120 research outputs found
XANES calculation with an efficient ``Non Muffin-Tin'' method: Application to the angular dependence of the AlK-edge in corundum
No full textWe present a reciprocal-space ``non mufin-tin'' scheme for calculating X-ray Absorption Near-Edge Structure (XANES). The method uses pseudopotentials and reconstructs all electron wave functions within the Projector Augmented Wave framework. The method incorporates a recursive method to compute absorption cross section as a continued fraction. The continued fraction formulation of absorption is advantageous in that it permits the treatment of core-hole-electron interaction through large supercells ( hundreds of atoms). This opens new fields of applications like surfaces, molecules, small aggregates or amorphous materials, for which large supercells are required. The method is applied to the natural linear dichroism at the Al K-edge in corundum (alpha-Al2O3). Details about the convergence process of the calculation are given. The influence of the core-hole effects is emphasized and comparison with ``muffin-tin'' multiple scattering calculation is made
Chemical Strategies and Characterization Tools for the Organization of Single Molecule Magnets on Surfaces
No full textAddressing individual bistable magnetic molecules, known as Single Molecule Magnets (SMMs), is a fascinating goal at the borderline between molecular magnetism and spin electronics. This tutorial review focuses on the first step towards single-molecule experiments, namely the organization of SMMs on surfaces. Both preparation and characterization of surface-supported SMMs prove to be quite demanding and a multidisciplinary approach is necessary, which is described here using selected examples. We first illustrate the chemical strategies devised to assemble SMMs and to control their orientation on surfaces. Then, we present characterization tools, which have been selected on the basis of their relevance to address specific points, i.e. the chemical composition of the deposited SMM films, the organization of the molecules on the surface, the intramolecular arrangement of the spins, the magnetic anisotropy of SMMs, and eventually the dynamics of their magnetization on surfaces. Particular attention is devoted to techniques exploiting synchrotron light
On-Surface Magnetometry: The Evaluation of Superexchange Coupling Constants in Surface-Wired Single-Molecule Magnets
No full textA comfortable environment: Interaction with a gold surface fully preserves the spin structure of a Fe3Cr single-molecule magnet of current interest in molecular spintronics. Magnetic polarization at metal sites is measured over a broad temperature range (from 10 to 300 K) by X-ray magnetic circular dichroism (XMCD), which provides the required monolayer sensitivity. The data are used to evaluate Fe-Cr superexchange coupling interactions, which compare well with bulk-phase values in both sign and strength
Magnetic Memory of a Single-molecule Quantum Magnet Wired to a Gold Surface
No full textIn the field of mol. spintronics, the use of magnetic mols. for information technol. is a main target and the observation of magnetic hysteresis on individual mols. organized on surfaces is a necessary step to develop mol. memory arrays. Although simple paramagnetic mols. can show surface-induced magnetic ordering and hysteresis when deposited on ferromagnetic surfaces, information storage at the mol. level requires mols. exhibiting an intrinsic remnant magnetization, like the so-called single-mol. magnets (SMMs). These have been intensively investigated for their rich quantum behavior but no magnetic hysteresis has been so far reported for monolayers of SMMs on various nonmagnetic substrates, most probably owing to the chem. instability of clusters on surfaces. Using X-ray absorption spectroscopy and X-ray magnetic CD synchrotron-based techniques, pushed to the limits in sensitivity and operated at sub-kelvin temps., we have now found that robust, tailor-made Fe4 complexes retain magnetic hysteresis at gold surfaces. Our results demonstrate that isolated SMMs can be used for storing information. The road is now open to address individual mols. wired to a conducting surface in their blocked magnetization state, thereby enabling investigation of the elementary interactions between electron transport and magnetism degrees of freedom at the mol. scale
XAS and XMCD Investigation of Mn12 Monolayers on Gold
No full textThe deposition of Mn12 single-molecule magnets on gold surfaces was studied for the first time using combined X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) methods at low temperature. The ability of the proposedapproach to probe the electronic structure and magnetism of Mn12 complexes without significant sample damage was successfully checked on bulk samples. Detailed information on the oxidation state and magnetic polarization of manganese ions in the adsorbates was obtained from XAS and XMCD spectra,respectively. Partial reduction of metal ions to MnII was clearly observed upon deposition on Au(111) of two different Mn12 derivatives bearing 16-acetylthio-hexadecanoate and 4-(methylthio)benzoate ligands. The average oxidationstate, as well as the relative proportions of MnII, MnIII and MnIV species, are strongly influenced by the deposition protocol. Furthermore, the local magnetic polarizations are significantlydecreased as compared with bulk Mn12 samples. The results highlight an utmost redox instability of Mn12 complexes at gold surfaces, presumably accompanied by structural rearrangements,which cannot be easily revealed by standard surface analysisbased on X-ray photoelectron spectroscopy and scanning tunnelling microscopy
Magnetic Bistability in a Submonolayer of Sublimated Fe4 Single-Molecule Magnets
No full textWe demonstrate that Fe4 molecules can be deposited on gold by thermal sublimation in ultra-high vacuum with retention of single molecule magnet behavior. A magnetic hysteresis comparable to that found in bulk samples is indeed observed when a submonolayer film is studied by X-ray magnetic circular dichroism. Scanning tunneling microscopy evidences that Fe4 molecules are assembled in a two-dimensional lattice with short-range hexagonal order and
coexist with a smaller contaminant. The presence of intact Fe4 molecules and the retention of their bistable magnetic behavior on the gold surface are supported by density functional theory calculations
XMCD of a single layer of single molecule magnets
No full textA detailed report on the X-ray Magnetic Circular Dichroism (XMCD) investigation of monolayers of Mn12-based single molecule magnets (SMMs) deposited on gold Au(111) is presented. A semi-quantitative analysis of data is provided in order to extract chemical and magnetic information on Mn ions, by comparison withXMCD on bulk samples. This work points that XMCD is a key-tool for the characterization of SMMs-based nanostructured systems. XMCD surface sensitivity and element-specificity will play a fundamental role in the identification of good candidates for SMMs based devices. © EDP Sciences and Springer 2009
Multiple Scattering Calculations Applied to XAS: A Structure and Electronic Sensitive Tool
No full textABSTRACTWe present an application of multiple scattering theory with “muffin-tin” potentials to the calculation of X-ray absorption cross section. We have measured and calculated the K-edge spectra of atoms in compounds with zincblende structure : SiC, ZnS. We show that some spectral features can be precisely related to the local environnement around the absorbing atom.</jats:p
Organization of Single-Molecule Magnets (SMMs) by coordination chemistry : toward a new generation of magnetic and photomagnetic materials
No full textDepuis leur découverte dans les années 90, les molécules-aimants constituent une classe de matériaux magnétiques qui a attiré l'attention du fait de leur bistabilité magnétique. Ces systèmes donnent l’espoir formidable de pouvoir stocker un bit d’information à l’échelle moléculaire. Ainsi, leur organisation dans des réseaux est devenue un enjeu essentiel en vue de leur intégration dans des dispositifs. Lors de cette thèse, l’organisation contrôlée de ces molécules par chimie de coordination en utilisant différents connecteurs s’est révélée être une stratégie de choix. Le chapitre I présente une approche théorique de ce projet de recherche. Dans ce chapitre, les propriétés de molécules-aimants, chaînes-aimants, conversion de spin et transfert d'électron sont décrits et discutés. Le chapitre II contient la bibliographie pertinente sur les réseaux de coordination à base de molécules-aimants et les systèmes photoactifs bimétalliques conténant des groupements cyanures. Le chapitre III présente l'organisation de molécules aimants[Mn4] en réseaux 1D et 2D par des liens diamagnétiques (ions chlorures) ou des liens paramagnétiques contenant des ions métalliques (NiII, MnII et CuII). Les études physiques (cristallographie par rayons X, mesuresmagnétiques et de chaleurs spécifiques) et des analyses théoriques sur ces nouveaux réseaux ont montré des propriétés magnétiques améliorées par rapport à la molécule-aimant [Mn4] isolée. Dans le chapitre IV, nous avonspréparé de nouveaux connecteurs commutables pour in fine concevoir des réseaux de molécules-aimants photomagnétiques. Une approche « building-block » a été utilisée pour obtenir un composé binucléaire de Fe et Co.Des études spectroscopiques, électrochimiques et magnétiques ont été effectuées et ont révélé sans ambiguïté une conversion de spin thermo-induite à l'état solide, et un transfert d'électron intramoléculaire assisté par protonation contrôlée en solution, accompagnés de changements optiques et magnétiques. Pour la première fois, ce nouveaucomplexe montre deux processus de commutation distincts selon son état physique et le stimulus externe utilisé.The beginning of the 1990’s marked the discovery of Single-Molecule Magnets (SMMs), which created the hope tostore information on a single molecule due to their magnetic bistability. However, it is becoming of strategicimportance to dedicate a part of our research to their organization in order to achieve devices for the potentialapplication. During this thesis work, our strategy was to exploit coordination chemistry to organize these moleculesin a controlled way by using different types of linkers.Chapter I covers theoretical backgrounds for this research project. In this chapter, Single-Molecule Magnets(SMMs), Single-Chain Magnets (SCMs), Spin Crossover (SC) and Electron Transfer (ET) systems are described anddiscussed. Chapter II contains relevant literature on SMM-based coordination networks and photoactive cyanidobasedbimetallic systems. Chapter III presents the organization of [Mn4] SMMs in 1D and 2D networks withdiamagnetic linkers (chlorido ions) or paramagnetic linkers containing NiII, MnII, and CuII ions. The extensivephysical studies (X-ray crystallography, magnetic and heat capacity measurements, and theoretical analysis) on thesenetworks demonstrated new magnetic behavior and enhanced energy barrier compared to the isolated [Mn4] SMMs.In Chapter IV, we prepared new switchable linkers based on the cyanido-bridged Fe/Co unit, to realizephotomagnetic networks of SMMs. A rational building-block approach has been used to design these dinuclearFe/Co complexes. Extensive spectroscopic, electrochemical and magnetic characterizations have been performed tounambiguously reveal in one of the synthesized complexes the presence of a spin crossover induced by temperaturein the solid-state, and an intramolecular electron transfer assisted by controlled protonation in solution, bothaccompanied by optical and magnetic changes. For the first time, this new complex shows two distinct switchingprocesses depending on its physical state and external stimuli
Etude de l'ordre chimique et magnétique d'oxydes spinelles de taille nanométrique par dichroïsme magnétique circulaire des rayons X
No full textPARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF
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