Дисертації: "Magnetic Exchange Interaction"

1

Inoue, Jun-ichiro. "Effective exchange interaction and Curie temperature in magnetic semiconductors." The American Physical Society, 2003. http://hdl.handle.net/2237/7112.

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2

Sapozhnik, Alexey [Verfasser]. "Magnetic properties of antiferromagnetic Mn2Au: exchange interaction and domain manipulation / Alexey Sapozhnik." Mainz : Universitätsbibliothek Mainz, 2018. http://d-nb.info/1170263666/34.

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3

Kalapos, Thomas Lawrence. "Interaction of Water with the Proton Exchange Fuel Cell Membrane." Case Western Reserve University School of Graduate Studies / OhioLINK, 2007. http://rave.ohiolink.edu/etdc/view?acc_num=case1175891061.

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4

Pinel, Lucas. "Probing the magnetic exchange interaction in agraphene-ferromagnetic insulator system usingQuantum Hall Effect and non-local resistancemeasurements." Thesis, KTH, Tillämpad fysik, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-162232.

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5

Tanaka, Hiroki. "Zeeman Splitting Caused by Localized sp-d Exchange Interaction in Ferromagnetic GaMnAs Observed by Magneto-Optical Characterization." Ohio University / OhioLINK, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1441982108.

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6

Vallobra, Pierre. "Effects of interfacial interactions on optical switching in magnetic heterostructures." Thesis, Université de Lorraine, 2019. http://www.theses.fr/2019LORR0015/document.

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Pendant les 20 dernières années, le nanomagnétisme a suscité un intérêt grandissant au sein de la communauté scientifique du fait de ses nombreuses applications pour les mémoires magnétiques. A l’échelle nanométrique beaucoup de propriétés des matériaux magnétiques découlent de leurs interfaces avec d’autres matériaux (magnétiques ou non). Cela explique l’omniprésence des hétérostructures composées de plusieurs couches d’épaisseur nanométrique dans le domaine du nanomagnétisme. Dans les hétérostructures que nous étudions, ces propriétés interfaciales sont le décalage d’échange, l’interaction Dzyaloshinskii-Moriya, l’anisotropie magnétique perpendiculaire et l’échange entre deux couches ferromagnétiques. D’abord nous étudions la modification du champ de décalage d’échange dans une bicouche [Pt/Co]xN/IrMn lorsque l’on l’expose à des impulsions laser de lumière polarisée circulairement. Nous montrons que le champ de décalage d’échange après exposition au laser résulte de la configuration du ferromagnétique [Pt/Co]xN. Nous étudions ensuite les conditions nécessaires à un retournement tout optique dépendant de l’hélicité d’un matériau ferrimagnétique de synthèse composé de deux couches de CoFeB /Pt /CoFeB et Co couplées antiferromagnétiquement et concluons que les facteurs clés qui gouvernent le renversement de l’aimantation totale sont les températures respectives des deux couches. Nous nous sommes aussi concentrés sur la propagation de parois de domaine de Néel de même chiralité stabilisées par interaction Dzyaloshinskii-Moriya dans des multicouches de [Pt/Co/Ni]N. Nous avons finalement démontré la possibilité de générer des bulles skyrmioniques par le laser femtoseconde
During the last 20 years, nanomagnetism has attracted a growing interest in the scientific community due to its multiple applications for magnetic memories. At the nanometer scale, many of the properties of the magnetic materials arise from their interfaces with other materials (magnetic or non-magnetic). This explains the omnipresence of heterostructures composed of several layers of thicknesses in the range of the nanometer in the field of nanomagnetism. In the heterostructures we study, those interfacial properties are the exchange bias, the Dzyaloshinskii-Moriya interaction, the perpendicular magnetic anisotropy and the interlayer exchange between two ferromagnetic layers. First we study the modification of the exchange bias field in a [Pt/Co]xN/IrMn bilayer when we expose it to laser pulses of a femtosecond circularly polarized light. We demonstrate that the final exchange bias field after laser pulses results from the magnetic configuration of the [Pt/Co]xN multilayer. We then study the conditions required for a helicity-dependent all optical switching of a synthetic ferromagnetic material composed of a CoFeB /Pt /CoFeB and a Co ferromagnetic layers coupled antiferromagnetically and conclude that the key factors that drive the switching of the total magnetization are the Curie temperatures of both layers. We focused also on the field-driven propagation of Néel domain walls of the same chirality stabilized by the Dzyaloshinskii-Moriya interaction in [Pt/Co/Ni]xN multilayers. We finally demonstrated the possibility to generate skyrmionic bubbles with the femtosecond laser

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7

Goryan, Alexander S. "Nuclear magnetic resonance studies on bentonite in complex mixed systems." Licentiate thesis, Luleå tekniska universitet, Industriell miljö- och processteknik, 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-18463.

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In this work 23Na MAS NMR was validated as a successful quantitative method for studies of exchanging sodium in bentonites useful, in particular, for studies of ion-exchange kinetics. Na-enriched bentonites equilibrated in a re-circulated process water at iron-oxide pelletizing plants may acquire properties of Ca-bentonites after already 20 minutes of the equilibration time, since >50 % of sodium ions will be exchanged by calcium ions during first minutes of bentonite placed in contact with the process water. It was shown that all sodium activated bentonites used in this study exchange >50% of sodium in Na+/Ca2+ and ca 20 % of sodium in binary Na+/Mg2+ systems with the same bentonite/solution ratio and same concentrations of these ions in aqueous solutions as in the process water at a pelletizing plant. In total, approximately 50 % of the exchangeable sodium in original bentonites was exchanged after equilibrating of bentonites in the process water already after 20 minutes. Experimental Na+/Ca2+ exchange curves for ‘model’ Ca2+(aq) solutions and for process water are very similar as Ca2+ is the dominant constituent in the process water. Since bivalent ions (Ca2+ and Mg2+) that present in the process water readily replace Na+ ions, Na-bentonite transforms into Ca- or Mg- bentonite, which have worse rheological, swelling and, therefore, binding properties. This ion-exchange process can influence the binder performance in the pelletizing process. Taking into account that fluorapatite is one of the components present in a blend of minerals processed, possible interactions between orthophosphate (the principal anionic component of apatites) and bentonites in aqueous suspensions are considered. It was found that sorption of orthophosphate on Ca-montmorillonite follows a different pattern from sorption of orthophosphate on aluminum oxides and kaolinite. While there is a small amount of sorption below pH 7, which may involve inner-sphere complexation and precipitation of AlPO4 to Al-OH edge sites on the montmorillonite crystals, most sorption of orthophosphate occurs at higher pH. Both macroscopic sorption measurements and solid-state 31P MAS NMR suggest that above pH 7 there is precipitation of proton depleted calcium phosphate phases. Based on both 31P chemical shifts and 31P chemical shift anisotropies it was concluded that the principal precipitated phased are most likely ‘brushite-like’ phases. Very short spin-lattice T2(31P) relaxation times (≤100 μs) for the orthophosphate/bentonite systems can possibly be explained by the presence of paramagnetic Fe in bentonites. Since there are insufficient concentrations of soluble Fe species in the supernatant solution that may give rise to the observed effects, it is likely that orthophosphate is precipitated as thin layers on the surfaces of montmorillonite crystals, where phosphorus may interact with Fe atoms present in the crystal lattice. PO4-tetrahedra in sorbed species can be also distorted giving rise to a larger 31P CSA than for pure ‘apatite-like phases’. 29Si MAS and 1H-29Si CP/MAS NMR experiments on bentonite samples also performed in this work provide information about impurities of quartz in bentonites, a level of substitution of aluminum by iron atoms in the structure of montmorillonite and about the degree of hydration of montmorillonite. 29Si NMR experiments on bentonite incubated with waterglass in aqueous suspensions at concentrations of sodium silicates as in the process water demonstrated that one can follow the process of polymerization of waterglass in solutions and also detect sodium silicates polymerized on surfaces of bentonites already after 1 hour of incubation. Polymerized waterglass sorbed on bentonite surfaces may also alter rheological, swelling and, therefore, binding properties of sodium-activated bentonites used in pelletization of iron-oxide ores.

Godkänd; 2012; 20121011 (alegor); LICENTIATSEMINARIUM Ämne: Gränsytors kemi/Chemistry of Interfaces Examinator: Professor Oleg N. Antzutkin, Institutionen för samhällsbyggnad och naturresurser, Luleå tekniska universitet Diskutant: Professor emeritus Willis Forsling, Institutionen för samhällsbyggnad och naturresurser, Luleå tekniska universitet Tid: Onsdag den 5 december 2012 kl 13.00 Plats: C305, Luleå tekniska universitet

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8

Kumar, Deepak. "Thin film growth by combinatorial epitaxy for electronic and energy applications." Thesis, Normandie, 2019. http://www.theses.fr/2019NORMC255.

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Les oxydes de métaux de transition à structure pérovskite ABO3 présentent des degrés de liberté structurels et électroniques fortement enchevêtrés. On s'attend donc à découvrir des phases et des propriétés exotiques en agissant sur le réseau par le biais de divers stimuli externes. L'ingénierie des contraintes épitaxiales dans les couches minces d'oxydes est un moyen important d'adapter la distorsion du réseau cristallin par l'effet coopératif de Jahn Teller. En utilisant les couches minces actives PrVO3 de Jahn Teller comme système modèle, la corrélation structurelle avec le magnétisme est établie. Nous imposons différentes contraintes de contrainte épitaxiale dans les films minces PrVO3 via différents moyens, tels que, en utilisant divers substrats monocristallins disponibles dans le commerce, l'épaisseur du film, des substrats avec des orientations de surface cristallines différentes, etc. En conséquence, des phases nouvelles et cachées, absentes du composé en vrac, commencent à apparaître. Notamment, la contrainte de compression dans les films de PrVO3 améliore l'interaction de super échanges menant à une augmentation de la température de Neel antiferromagnétique, une forte anisotropie magnétique dans les films minces de PrVO3 cultivés sur des substrats SrTiO3 orientés (001), 110 et 111, sont quelques exemples
Transition-metal oxides with an ABO3 perovskite structure exhibit strongly entangled structural and electronic degrees of freedom and thus, one expects to unveil exotic phases and properties by acting on the lattice through various external stimuli. The epitaxial strain engineering in oxide thin films is an important mean to tailor the crystal lattice distortion through cooperative Jahn Teller effect. Using the Jahn Teller active PrVO3 thin films as a model system, the structural correlation with the magnetism is established. We impose different strength of epitaxial strain in PrVO3 thin films via different means, such as, using various commercially available single crystal substrates, film thickness, substrates with different crystal surface orientations, etcetera. As a result, new and hidden phases that are absent in the bulk compound, begin to appear. Namely, the compressive strain in PrVO3 films enhances the super-exchange interaction leading to an increased antiferromagnetic Neel temperature, a strong magnetic anisotropy in PrVO3 thin films grown on (001)-, (110)- and (111)-oriented SrTiO3 substrates, are few examples

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9

Smith, Craig David. "Synthesis and properties of novel free radicals with potential as molecular magnetic materials and spin probes." Thesis, Queensland University of Technology, 2002.

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10

Ma, Xiaozhou. "Synthesis and study of redox-active molecular nanomagnets." Thesis, Bordeaux, 2019. http://www.theses.fr/2019BORD0128.

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Ce travail de thèse portait sur la synthèse et l'étude de complexes magnétiques redox-actifs comme prototypes pour la conception d'aimants moléculaires à haute température. L'activité redox est assurée par le ligand pontant, qui peut moduler et parfois améliorer significativement les propriétés magnétiques. Après un chapitre d'introduction présentant les derniers développements dans le domaine des matériaux magnétiques moléculaires, un accent particulier est mis sur l'importance d'avoir un fort couplage d'échange magnétique J entre les porteurs de spin. Une étude bibliographique présentant deux approches émergentes pour augmenter J dans les composés polynucléaires est également présentée et discutée. Le chapitre 2 présente les synthèses et caractérisations de complexes dinucléaires [M2(tphz)(tpy)2](PF6)n (M = Co(II) ou Ni(II); n = 4, 3, 2, tphz = tétrapyridophénazine, tpy = terpyridine) construits à partir de ligands pontant (tphz) et bloquant (tpy) fortement coordinants et redox-actifs. Les études approfondies de ces composés montrent que le ligand pontant redox-actif peut être utilisé comme un outil de choix pour promouvoir une délocalisation des spins, de forts couplages magnétiques, ainsi que de la commutabilité. L’analyse des résultats obtenus permet également de mieux comprendre les paramètres clés pour l’élaboration de systèmes fortement couplés magnétiquement. Dans le prolongement de ce travail visant à sélectionner les meilleurs composants pour la conception rationnelle d'aimants moléculaires à haute température, le chapitre 3 décrit une nouvelle série de complexes mononucléaires [Cr(III)(tphz)(tpy)](CF3SO3)n (n = 3, 2, 1). Les complexes mono- et doublement réduits présentent des interactions magnétiques remarquablement fortes entre les ions métalliques et les ligands radicalaires, et pourraient servir d'unités magnétiques intéressantes pour la conception d'aimants de plus hautes nucléarités
The thesis work aims at the synthesis and study of redox-active magnetic molecules as prototypes towards the design of molecule-based magnets with high operating temperature, a prerequisite for technological applications. The redox activity is provided by the bridging ligand, which could tune and sometimes enhance significantly the magnetic properties of the resulting molecular architectures. After an introduction chapter presenting the latest developments in the field of molecule-based magnetic materials, special emphasis is given on the importance of having large magnetic exchange coupling J between the spin carriers to reach high operating temperature. This is supported by a bibliographic study concerning two emerging approach to enhance J values in polynuclear compounds. Chapter 2 presents the syntheses and characterizations of dinuclear M(II) complexes [M2(tphz)(tpy)2](PF6)n (M = Co or Ni; n = 4, 3, 2, tphz = tetrapyridophenazine) built by using strongly complexing, redox-active bridging ligand (tphz), and terpyridine (tpy) as capping ligands. The extensive studies on these compounds show that the redox-active bridging ligand can be used as a tool to promote spin delocalization, high spin complexes and magnetic multi-switchability. Importantly the work reveals the key parameters towards building strongly magnetically coupled systems. As a continuation research of finding the best magnetic components for the rational design of high temperature molecule-based magnets, Chapter 3 describes a new series of [Cr(III)(tphz)(tpy)](CF3SO3)n (n = 3, 2, 1) mononuclear complexes. Both the mono and doubly-reduced complexes show remarkable magnetic interactions between metal center and radical ligands, which could further act as interesting magnetic units for the design of higher nuclearities magnets

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11

Nehme, Zeinab. "Nanoparticules magnétiques d’architecture complexe core-shell : couplage d'échange bias et interaction dipolaire." Thesis, Le Mans, 2016. http://www.theses.fr/2016LEMA1019.

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Le travail de thèse est consacré à l'étude numérique de nanoparticules (NPs) magnétiques core@shell Fe3O4@CoO présentant des propriétés d'échange bias (EB) en utilisant la méthode Monte Carlo (MC). En particulier, nous nous sommes concentrés sur l'étude de l'effet des réponses collectives (interactions inter-particules telles que les interactions dipolaires (ID)) sur les propriétés magnétiques de ces structures. Des résultats expérimentaux préliminaires, montrant l'existence d'une relation entre le décalage du cycle d'hystérésis et l'interaction entre NPs, ont motivé le travail numérique mené dans le cadre de cette thèse.La première partie de ce mémoire est une étude méthodologique visant à trouver les conditions optimales pour simuler les cycles d'hystérésis d'une façon correcte par MC.Les résultats révèlent une dépendance linéaire entre le champ coercitif Hc et la constante d'anisotropie effective pour des conditions non biaisées (algorithme libre, algorithme du cône, algorithme mixte). La deuxième partie est consacrée à l'étude, à l'échelle atomique, des nanostructures présentant l'EB dont nous avons reproduit les deux caractéristiques (un décalage du cycle d'hystérésis, une augmentation importante de Hc).Nous avons également proposé une méthode permettant l'évaluation de la valeur de l'anisotropie effective.En passant à l'échelle d'une assemblée de NPs, plusieurs modèles furent étudiés. Nous arrivons à interpréter les résultats expérimentaux selon le degré d'agrégation des NPs. Nous montrons que l'agrégation (interactions d'échanges entre les NPs) a un effet direct sur le champ d'échange bias, mais le rôle d'ID sur le champ d'échange mérite des études complémentaires
This thesis is dedicated to the numerical study by means of Monte Carlo (MC) simulations of core@shell Fe3O4@CoO magnetic nanoparticles (NPs) presenting exchange bias properties (EB). In particular, we focused our study on the effect of collective responses (inter-particle interactions as dipolar interactions (DI)) on the magnetic properties of these structures. Our numerical work is motivated by some preliminary experimental results showing the existence of a relationship between the hysteresis loop shift (exchange bias field) and the interaction between NPs. The first part of this thesis is a methodological study to figure out the optimal conditions to simulate hysteresis loops correctly by MC. The results reveal that the coercive field Hc is linearly related to the effective anisotropy constant for non-biased conditions (free algorithm, cone algorithm, mixed algorithm). The second part is dedicated to the study of exchange-biased nanostructures at the atomic scale. We have been able to reproduce both characteristics of EB (hysteresis loop shift, significant increase in Hc). A method allowing the evaluation of the effective anisotropy has been proposed. Considering an assembly of nanoparticles, several models are studied. The experimental results are interpreted according to the degree of aggregation of NPs. It was shown that the aggregation (exchange interactions between NPs) has a direct effect on the exchange bias field, but the role of the ID on the exchange field requires complimentary calculations to be clarified

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12

Tang, Chiu C. "The magnetic exchange interactions in chromium chalcogenide spinels." Thesis, Aston University, 1988. http://publications.aston.ac.uk/8235/.

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The chromium chalcogenide spinels, MCr2X4 (M = Zn, Cd, Hg; X = O, S, Se), have been the subject of considerable interest in recent years. In each case the crystal structure is that of a normal spinel with the chromium ions exclusively occupying the octahedral (B) sites, so that when diamagnetic ions are located at the tetrahedral (A) sites the only magnetic interactions present are those between B-site ions. Despite such apparently simple circumstances a rich variety of magnetic behaviour is exhibited. For the oxides the ground state spin configurations are antiferromagnetic whilst for the selenides ferromagnetic interactions dominate and several authors have drawn attention to the fact that the nature of the dominant interaction is a function of the nearest neighbour chromium - chromium separation. However, at least two of the compounds exhibit spiral structures and it has been proved difficult to account for the various spin configurations within a unified theory of the magnetic interactions involved. More recently, the possibility of formulating a simplified interpretation of the magnetic interactions has been provided by the discovery that the crystal struture of spinels does not always conform to the centrosymmetrical symmetry Fd3m that has been conventionally assumed. The deviation from this symmetry is associated with small < 111> displacements of the octahedrally coordinated metal ions and the structures so obtained are more correctly referred to the non-centrosymmetrical space group F43m. In the present study, therefore, extensive X-ray diffraction data have been collected from four chromium chalcogenide specimens and used to refine the corresponding structural parameters assuming F43m symmetry and also with conventional symmetry. The diffracted intensities from three of the compounds concerned cannot be satisfactorily accounted for on the basis of conventional symmetry and new locations have been found for the chromium ions in these cases. It is shown, however, that these displacements in chromium positions only partially resolve the difficulties in interpreting the magnetic behaviour. A re-examination of the magnetic data from different authors indicates much greater uncertainty in their measurements than they had claimed. By taking this into consideration it is shown that a unified theory of magnetic behaviour for the chromium chalcogenide spinels is a real possibility.

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13

Coulaud, Esther. "Analyse théorique de l'interaction d'échange magnétique : effets de solvant et décomposition." Thesis, Aix-Marseille, 2012. http://www.theses.fr/2012AIXM4740.

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La Théorie de la Fonctionnelle de la Densité (DFT) combinée avec le formalisme de la Symétrie Brisée (BS) est aujourd'hui très utilisée dans le domaine du magnétisme moléculaire pour le calcul des constantes d'échange magnétique. Dans le but d'améliorer la compréhension des contributions qui participe à l'interaction d'échange caractérisant les systèmes magnétiques, nous avons mené une étude théorique systématique basée sur l'étude de deux types de modèles. Ces complexes binucléaires de cuivre(II) reliés par divers groupements pontant de type hydroxo, azoture, et chloro, ainsi que des bis-nitroxydes modèles constitués de deux groupements nitroxydes reliés par un système conjugué sont caractérisés par des couplages ferro- et anti-ferromagnétiques variés. Nous avons développé une approche théorique basée sur l'utilisation d'orbitales gelées permettant d'étudier la décomposition de l'interaction d'échange magnétique en différentes contributions telles que l'échange direct, l'échange cinétique et le mécanisme de polarisation de spin. Dans un second temps, nous avons étendu ce travail de rationalisation aux effets de solvant à l'aide de modèles implicites ou explicites. Grâce à une analyse basée sur l'approche de Hoffmann, nous avons pu déterminer les paramètres caractérisant dans les mécanismes d'échange direct et d'échange cinétique
Density Functional Theory (DFT) combined with the Broken Symmetry (BS) method is today widely used in the field of molecular magnetism for the computation of magnetic exchange coupling constants. In order to improve the understanding of the contributions involved in the exchange phenomena characterizing some magnetic systems, we propose a study based on two types of models. Copper(II) binuclear complexes connected with various bridging groups like hydroxo, azide, and chlorine, as well as small bis-nitroxide composed of two nitroxide moieties linked by a conjugated system show various ferro- and anti-ferromagnetic couplings. We have developed a theoretical approach based on the use of frozen orbitals to decompose the magnetic exchange interaction in different contributions such as the direct exchange, the kinetic exchange and mechanism of spin polarization. In a second part, we have extended this work to rationalize the solvent effects using implicit or explicit models. Through an analysis based on the Hoffmann's approach, we have determined some parameters characterizing the mechanisms of direct exchange and kinetic exchange

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14

Kolanji, Kubandiran [Verfasser]. "Organic Radicals for Magnetic Materials through Controlling of Magnetic Exchange Interactions / Kubandiran Kolanji." Mainz : Universitätsbibliothek Mainz, 2019. http://d-nb.info/1175536792/34.

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15

Kamenskyi, Dmytro. "Electron spin resonance studies of frustrated quantum spin systems." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2013. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-108777.

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Since the last few decades frustrated spin systems have attracted much interest. These studies are motivated by the rich variety of their unusual magnetic properties and potential applications. In this thesis, excitation spectra of the weakly coupled dimer system Ba3Cr2O8, the spin-1/2 chain material with distorted diamond structure Cu3(CO3)2(OH)2 (natural mineral azurite), and the quasi-twodimensional antiferromagnet with triangle spin structure Cs2CuBr4 have been studied by means of high-field electron spin resonance. Two pairs of gapped modes corresponding to transitions from a spin-singlet ground state to the first excited triplet state with zero-field energy gaps, of 19.1 and 27 K were observed in Ba3Cr2O8. The observation of ground-state excitations clearly indicates the presence of a non-secular term allowing these transitions. Our findings are of crucial importance for the interpretation of the field-induced transitions in this material (with critical fields Hc1 = 12.5 T and Hc2 = 23.6 T) in terms of the magnon Bose-Einstein condensation. The natural mineral azurite, Cu3(CO3)2(OH)2, has been studied in magnetic fields up to 50 T, revealing several modes not observed previously. Based on the obtained data, all three critical fields were identified. A substantial zero-field energy gap, Δ = 9.6 K, has been observed in Cs2CuBr4 above the ordering temperature. It is argued that contrary to the case for the isostructural Cs2CuCl4, the size of the gap can not be explained solely by the uniform Dzyaloshinskii-Moriya interaction, but it is rather the result of the geometrical frustration stabilizing the spin-disordered state in Cs2CuBr4 in the close vicinity of the quantum phase transition between a spiral magnetically ordered state and a 2D quantum spin liquid.

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16

Rouault, Philippe. "Les matériaux intermétalliques terres rares - métaux de transition et instabilité de l'antiferromagnétisme de bande." Grenoble 1, 1989. http://www.theses.fr/1989GRE10105.

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Nous presentons dans la premiere partie les avantages d'une fusion en levitation pour la preparation de composes intermetalliques terres rares-metaux de transition destines, a l'etude du magnetisme itinerant. Viennent ensuite les caracteristiques techniques du four a levitation que nous avons realise puis des exemples d'applications d'un tel procede d'elaboration. Dans la seconde partie, nous contribuons a l'etude de l'antiferromagnetisme de bande proche de l'instabilite a travers certaines phases de laves rmn#2. Grace aux informations experimentales sur les interactions d'echange extraites des diagrammes de diffraction neutronique et sur l'anisotropie, apprehendee a partir de spectres de resonance magnetique nucleaire, nous proposons un nouvel arrangement des moments magnetiques des atomes de manganese au sein de la structure magnetique du compose ymn#2. Nous discutons des effets de la frustration des interactions d'echange et de la forte anisotropie du manganese dans ce compose. Nous etudions egalement les proprietes magnetiques des composes pseudo-binaires y#1##xtb#xmn#2: la substitution d'atomes de terbium a ceux d'yttrium introduit, en sus de la tres forte frustration des interactions d'echange qui existent deja dans le compose ymn#2, une competition des energies d'anisotropie du terbium et du manganese. Il en resulte un ordre magnetique homogene complexe ainsi que l'apparition d'un ordre a courte distance que nous avons plus particulierement etudie dans le compose tbmn#2. Le terbium joue dans ce cas, le role d'une sonde des correlations de densite d'aimantation du manganese

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17

GARNIER, AMALIA. "Processus metamagnetiques dans les systemes frustres rm#2x#2 (r = terre rare, m = metal de transition, x = si ou ge)." Université Joseph Fourier (Grenoble), 1996. http://www.theses.fr/1996GRE10129.

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Le but de ce travail est de montrer la richesse des comportements magnetiques des composes rm#2x#2 (r = terre rare, m = co, ni, ru, x = si, ge). Cette richesse ne necessite pas d'autres ingredients que la presence d'interactions magnetiques a longue portee et antagonistes (frustration), et d'anisotropie magnetocristalline: l'etude de systemes de type ising, x-y ou de faible anisotropie a permis d'illustrer les differents comportements associes. En ce qui d'un moment magnetique de l'etat o vers +m ou passage de l'etat -m vers l'etat o. Les processus concerne les systemes de type ising, nous avons mis en evidence, pour la premiere fois, l'existence de phases magnetiques mixtes ou coexistent des moments nuls et non nuls dans la structure magnetique. Un nouveau type de processus metamagnetique est associer a ces phases: passage metamagnetiques dans les systemes de type ising sont analyses quantitativement par le modele de champ moyen periodique: ce modele permet d'expliquer l'origine des sauts d'aimantation associes a des retournements d'une partie des moments, avec ou sans changement de periodicite. Les effets thermiques sont analyses et montrent l'evolution du vecteur de propagation lorsque la temperature decroit: il y a blocage du vecteur de propagation vers un vecteur de propagation decrivant une structure magnetique de plus courte periode pour les transitions de phase avec changement de periodicite. L'etude de composes a base de gadolinium a permis de montrer l'existence d'une anisotropie geante des interactions d'echange. L'etude comparative des series en fonction de la substitution de la terre rare, du metal de transition ou de x a permis d'isoler les parametres pertinents qui permettent de comprendre l'evolution des proprietes magnetiques dans les series etudiees

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Barron, Louise Lillias Margaret. "Effect of exchange and magnetostatic interactions on grain boundaries." Thesis, University of Edinburgh, 2011. http://hdl.handle.net/1842/5023.

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Magnetic minerals are abundant within our Earth's crust and can retain, through one of a number of processes, a remanent magnetisation induced by the Earth's magnetic field. Analyses of palaeomagnetic samples have been used for the past fifty years to improve our understanding of many of the Earth's major processes. Recent studies utilising newly developed imaging techniques, namely holographic transmission electron microscopy, have for the first time allowed direct observations of the magnetic structure in palaeomagnetic samples on a nanoscale. It is commonly observed that igneous rocks contain closely packed magnetic lamellae with a non-magnetic matrix, a result of the chemical process of exsolution. However, the results of current micromagnetic models, generated to predict the magnetic structure within such samples, are not in agreement with these direct observations. The results do, however, show strong similarities to the direct observations. The discrepancies between the direct observations and micromagnetic models indicate a lack of understanding of the magnetic interactions within such samples. To examine this two distinct hypotheses have been tested. Firstly, the geometry of the system has been altered to examine the effect of this on the magnetic structure of the grains. Secondly, a multiphase model has been produced. This multiphase model allows the simulation of more complicated systems that include more than one magnetic material in direct contact. This multiphase model has allowed us to examine the effect of varying the exchange in these multiphase structures and its effect on the modelled magnetic structure. Further, this multiphase model has allowed us to examine theoretical systems involving combinations of magnetic materials commonly found in palaeomagnetic samples.

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Ball, Andrew. "Commensurabilité magnétique à longue période et anisotropie dans la série hexagonale RGa2 (R=Pr, Nd, Gd)." Grenoble 1, 1993. http://www.theses.fr/1993GRE10075.

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Les composes hexagonaux simples rga(r=gd, pr et nd) sont d'excellents candidats pour l'etude de structures magnetiques frustrees complexes resultant des interactions d'echange a longue portee. Par l'utilisation de techniques experimentales variees, nous avons mis en evidence sur ces composes des proprietes magnetiques originales et hautement complexes. Dans le cas de gdga#2, un champ magnetique induit des changements subtils dans la configuration antiphase incommensurable de la phase en champ nul. Ces processus peuvent se resumer en trois diagrammes de phases champ-temperature d'une complexite etonnante vue l'absence d'anisotropie magnetocristalline. Un modele de champ moyen periodique montre que les structures sous champ sont de type x-y en eventail, mettant ainsi en evidence l'existence d'une faible anisotropie d'echange. Les composes prga#2 et ndga#2 presentent des structures magnetiques en champ nul de type ising, confinees dans le plan hexagonal. Pres de t#n, ces structures sont incommensurables et evoluent a basse temperature vers des structures commensurables a longue periode. Dans prga#2, le champ applique induit des transitions metamagnetiques de type spin-slip et spin-flip entre differentes configurations magnetiques commensurables a longue periode. Apres une etude approfondie du champ cristallin, le modele du champ moyen periodique nous a permis d'analyser quantitativement les proprietes magnetiques principales de ce compose: chaleur specifique, aimantation, susceptibilite. . .

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20

Ferré, Giménez Ricardo. "Etude de nanoparticules magnétiques par simulation numérique." Université Joseph Fourier (Grenoble), 1995. http://www.theses.fr/1995GRE10224.

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Nous avons etudie par simulation numerique les proprietes magnetiques d'une petite particule magnetique aussi bien que les proprietes d'un ensemble de telles particules. Dans la premiere partie de ce travail, un modele de petites particules est developpe. Ce modele, base sur les equations micromagnetiques, tient compte des interactions d'echange et dipolaire entre les moments magnetiques dans la particule aussi bien que de l'anisotropie magnetocristalline. Ce modele nous a permis d'etudier les configurations d'aimantation a l'equilibre pour des petites particules magnetiques. Nous avons aussi etudie plus en detail le retournement d'aimantation d'une particule isolee de ferrite de baryum d eforme aplatie. La deuxieme partie de ce travail est dediee a la modelisation d'un ensemble de petites particules en interaction dipolaire. Nous avons mis en evidence l'effet cruciale de la concentration de particules sur les proprietes magnetiques d'un tel agregat. Nous montrons que l'effet de l'augmentation de la concentration combine avec l'anisotropie de l'interaction dipolaire conduit a l'apparition d'une valeur maximale du champ coercitif pour les concentrations intermediaires. Nous avons aussi considere un terme d'anisotropie magnetique qui a comme effet la reduction de la portee des interactions dipolaires. La derniere partie de ce travail est dediee a une etude en temperature (monte carlo) de ces systemes de particules. Une transition de gel dynamique est observee comme a resultat de l'evaluation du parametre d'ordre d'edwards anderson pour notre modele ainsi que nombreux etats metastables sont possibles a basse temperature. Ces signatures, neanmoins, ne nous permettent pas d'affirmer l'existence d'une transition du type verre de spins

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Janssen, Yuri. "Interplay between magnetic anisotropy and exchange interactions in rare-earth - transition-metal ferrimagnets." [S.l. : Amsterdam : s.n.] ; Universiteit van Amsterdam [Host], 2003. http://dare.uva.nl/document/69269.

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22

Pedersen, Anders Hjordt. "Molecule-based magnetic materials of the ReIV ion." Thesis, University of Edinburgh, 2017. http://hdl.handle.net/1842/28885.

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The [ReCl6]2-, [ReBr6]2- and [ReCl4(ox)]2- anions are crystallised with the organic 4,4’- bipyridinium dication (4,4-H2bipy). Magnetometry reveals exotic behaviour of the [4,4’- H2bipy][ReCl6] and [4,4’-H2bipy][ReBr6] salts which demonstrate spin-canting, antiferromagnetic exchange interactions and metamagnetism. Single crystal X-ray structures at T = 3, 14 and 20 K of the [4,4’-H2bipy][ReBr6] salt reveal the behaviour to be purely of magnetic origin as no structural changes are observed. For the [4,4’-H2bipy][ReCl4(ox)] compound an antiferromagnetic exchange interaction of 10.2 cm-1 between the anions is observed (Chapter 2). The complexes (NBu4)2[(ReCl5)2(μ-pyrazine)], (NBu4)2[(ReBr5)2(μ-pyrazine)], (NBu4)2[(ReBr5)2(μ-pyrimidine)] and (NBu4)2[(ReBr5)2(μ-triazine)] are structurally and magnetically characterised in Chapter 3. Magnetic measurements reveal the ReIV ions bridged by a 1,4-heterocyclic amine to exhibit strong antiferromagnetic coupling induced by the linearity of the bridging ligand. The two dimers bridged by a 1,3-heterocyclic amine exhibit intramolecular ferromagnetic exchange and at low temperature an intermolecular antiferromagnetic coupling is observed for the (NBu4)2[(ReBr5)2(μ-triazine)] complex due to the presence of short intermolecular Br···Br distances. Six molecular ReIVCuII chains of formula {[Cu(L)4][ReCl6]}n (L = imidazole, 1- methylimidazole, 1-vinylimidazole, 1-butylimidazole, 1-vinyl-1,2,4-triazole or dimethylformamide) are characterised structurally and magnetically in Chapter 4. SQUID magnetometry and theoretical calculations reveal the chains to exhibit ferromagnetic exchange interactions, which increase as the Re–Cl–Cu bond angle decreases. The {[Cu(vinylimidazole)4][ReCl6]}n chain exhibit magnetic order at TC = 2.4 K, and the {[Cu(imidazole)4][ReCl6]}n network exhibits ferrimagnetic behaviour. Eight complexes of the [ReCl6]2- and [ReBr6]2- anions crystallised with the [MII(L•)2]2+ (M = Fe, Co or Cu) or [Ni(L•)(CH3CN)3]2+ cations (L• = 4-dimethyl-2,2-di(2-pyridyl)oxazolidine N-oxide) are characterised structurally and magnetically in Chapter 5. The [Co(L•)2]2+ cation shows evidence of a gradual, thermally induced spin-crossover transition in variable-temperature magnetic and structural experiments. The [Ni(L•)(CH3CN)3]2+ cation show exchange of the coordinated acetonitrile molecules for atmospheric water upon drying. The nickel-radical magnetic coupling is ferromagnetic in all cases, demonstrating spin-canting behaviour with an ordering temperature of T = 2.7 K for the [ReCl6]2- based compound, and intermolecular antiferromagnetic exchange interactions for the [ReBr6]2- based complex.

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23

Ravat, Prince [Verfasser]. "Tuning of magnetic exchange interactions between organic radicals through bond and space / Prince Ravat." Mainz : Universitätsbibliothek Mainz, 2014. http://d-nb.info/1060526719/34.

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Iwamoto, Wellington Akira 1979. "Ressonância de spin eletrônico (ESR) em sistemas de dimensões reduzidas." [s.n.], 2011. http://repositorio.unicamp.br/jspui/handle/REPOSIP/278522.

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Orientador: Pascoal José Giglio Pagliuso
Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin
Made available in DSpace on 2018-08-18T12:30:25Z (GMT). No. of bitstreams: 1 Iwamoto_WellingtonAkira_D.pdf: 11664469 bytes, checksum: 8790fc8d822330998a201e2b73fea42c (MD5) Previous issue date: 2011
Resumo: Sistemas de dimensões reduzidas possuem muitas aplicações tecnológicas. Há uma corrida para o desenvolvimento de dispositivos cada vez menores assim como para alcançar o controle e manipulação de dispositivos na escala nanométrica. Isto requer estudos sistemáticos de propriedades físicas em sistemas de tamanhos reduzidos. O foco deste trabalho é o estudo de Ressonância de Spin Eletrônico (ESR) em sistemas de dimensões reduzidas. Os materiais escolhidos para esse estudo foram filmes finos de GaAs, GaN dopados com Mn (GaMnAs e GaMnN), filmes finos amorfos de silício dopados com terras-raras a-Si:RE (RE = Y, Gd, Er e Lu) e nanopartículas (NPs) dopadas com impurezas magnéticas de terras-raras e metal de transição em matrizes metálicas Ag:R (R = Er, Yb e Mn) e em isolantes NaYF4:Gd. A finalidade desse estudo é explorar as propriedades magnéticas microscópicas destes sistemas. Os resultados das medidas em filmes de GaMnAs e GaMnN indicaram ausência de ferromagnetismo de longo alcance. Os experimentos de ESR mostraram ausência da relação entre largura de linha (?H) de ESR e a concentração de íons de Mn2+, mas foi observado que ?H aumenta conforme o nível de cristalinidade das amostras aumenta. Além disso, há um aumento de ?H em baixas temperaturas para os filmes com maior nível de cristalinidade, sugerindo uma correlação magnética de curto alcance entre os íons magnéticos se estabelecendo nessas amostras. Resultados similares foram encontrados para os filmes de GaMnN, exceto na medida de magnetização em função do campo magnético para GaMnN, a qual podemos observar ¿loops¿ ferromagnéticos abaixo de T ? 50 K ao contrário dos filmes de GaMnAs que não observamos nenhum ¿loop¿ ferromagnético em T = 2 K. Para outro grupo de filmes de a-Si:RE, foi estudado o efeito de redução na densidade de estados ligações pendentes (D0) nos filmes Si dopados com diferentes espécies de terras-raras (RE¿s) em função das diferentes concentrações. De acordo com nossos resultados, a dopagem com RE reduz a intensidade do sinal de ESR dos estados D0 com uma dependência exponencial das concentrações de RE¿s. As NPs de Ag:R e de NaYF4:Gd foram preparadas pelo método químico. Nós observamos uma forma de linha de ESR tipicamente Lorentziana consistente com os estados fundamentais dos íons de Er3+, Yb3+ e Mn2+ em simetria cúbica. O fator g encontrado para esses íons nas NPs é muito próximo ao observado em sistemas isolantes cúbicos, ao contrário do encontrado em metais, onde é observado um deslocamento de g. Além disso, não foi possível observar a relaxação Korringa para as linhas de ESR de Er3+, Yb3+ e Mn2+ no sistema de NPs que é observada, tipicamente, em metais. Logo, esses resultados nos levam a acreditar que a interação de troca (Jfs) entre os momentos localizados dos íons magnéticos (ML) e os elétrons de condução (c-e) está ausente no sistema de NPs de Ag:R, indicando que a natureza desta interação deveria ser reexaminada na escala nanométrica. Para as NPs de NaYF4:Gd, o controle do tamanho da partícula foi adquirido segundo a quantidade de rps = precursor/surfactantes. Não foi observada nenhuma evidência de cluster de Gd e observamos o espectro de ESR com as mesmas características já observadas no sistema bulk : três linhas de ressonância, sendo essas linhas com origens bem controvérsias, das quais não sabemos se é de origem de um campo cristalino de combinações de simetria cúbica com tetragonal ou mesmo rômbica ou mesmo de sítios de Gd3+ com simetrias mais baixas
Abstract: Reduced dimensions systems present many potencial technological applications. There is great interest in the development of small scale devices as well as in the control and manipulation at the nanoscale and in study of finite size on physical properties. The main goal of this work is the study of Electron Spin Resonance (ESR) in systems of reduced dimensions. The materials chosen for this study were thin films of GaAs, GaN doped with Mn (GaMnAs and GaMnN), amorphous silicon thin films doped with rare-earth a-Si:RE (RE = Y, Gd, Er and Lu) and nanoparticles (NPs) doped with magnetic impurities such as of rare earth and transition metal doped Ag:R (R = Er, Yb and Mn) and insulating NaYF4:Gd. The purpose of this study is to explore the microscopic magnetic properties of these systems. The results of the measurements in GaMnAs and GaMnN films indicated absence of long range ferromagnetism. the ESR results reveal no relationship between ESR linewidth (?H) and the Mn2+ concentration in this films. Instead, a broadening of the ESR ?H was found as a function of the increasing in the crystallinity level of the films. Furthermore, for the films with higher level of crystallinity, a significant broadening of the ESR ?H is observed as the temperature is decreased, suggesting the development of short-range magnetic correlations between the Mn2+ ions. Similar results were found for films GaMnN, except in the magnetization versus magnetic field experiments for GaMnN, which we could observe ferromagnetic loops in T < 50 K, in constract of GaMnAs films where no ferromagnetic loop in T = 2 K was found for all measured films. For the other group of films, a-Si, we studied the suppresion effects in the density of dangling bonds species D0 states as function concentration for different Rare-Earth (RE¿s) species. According to our data, the RE-doping reduces the ESR signal intensity of the D0 states with an exponential dependence on the Re¿s concentrations. Ag:R and NaYF4:Gd NPs were prepared by chemical method. We observed a typical Lorentzian line-shape ESR lines for all studied dopants (R = Er, Yb e Mn). The gfactor found for Er3+, Yb3+ e Mn2+ in the nanoparticles is very close to g-value found in ground-states of these ions in insulating cubic systems, in contrast that what was found in metals, where it is observed a g-shift for the metallic system. Furthermore, it was not possible to observe the Korringa relaxation for the ESR lines of Er3+, Yb3+ e Mn2+ in the NPs system typically observed in metals. Therefore, the results suggest that the exchange interaction (Jfs) between localized magnetic moments (ML) and conduction electrons (c-e) is absent in Ag:R NPs, indicating that the nature of this interaction needs to be reexamined at the nanoscale range. For NaYF4:Gd NPs, the particle control size was obtained by the amount of rps = precursor/surfactant. There was no evidence of Gd clusters in our results and we found the same characteristics observed in the bulk system: three resonance lines, with the controversies origins. It is still unknown the source of crystalline field of cubic symmetry with tetragonal combinations or orthorhombic or even Gd3+ sites with lower symmetries
Doutorado
Física da Matéria Condensada
Doutor em Ciências

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25

Kitaura, R., H. Okimoto, H. Shinohara, T. Nakamura, and H. Osawa. "Magnetism of the endohedral metallofullerenes M@C_82 (M=Gd,Dy) and the corresponding nanoscale peapods: Synchrotron soft x-ray magnetic circular dichroism and density-functional theory calculations." American Physical Society, 2007. http://hdl.handle.net/2237/11305.

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Moraes, Flávio Campopiano Dias de. "Indução ótica de magnetização em semicondutores magnéticos." Universidade de São Paulo, 2017. http://www.teses.usp.br/teses/disponiveis/43/43134/tde-04012018-120905/.

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Nesta tese, analisamos dois sistemas de semicondutores magnéticos: um semicondutor magnético cristalino de EuTe e uma heteroestrutura formada por um poço quântico de GaAs/AlGaAs ao lado uma barreira tipo delta de Mn, que, ao difundir-se, forma o semicondutor magnético diluído de (Ga,Mn)As. Nossos estudos foram focados na possibilidade de manipularmos oticamente a orientação magnética de ambos os sistemas. No semicondutor magnético de EuTe, a indução de magnetização se dá pela formação de polarons magnéticos ao redor de elétrons fotoexcitados. Para o estudo dos polarons, um modelo teórico elaborado foi adaptado para a construção de um sistema computacional baseado no método de Monte Carlo. Essa sistema permitiu o cálculo do momento magnético e do raio do polaron em temperaturas finitas, muito acima da temperatura de Néel. O modelo foi elaborado para reproduzir tanto as propriedades do EuTe sem o polaron (temperatura de Néel e campo crítico), quanto o deslocamento da linha de fotoluminescência devido a formação do polaron. Além do desenvolvimento do próprio método computacional, que pode ser utilizado para estudar outros sistemas, o conhecimento adquirido com o estudo do EuTe serviu como base para o estudo de um sistema mais complexo, que é a heteroestrutura de GaAs/AlGaAs + dMn. O estudo da heteroestrutura de GaAs/AlGaAs + dMn foi feito em cima de medidas experimentais de rotação de Kerr com resolução temporal. O sistema de medição construído permite, também, medidas de rotação de Kerr com resolução espacial, que servem para o estudo de transporte e hélice de spin em semicondutores, e está detalhadamente descrito em um dos capítulo desta tese. Na amostra estudada, o controle da magnetização dos íons de Mn é feito através da interação de troca com o elétron fotoexcitado no poço quântico. Os resultados obtidos das medidas de rotação de Kerr mostram uma frequência de precessão dependente do tempo, que revela a existência de dois processos com dinâmicas diferentes: uma primeira orientação do spin dos íons de Mn devido à polarização do par elétron-buraco no poço quântico, seguida por um realinhamento desses spins com o campo magnético externo, a partir do momento em que a coerência dos spins dos buracos desaparece. Esse resultado sugere que a interação entre os elétrons fotoexcitados e os íons de Mn ocorre por intermédio dos buracos fotoexcitados, ao contrário do que havia sido proposto em estudos anteriores de estruturas similares, mas de acordo com o modelo de interação sp-d, utilizado para explicar o ferromagnetismo do (Ga,Mn)As.
In this thesis we analyzed two magnetic semiconductor systems: one intrinsic magnetic semiconductor crystal of EuTe and one GaAs-based heterostructure with a GaAs/AlGaAs quantum well close to delta-type Mn barrier, that forms a diluted magnetic semiconductor of (Ga,Mn)As after diffusion. Our studies on both systems were focused on the possibility of optical manipulation of magnetic order. In EuTe pure semiconductor, the magnetization control occurs due to de formation od magnetic polarons around photo-excited electrons. To study magnetic polarons we adapted a theoretical model to build a computer simulation system based on Monte Carlo\'s method. This system allowed us to calculate the magnetic moment and radius of the polaron at finite temperatures fair above Néel Temperature. The computational model was tested to reproduce EuTe properties without polarons (Néel Temperature and critical magnetic field) and with polarons (photoluminescence line shift). Beside the development of this computational model, that can be used to study other systems, the knowledge acquired during the studies on EuTe helped us to better understand the more complex system of the GaAs/AlGaAs +dMn heterostructure. The studies about the GaAs/AlGaAs + dMn heterostructure were based on experimental measurements of time-resolved Kerr rotation. The measurement system we built also allows us to perform spatial-resolved Kerr rotation measurements to study spin transport and spin helix on semiconductors and it is described in details in one chapter of this thesis. The optical manipulation of Mn ions magnetization on the studied sample is consequence of the exchange interaction with the photoexcited electron inside the quantum well. The results of Kerr rotation measurements show a time-dependent precession frequency that reveals the existence of two process with distinct dynamics: the initial orientation of Mn ions spins with the photoexcited electron-hole pair, followed by the realignment of these spins with the external magnetic field, as soon as the photoexcited hole spins loose its coherence. These results indicate that the exchange interaction between the photoexcited electron inside the quantum well and the Mn ions is mediated by the photoexcited holes, in opposition of what was being proposed in previous studies of similar structures, but in agreement with the sp-d model, used to explain the (Ga,Mn)As ferromagnetism.

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Afsharnaderi, M. "Effective exchange interactions for magnetic surfaces, overlayers and surface impurities in the itinerant model of ferromagnetism." Thesis, City University London, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.378864.

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Barilero, Gilles. "Etude de quelques proprietes magnetooptiques et magnetiques des semiconducteurs semimagnetiques zn : :(1-x)mn::(x)te et hg::(1-x)mn::(x)te." Paris 6, 1987. http://www.theses.fr/1987PA066084.

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L'effet d'echange entre les electrons de bande et les electrons d des ions manganese modifie les produits electroniques des alliages de zn dans un champ magnetique. L'effet zeeman de l'exciton permet de determiner la valeur des integrales d'echange de la bande de valence et de la bande de conduction dans ces composes. La presence d'interactions magnetiques entre les ions manganese est responsable des proprietes magnetiques. Les courbes d'aimantation des alliages dilues pour x5% en champ intense, a basse temperature, permettent de determiner la constante d'echange entre ions mn 1er voisins. Les alliages concentres presentent une phase de verre spin a basse temperature

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29

Abdulmalic, Mohammad A., Azar Aliabadi, Andreas Petr, Yulia Krupskaya, Vladislav Kataev, Bernd Büchner, Ruslan Zaripov, et al. "Magnetic superexchange interactions: trinuclear bis(oxamidato) versus bis(oxamato) type complexes." Universitätsbibliothek Chemnitz, 2015. http://nbn-resolving.de/urn:nbn:de:bsz:ch1-qucosa-165666.

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The diethyl ester of o-phenylenebis(oxamic acid) (opbaH2Et2) was treated with an excess of RNH2 in MeOH to cause the exclusive formation of the respective o-phenylenebis(N(R)-oxamides) (opboH4R2, R = Me 1, Et 2, nPr 3) in good yields. Treatment of 1–3 with half an equivalent of [Cu2(AcO)4(H2O)2] or one equivalent of [Ni(AcO)2(H2O)4] followed by the addition of four equivalents of [nBu4N]OH resulted in the formation of mononuclear bis(oxamidato) type complexes [nBu4N]2[M(opboR2)] (M = Ni, R = Me 4, Et 5, nPr 6; M = Cu, R = Me 7, Et 8, nPr 9). By addition of two equivalents of [Cu(pmdta)(NO3)2] to MeCN solutions of 7–9, novel trinuclear complexes [Cu3(opboR2)(L)2](NO3)2 (L = pmdta, R = Me 10, Et 11, nPr 12) could be obtained. Compounds 4–12 have been characterized by elemental analysis and NMR/IR spectroscopy. Furthermore, the solid state structures of 4–10 and 12 have been determined by single-crystal X-ray diffraction studies. By controlled cocrystallization, diamagnetically diluted 8 and 9 (1%) in the host lattice of 5 and 6 (99%) (8@5 and 9@6), respectively, in the form of single crystals have been made available, allowing single crystal ESR studies to extract all components of the g-factor and the tensors of onsite CuA and transferred NA hyperfine (HF) interaction. From these studies, the spin density distribution of the [Cu(opboEt2)]2− and [Cu(opbonPr2)]2− complex fragments of 8 and 9, respectively, could be determined. Additionally, as a single crystal ENDOR measurement of 8@5 revealed the individual HF tensors of the N donor atoms to be unequal, individual estimates of the spin densities on each N donor atom were made. The magnetic properties of 10–12 were studied by susceptibility measurements versus temperature to give J values varying from −96 cm−1 (10) over −104 cm−1 (11) to −132 cm−1 (12). These three trinuclear CuII-containing bis(oxamidato) type complexes exhibit J values which are comparable to and slightly larger in magnitude than those of related bis(oxamato) type complexes. In a summarizing discussion involving experimentally obtained ESR results (spin density distribution) of 8 and 9, the geometries of the terminal [Cu(pmdta)]2+ fragments of 12 determined by crystallographic studies, together with accompanying quantum chemical calculations, an approach is derived to explain these phenomena and to conclude if the spin density distribution of mononuclear bis(oxamato)/bis(oxamidato) type complexes could be a measure of the J couplings of corresponding trinuclear complexes
Dieser Beitrag ist aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich

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30

Wikberg, Magnus. "Fundamental Properties of Functional Magnetic Materials." Doctoral thesis, Uppsala universitet, Teknisk-naturvetenskapliga fakulteten, 2010. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-133257.

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Magnetic properties of powders, thin films and single crystals have been investigated using magnetometry methods. This thesis provides analysis and conclusions that are supported by the results obtained from spectroscopic and diffraction measurements as well as from theoretical calculations. First, the magnetic behavior of transition metal (TM) doped ZnO with respect to doping, growth conditions and post annealing has been studied. Our findings indicate that the magnetic behavior stems from small clusters or precipitates of the dopant, with ferromagnetic or antiferromagnetic interactions. At the lowest dopant concentrations, the estimated cluster sizes are too small for high resolution imaging. Still, the clusters may be sufficiently large to generate a finite spontaneous magnetization even at room temperature and could easily be misinterpreted as an intrinsic ferromagnetic state of the TM:ZnO compound. Second, influence of lattice strain on both magnetic moment and anisotropy has been investigated for epitaxial MnAs thin films grown on GaAs substrates. The obtained magnetic moments and anisotropy values are higher than for bulk MnAs. The enhanced values are caused by highly strained local areas that have a stronger dependence on the in-plane axis strain than out-of plane axis strain. Finally, spin glass behavior in Li-layered oxides, used for battery applications, and a double perovskite material has been investigated. For both Li(NiCoMn)O2 and (Sr,La)MnWO6, a mixed-valence of one of the transition metal ions creates competing ferromagnetic and antiferromagnetic interactions resulting in a low temperature three-dimensional (3D) spin glass state. Additionally, Li(NiCoMn)O2 with large cationic mixing exhibits a percolating ferrimagnetic spin order in the high temperature region and coexists with a two-dimensional (2D) frustrated spin state in the mid temperature region. This is one of the rare observations where a dimensional crossover from 2D to 3D spin frustration appears in a reentrant material.
Felaktigt tryckt som Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 720

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31

Fiore, Mosca Dario. "Quantum magnetism in relativistic osmates from first principles." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2019. http://amslaurea.unibo.it/17982/.

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The interplay between electron correlation, local distortions and Spin Orbit Coupling is one of the most attractive phenomena in condensed matter Physics and have stimulated much attention in the last decade. In Osmates double perovskites the coupling between electronic, structural and orbital degrees of freedom leads to the formation of an unconventional magnetic phase, whose precise origin and characteristics are still not understood. In particular strong Spin Orbit Coupling effect is believed to occur and have a crucial role in enhancing multipolar exchange interactions in a fashion similar to the more studied 4f electron systems. In this thesis, by means of first principles calculations, we study the structural, electronic and magnetic proprieties of the Mott insulating Ba2NaOsO6 with Osmium in 5d1 electron configuration within the fully relativistic Density Functional Theory plus on site Hubbard U (DFT + U) scheme. We find that the system is subjected to local symmetry breaking and that the magnetic ground state is strongly dependent on the on site Coulomb interaction. Furthermore, by mapping the energy onto a Pseudospin Hamiltonian, we are capable to prove that quadrupolar and octupolar exchanges play a significant role. We repeated the study for Ba2CaOsO6 with Os in 5d2 electronic configuration as a preliminary step for understanding if phase transitions are possible when Ba2NaOsO6 is doped.

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32

Pontillon, Yves. "Étude de la densité de spin de composés magnétiques moléculaires par diffraction de neutrons polarisés." Université Joseph Fourier (Grenoble ; 1971-2015), 1997. http://www.theses.fr/1997GRE10220.

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Au cours de ce travail, nous avons applique la technique de la diffraction de neutrons polarises a l'etude de la densite de spin dans des composes magnetiques moleculaires. Nous avons tout d'abord etudie deux nitronyl nitroxydes presentant une structure en chaine via des liaisons hydrogenes : la densite de spin est majoritairement localisee dans une orbitale antiliante * construite sur les fonctions 2#p des atomes d'azote et d'oxygene des groupes nitroxydes. Par ailleurs, nous avons mis en avant le role primordial des liaisons hydrogenes dans le couplage ferromagnetique intermoleculaire, role confirme par des calculs theoriques ab initio (methode de la fonctionnelle de la densite). Nous nous sommes ensuite interesses a un complexe cuivrique d'une enaminocetone nitroxyde. Nous avons pu determiner qu'une dimerisation quasi totale avait lieu a basse temperature entre deux fonctions nitroxydes, de deux molecules differentes, se faisant face et distantes de 3. 40a. Puis, nous presentons l'etude d'un ferroaimant moleculaire (radical nitronyl nitroxyde). La densite de spin est en grande partie concentree sur le fragment oncno du cycle nit dans une orbitale magnetique moleculaire antiliante *, le couplage ferromagnetique intermoleculaire modifiant l'orientation relative des orbitales magnetiques des deux atomes d'oxygene des fonctions -no, rotation s'accompagnant d'une hybridation sp. La derniere etude presentee dans ce manuscrit a trait a un alkyl nitroxyde presentant un ordre ferromagnetique. Nous avons verifie la localisation majoritaire de l'electron non apparie sur le groupe -no et la nature antiliante * de l'orbitale moleculaire magnetique. Les populations individuelles de spin obtenues sur l'ensemble de la molecule, nous ont permis de proposer un mecanisme expliquant les couplages ferromagnetiques intermoleculaires.

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33

Filho, Roberto Ferreira Sena. "Teoria microscÃpica de ondas de spin em nanofios magnÃticos." Universidade Federal do CearÃ, 2007. http://www.teses.ufc.br/tde_busca/arquivo.php?codArquivo=428.

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Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico
O comportamento dinÃmico de spins em materiais magnÃticos Ã influenciado pela geometria que eles apresentam. AlÃm disso, outro aspecto relevante Ã a dimensionalidade do sistema. Trabalhos recentes comprovam o interesse do estudo das propriedades magnÃticas em sistemas de baixa dimensionalidade, que Ã devido em grande parte as aplicaÃÃes tecnolÃgicas, tais como: nanosensores, gravadores magnÃticos de alta densidade, dispositivos magneto-eletrÃnicos, etc. Neste trabalho estudamos a propagaÃÃo de ondas de spin em nanofios magnÃticos cilÃndricos, onde a abordagem Ã feita utilizando teoria microscÃpica, atravÃs do Hamiltoniano de Heisenberg, em que os spins sÃo considerados fixos nos sÃtios da rede e cuja geometria da seÃÃo transversal dos cilindros Ã hexagonal. Entre as interaÃÃes magnÃticas estudadas consideramos: a interaÃÃo de troca que pode ser ferromagnÃtica se os primeiros vizinhos dos spins estÃo numa configuraÃÃo paralela, ou antiferromagnÃtica se estiverem antiparelelos; a interaÃÃo Zeeman que Ã devido ao campo magnÃtico externo aplicado ao sistema; a interaÃÃo de Anisotropia, esta sendo responsÃvel pela direÃÃo de magnetizaÃÃo preferida que diversos sistemas magnÃticos reais apresentam e a interaÃÃo dipolar de natureza magnetostÃtica, presente em todos os materiais. O formalismo leva em consideraÃÃo a dependÃncia espacial dos spins no sistema, onde os operadores de spin do hamiltoniano sÃo escritos em termos de operadores bosÃnicos de criaÃÃo e aniquilaÃÃo atravÃs da RepresentaÃÃo de Holstein-Primakoff. Em seguida, aproveitando-se da simetria translacional em uma direÃÃo devido a periodicidade da rede, realizamos a transformada de Fourier para estes operadores fornecendo um sistema de equaÃÃes matriciais no espaÃo dos vetores de onda. A partir desse sistema de equaÃÃes obtemos vÃrios espectros de excitaÃÃo como: a relaÃÃo de dispersÃo para as ondas de spin, que Ã o grÃfico onde mostra como a frequÃncia de ondas de spin varia em funÃÃo do vetor de onda e a variaÃÃo da energia do sistema com o campo aplicado.
The dynamical behavior of spins in magnetic materials is affected by its geometry and dimensionality. One can find several new results in the literature exploiting the magnetic properties of low dimension systems with different geometries, since the development of new devices such as: nanosensors, high density magnetic storage, etc., is closely related to new geometries. In this piece of work, we study the propagation of spin waves on cylindrical magnetic nanowires described by a microscopic theory through the Heisenberg Hamiltonian, where we consider the spins fixed at the sites lattice and the transversal section of the wire is hexagonal. Our model takes into account the exchange interaction between the spins that can be ferromagnetic or antifferomagnetic, the interaction of an external field with the spins (Zeeman interaction), anisotropic interactions due to a preferred direction of magnetization, and finally dipole-dipole interactions. The spins are described by boson operators through Holstein-Primakoff representation. The equations of motion for the spins are written in terms of these operators and translational symmetry in a preferential direction allows us to calculate several excitations spectra.

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34

Ballou, Rafik. "Anisotropies magnétiques du cobalt dans les composés intermétalliques lanthanide-cobalt." Grenoble 1, 1987. http://www.theses.fr/1987GRE10114.

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Analyse phénoménologique des processus d'aimantation des composes YCo5 et GDCo5 mettant en évidence une anisotropie des interactions d'échangé Ln-Co liée à l'anisotropie de polarisation des électrons 5d de l'atome Ln. Etude de l'incidence de l'instabilité du magnétisme de bande 3d sur l'anisotropie magnétique dans les phases Ln2Co7 et LnCo3. Etude de l'anisotropie des systèmes à empilement quasi-unidimensionnel LnCo(1-epsilon ). Analyse de la sélection orbitale induite par les intégrales de transfert

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35

Iusan, Diana Mihaela. "Density Functional Theory Applied to Materials for Spintronics." Doctoral thesis, Uppsala universitet, Materialteori, 2010. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-119887.

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The properties of dilute magnetic semiconductors have been studied by combined ab initio, Monte Carlo, and experimental techniques. This class of materials could be very important for future spintronic devices, that offer enriched functionality by making use of both the spin and the charge of the electrons. The main part of the thesis concerns the transition metal doped ZnO. The role of defects on the magnetic interactions in Mn-doped ZnO was investigated. In the presence of acceptor defects such as zinc vacancies and oxygen substitution by nitrogen, the magnetic interactions are ferromagnetic. For dilute concentrations of Mn (~ 5%) the ordering temperature of the system is low, due to the short ranged character of the exchange interactions and disorder effects. The clustering tendency of the Co atoms in a ZnO matrix was also studied. The electronic structure, and in turn the magnetic interactions among the Co atoms, is strongly dependent on the exchange-correlation functional used. It is found that Co impurities tend to form nanoclusters and that the interactions among these atoms are antiferromagnetic within the local spin density approximation + Hubbard U approach. The electronic structure, as well as the chemical and magnetic interactions in Co and (Co,Al)-doped ZnO, was investigated by joined experimental and theoretical techniques. For a good agreement between the two, approximations beyond the local density approximation must be used. It is found that the Co atoms prefer to cluster within the semiconducting matrix, a tendency which is increased with Al co-doping. We envision that it is best to describe the system as superparamagnetic due to the formation of Co nanoclusters within which the interactions are antiferromagnetic. The magnetic anisotropy and evolution of magnetic domains in Fe81Ni19/Co(001) superlattices were investigated both experimentally, as well as using model spin dynamics. A magnetic reorientation transition was found.

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36

Bordel, Catherine. "Etude de la modification par nitruration des propriétés magnétiques des alliages amorphes TbFe et TbFeCo." Université Joseph Fourier (Grenoble), 1997. http://www.theses.fr/1997GRE10174.

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Nous avons elabore par pulverisation cathodique des couches minces amorphes de tbfe et tbfeco nitrurees, dans le but d'etudier le role de l'azote sur les proprietes magnetiques de ces alliages. Des experiences de spectroscopie d'absorption x mettent en evidence l'existence d'un ordre structural local dans les echantillons tbfe(co), que nous considerons comme l'origine de l'anisotropie magnetique perpendiculaire presente dans ces alliages. Les mesures magnetiques macroscopiques montrent une modification tres importante des proprietes magnetiques, notamment une forte reduction de la temperature de compensation et de l'aimantation a saturation de ces materiaux en fonction de leur concentration en azote. L'utilisation de sondes magnetiques chimiquement selectives, telles que le dichroisme circulaire magnetique de rayons x et la spectroscopie mossbauer, permet d'analyser le role de l'azote sur chacun des deux sous-reseaux magnetiques. Ces experiences montrent que l'insertion d'azote provoque essentiellement une modification du sous-reseau de tb, qui consiste en un desalignement croissant des moments magnetiques de tb par rapport a la direction de l'aimantation resultante lorsque la concentration en azote augmente. Un calcul de champ moyen permet de confirmer que ces changements resultent de l'affaiblissement des interactions d'echange tb-fe sous l'effet de la nitruration. Nous avons par ailleurs effectue des experiences de relaxation magnetique qui suggerent que le processus de renversement de l'aimantation de nos alliages tbfe(co) est domine par la propagation de parois de domaines magnetiques. Nous avons enfin complete ce travail par la simulation numerique de courbes d'aimantation a temperature nulle d'un systeme tbfe, qui constitue un premier pas vers l'etude du comportement magnetique de bicouches tbfe-tbfen couplees par echange.

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37

Bergqvist, Lars. "Electronic Structure and Statistical Methods Applied to Nanomagnetism, Diluted Magnetic Semiconductors and Spintronics." Doctoral thesis, Uppsala University, Department of Physics, 2005. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-5732.

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This thesis is divided in three parts. In the first part, a study of materials aimed for spintronics applications is presented. More specifically, calculations of the critical temperature in diluted magnetic semiconductors (DMS) and half-metallic ferromagnets are presented using a combination of electronic structure and statistical methods. It is shown that disorder and randomness of the magnetic atoms in DMS materials play a very important role in the determination of the critical temperature.

The second part treats materials in reduced dimensions. Studies of multilayer and trilayer systems are presented. A theoretical model that incorporates interdiffusion in a multilayer is developed that gives better agreement with experimental observations. Using Monte Carlo simulations, the observed magnetic properties in the trilayer system Ni/Cu/Co at finite temperatures are qualitatively reproduced.

In the third part, electronic structure calculations of complex Mn-based compounds displaying noncollinear magnetism are presented. The calculations reproduce with high accuracy the observed magnetic properties in these compounds. Furthermore, a model based on the electronic structure of the necessary conditions for noncollinear magnetism is presented.

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38

De, paz Aurelie. "Échange de spin et dynamique d’aimantation d’un gaz quantique dipolaire." Thesis, Sorbonne Paris Cité, 2015. http://www.theses.fr/2015USPCD096/document.

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Dans ce mémoire nous présentons plusieurs études expérimentales des propriétés magnétiques d’un condensat de Bose-Einstein de Chrome chargé dans un réseau 3D, en nous focalisant sur les effets associés aux interactions dipolaires. Nous montrons que dans un réseau 3D, la relaxation dipolaire est un processus résonant du fait de la réduction de la densité d’états orbitaux accessibles. Les résonances sont observées à des champs magnétiques Bres tels que l’énergie Zeeman relâchée soit égale à l’énergie nécessaire à exciter les atomes dans une bande d’énergie supérieure du réseau. Nous pouvons inhiber ce processus en appliquant un champ différent de Bres. L’analyse des résonances a permis de sonder la structure de bande 3D du réseau, ainsi que la mise en évidence de l’effet des interactions entre atomes. Nous avons étudié la dynamique d’échange de spin dans un réseau 3D. Nous présentons en particulier la première observation d’échange de spin entre atomes localisés dans des sites séparés. Ces études permettent une exploration nouvelle du magnétisme en réseau. En variant la profondeur du réseau, nous étudions ces effets dans le régime superfluide, bien décrit par une théorie de champ moyen, et dans le régime fortement corrélé, dont la description théorique est difficile. Enfin, nous étudions l’évolution de deux spins géants interagissant par interaction dipolaire. Le condensat initialement divisé en deux, les atomes des deux nuages sont préparés dans des états de spin opposés formant ainsi deux spins géants ±3xN. Nous montrons que toute dynamique de spin est énergétiquement inhibée pour de grands spins ce qui est bien reproduit par une théorie classique
This Thesis reports on several experimental studies of magnetic properties of a Chromium Bose-Einsteincondensate loaded into a 3D optical lattice, focusing on the effects induced by dipolar interactions.We show that in a 3D lattice dipolar relaxation is a resonant process due to the reduction of the density ofaccessible orbital states. These resonances are observed for magnetic fields Bres such that the Zeeman energyreleased matches an excitation towards higher-energy bands of the lattice. We can thus inhibit those processes byapplying a field different from Bres. Analyses of the resonances allowed us to probe the lattice 3D band structureas well as to demonstrate the effects of local interactions between atoms.We study spin exchange dynamics in a 3D lattice. We especially observed for the first time spin exchangebetween atoms localized in different lattice sites mediated by dipolar interactions. These studies are the firststep toward a new exploration of magnetism in lattice. Varying the depth of the lattice we study these effects inthe superfluid regime, well described by mean filed theories, as well as in the strongly correlated regime, whosetheoretical description is still challenging.Finally, we study the evolution dynamics of two giant spins interacting through dipolar interactions. Thecondensate being initially splitted in half, atoms from the two clouds are prepared in opposite spin states thusproducing two giant spins ±3×N. We show that any spin dynamics is energetically inhibited for large spinswhich is well accounted for by a classical theory

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39

Cardoso, Marcus Vinicíus Cangussu 1981. "Água e carboidratos : aspectos macroscópicos e moleculares de suas interações." [s.n.], 2012. http://repositorio.unicamp.br/jspui/handle/REPOSIP/250168.

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Orientadores: Edvaldo Sabadini, Munir Salomão Skaf
Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Química
Made available in DSpace on 2018-08-20T16:17:25Z (GMT). No. of bitstreams: 1 Cardoso_MarcusViniciusCangussu_D.pdf: 5363072 bytes, checksum: 77f8ee9c80c8ce3514b001281dfddae6 (MD5) Previous issue date: 2012
Resumo: Soluções aquosas de mono, di, oligo, e polissacarídeos foram estudas nos níveis macroscópico e molecular, empregando-se enfoques termodinâmicos e espectroscópicos. A influência da intensidade da ligação de hidrogênio sobre a solubilidade dos carboidratos (lineares e cíclicos) mostrou-se fortemente dependente de suas solubilidades. Quanto menos solúvel o carboidrato, maior é o efeito da substituição isotópica do solvente (H2O por D2O). Este efeito sugere que carboidratos menos solúveis (e maiores) perturbam mais fortemente a estruturação das moléculas de água. Devido ao efeito cooperativo da transição coil-helix da k-carragena, o efeito isotópico sobre a gelificação é bastante intensificado. Segundo um perspectiva mais molecular, as taxas de troca protônicas, kb, entre os prótons da água e os grupos OH dos carboidratos dependem da natureza do açúcar, sendo os maiores valores observados para a forma linear, seguida pela forma piranosidea e por último pela forma furanosidea. Já as transferências de magnetização entre as populações de prótons da água e dos grupos CH-carboidratos são moduladas pelos movimentos moleculares e intermediadas pelas trocas protônicas com os grupos OH. Propõe-se que os prótons das moléculas de água interagem preferencialmente com os prótons OH e negligenciavelmente com os prótons CH. Estudos de relaxação H mostraram-se ricos para o estudo de processos moleculares de agregação micelar dos n-alquil-glicosídeos sendo possível demonstrar experimentalmente que a agregação leva a indisponibilização das hidroxilas ao interagirem com as moléculas de água
Abstract: Aqueous solutions of mono, di, oligo, and polysaccharides were studied on the macroscopic and molecular standpoints through thermodynamic and spectroscopic approaches. The effect of hydrogen-bonding strength on the solubility of a series of (linear and cyclic) saccharides showed to be strongly dependent of the solubility of the carbohydrate. As lower is the solubility of the carbohydrate, greater will be the deuterium isotopic effect of the solvent (H2O for D2O) on the carbohydrate solubilities. These results suggest that low soluble carbohydrates (and larger ones) perturb more strongly the water structure. Owing to the cooperativity of the coil-helix transition, the deuterium isotope effect on the gelling of k-carrageenan is intensified leading to stronger gels and double-helices more stable in D2O. Looking deeper onto a molecular perspective and based on spin-spin nuclear magnetic relaxation, the proton exchange rates, kb, between water and OH-carbohydrate, are dependent of the nature of the saccharide. The kb values are higher for linear than for pyranoside form, and the slowest value is found for fructofuranoside form. The transferring of magnetization between proton pools of water and CH-carbohydrates are modulated by molecular motions and intermediated by proton exchanging process between water and OH-carbohydrate protons. H NMR relaxation experiments of exchangeable protons provide to be rich in probing the micelar aggregation of n-alkyl-glucosides. It was possible to demonstrate experimentally that the aggregation of the surfactant molecules provoke a drastic reduction on the interactions between water and OH-saccharide head groups
Doutorado
Físico-Química
Doutor em Ciências

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40

Haury, Axel. "Hétérostructures à puits quantique Cd(Mn)Te/CdMgZnTe dopées pgaz de trous et phase ferromagnétique." Université Joseph Fourier (Grenoble), 1997. http://www.theses.fr/1997GRE10205.

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Nous presentons la premiere mise en evidence d'une phase ferromagnetique dans des heterostructures a puits quantique semimagnetique cdmnte/cdmgznte dopees p par modulation (azote) dans les barrieres. Dans de tels echantillons, le transfert de trous des barrieres dans le puits cree un gaz de trous quasi-bidimensionnel dans la couche de cdmnte. Pour optimiser les conditions de dopage, une etude preliminaire a ete menee sur une serie d'echantillons a puits quantiques non magnetiques cdte/cdmgznte montrant que, dans des puits de 80 a, le gaz de trous devient metallique pour des densites superieures a 1. 5 x 10#1#1 cm#-#2. En outre, ces echantillons nous ont permis d'observer les deux types d'excitons charges x#+ et x#-. Dans les echantillons a puits semimagnetique cdmnte, l'interaction d'echange entre les ions magnetiques mn#+#+ et le gaz de trous en phase metallique ( 2 x 10#1#1 cm#-#2) cree un couplage ferromagnetique du type rkky entre les spins localises des ions mn#+#+. Dans des puits d'une largeur de 80 a et pour des concentrations de mn d'environ 3%, l'interaction ferromagnetique induite par le systeme de trous bidimensionnel devient superieure au couplage antiferromagnetique de superechange entre les ions magnetiques mn#+#+. Ceci se traduit par une transition de phase ferromagnetique a la temperature critique t#c d'environ 2 k. La phase paramagnetique au dessus de t#c est caracterisee par une augmentation spectaculaire de la susceptibilite magnetique.

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41

Bergman, Anders. "A Theoretical Study of Magnetism in Nanostructured Materials." Doctoral thesis, Uppsala : Acta Universitatis Upsaliensis : Universitetsbiblioteket [distributör], 2006. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-6763.

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42

Weheabby, Saddam [Verfasser], Heinrich [Akademischer Betreuer] Lang, Heinrich [Gutachter] Lang, and Harald [Gutachter] Krautscheid. "Coordination compounds with fused oxamato/oxamidato ligands: A new approach to strengthen and tailor magnetic exchange interactions / Saddam Weheabby ; Gutachter: Heinrich Lang, Harald Krautscheid ; Betreuer: Heinrich Lang." Chemnitz : Technische Universität Chemnitz, 2019. http://d-nb.info/1215909950/34.

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43

Terrier, Erwan. "Désaimantation induite par impulsions laser femtosecondes dans des nanostructures d'oxyde de fer." Thesis, Strasbourg, 2016. http://www.theses.fr/2016STRAE010/document.

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Ce travail de thèse traite de la dynamique ultrarapide de spins et de charges dans des oxydes de fer. Dans un premier temps, on montre à l'aide d'un montage pompe-sonde résolu en temps et exploitant l'effet Faraday magnéto-optique, que le temps de désaimantation dans une assemblée de nanoparticules de maghémite est plus rapide que le temps de désaimantation dans une assemblée de nanoparticules de magnétite. Une superposition des temps de thermalisation des électrons et de désaimantation est observée dans la maghémite. Cette accélération du temps de désaimantation est interprétée comme étant la conséquence d'un renforcement des interactions antiferromagnétiques dans la maghémite. La seconde partie prouve qu'il est possible de caractériser la transition de Verwey dans un film de magnétite grâce à des signaux de dynamique de charges et de spins. La dynamique ultrarapide d'aimantation se caractérise par un mouvement de précession dépendant de la température. D'importantes modifications des oscillations sont visibles de part et d'autre de la température de Verwey, reflétant un changement d'anisotropie caractéristique de cette transition
This work deals with spins and charges ultrafast dynamics in iron oxide. Thanks to a time-resolved magneto-optical Faraday effect measurements, we show the demagnetization time in an assembly of maghemite nanoparticles is faster than the demagnetization time in an assembly of magnetite nanoparticles. A superposition of thermalization times of electron and demagnetization times is observed in maghemite. This acceleration of the demagnetization time is interpreted as the effect of an enhancement of antiferromagnetic interactions in maghemite. The second part demonstrates the possibility to characterize the Verwey transition in a thin film of magnetite thanks to charges and spins dynamics signals. The ultrafast magnetization dynamic shows a temperature-dependent precession motion. Huge modifications of oscillations are visible on both side of Verwey temperature, reflecting an anisotropy change typical of this transition

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44

Bhandary, Sumanta. "First Principles Studies of Functional Materials Based on Graphene and Organometallics." Doctoral thesis, Uppsala universitet, Materialteori, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-217175.

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Graphene is foreseen to be the basis of future electronics owing to its ultra thin structure, extremely high charge carrier mobility, high thermal conductivity etc., which are expected to overcome the size limitation and heat dissipation problem in silicon based transistors. But these great prospects are hindered by the metallic nature of pristine graphene even at charge neutrality point, which allows to flow current even when a transistor is switched off. A part of the thesis is dedicated to invoke electronic band gaps in graphene to overcome this problem. The concept of quantum confinement has been employed to tune the band gaps in graphene by dimensional confinement along with the functionalization of the edges of these confined nanostructures. Thermodynamic stability of the functionalized zigzag edges with hydrogen, fluorine and reconstructed edges has been presented in the thesis. Keeping an eye towards the same goal of band gap opening, a different route has been considered by admixing insulating hexagonal boron nitride (h-BN) with semimetal graphene. The idea has been implemented in two dimensional h-BN-graphene composites and three dimensional stacked heterostructures. The study reveals the possibility of tuning band gaps by controlling the admixture. Occurrence of defects in graphene has significant effect on its electronic properties. By random insertion of defects, amorphous graphene is studied, revealing a semi-metal to a metal transition. The field of molecular electronics and spintronics aims towards device realization at the molecular scale. In this thesis, different aspects of magnetic bistability in organometallic molecules have been explored in order to design practical spintronics devices. Manipulation of spin states in organometallic molecules, specifically metal porphyrin molecules, is achieved by controlling surface–molecule interaction. It has been shown that by strain engineering in defected graphene, the magnetic state of adsorbed molecules can be changed. The spin crossover between different spin states can also be achieved by chemisorption on magnetic surfaces. A significant part of the thesis demonstrates that the surface-molecule interaction not only changes the spin state of the molecule, but allows to manipulate magnetic anisotropies and spin dipole moments via modified ligand fields. Finally, in collaboration with experimentalists, a practical realization of switching surface–molecule magnetic interactions by external magnetic fields is demonstrated.

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45

Lou, Ming. "Exotic states in condensed matter I. Mesoscopic magnetism in integrable systems; II. Cooper pairing mediated by multiple-spin exchanges /." Cincinnati, Ohio : University of Cincinnati, 2008. http://rave.ohiolink.edu/etdc/view.cgi?acc_num=ucin1218656392.

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46

Courtois, Denis. "Contribution à l'étude des propriétés cristallographiques et magnétiques des composés R3(Fe,T')29 (R=terre rare, T'=Ti ou V)." Université Joseph Fourier (Grenoble ; 1971-2015), 1997. http://www.theses.fr/1997GRE10247.

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Le but de ce travail est d'appronfondir la comprehension des proprietes cristallographiques et magnetiques de la nouvelle famille de composes r#3(fe,t)#2#9 (r = terre rare et t = ti ou v) et de comparer ces proprietes avec celles des autres familles de composes intermetalliques r-fe riches en fer, qui trouvent leur application principale dans les materiaux pour aimants permanents. Nous montrons que les composes 3 : 29 peuvent se former par reaction a l'etat solide avec les elements de terres rares lourdes r = gd, tb, dy et y. Une analyse cristallographique detaillee du cristal y#3(fe,v)#2#9 obtenu par la methode de czochralski revele qu'il est forme de plusieurs domaines cristallographiques de structure monoclinique de type nd#3(fe,ti)#2#9, chaque domaine etant relie aux autres par rotation d'angles = n. 60 autour de la direction 1 0 2. La contribution 3d au magnetisme des composes 3:29 est determinee par la caracterisation du cristal d'yttrium. L'analyse quantitative de l'anisotropie magnetocristalline, de l'aimantation a saturation et de la temperature d'ordre rend compte du caractere intermediaire de la structure cristallographique des composes 3:29 entre les structures des composes 2:17 et 1:12. La derniere partie porte sur l'analyse des composes ou l'element r est magnetique. Les coefficients d'echange n#t#b#f#e et de champ cristallin a#k#q# sur les sites 2a et 4i de l'ion tb#3#+ sont calcules a partir des mesures d'aimantation effectuees sur un cristal de terbium et sont compares avec ceux qui caracterisent les composes 2:17 et 1:12. Les interactions d'echange r-fe sont dans les composes 3:29 du meme ordre de grandeur que dans les autres series de composes r-fe. Nous notons que la decroissance de n#r#f#e a travers la serie des lanthanides est differente pour les composes riches et pauvres en fer. Ce phenomene est attribue a une difference de la polarisation de la bande 5d selon la stchiometrie des composes.

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47

Giroud, Monique. "Etude expérimentale de supraconducteurs magnétiques : diagramme de phase de HoMo6S8 monocristallin et supraconductivité induite dans l'état ferromagnétique." Grenoble 1, 1987. http://www.theses.fr/1987GRE10123.

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Etude par des mesures de susceptibilite, de conductivite electrique et d'aimantation. Etude detaillee du diagramme de phase de homo::(6)s::(8); observation de transitions supraconductrices lors de cyclages en champ et en temperature, bien au-dessous du point de curie. Mise en evidence d'effet exceptionnellement faibles de l'echange sur la supraconductivite a partir de l'etude des champs critiques et de leur anisotropie; les transitions dans l'etat ferromagnetique sont expliquees par la reduction de l'induction magnetique lorsque le champ est antiparallele a l'aimantation spontanee. Recherche des effets des interactions d'echange sur la pente du champ critique a tc dans errh::(4)b::(4); mise en evidence d'une supraconductivite residuelle dans l'etat ferromagnetique, associee aux parois de bloch, dans le cas de sner::(4)rh::(6)sn::(18)

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48

Sena, Filho Roberto Ferreira. "Teoria microscópica de ondas de spin em nanofios magnéticos." reponame:Repositório Institucional da UFC, 2007. http://www.repositorio.ufc.br/handle/riufc/12372.

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SENA FILHO, Roberto Ferreira. Teoria microscópica de ondas de spin em nanofios magnéticos. 2007. 74 f. Dissertação (Mestrado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2007.
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The dynamical behavior of spins in magnetic materials is affected by its geometry and dimensionality. One can find several new results in the literature exploiting the magnetic properties of low dimension systems with different geometries, since the development of new devices such as: nanosensors, high density magnetic storage, etc., is closely related to new geometries. In this piece of work, we study the propagation of spin waves on cylindrical magnetic nanowires described by a microscopic theory through the Heisenberg Hamiltonian, where we consider the spins fixed at the sites lattice and the transversal section of the wire is hexagonal. Our model takes into account the exchange interaction between the spins that can be ferromagnetic or antifferomagnetic, the interaction of an external field with the spins (Zeeman interaction), anisotropic interactions due to a preferred direction of magnetization, and finally dipole-dipole interactions. The spins are described by boson operators through Holstein-Primakoff representation. The equations of motion for the spins are written in terms of these operators and translational symmetry in a preferential direction allows us to calculate several excitations spectra.
O comportamento dinâmico de spins em materiais magnéticos é influenciado pela geometria que eles apresentam. Além disso, outro aspecto relevante é a dimensionalidade do sistema. Trabalhos recentes comprovam o interesse do estudo das propriedades magnéticas em sistemas de baixa dimensionalidade, que é devido em grande parte as aplicações tecnológicas, tais como: nanosensores, gravadores magnéticos de alta densidade, dispositivos magneto-eletrônicos, etc. Neste trabalho estudamos a propagação de ondas de spin em nanofios magnéticos cilíndricos, onde a abordagem é feita utilizando teoria microscópica, através do Hamiltoniano de Heisenberg, em que os spins são considerados fixos nos sítios da rede e cuja geometria da seção transversal dos cilindros é hexagonal. Entre as interações magnéticas estudadas consideramos: a interação de troca que pode ser ferromagnética se os primeiros vizinhos dos spins estão numa configuração paralela, ou antiferromagnética se estiverem antiparelelos; a interação Zeeman que é devido ao campo magnético externo aplicado ao sistema; a interação de Anisotropia, esta sendo responsável pela direção de magnetização preferida que diversos sistemas magnéticos reais apresentam e a interação dipolar de natureza magnetostática, presente em todos os materiais. O formalismo leva em consideração a dependência espacial dos spins no sistema, onde os operadores de spin do hamiltoniano são escritos em termos de operadores bosônicos de criação e aniquilação através da Representação de Holstein-Primakoff. Em seguida, aproveitando-se da simetria translacional em uma direção devido a periodicidade da rede, realizamos a transformada de Fourier para estes operadores fornecendo um sistema de equações matriciais no espaço dos vetores de onda. A partir desse sistema de equações obtemos vários espectros de excitação como: a relação de dispersão para as ondas de spin, que é o gráfico onde mostra como a frequência de ondas de spin varia em função do vetor de onda e a variação da energia do sistema com o campo aplicado.

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49

Ravach, Gwennola. "Etude structurale, magnétique et thermique de rubans amorphes Fe-Tr-B (Tr, Nd, Ho, Dy) à anisotropie magnétique aléatoire." Rouen, 1997. http://www.theses.fr/1997ROUES056.

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Les alliages amorphes métal de transition-terre rare peuvent présenter de l'anisotropie magnétique aléatoire due au désordre structural et au couplage spin-orbite de la terre rare. Les propriétés structurales et magnétiques de rubans amorphes hypertrempés Fe80-xTrxB20 (Tr = Nd, Ho, Dy ; x 16 % at. ) Ont été étudiées par spectrométrie Mössbauer, spectroscopie d'absorption X, analyse thermique différentielle et diffraction X aux grands angles. On montre par Mössbauer que les propriétés structurales des alliages ne dépendent pas de la nature de la terre rare. Une discontinuité des variations des paramètres hyperfins a lieu dans le domaine de concentration X#T#R#C 8-9% at. . Elle a été attribuée à une modification structurale autour du fer. Au-dessous de X#T#R#C , les sites de fer apparaissent similaires à ceux de Fe#8#0B#2#0. Un autre type d'environnement, plus magnétique, est suggère au-delà de X#T#R#C. La discontinuité et son origine structurale sont confirmées par spectroscopie d'absorption X. La modification intervient déjà dans la première sphère de coordination du fer. Les caractéristiques des paires Fe-Fe, identiques pour X#T#R < X#T#R#C, sont analogues à celles de Fe#8#0B#2#0. Au voisinage de X#T#R#C, un atome de fer supplémentaire apparait dans la première sphère de coordination et la distance moyenne Fe-Fe augmente. Au-delà de X#T#R#C, la première sphère de coordination du fer se contracte progressivement. D'après l'étude de cristallisation de deux alliages situés de part et d'autre de X#T#R#C, les premiers composés formés sont en accord avec des environnements locaux différents dans les deux domaines. L'étude Mössbauer sous champ des alliages les plus concentrés en terre rare indique un rapport anisotropie/échange plus faible dans l'alliage à base de Ho que dans ceux à base de Nd et Dy. Un modèle décrivant l'évolution sous champ de la structure magnétique du fer en termes de déformation angulaire a été développé. Il explique quantitativement les résultats dans le cas de Ho et Dy.

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Onofrio, Nicolas. "Modélisation de l'interaction d'échange par théorie de la fonctionnelle de la densité couplée au formalisme de la symétrie brisée. Application aux dimères de cuivre." Phd thesis, Université de Grenoble, 2011. http://tel.archives-ouvertes.fr/tel-00685279.

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La Théorie de la Fonctionnelle de la Densité (DFT) combinée avec la méthode de la Symétrie Brisée (BS) est aujourd'hui très utilisée dans le domaine du magnétisme moléculaire pour le calcul des constantes d'échange. Cette méthode (DFT-BS) reste cependant semi-quantitative et elle souffre de défauts déjà discutés dans la littérature. Dans le but de mieux en comprendre l'origine, nous avons réexaminé les contributions physiques qui participent au mécanisme d'échange. Nous proposons alors plusieurs formules analytiques construites suivant deux approches complémentaires (orbitales moléculaires et liaison de valence). Au cours de notre analyse, nous avons soulevé un problème inédit relatif à l'état de symétrie brisée tel que livré par le calcul DFT. Nos modèles seront appliqués au cas des dimères de cuivre(II) et nous verrons comment quantifier les différents paramètres afin de reconstruire les constantes d'échange. Qui plus est, notre travail permet d'établir une correspondance quantitative originale entre les deux approches pré-citées.

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