. Here we show a strong Coulomb mixing of the Rydberg and valence final states, which opens the forbidden RIXS channels to the "dark" final Rydberg states and drastically changes the RIXS profile. Third, using a combination of high-resolution experiment and high-level theory, we improve the |4σ-12π1) final-state potential-energy curve by fitting its bottom part with the experiment. Also, the coupling constants between Rydberg and valence states were refined via comparison with the experiment. Our results illustrate the large potential of the RIXS technique for advanced studies of highly excited states of neutral molecules. (Less)

AIP Conference Proceedings, 2007

ABSTRACT The Ti-K edge X-ray Absorption Near Edge Structure (XANES) for CaTiO3 and PbTiO3 have be... more ABSTRACT The Ti-K edge X-ray Absorption Near Edge Structure (XANES) for CaTiO3 and PbTiO3 have been measured under high pressure in a diamond anvil cell at room temperature. Despite the huge absorption from the diamond cell and the sample high quality XANES allows us to observe that in CaTiO3 no change occurs when applying pressure, at the opposite of PbTiO3 in which the pre-edge features vary strongly. It allows studying the phase transition from ferroelectric to paraelectric phase in PbTiO3 from the local point of view. Under pressure the change in intensity of the pre-edge indicates qualitatively that the Ti atom is moving toward the centre of the oxygen octahedron along the c-axis.

IEEE Transactions on Magnetics, 2016

Time-averaged images of the magnetization within single layer spin transfer oscillators have been... more Time-averaged images of the magnetization within single layer spin transfer oscillators have been obtained using the holography with extended reference by autocorrelation linear differential operator (HERALDO) technique. Transport measurements on a Pd(5)-Cu(20)-Ni81Fe19(7)-Cu(2)-Pd(2) (in nm) stack with a 100 nm diameter nano-contact reveal the presence of vortex dynamics. Magnetic images of the device for injected current values of 24mA and-24mA suggest that a vortex has been ejected from the nano-contact and become pinned at the edge of the region that is visible through the Au mask.

Scientific Reports, 2016

The original version of this Article contained a typographical error in the spelling of the autho... more The original version of this Article contained a typographical error in the spelling of the author Bharati Tudu, which was incorrectly given as Bahrati Tudu. This has now been corrected in the PDF and HTML versions of the Article.

Scientific Reports, 2016

The unique opportunity to study and control electron-nuclear quantum dynamics in coupled potentia... more The unique opportunity to study and control electron-nuclear quantum dynamics in coupled potentials offered by the resonant inelastic X-ray scattering (RIXS) technique is utilized to unravel an anomalously strong two-electron one-photon transition from core-excited to Rydberg final states in the CO molecule. High-resolution RIXS measurements of CO in the energy region of 12-14 eV are presented and analyzed by means of quantum simulations using the wave packet propagation formalism and ab initio calculations of potential energy curves and transition dipole moments. The very good overall agreement between the experimental results and the theoretical predictions allows an in-depth interpretation of the salient spectral features in terms of Coulomb mixing of "dark" with "bright" final states leading to an effective two-electron one-photon transition. The present work illustrates that the improved spectral resolution of RIXS spectra achievable today may call for more advanced theories than what has been used in the past.

Scientific reports, Jan 6, 2016

Does the excitation of ultrafast magnetization require direct interaction between the photons of ... more Does the excitation of ultrafast magnetization require direct interaction between the photons of the optical pump pulse and the magnetic layer? Here, we demonstrate unambiguously that this is not the case. For this we have studied the magnetization dynamics of a ferromagnetic cobalt/palladium multilayer capped by an IR-opaque aluminum layer. Upon excitation with an intense femtosecond-short IR laser pulse, the film exhibits the classical ultrafast demagnetization phenomenon although only a negligible number of IR photons penetrate the aluminum layer. In comparison with an uncapped cobalt/palladium reference film, the initial demagnetization of the capped film occurs with a delayed onset and at a slower rate. Both observations are qualitatively in line with energy transport from the aluminum layer into the underlying magnetic film by the excited, hot electrons of the aluminum film. Our data thus confirm recent theoretical predictions.

Applied Physics Letters, 2015

Ring-shaped multilayered sub-micron dots have the potential for the development of non-volatile m... more Ring-shaped multilayered sub-micron dots have the potential for the development of non-volatile multi-bit devices. We show that a Co/Cu/FeNi asymmetric ring can take four distinct remanent magnetic states, each one stabilized by applying a magnetic field pulse along one of four in-plane orthogonal directions. We use element selective x-ray holography for imaging the Co magnetic configuration following a magnetic pulse. Micro-magnetic simulations support our experimental findings; they also provide an estimate of the system magnetization dynamics, setting out the conditions for further time-resolved experiments.