Théorie du dichroïsme circulaire de rayons X et applications à des matériaux sous pression

The main purpose of this thesis was to compute X-ray magnetic circular dichroism spectra at the K-edge in order to provide a tool to interpret the, so far very puzzling, experimental spectra. Computation of circular dichroism requires precise calculations of X-ray absorption spectra (XAS) for circularly polarized light. We have found that there is an incompatibility of the semi-classical time-dependent perturbation theory commonly used to calculate light absorption and scattering cross-sections with both gauge invariance and semi-relativistic descriptions of the electron dynamics. The problems are solved by applying a Foldy-Wouthuysen transformation to the fully relativistic cross-sections given by quantum electrodynamics. In the process, a new light-matter interaction term emerges, that we named the "spin-position" interaction. An efficient first-principles approach was developed to compute the absorption cross-section in order to obtain X-ray magnetic circular dichroism (XMCD) and X-ray natural circular dichroism (XNCD). The numerical method relies on density-functional theory with plane waves and pseudopotentials. We find that the term coupling the electric dipole operator with the spin-position operator contributes significantly to the XMCD at the K-edge of ferromagnetic iron, cobalt, and nickel. We obtain a sum rule relating this term to the spin magnetic moment of the p states. We also applied the method to calculations of K-edge XMCD in FeH and CrO2. In both cases, the combination of experiment and theory leads to mutual enrichment.Le but principal de cette thèse était de calculer les spectres de dichroïsme circulaire magnétique de rayons~X au seuil K afin de fournir un outil pour interpréter les spectres expérimentaux, jusqu'ici très déroutants. La détermination du dichroïsme circulaire nécessite le calcul précis des spectres d'absorption des rayons~X polarisés circulairement. Nous avons constaté que la théorie des perturbations semi-classique dépendante du temps, communément utilisée pour calculer les sections efficaces d'absorption et de diffusion, est incompatible à la fois, avec l'invariance de jauge et avec les descriptions semi-relativistes de la dynamique des électrons. Pour résoudre ces problèmes, on applique une transformation de Foldy-Wouthuysen aux sections efficaces relativistes données par l'électrodynamique quantique. Ainsi, un nouveau terme d'interaction lumière-matière émerge, que nous avons appelé "spin-position". Une approche performante a été développée pour calculer la section efficace d'absorption afin d'obtenir le dichroïsme circulaire magnétique de rayons~X (XMCD) et le dichroïsme circulaire naturel de rayons~X (XNCD). La méthode numérique repose sur la théorie de la fonctionnelle de la densité en ondes planes avec des pseudopotentiels. Nous constatons que le terme couplant l'opérateur dipolaire électrique avec l'opérateur spin-position contribue significativement au XMCD au seuil K du fer, du nickel et du cobalt ferromagnétiques et nous l'expliquons grâce aux règles de somme. Nous avons également appliqué la méthode aux calculs du XMCD dans FeH et CrO2. Dans les deux cas, la combinaison de l'expérience et de la théorie conduit à un enrichissement mutuel

Gauge invariance and relativistic effects in photon absorption and scattering by matter

The plan of the paper has been modified to make it more pedagogical.There is an incompatibility between gauge invariance and the semi-classical time-dependent perturbation theory commonly used to calculate light absorption and scattering cross-sections. There is an additional incompatibility between perturbationtheory and the description of the electron dynamics by a semi-relativistic Hamiltonian.In this paper, the gauge-dependence problem of exact perturbation theory is described, the proposed solutions are reviewed and it is concluded that none of them seems fully satisfactory. The problem is finally solved by using the fully relativistic absorption and scattering cross-sections given by quantum electrodynamics. Then, a new many-body Foldy-Wouthuysen transformation is presented to obtain correct semi-relativistic transition operators.This transformation considerably simplifies the calculation of relativistic corrections. In the process, a new light-matter interaction term emerges, called the spin-position interaction, that contributes significantly to the magnetic x-ray circular dichroism of transition metals.We compare our result with the ones obtained by using several semi-relativistic time-dependent Hamiltonians. In the case of absorption, the final formula agrees with the result obtained from one of them. However, the correct scattering cross-section is not given by any of the semi-relativistic Hamiltonians

Electronic and magnetic properties of iron hydride under pressure: An experimental and computational study using x-ray absorption spectroscopy and x-ray magnetic circular dichroism at the Fe K edge

International audienceThe application of a 3.5 GPa pressure on Fe in a H 2 environment leads to the formation of iron hydride FeH. Using a combination of high pressure x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) at the Fe K edge, we have investigated the modification of electronic and magnetic properties induced (i) by the transition from bcc-Fe to dhcp (double hexagonal)-FeH under pressure and (ii) by the compression of FeH up to 28 GPa. XAS and XMCD spectra under pressure have been computed in bcc-Fe and dhcp-FeH within a monoelectronic framework. Our approach is based on a semirelativistic density-functional theory (DFT) calculation of the electron density in the presence of a core hole using plane waves and pseudopotentials. Our method has been successful to reproduce the experimental spectra and to interpret the magnetic and electronic structure of FeH. In addition, we have identified a transition around 28 GPa, which is a purely magnetic transition from a ferromagnetic state to a paramagnetic state

1s2p Resonant Inelastic X-ray Scattering Magnetic Circular Dichroism as a probe for the local and non-local orbitals in CrO 2

International audienceWe have determined the magnetic ground state of the half-metal CrO 2 based on 1s2p Resonant Inelastic X-ray Scattering Magnetic Circular Dichroism (RIXS-MCD) experiments. The two-dimensional RIXS-MCD map displays the 1s X-ray absorption spectrum combined with the 1s2p X-ray emission decay, where there is a large MCD contrast in the final state involving the 2p core hole. Our measurements show that the Cr K pre-edge structure is dominated by dipolar contributions and the quadrupole peak is invisible in direct K pre-edge absorption. Using RIXS-MCD, we reveal that the quadrupole 1s3d pre-edge has a large MCD contrast, which appears at lower energy with respect to the K pre-edge maximum. We use crystal field multiplet calculations to model the excitonic RIXS-MCD spectral shape in tetragonal (D 4h) symmetry. The RIXS-MCD is strongly sensitive to the ground state distortion of the Cr 4+ sites. The calculations of the RIXS-MCD maps suggest that the 3d spin–orbit interaction is fully quenched (3d = 0 meV) and the ground state electron configuration must contain a 3 B 2g (D 4h) contribution, which is required to explain the appearance of the Magnetic Circular Dichroism (MCD) in the Cr K pre-edge. This is in apparent contrast with the compressed tetragonal distortion

Bad Neighbour, Good Neighbour: How Magnetic Dipole Interactions Between Soft and Hard Ferrimagnetic Nanoparticles Affect Macroscopic Magnetic Properties in Ferrofluids

International audienceFluids responding to magnetic fields (ferrofluids) offer a scene with no equivalent in nature to explore long-range magnetic dipole interactions. Here, we studied the very original class of binary ferrofluids, embedding soft and hard ferrimagnetic nanoparticles. We used a combination of X-ray magnetic spectroscopy measurements supported by multi-scale experimental techniques and Monte-Carlo simulations to unveil the origin of the emergent macroscopic magnetic properties of the binary mixture. We found that the association of soft and hard magnetic nanoparticles in the fluid has a considerable influence on their inherent magnetic properties. While the ferrofluid remains in a single phase, magnetic interactions at the nanoscale between both types of particles induce a modification of their respective coercive fields. By connecting the microscopic properties of binary ferrofluids containing small particles, our findings lay the groundwork for the manipulation of magnetic interactions between particles at the nanometer scale in magnetic liquids

XAS and XMCD Reveal a Cobalt(II) Imide Undergoes High-Pressure-Induced Spin Crossover

International audienceThe spin crossover (SCO) transition is investigated in two molecular Co 2+ bisimide compounds, Co(dpzca) 2 and Co(pypzca) 2 (where Hdpzca = N-(2-pyrazylcarbonyl)-2pyrazinecarboxamide and Hpypzca = N-(2-pyrazylcarbonyl)-2-pyridinecarboxamide). They crystallize solvent-free with similar crystal structures but have been reported to exhibit different temperature-dependent magnetic behaviors. Using temperature-and pressure-dependent element selective X-ray absorption spectroscopy (XAS and XMCD) measurements, it is revealed herein that although Co(pypzca) 2 does not afford a temperature-induced SCO, it undergoes a reversible pressure-induced SCO transition that is less abrupt, and is complete at a higher pressure (3.5 GPa), than for Co(dpzca) 2 (0.5 GPa). Wave function-based calculations performed on isolated complexes confirm the LS state nature of Co(dpzca) 2 at low temperatures, and the values of spin and orbital magnetic moments are determined. Calculations show the similarity of groundstate properties for HS Co(dpzca) 2 and HS Co(pypzca) 2 and the existence of a double-well HS-LS in the potential energy surface for both compounds. It is concluded that the observed, significant differences in pressure-and temperature-induced SCO transitions of Co(dpzca) 2 versus Co(pypzca) 2 are probably due to different intermolecular interactions between Co(dpzca) 2 and Co(pypzca) 2 , which would hamper the temperature-induced SCO in Co(pypzca) 2

XAS and XMCD Reveal a Cobalt(II) Imide Undergoes High-Pressure-Induced Spin Crossover

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XAS and XMCD Reveal a Cobalt(II) Imide Undergoes High-Pressure-Induced Spin Crossover

International audienceThe spin crossover (SCO) transition is investigated in two molecular Co 2+ bisimide compounds, Co(dpzca) 2 and Co(pypzca) 2 (where Hdpzca = N-(2-pyrazylcarbonyl)-2pyrazinecarboxamide and Hpypzca = N-(2-pyrazylcarbonyl)-2-pyridinecarboxamide). They crystallize solvent-free with similar crystal structures but have been reported to exhibit different temperature-dependent magnetic behaviors. Using temperature-and pressure-dependent element selective X-ray absorption spectroscopy (XAS and XMCD) measurements, it is revealed herein that although Co(pypzca) 2 does not afford a temperature-induced SCO, it undergoes a reversible pressure-induced SCO transition that is less abrupt, and is complete at a higher pressure (3.5 GPa), than for Co(dpzca) 2 (0.5 GPa). Wave function-based calculations performed on isolated complexes confirm the LS state nature of Co(dpzca) 2 at low temperatures, and the values of spin and orbital magnetic moments are determined. Calculations show the similarity of groundstate properties for HS Co(dpzca) 2 and HS Co(pypzca) 2 and the existence of a double-well HS-LS in the potential energy surface for both compounds. It is concluded that the observed, significant differences in pressure-and temperature-induced SCO transitions of Co(dpzca) 2 versus Co(pypzca) 2 are probably due to different intermolecular interactions between Co(dpzca) 2 and Co(pypzca) 2 , which would hamper the temperature-induced SCO in Co(pypzca) 2