Probing the spin dimensionality in single-layer CrSBr van der Waals heterostructures by magneto-transport measurements

Two-dimensional (2D) magnetic materials offer unprecedented opportunities both in terms of fundamental concepts and applied devices with special relevance in the fields of spintronics and magnonics. Beyond the pioneering studies on CrI3 and Cr2Ge2Te6, the family of 2D magnets has expanded to layered antiferromagnets with different spin anisotropies and spin textures. However, all these compounds are highly insulating, thus limiting their possibilities for being integrated into devices. Of particular interest is the case of the layered metamagnet CrSBr, a 2D semiconductor formed by ferromagnetic layers (Tc ~ 150 K) coupled antiferromagnetically between them. This material exhibits a rich physical scenario, including thermal spin dimensionality crossovers and low-temperature hidden order (T* ~ 40 K). Here, we inspect the magneto-transport properties of monolayer, bilayer and trilayer CrSBr integrated into vertical van der Waals heterostructures. Our results in the monolayer limit demonstrate (1) the marked low dimensional character of the ferromagnetic layer, with short-range correlations extending at temperatures well above Tc, (2) a spin anisotropy, with the spins spontaneously aligned along the easy axis (b) of the plane, (3) a reorientation of these spins along a and c upon applying a moderate magnetic field in these directions, and (4) the appearance of field-induced phases in these two directions below ca. 30-40 K due to a cooperative freezing of the spins. In the multilayer case, a spin valve-like behavior is also observed characterized by a negative MR strongly enhanced below T*. Overall, the present results, supported by first-principles calculations, show that the monolayer and bilayer of CrSBr capture most of the physics present in the bulk, offering new insights into the physics of 2D magnets and the integration of these layers into vertical spintronic devices.Comment: 4 figure

Conducting Anilate-Based Mixed-Valence Fe(II)Fe(III) Coordination Polymer: Small-polaron Hopping Model for Oxalate-Type Fe(II)Fe(III) 2D Networks

The mixed-valence FeIIFeIII 2D coordination polymer formulated as [TAG][FeIIFeIII(ClCNAn)3]·(solvate) 1 (TAG = tris(amino)-guanidinium, ClCNAn2− = chlorocyanoanilate dianionic ligand) crystallized in the polar trigonal space group P3. In the solid-state structure, determined both at 150 and at 10 K, anionic 2D honeycomb layers [FeIIFeIII(ClCNAn)3]− establish in the ab plane, with an intralayer metal−metal distance of 7.860 Å, alternating with cationic layers of TAG. The similar Fe−O distances suggest electron delocalization and an average oxidation state of +2.5 for each Fe center. The cation imposes its C3 symmetry to the structure and engages in intermolecular N−H···Cl hydrogen bonding with the ligand. Magnetic susceptibility characterization indicates magnetic ordering below 4 K and the presence of a hysteresis loop at 2 K with a coercive field of 60 Oe. Mössbauer measurements are in agreement with the existence of Fe(+2.5) ions at RT and statistic charge localization at 10 K. The compound shows semiconducting behavior with the in-plane conductivity of 2 × 10−3 S/cm, 3 orders of magnitude higher than the perpendicular one. A small-polaron hopping model has been applied to a series of oxalate-type FeIIFeIII 2D coordination polymers, providing a clear explanation on the much higher conductivity of the anilate-based systems than the oxalate ones

Photoinduced HS state in the first spin-crossover chain containing a cyanocarbanion as bridging ligand

A new polymeric approach, based on cyanocarbanion ligands, for the design of spin crossover (SCO) compounds led us to the compound [Fe(abpt)2(tcpd)] (1) (tcpd^2 = (C[C(CN)2|3)^2 , abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole) which has been characterised as the first SCO molecular chain involving a cyanocarbanion as bridging ligand.Gomez Garcia, Carlos Jose, [email protected]

Chemins d'intéraction dans les composés magnétiques moléculaires à partir d'études de diffraction X haute résolution

Nous avons utilisé la diffraction de rayons X à haute résolution afin de modéliser la distribution de densité électronique dans différents composés magnétiques moléculaires, dont les mécanismes d'interaction reposent sur des phénomènes de transfert de charges, recouvrements directs ou indirects d'orbitales, liaisons hydrogènes, polarisation ou délocalisation électroniques. Nous avons ainsi relié la structure électronique atomique de ces systèmes à leur comportement magnétique et déterminé les chemins d'échange par l'analyse topologique de la densité électronique. Nous avons confirmé l'implication d'un transfert de charge de l'orbitale SOMO vers l'orbitale LUMO, selon le mécanisme de McConnell l, par délocalisation intramoléculaire et liaison hydrogène intermoléculaire dans le composé Nit(SMe)Ph. Trois points critiques de liaison dans les trois liaisons hydrogène intermoléculaires démontrent de plus la localisation des chemins d'échange entre radicaux, assurant la coopérativité du système. Des interactions 2D par superéchange ont été caractérisées dans le composé CU2(OH)3N03, notamment par l'absence de point critique de liaison entre atomes de cuivre. Des interactions à fort et faible recouvrement d'orbitales résultent de la participation de deux ions hydroxo ou d'un ion hydroxo et d'un ion nitrate au chemin d'échange respectivement. Trois liaisons hydrogène, caractérisées par l'analyse topologique, assurent les couplages inter-plans. Une délocalisation électronique a été mise en évidence depuis les deux centres métalliques sur le pont oxamato du système ferrimagnétique ID MnCu(pba). Les liaisons Cu-N et Cu-O, a plus fort recouvrement d'orbitales que lV1n-O rendent la délocalisation ainsi que la polarisation du pont oxamato à partir de l'atome de cuivre plus efficace. L'analyse topologique a révélé l'implication de quatre liaisons hydrogène dans les couplages inter-chaînes, expliquant la déviation au comportement purement ID à très basse température.NANCY1-SCD Sciences & Techniques (545782101) / SudocSudocFranceF

Prévention et dépistage du VIH et des hépatites B et C chez les migrants des Alpes-Maritimes (intérêt et évaluation d'une consultation de prévention au Centre d'acceuil, de Soins et d'Orientation de Médecins du Monde à Nice)

NICE-BU Médecine Odontologie (060882102) / SudocSudocFranceF

The Piloting of E-Commerce Performance: Development of a Model of Assistance to Piloting by Objectives

International audienc

Proposition d’une méthode de qualification et de sélection d’un logiciel d’analyse et de suivi du référencement dans les moteurs de recherche

In order to measure website visibility in search engines, there are softwares for analytics and referencing follow-up. They permit to quantify website's efficacity of referencing and optimize its positionning in search engines. With regard to search engines' algorithms' evolution and centralization of Key Performance Indicators for Marketing decision making, it becomes hard to find solutions to effectively lead a lot of projects for referencing. That's why we have built a methodology in order compare, evaluate and choose a software for analytics and referencing follow-up in search engines

Modélisation des processus à l'équilibre et hors équilibre de matériaux à transition de spin (application à la simulation des diagrammes de diffraction des rayons X)

La modélisation des processus à l'équilibre et hors de l'équilibre observés expérimentalement dans les composés à transition de spin est indispensable à la compréhension des mécanismes de transition existants chez ces matériaux bistables, très prometteurs pour de futures applications dans le stockage informatique. Les mesures suivant le changement d'état moléculaire (magnétiques, optiques) ont été reproduites avec succès à l'aide de diverses modèles, en particulier les modèles de type Ising. En revanche, le changement de phase structurale, observé par diffraction des rayons X ou neutrons, accompagnant le changement d'état de spin, ne peut être simulé avec ces modèles. L'objectif de ce travail est de réconcilier les deux types de mesures : magnétiques, optiques et la diffraction. Un nouveau modèle de spin a dû être introduit, dans lequel sont pris en compte de manière explicite les variables de réseau. Le modèle a été étudié analytiquement dans le cas unidimensionnel et numériquement dans le cas bidimensionnel par simulation Monte Carlo des processus à l équilibre et hors équilibre. Le modèle retrouve d une part les résultats des modèles de type Ising et d autre part la contraction ou la dilatation du réseau à la transition de spin. Afin de comparer avec les mesures de diffraction, la mise en conditions expérimentales a été simulée à l'aide du modèle anharmonique couplé avec un programme (DISCUS) pour calculer l'intensité diffractée. Les clichés de diffraction ont été reproduits avec succès dans les différents cas observés expérimentalement. Les conditions d'apparition de la coexistence des pics de Bragg HS et BS sont analysées. La notion de "domaines de spin" est alors complétée par l'introduction "des domaines de spin moléculaires" (clusters de molécules) et "des domaines de spin structuraux".Modelisation of Equilibrium and non equilibrium processes experimentally observed in spin crossover compounds is essential for a better understanding of mechanisms of the transition existing in those bistable solids which are good candidates for future industrial applications in computing data storage. Measurements following the molecular spin state change (magnetic, optical ) have been successfully reproduced, using various models, in particular Ising-like models. However, structural phase changes, occurring during the spin state change and only accessible with X-ray or neutron diffraction experiments, can t be reproduce with Ising-like models. The aim of this work is to interpret both kinds of measures: magnetic and diffraction. We have also introduced a new microscopic spin model in which lattice degrees of freedom have been explicitly taken into account. This anharmonic model has been solved analytically in the one dimensional case and numerically in higher dimensions using Monte Carlo simulation of equilibrium and non equilibrium processes. This model retrieves most of results or Ising-like models and simulates lattice expansion or contraction during the spin transition. We have coupled this anharmonic model with a soft called DISCUS, in order to simulate diffraction experimental conditions and to calculate the intensity diffracted by the lattice. Diffraction patterns have been reproduced in the different cases (thermal transition, thermal relaxation, photoexcitation ) observed experimentally. Conditions for the observation of the coexistence of HS and LS Bragg peak are discussed and analyzed. The concept of like spin domains (LSD) is then defined more precisely by introducing the concept of molecular like spin domains (MLSD) and structural like spin domains (SLSD).NANCY1-Bib. numérique (543959902) / SudocSudocFranceF

Out of equilibrium charge density studies of light induced metastable states

The experimental electron density analysis is becoming a very mature field of research with outstanding applications in many areas of solid state physics and chemistry. This approach has allowed gaining new insights on bonding interactions in ground state systems often from joined experimental and theoretical methods. The extension of this technique to out of equilibrium systems is very promising and might surely lead to a better fundamental understanding of dynamic processes in solids such as phase transition or chemical reactivity [1]. Steady-state and time resolved x-ray diffraction techniques under external perturbation, like electric field [2], pressure or optical excitation [3], are more and more developed, mainly for structural purposes. We have shown that under appropriate and well controlled experimental conditions, accurate electron density distribution of metastable states can be derived [4]. State of the art in light-induced high resolution x-ray diffraction experiments and charge density analysis of molecular metastable states will be presented. Applications will cover light-induced phase transitions in Fe(II) spin-crossover complexes and solid state chemical reactivity

Kinetics of light-induced first-order phase transformation in molecular solids: Fe(btr)2(NCS)2∙H2O

International audienceThe mechanism and kinetics of the thermally and light-induced spin transition of the highly cooperative spin crossover material Fe(btr)2(NCS)2∙H2O (btr=4,4'–bis–1,2,4–triazole) have been investigated by single crystal x-ray diffraction techniques. The key role of like-spin domains is quantitatively analyzed through a nucleation, growing, and coarsening mechanism whose kinetics follow the Kolmogorov-Johnson-Mehl-Avrami model with low dimensional characteristics