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Papers by h magnan

LURE, CNRS-MENR, Bât. 209d, Université Paris Sud, BP 34, 91898 Orsay cedex, France SPCSI, CEA, 91... more LURE, CNRS-MENR, Bât. 209d, Université Paris Sud, BP 34, 91898 Orsay cedex, France SPCSI, CEA, 91191 Gif sur Yvette cedex, France SOLEIL, L’Orme des Merisiers, Saint Aubin, BP48, 91192 Gif sur Yvette cedex, France GPS, CNRS-Universités Paris 6 et 7, 2 place Jussieu, 75251 Paris cedex 05, France LRRS,CNRS-Université de Bourgogne, BP47870, 21078 Dijon cedex, France HASYLAB / DESY, Notkestrasse 85, 22603 Hamburg, Germany

AIP Conference Proceedings

Quadrupolar transitions can play an important role in X-ray absorption spectroscopy, especially w... more Quadrupolar transitions can play an important role in X-ray absorption spectroscopy, especially when it is used for magnetic measurements, like in X-ray Magnetic Circular Dichroism or Resonant Magnetic Scattering. We show here that resonantly excited Ti KL2,3L2,3 Auger spectra of TiO2 (110) carry a clear signature of quadrupolar transitions from the 1s to localized eg and t2g d-like states. The

We studied the growth of epitaxial ferroelectric layers of BaTiO3/Pt(001) in the framework of sol... more We studied the growth of epitaxial ferroelectric layers of BaTiO3/Pt(001) in the framework of solar water splitting. The stoichiometry, that is, the Ti/Ba ratio, appears as a crucial parameter not ...

Thin Solid Films

Abstract Ta3N5 belongs to the group of transition metal nitrides with the cation in a high oxidat... more Abstract Ta3N5 belongs to the group of transition metal nitrides with the cation in a high oxidation state. These are typically challenging to synthesize owing to the low reactivity of nitrogen. This applies similarly to Ta3N5 that crystallizes only in the presence of oxygen during synthesis. Typical preparation methods are ammonolysis of oxidized Ta or magnetron sputtering of a Ta target in an atmosphere of Ar, N2 and O2. However, the material typically obtained by either synthesis method is of varying degrees of crystallinity and the key parameter affecting the crystallinity remains elusive. In this study, we examine the role of oxygen for the crystallinity of Ta3N5 samples by studying. Thin film samples prepared by magnetron sputtering reveal that oxygen is indeed the central driver for Ta3N5 crystallinity. While little oxygen in the films yields the metallic δ-TaN phase, excess oxygen results in low crystallinity Ta3N5 or Ta-O-N films. Ta3N5 samples with a high degree of crystallinity are obtained by limiting the oxygen supply to the sample during the deposition. A comparison with other studies suggests a fundamental oxygen incorporation limit above which the crystallinity of Ta3N5 is compromised. The most crystalline sample from this study contains 4.4 at.% of oxygen. It is grown onto a Si(100) substrate, covered with a 30 nm-thick metallic diffusion layer. For this sample, we observe Ta3N5 grains between 80 and 120 nm in size.

Revue de Métallurgie

Dans cet article, nous presentons comment utiliser les oscillations du coefficient d’absorption X... more Dans cet article, nous presentons comment utiliser les oscillations du coefficient d’absorption X (EXAFS) pour determiner la structure et l’ordre local autour d’une espece chimique precise dans un materiau. Une description theorique du phenomene est donnee. Cette technique est particulierement bien adaptee a l’etude d’adsorbats ou de films minces epitaxies.

MRS Proceedings

ABSTRACTIn this paper, the interrelation between structural and magnetic properties of ultra-thin... more ABSTRACTIn this paper, the interrelation between structural and magnetic properties of ultra-thin Mn films epitaxially grown on (001) bcc Fe is studied. The Mn growth and in-plane structure were controlled by electron diffraction (RHEED). The structures of the Mn films were determined by using X-Ray absorption spectroscopy (SEXAFS). Finally, the magnetic properties were studied by using X-Ray magnetic circular dichroïsm (XMCD). All the experiments were performed under ultra-high vacuum. As shown by XMCD experiments, a magnetic transition is observed at 2 Mn monolayers. The analysis of RHEED and SEXAFS experimental results clearly demonstrates that a structural transition comes with this magnetic transition.

Physical Review B

The growth of cobalt on a Cu(111) single crystal is studied using x-ray-absorption spectroscopy. ... more The growth of cobalt on a Cu(111) single crystal is studied using x-ray-absorption spectroscopy. A polarization-dependent multiple-scattering analysis up to the fourth shell of neighbors using the FEFF code clearly shows that the thick films have a local hcp stacking (ABAB planes) and not the fcc stacking (ABCABC) of the substrate. Moreover, we determine that on a C surface type of the ABC Cu substrate, the Co atoms are first adsorbed on A sites, the second layer on B sites continues the Cu lattice, but the third Co layer already starts a hcp stacking, by mainly choosing A sites again. ' C. R. Natoli et al. , Phys. Rev. A 22, 1104 {1980).

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment

Surface and Coatings Technology, 2016

Ta 3 N 5 has ideal properties for the efficient water splitting under sunlight illumination, but ... more Ta 3 N 5 has ideal properties for the efficient water splitting under sunlight illumination, but its formation is only triggered by the incorporation of oxygen in its lattice. As a result, material properties may deviate from those of an ideal Ta 3 N 5 crystal, which can promote or impede the water splitting. This paper is to quantify variations in key properties relevant for the photoelectrolysis as a function of oxygen incorporation and to determine an optimum oxygen concentration. Thin films were prepared by direct current magnetron sputtering in an Ar/N 2 /O 2 gas mixture at two different N 2 and varying O 2 concentrations. The films were characterized by grazing-incidence x-ray diffraction (GI-XRD), total transmission and reflectance spectroscopy (TTRS) from which the band gap is deduced, photocurrent measurements on which the calculation of the efficiency-mobility-lifetime (ημτ) product is based and photoelectrolysis experiments. Finally, the stability of films was determined by x-ray photoelectron spectroscopy (XPS) measured before and after the photoelectrolysis. We show that material properties degrade with oxygen incorporation during the film growth. At the highest oxygen concentrations investigated, the material becomes amorphous and shows strong surface oxidation and a possible reaction with the electrolyte during the photoelectrolysis. Samples deposited at an oxygen concentration below 1%, exhibit the Ta 3 N 5 phase, a band gap below 2.0 eV, a high mobility-lifetime product, photoelectrolytic activity and a good relative stability against surface oxidation during the photoelectrolysis.

Thin Solid Films, 2016

Controlling the ferroelectric polarization at macroscopic or microscopic levels is crucial in the... more Controlling the ferroelectric polarization at macroscopic or microscopic levels is crucial in the framework of the development of ferroelectric materials used in yet challenging photo-electrochemical (PEC) cells and spintronic applications. We report here on polarization methods allowing to electrically polarize prototypical samples of BaTiO 3 (001) films. Epitaxial single crystalline layers were grown up to a thickness of 25 nm by atomic oxygen assisted molecular beam epitaxy on 1 at.% Nb doped SrTiO 3 (001) single crystals. The samples were both microscopically and macroscopically polarized using Piezoresponse Force Microscopy and electrochemical poling in an electrolyte respectively. In addition we demonstrate the possibility to retrieve a quasi-native mixed ferroelectric polarization state after annealing. These polarization methods may be applied to many other ferroelectric thin films.

Physical Review B, 1998

The determination of the atomic structure of a Mn monolayer deposited at room temperature on Ag(0... more The determination of the atomic structure of a Mn monolayer deposited at room temperature on Ag(001) was achieved by using surface extended x-ray absorption fine structure. Keeping in mind the previous results of x-ray photoelectron diffraction, several structural models involving the two topmost layers of the Ag substrate were tested in ab initio polarization-dependent XAFS calculations. Among these models, the

$Research paper thumbnail of Resonant photoelectron and photoelectron diffraction across the Fe L 3 edge of Fe 3 O 4$

Physical Review B, 2010

We combined measurements of valence-band photoemission and valence-band photoelectron diffraction... more We combined measurements of valence-band photoemission and valence-band photoelectron diffraction on textFe3textO4{\text{Fe}}_{3}{\text{O}}_{4}textFe3textO4 at the textFetext2ptextensuremath−3d\text{Fe}\text{ }2p\text{\ensuremath{-}}3dtextFetext2ptextensuremath−3d resonance. The different structures in the valence band of magnetite due to the different sites of iron were identified experimentally. Specifically the structure near the Fermi level is unambiguously attributed only to octahedral textFe2+{\text{Fe}}^{2+}textFe2+ sites (Btext−Fe2+)(B{\text{-Fe}}^{2+})(Btext−Fe2+). We showed that tuning the photon energy to the resonance of Btext−Fe2+B{\text{-Fe}}^{2+}Btext−Fe2+, the whole valence band is dominated by signal coming from BBB sites of iron. Moreover this work shows how resonant photoelectron diffraction is a powerful tool for the study of mixed valence oxides.

Applied Physics Letters, 2015

HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific re... more HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Physical Review Letters, 1991

Surface extended x-ray-absorption fine structure is used to determine the crystallographic struct... more Surface extended x-ray-absorption fine structure is used to determine the crystallographic structure of iron films grown on Cu(100). We show that these metastable films have structural and vibrational properties strongly dependent on their thickness and their growth temperature: Either a well-defined fcc structure or an anisotropically distorted fcc structure is observed. A strong reordering of the thin iron films prepared

Physical Review B, 1999

... 14 N. Marsot, R. Belkhou, F. Scheurer, B. Bartenlian, N. Barrett, and C. Guillot, Surf. Sci. ... more ... 14 N. Marsot, R. Belkhou, F. Scheurer, B. Bartenlian, N. Barrett, and C. Guillot, Surf. Sci. 377?379, 225 1997 . 15 A. Clarke, G. Jennings, RF Willis, JP Rous, and JB Pendry, Surf. Sci. 187, 327 1987 . 16 ` O. Heckmann, H. Magnan, P. Le Fevre, D. Chandesris, and JJ Rehr, Surf. ...

NATO ASI Series, 1993

The knowledge of the crystallography of metastable magnetic films prepared by deposition on suita... more The knowledge of the crystallography of metastable magnetic films prepared by deposition on suitable substrates is the key for the understanding of their unusual magnetic properties. Surface EXAFS1 is a very attractive technique to measure it. It gives to a high precision the shape of the first neighbour shell2, including its possible asymmetry3 and its thermal broadening4,5 which is related to the elastic force constant between nearest neighbours (nn) in the film. Moreover, the linear polarization of the synchrotron radiation reveals information about the anisotropy of the crystallographic structure. Moreover EXAFS is a selective method: by measuring the EXAFS oscillations above the K edge of iron (or cobalt) we are sure to be sensitive only to the local order in the iron (or cobalt) film. Then, films of any thickness, coated films and multilayers can be characterized with the same precision. This can be illustrated by the examples of iron on copper and cobalt on copper metastable structures.