Le domaine de l’enregistrement magnétique est en constante évolution pour repousser davantage les limites de stockage de l’information, une approche prometteuse étant l’enregistrement perpendiculaire. Le matériau faisant l’objet de ce manuscrit est le ferrite de cobalt CoFe2O4 (= CFO). Ses propriétés font de lui un candidat prometteur pour la réalisation de dispositif à enregistrement perpendiculaire, cela passant par le contrôle de sa direction de facile aimantation.Ce travail de thèse traite ainsi de la modification de l’anisotropie magnétocristalline du CFO en couche mince par dopage aux éléments de terres rares. Nous avons démontré la possibilité de moduler la direction de facile aimantation du CFO non dopé, en modifiant la pression partielle en O2/N2 lors de l’élaboration. Nous avons également mis en évidence l’insertion des éléments lanthanides dans la structure du CFO ainsi que l’impact de l’anisotropie de la terre rare sur les propriétés magnétiques du matériau. / The field of magnetic storage is in constant progress to constantly push further the storage capacity of the device. A promising approach is the perpendicular magnetic recording of datas. The material presented in this manuscript is cobalt ferrite. It is an excellent candidate for the realization of perpendicular storage device due to its properties. The present work deals with the modification of the magnetic anisotropy by doping the ferrite cobalt thin films with rare earth elements. We have demonstrated the possibility to modulate the easy magnetization axis of undoped cobalt ferrite by changing the partial pressure of O2/N2 during the elaboration of the thin films. We have also highlighted the insertion of rare earth elements into the structure of the cobalt ferrite although their important ionic radii. The impact of the rare earth anisotropy on the magnetic properties of the ferrite cobalt has also been observed.

La spintronique moléculaire et la photocatalyse sont deux domaines en constante évolution. Le premier s’attache à exploiter la possibilité de coupler deux phénomènes physiques, à savoir le transport d’un flux de porteurs de charges et le spin de l’électron, tandis que le second se concentre sur l’exaltation des propriétés chimiques de transfert d’électrons d’une espèce donnée grâce au phénomène physique d’irradiation lumineuse. Depuis quelques années, les nanotubes de carbone ont suscité un grand intérêt à la fois en tant que composant pour la spintronique moléculaire, en raison de leur grande cohérence de spin, et en tant que support idéal pour la catalyse moléculaire, grâce à leurs exceptionnelles propriétés électroniques de surface. Au cours de ce travail de thèse, nous nous sommes attachés à concevoir des complexes inorganiques possédant des propriétés physiques, (magnétiques ou optiques) et des propriétés chimiques (permettant leur assemblage non-covalent sur des nanotubes de carbone monoparoi) de manière à former des adduits complexes inorganiques-nanotubes aux propriétés exploitables en spintronique moléculaire et en photocatalyse. Les propriétés des complexes synthétisés ont été extensivement caractérisées (Chapitre 2), et les plus prometteurs de ces composés ont été assemblés avec succès sur les nanotubes de carbone (Chapitre 3), comme en attestent les mesures spectroscopiques réalisées. Enfin, les deux domaines d’applications concernés par nos travaux faisant intervenir des phénomènes de transport électronique, des études spécifiques sur des dispositifs électriques de type transistor à effet de champ dont le canal de conduction est constitué de nanotubes de carbone ont été réalisées (Chapitre 4). Celles-ci mettent à chaque fois en évidence l’existence d’une communication électronique entre les complexes inorganique et les nanotubes de carbone sur lesquels ils sont assemblés au sein des dispositifs. Bien qu’au final un couplage entre les propriétés magnétiques des complexes synthétisés et les propriétés de transport des nanotubes n’ait pas pu être mis en évidence, de nombreux phénomènes inattendus et extrêmement intéressants tels que des effets ambipolaires, des transferts de charge ou des ruptures de liaisons ont été observés. Par contre, un fort couplage opto-électronique a pu être obtenu entre un complexe et le flux de porteurs de charge des dispositifs, ce qui s’avère être de très bon augure pour des futures applications en photocatalyse. / Molecular spintronic and photocatalysis are two fields in constant evolution. While the first deals with the coupling of two physical properties, the flux of charge carriers and the spin of the electron, the second is focusing on the enhancement of the electron transfer of chemical species under light irradiation. Recently, there has been an increasing interest in carbon nanotubes as new components for molecular spintronics, since they possess high spin coherence, and as ideal materials for molecular catalysis, for their tremendous electronic surface properties. Our work consisted in conceiving inorganic complexes with both physical (magnetic or optic) and chemical (ability of realizing non covalent assembly on single-walled carbon nanotubes) properties, in order to create new nanotube-complex nanohybrids which could be exploited for molecular spintronics or photocatalysis applications. The properties of the synthesized complexes were extensively characterized (Chapter 2), and the most promising molecules were successfully assembled onto carbon nanotubes, as is proven by the spectroscopic measurement which were performed (Chapter 3). Finally, since both domains of applications we considered involve electronic transportation, specific studies were realized on field effect transistor devices with carbon nanotubes as the conduction channel (Chapter 4). They evidence strong electronic communications between the inorganic complexes and the carbon nanotubes onto which they are assembled in the devices. Even if in the end no coupling was observed between the magnetic properties of the inorganic complexes and the transport ones of the carbon nanotubes, numerous unexpected and very interesting phenomena such as ambipolar behavior, charge transfer effect or bond cleavage were evidenced. As for the optoelectronic coupling which was investigated for photocatalytic applications, a first step was made as the transport of the carbon nanotube field effect transistor devices onto which a complex was assembled shows a strong dependence with the applied light irradiation.

Dans le cadre de l'augmentation de la densité de stockage des mémoires magnétorésistives à accès direct (MRAM), les matériaux à anisotropie magnétique perpendiculaire sont particulièrement intéressants car ils possèdent une très forte anisotropie. Cependant, cette augmentation d'anisotropie induit également un accroissement de la consommation d'écriture. Un nouveau concept d'écriture assistée thermiquement a été proposé par le laboratoire SPINTEC. Le principe est de concevoir une structure très stable à température ambiante, mais qui perd son anisotropie lorsqu'elle est chauffée, facilitant ainsi l'écriture. Le but de cette thèse est de valider expérimentalement ce concept. Les premiers chapitres sont consacrés à l'optimisation des matériaux à anisotropie perpendiculaire que sont les multicouches (Co/Pt), (Co/Pd) et (Co/Tb). Leur intégration dans une jonction tunnel magnétique est ensuite présentée. L'évolution de l'anisotropie en température, paramètre crucial au bon fonctionnement de l'assistance thermique, a également été étudiée. Enfin, il est démontré que l'écriture thermiquement assistée est particulièrement efficace : les structures développées présentent une consommation d'écriture réduite par rapport aux structures classiques et une forte stabilité à température ambiante. / In order to increase the storage density of magnetoresistive random access memories (MRAM), magnetic materials with perpendicular anisotropy are very appealing thanks to high anisotropy. However, the enhancement of anisotropy induces an increase of writing consumption as well. A new thermally assisted switching concept has been proposed by SPINTEC laboratory. The principle is to design a highly stable structure at stand-by temperature which loses its anisotropy when heated, making thus the switching easier. The aim of this thesis is to validate experimentally this concept. The first chapters describe the optimisation of out-of-plane magnetic materials such as (Co/Pt), (Co/Pd) and (Co/Tb) multilayers. Their integration in magnetic tunnel junctions is then presented. The evolution of anisotropy with temperature is a critical parameter for thermally assisted writing and has been therefore studied. Finally, the efficiency of this thermally assisted writing is demonstrated: the developed structures present a reduced consumption compared to standard structures and high stability at room temperature.

Multiferroic materials exhibit unique properties such as simultaneous existence of two or more of coupled ferroic order parameters (ferromagnetism, ferroelectricity, ferroelasticity or their anti-ferroic counterparts) in a single material. Recent years have seen a huge research interest in multiferroic materials for their potential application as high density non-volatile memory devices. However, the scarcity of these materials in single phase and the weak coupling of their ferroic components have directed the research towards multiferroic heterostructures. These systems operate by coupling the magnetic and electric properties of two materials, generally a ferromagnetic material and a ferroelectric material via strain. In this work, horizontal heterostructures of composite multiferroic materials were grown and characterized using pulsed laser ablation technique. Alternate magnetic and ferroelectric layers of cobalt ferrite and lead zirconium titanate, respectively, were fabricated and the coupling effect was studied by X-ray stress analysis. It was observed that the interfacial stress played an important role in the coupling effect between the phases. Doped zinc oxide (ZnO) heterostructures were also studied where the ferromagnetic phase was a layer of manganese doped ZnO and the ferroelectric phase was a layer of vanadium doped ZnO. For the first time, a clear evidence of possible room temperature magneto-elastic coupling was observed in these heterostructures. This work provides new insight into the stress mediated coupling mechanisms in composite multiferroics.

No description available.

The search for alternate and renewable energy resources as well as the efficient use of energy and development of such systems that can help to save the energy consumption is needed because of exponential growth in world population, limited conventional fossil fuel resources, and to meet the increasing demand of clean and environment friendly substitutes. Hydrogen being the simplest, most abundant and clean energy carrier has the potential to fulfill some of these requirements provided the development of efficient, safe and durable systems for its production, storage and usage. Chemical hydrides, complex hydrides and nanomaterials, where the hydrogen is either chemically bonded to the metal ions or physiosorbed, are the possible means to overcome the difficulties associated with the storage and usage of hydrogen at favorable conditions. We have studied the structural and electronic properties of some of the chemical hydrides, complex hydrides and functionalized nanostructures to understand the kinetics and thermodynamics of these materials. Another active field relating to energy storage is rechargeable batteries. We have studied the detailed crystal and electronic structures of Li and Mg based cathode materials and calculated the average intercalation voltage of the corresponding batteries. We found that transition metal doped MgH2 nanocluster is a material to use efficiently not only in batteries but also in fuel-cell technologies. MAX phases can be used to develop the systems to save the energy consumption. We have chosen one compound from each of all known types of MAX phases and analyzed the structural, electronic, and mechanical properties using the hybrid functional. We suggest that the proper treatment of correlation effects is important for the correct description of Cr2AlC and Cr2GeC by the good choice of Hubbard 'U' in DFT+U method. Hydrogen is fascinating to physicists due to predicted possibility of metallization and high temperature superconductivity. On the basis of our ab initio molecular dynamics studies, we propose that the recent claim of conductive hydrogen by experiments might be explained by the diffusion of hydrogen at relevant pressure and temperature. In this thesis we also present the studies of phase change memory materials, oxides and amorphization of oxide materials, spintronics and sulfide materials.

We have studied by STM, SQUID, X-ray reflectivity, X-ray diffraction, optical absorption and XAS Fe(phen)2(NCS)2 and Fe{[3,5-dimethylpyrazolyl]3BH}2 samples deposited by thermal evaporation on Cu(100), Co(100) and SiO2 substrates, and compared with results on powder samples. We have confirmed the existence of the soft X-ray induced excited spin state trapping (SOXIESST), and investigated its properties, in particular dynamic aspects. The effect is sensitive to the intensity and the structure of the applied X-ray beam, and is non-resonant. We suggest that its efficiency is also governed by metal-ligand charge transfer states (MLCT). The study of single molecules has revealed that they could be switched by voltage pulses, and by this way building memristive devices, but only if the influence of the substrate is sufficiently reduced. We have then investigated thin films with the help from a simple thermodynamic model, and evidenced that the cooperativity was reduced and the transition temperature is modified (higher for Fe-phen, and lower for Fe-pyrz). Finally, we use these results to build multilayer vertical devices Au/Fe-phen/Au, and its electrical properties depends, according to our preliminary results, on the external stimuli (temperature, magnetic field). Notably, they present a "diode" effect at the spin transition.

I studied organic semiconductor/ferromagnet interfaces by characterizing them by spin-polarized electron scattering and photoemission spectroscopy experiments. In the first part, a completely unexpected behaviour of the spin-dependent electron reflection properties of these interfaces is observed. In fact, sub-monolayer coverage of the organic molecules makes the electron reflection amplitude independent of the spin, i.e. both the reflectivity and the reflection phase become independent of the spin orientation of the incident electrons. Although I am not able at the moment to identify the cause of this phenomenon, I show that it is a very general phenomenon which is independent of the energy of the primary electrons, the choice of the ferromagnetic substrate, the choice of the organic molecule, and of the orientation of the initial spin polarization. It is not due to a change of the surface magnetization, a depolarization of the primary electrons, or a direct interaction of the molecules with the ferromagnetic substrate. Moreover, theory does not predict so far the experimental results and further research is required to unveil the physics behind these observations. In the second part of my thesis, spin-resolved photoemission experiments have been performed at the synchrotron SOLEIL. The main result is the observation of a highly spin-polarized molecule-induced electronic state close to the Fermi level. Measurements as a function of the organic layer thickness allow us to determine the interfacial character of this electronic state. Finally, these results are compared with theoretical calculations performed at the institute.

Time-resolved Faraday rotation (TRFR) is a pulsed laser pump/probe optical measurement used to characterize electron spin dynamics in semiconductor materials. A Mach-Zehnder type interferometer with orthogonally polarized arms is presented as a device for TRFR measurement that is superior to optical bridge detection, the traditional measuring technique, since Faraday rotation can be passively optically amplified via interference. Operation of the interferometer is analyzed under ideal conditions. Corrections to the ideal case stemming from imperfectly aligned optics, finite polarization extinction ratios, and an imperfect recombination optic are analyzed using a matrix transformation approach. The design of the interferometer is presented and chronicled. A description of the single-beam active control system utilized to stabilize the interferometer by continuous corrections to the optical path length of one arm with a piezoelectric actuator is given. Optical amplification by increasing the power in either arm of the interferometer is demonstrated and TRFR measurements taken with the interferometer at ambient temperatures are compared with measurements taken with the optical bridge. We find the interferometer to offer a detection limit on the order of 50 mrad at room temperature, which is five times more sensitive than the optical bridge. Isolation and stabilization of the interferometer were also successful in reducing signal noise to a level comparable with the optical bridge. Our results demonstrate that the interferometer is a better detection device for Faraday rotation under ambient conditions. In the immediate future, improvements to the control system should be made and experiments should be performed with high-quality samples at cryogenic temperatures to confirm that the interferometer performs as favorably under those conditions.

Cette thèse s'inscrit dans le contexte de l'électronique de spin et traite plus particulièrement de l'interaction réciproque entre un courant polarisé en spin et l'aimantation des structures magnétiques. Au cours de ce travail, les équations du transport diffusif dépendant du spin ont été couplées de façon auto-cohérente à l'équation de la dynamique d'aimantation dans l'approche micromagnétique au sein du code éléments finis. Cet outil numérique est appliqué à l'étude de la dynamique de parois de domaines dans différentes géométries sous l'action d'un courant polarisé. Il a permis de mettre en évidence plusieurs nouveaux phénomènes liés à l'interaction mutuelle entre l'aimantation et les spins des électrons. Pour des rubans à section rectangulaire, l'impact de cette interaction, habituellement négligée dans les modèles simplifiés, est quantifié via le calcul de la vitesse de déplacement de parois et du courant critique de Walker. Ces paramètres ont été étudiés en fonction de la largeur de paroi, du courant appliqué et des longueurs caractéristiques du transport polarisé en spin. L'augmentation du paramètre de non-adiabaticité du système, liée à l'augmentation du gradient d'aimantation ainsi qu'à une forte non-localité du modèle couplé, a été démontrée. Pour des fils à section circulaire et à diamètre modulable, une contribution supplémentaire à la non-adiabaticité du système liée, à la géométrie confinée, a été mise en évidence. Puis, les différents régimes dynamiques ainsi que les conditions de dépiégage de la paroi ont été caractérisés en fonction de la taille de constrictions. / In the context of spintronics this thesis studies the mutual interaction between a spin polarised current and the magnetization of magnetic structures. During this work, the diffusive spin transport equations were coupled in a self-consistent manner with the magnetization dynamics equations in the micromagnetic approach in our homemade finite element code. This numerical tool applied to the study of domain walls dynamics in different geometries under the action of spin polarized current highlighted several new phenomena related to the mutual interaction between the magnetization and the spins of electrons. For rectangular cross section stripes, the impact of this interaction, usually neglected in simplified models, is quantified by the computation of the domain wall velocity and the Walker critical current. These quantities were studied as a function of the domain wall width, the applied current, and the spin polarised transport characteristic lengths. Increasing the non-adiabatic parameter of the system related to the increase in the magnetization gradient and a strong non-locality of the coupled model was demonstrated. For circular cross section wires with a modulated diameter, an additional contribution to the non-adiabaticity of the system related to the confined geometry is highlighted. Then the different dynamic regimes and domain wall unpinning conditions are characterised according to the constriction size.