In recent years multiferroic materials have great application potential in the ferroelectric random access memory and emerging spintronics development setting off the boom of the multiferroic materials research. It was assume that the ferroelectric and magnetic properties cannot coexist at the same temperature range or their coupling is weak making the searching for multiferroics in dream. Multiferroic materials have been discovered in recent years. Multiferroics may even be induced due to the strong coupling between artificial layers by growing multilayer technique. [1] [2] Among of which the BiFeO3 compound contains of antiferromagnetic and ferroelectric properties at room temperature. The BiFeO3 thin films manifests a larger spontaneous porlarization than that of the bulk and is highly capable of industrial application. In this study, Pb doped BFO (BPFO) this films were grown on top the conductive SrRuO3 layers. By varying BPFO¡¦s thickness, the strain effect the relative physical properties were studied. It is found that the lattice constants of SRO is totally different to that of STO substrate indicating strain relaxation. Similar phenomena are also discovered for BPFO thin film which is believed due to the tilting effect on the SRO/STO interface.

No description available.

For many decades, the revolution in semiconductor industry has continuously been powered by the successful down scaling of complementary metal-oxide semiconductor (CMOS) technology to produce integrated circuits with improved performance at lower cost. However, current charge-based CMOS technology is already approaching physical limits and, thus, encounters a number of technological challenges. Spintronics is an emerging and rapidly evolving research field that has a great potential to overcome these challenges confronting CMOS by introducing the electron spin, in addition to electron charge, as an extra degree of freedom. Traditional spintronic devices are based on the alignment of spins in magnetic layers, manipulated by spin-polarized currents. Thus, employing the non-volatile nature of layer magnetization and its direction to represent the bit state, spintronics provides power-efficient devices that are attractive for memory and logic applications. Magnetoresistive random access memory (MRAM) is one of the most essential applications of spin based electronics, which has already been recognized as the leading candidate for future universal memory. MRAM cells use spin-based magnetic tunnel junctions (MTJs) as the fundamental storage blocks. These conventional MTJs employ the use of magnetic elements with a single axis of magnetization, which provide two resistance states, capable of storing one bit of information. Enhancing the memory density is one of the major challenges encountered by MRAM industry, as the straightforward approach of reducing the magnetic bit size is unfeasible with magnetic devices due to intrinsic superparamagnetism effects. In this thesis, we propose increasing the bit density in MRAM by implementing shape anisotropy induced multistate MTJs. By patterning the free ferromagnetic layer of MTJs in the shape of four intersecting ellipses we achieve four in-plane stable axes of magnetization, capable of providing eight resistance states in total, the switching between which is performed by spin-orbit torques (SOT) in spin Hall metals (SHM). We initially verify the proposed concept with micromagnetic simulations followed by fabrication and, consequent, room temperature characterization of the first experimental prototypes.

Le remplacement des technologies DRAM et SRAM des mémoires caches est un enjeu pour l’industrie microélectronique qui doit faire face à des demandes de miniaturisation, de réduction des amplitudes et des durées des courants d’écriture et de lecture des données. Les mémoires à accès direct magnétiques (MRAM) sont des candidates pour une future génération de mémoires et la découverte des couples de spin-orbite (SOT) a ouvert la voix à une combinaison des deux technologies appelée SOT-MRAM. Ces mémoires sont très prometteuses car elles allient non-volatilité et bonne fiabilité, mais de nombreux défis techniques et théoriques restent à relever.L’objectif de ce travail de thèse est d’étudier le retournement de l’aimantation par couple de spin-orbite avec des impulsions de courant sub-nanoseconde et de diminuer les courants d’écriture à couple de spin-orbite. Ce travail est préliminaire à la preuve de concept d’une mémoire SOT-MRAM écrite avec des impulsions de courant électrique ultra-courtes et des amplitudes relativement faibles.Pour cela nous avons étudié des cellules mémoire à base de Ta-CoFeB-MgO. Nous avons vérifié les dépendances du courant critique en durées d’impulsions et en un champ magnétique extérieur. Nous avons ensuite, sur une cellule type SOT-MRAM, prouvé l’écriture ultrarapide avec des impulsions de courant inférieures à la nanoseconde. Puis nous nous sommes intéressés à la diminution du courant d’écriture de SOT-MRAM à l’aide d’un champ électrique. Nous avons démontré que ce dernier permet de modulerl’anisotropie magnétique. Sa diminution lors d’une impulsion de courant dans la liste de tantale montre que la densité de courant critique pour le retournement de l’aimantation du CoFeB par SOT est réduite. Ces résultats sont très encourageants pour le développement des SOT-MRAM et incitent à approfondir ces études. Le mécanisme de retournement de l’aimantation semble être une nucléation puis une propagation de parois de domaines magnétiques. Cette hypothèse se fonde sur des tendances physiques observées lors des expériences ainsi que sur des simulations numériques. / They require smaller areas for bigger storage densities, non-volatility as well as reduced and shorter writing electrical currents. Magnetic Random Access Memory (MRAM) is one of the best candidates for the replacement of SRAM and DRAM. Moreover, the recent discovery of spin-orbit torques (SOT) may lead to a new technology called SOT-MRAM. These promising technologies combine non-volatility and good reliability but many challenges still need to be taken up.This thesis aims at switching magnetization by spin-orbit torques with ultra-fast current pulse and at reducing their amplitude. This preliminary work should enable one to proof the concept of SOT-MRAM written with short current pulses and low electrical consumption to write a memory cell.To do so, we studied Ta-CoFeB-MgO-based memory cells for which we verified current dependencies on pulse lengths and external magnetic field. Then we proved the ultrafast writing of a SOT-MRAM cell with pulses as short as 400 ps. Next, we focused on reducing the critical writing currents by SOT with the application of an electric field. We showed that magnetic anisotropy can be modulated by an electricfield. If it can be lowered while a current pulse is injected through the tantalum track, we observed a reduction of the critical current density for the switching of the CoFeB magnetization. Those results are very promising for the development of SOT-MRAM and encourage one to delve deeper into this study. The magnetization switching mechanism seems to be a nucleation followed by propagations of magneticdomain walls. This assumption is based on many physical tendencies we observed and also on numerical simulations.

abstract: I studied the properties of novel Co2FeAl0.5Si0.5 (CFAS), ZnGeAs2, and FeS2 (pyrite) thin films for microelectronic applications ranging from spintronic to photovoltaic. CFAS is a half metal with theoretical spin polarization of 100%. I investigated its potential as a spin injector, for spintronic applications, by studying the critical steps involved in the injection of spin polarized electron populations from tunnel junctions containing CFAS electrodes. Epitaxial CFAS thin films with L21 structure and saturation magnetizations of over 1200 emu/cm3 were produced by optimization of the sputtering growth conditions. Point contact Andreev reflection measurements show that the spin polarization at the CFAS electrode surface exceeds 70%. Analyses of the electrical properties of tunnel junctions with a superconducting Pb counter-electrode indicate that transport through native Al oxide barriers is mostly from direct tunneling, while that through the native CFAS oxide barriers is not. ZnGeAs2 is a semiconductor comprised of only inexpensive and earth-abundant elements. The electronic structure and defect properties are similar in many ways to GaAs. Thus, in theory, efficient solar cells could be made with ZnGeAs2 if similar quality material to that of GaAs could be produced. To understand the thermochemistry and determine the rate limiting steps of ZnGeAs2 thin-film synthesis, the (a) thermal decomposition rate and (b) elemental composition and deposition rate of films were measured. It is concluded that the ZnGeAs2 thin film synthesis is a metastable process with an activation energy of 1.08±0.05 eV for the kinetically-limited decomposition rate and an evaporation coefficient of ~10-3. The thermochemical analysis presented here can be used to predict optimal conditions of ZnGeAs2 physical vapor deposition and thermal processing. Pyrite (FeS2) is another semiconductor that has tremendous potential for use in photovoltaic applications if high quality materials could be made. Here, I present the layer-by-layer growth of single-phase pyrite thin-films on heated substrates using sequential evaporation of Fe under high-vacuum followed by sulfidation at S pressures between 1 mTorr and 1 Torr. High-resolution transmission electron microscopy reveals high-quality, defect-free pyrite grains were produces by this method. It is demonstrated that epitaxial pyrite layer was produced on natural pyrite substrates with this method. / Dissertation/Thesis / Ph.D. Materials Science and Engineering 2013

La découverte de la magnétorésistance géante en 1988 est considérée comme la date de naissance d’un nouveau et dynamique champ de recherche appelé l’électronique de spin. La riche physique associée au transport de spin devrait révolutionner le futur de la nanoélectronique. Dans ce cadre les nano-oscillateurs à transfert de spin (STOs) se sont positionnés comme des candidats sérieux pour le développement d’une nouvelle génération de dispositifs rf basés sur l’électronique de spin.Au début de ma thèse, l’important bruit de phase des STOs restait une contrainte majeure limitant les perspectives technologiques à ce type d’oscillateurs. Dans cette thèse nous avons cherché à contrôler la dynamique des STOs et à réduire leur bruit phase en développant différentes stratégies : (i) l’optimisation des propriétés des matériaux magnétiques utilisés (ii) l’excitation de modes couplés dans des systèmes hybridés (iii) la stabilisation de la dynamique de la phase d’un STO avec un signal extérieur de référence (iv) la synchronisation mutuelle de différents oscillateurs pour améliorer la cohérence spectrale et la puissance des STOs. Nous focalisons en particulier sur le cas de STO à base de vortex magnétique qui présentent intrinsèquement des cohérences spectrales plus élevées que celles d’autres types d’oscillateurs.Dans une première partie, nous nous proposons d’identifier et d’étudier les différents mécanismes qui régissent et contrôle la dynamique d’un STO à base de vortex magnétique dans les régimes auto-oscillant et non-autonomes. Nous mettons tout d’abord en évidence que l’excitation de modes couplés permet de contrôler les propriétés rf d’un oscillateur unique en prenant l’exemple d’un STO à base de deux vortex magnétiques couplés. Par la suite, nous étudions la synchronisation « parfaite » de ces STOs à base de vortex avec un courant rf de référence. Nous corrélons l’observation d’une largeur de raie d’un hertz et d’un bruit de phase minimum de -90 dBc/Hz à 1kHz de la porteuse dans l’état synchronisé à une absence de glissement de phase, i.e. à l’absence de phénomène de désynchronisation. Le fort couple de Field-like planaire mesuré dans ces STOs représente un outil précieux pour contrôler le processus de synchronisation. Dans le cas des STOs à double vortex, un tel contrôle nous permet d’observer des phénomènes physiques exotiques allant de la synchronisation simultanée de plusieurs modes, à de l’auto-synchronisation en passant par des dynamiques de synchronisation incohérentes.Dans une seconde partie, nous proposons différents concepts innovants de dispositifs rf à base de vortex magnétique. Nous présentons tout d’abord les bases d’une boucle à retard de phase permettant d’asservir un STO. En prenant avantage du fort couple de Field Like, nous développons un nouveau schéma de détection rf, plus efficace que les actuelles diodes Schottky, basé sur un renversement d’aimantation en expulsant réversiblement le cœur de vortex à l’aide d’un courant rf. Finalement, nous démontrons qu’il est possible de synchroniser électriquement deux STOs connectés directement en parallèle ou en série, ou à l’aide d’une ligne à retard. Nous montrons ainsi qu’une forte amélioration de la cohérence spectrale (d’un facteur 2) et de la puissance (d’un facteur 4 pour un maximum de 1.6 μW) peut être obtenues dans l’état synchronisé. A l’aide de la ligne à retard, nous mettons par ailleurs en évidence le rôle crucial du déphasage entre les deux STOs sur les propriétés de l’état synchronisé. Ces résultats prometteurs ouvrent la voie vers la synchronisation de réseaux de STOs sans champ appliqué et sans ligne à retard entre les oscillateurs.Dix ans après leur découverte, les oscillateurs à transfert de spin n’ont toujours pas dévoilé tout leur potentiel et de nouvelles applications sont maintenant envisagées, allant de dispositif rf classiques à des circuits logiques et dispositifs bio-inspirés basés sur les STOs. / The discovery of the giant magnetoresistance in 1988 is considered as the birth date of a new and dynamic research field called spintronics. The rich physics associated with spin transport has created a breakthrough for the future of nano-electronics. In the magnetism roadmap, spin-torque oscillators (STOs) are candidates for future generation of spintronic based rf-devices.At the beginning of this thesis, one major issue of spin-torque oscillators remained their poor spectral coherence. To overcome this issue, we have investigated different approaches: (i) the development of magnetic materials with a low damping and large spin-polarization, (ii) the study of collective mode dynamics in hybridized magnetic systems (iii) the stabilization of the STO dynamics with a reference external signal (iv) the synchronization of multiple STOs to enhance both their power and spectral coherence. We focus our work on vortex based STOs which present higher spectral coherences than other kinds of STOs.In a first part, we study the different mechanisms that can drive and stabilize the dynamics of a vortex based STO in the autonomous and non-autonomous regimes. We first highlight that the excitation of collective modes allows the harnessing the rf-properties of a single and isolated in a double vortex based STO. Then we report the ``perfect'' phase-locking of a STO with an external rf-current. To go beyond this analysis, we notice that a 1 Hz minimum linewidth and a flat phase noise level of -90 dBc/Hz at 1 kHz from the offset frequency in the locked state could be associated with the absence of phase slips, i.e desynchronization events. We demonstrate that the locking process is driven by a Field-like in-plane torque which gives the possibility to control with precision the STO locking process. In our double vortex based STO, we can even observe exotic behaviors such as multi-mode synchronization, self-resonance and eventually incoherent motion. Such a degree of control, unexpected for a nano-scale oscillator, is particularly promising for the development of STO based nanodevices.In a second part, we propose different concepts of spintronic rf-devices based on vortex STOs. We describe the basis of an on-chip STO based phase locked loop. By taking advantage of the large Field-like torque in our STOs, we develop a new radio-frequency detection scheme, more efficient than the state of the art Schottky diode, based on magnetization switching through the resonant and reversible expulsion of the vortex core. Finally, we show the first experimental observation of the electrical synchronization of two STOs connected directly in parallel or in series, or with an electrical delay line. In the synchronized state, we show a strong improvement of both the spectral coherence (by a factor 2) and the output power (by a factor 4, up to 1.6 μW). We also demonstrate, with an electrical delay line, the strong impact of the phase shift between the two STOs on the synchronized regime. These promising results open the way for the synchronization of STO arrays at zero field and without electrical delays.Ten years after their discovery, spin-torque oscillators have thus not yet revealed all their potential and promising applications could be soon targeted, in order to realize a spin logic circuit, bio-inspired spintronic devices and more classical rf-applications.

In modern technology exciting developments are related to the ability to understand and control interfaces. Particularly, magnetic interfaces revealing spindependent electron transport are of great interest for modern spintronic devices, such as random access memories and logic devices. From the technological point of view, spintronic devices based on magnetic interfaces enable manipulation of the magnetism via an electric field. Such ability is a result of the different quantum effects arising from the magnetic interfaces (for example, spin transfer torque or spin-orbit torque) and it can reduce the energy consumption as compared to the traditional semiconductor electronic devices. Despite many appealing characteristics of these materials, fundamental understanding of their microscopic properties and related phenomena needs to be established by thorough investigation. In this work we implement first principles calculations in order to study the structural, electric, and magnetic properties as well as related phenomena of two types of interfaces with large potential in spintronic applications: 1) interfaces between antiferromagnetic 3d-metal-oxides and ferromagnetic 3d-metals and 2) interfaces between non-magnetic 5d(4d)- and ferromagnetic 3d-metals. A major difficulty in studying such interfaces theoretically is the typically large lattice mismatch. By employing supercells with Moir e patterns, we eliminate the artificial strain that leads to doubtful results and are able to describe the dependence of the atomic density at the interfaces on the component materials and their thicknesses. After establishing understanding about the interface structures, we investigate the electronic and magnetic properties. A Moir e supercell with transition layer is found to reproduce the main experimental findings and thus turns out to be the appropriate model for simulating magnetic misfit interfaces. In addition, we systematically study the magnetic anisotropy and Rashba band splitting at non-magnetic 5d(4d) and ferromagnetic 3d-metal interfaces and their dependences on aspects such as interdiffusion, surface oxidation, thin film thickness and lattice mismatch. We find that changes of structural details strongly alter the electronic states, which in turn influences the magnetic properties and phenomena related to spin-orbit coupling. Since the interfaces studied in this work have complex electronic structures, a computational approach has been developed in order to estimate the strength of the Rashba band splitting below and at the Fermi level. We apply this approach to the interfaces between a Co monolayer and 4d (Tc, Ru, Rh, Pd, and Ag) or 5d (Re, Os, Ir, Pt, and Au) transition metals and find a clear correlation between the overall size of the band splitting and the charge transfer between the d-orbitals at the interface. Furthermore, we show that the spin splitting at the Fermi surface scales with the induced orbital moment weighted by the strength of the spin-orbit coupling.

<p dir="ltr">Molecular materials exhibiting bistability between two states are intriguing candidates for next generation electronic devices. Two similar classes of materials, known as spin crossover (SCO) and valence tautomers (VT) respectively, are of particular interest due to their multifunctional properties, which are controllable via several external parameters, such as temperature, light irradiation, pressure, magnetic field, and electric field. In recent years, considerable research has been dedicated to better understanding the underlying principles that govern the behavior of these materials, so that their implementation into nano-based devices might be achieved.</p><p dir="ltr">In this report, a systematic study of the valence tautomer molecule [Co(sq)(cat)(3-tpp)₂] is presented. In the first chapter, the phenomenon of valence tautomerism (VT) occurring in coordination compounds is introduced and described from the perspective of Crystal Field Theory (CFT). Further, the molecular structure and physical properties of the [Co(sq)(cat)(3-tpp)₂] molecule are explored. The properties of the ferroelectric material Polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), and the 2-D Mxene Ti₃C₂ are also discussed.</p><p dir="ltr">The next section details equipment development and experimental methods. Thin films of VT molecules were prepared from solution via a drop-casting approach. For thin film analysis, we have developed a custom made, fully automated Vibrating Sample Magnetometer (VSM) with a sensitivity on the order of 1 × 10^-5 emu, as well as a fully automated, variable temperature, under vacuum electron transport stage, and a magneto-optic Kerr effect apparatus (MOKE). Additional experimental methods used to characterize the VT thin films include X-ray Absorption Spectroscopy (XAS), UV-visible Spectrometry (UV-Vis) and Differential Scanning Calorimetry. Experimental results obtained from these techniques are discussed and analyzed in the third section. PVDF-HFP polarization dependent isothermal spin state switching of [Co(sq)(cat)(3-tpp)₂] is also discussed as well as the effects of doping [Co(sq)(cat)(3-tpp)₂] with Ti₃C₂, followed by a conclusion and an outline of future work.</p>

In this thesis we present three different aspects of spin and spin-dependent transport properties in novel materials. Spurred by the prospect of spintronic devices, which use the spin degree of freedom of electrons instead of, or combined with, the charge degree of freedom, we analyse the spin properties of quantum wires, organic conducting polymers and sheets of graphene. First, we examine a quantum wire that is embedded in a two dimensional electron gas. We consider the Rashba spin-orbit coupling, and include the effect of interaction between the conduction electrons. We construct an analytically solvable low-energy theory for the wire, and explore the interaction between two magnetic impurities in the wire. We find that both the spin-orbit coupling and the electron-electron interaction have an effect on the magnetic interaction, and find the magnetic interaction to be tunable by an electric field. Next, we study an organic conducting polymer, which is contacted to magnetised ferromagnetic leads. In semiconducting organic polymers the current is transported by spinful polarons and spinless bipolarons. We simulate the transport through the system, including both types of charge carriers, and find the current to be insensitive to the presence of bipolarons. In addition, we find the bipolaron density to depend on the relative magnetisation of the ferromagnetic contacts. This constitutes an optical way of measuring the spin accumulation in conducting polymers. Finally, we investigate the optical conductivity of graphene. Symmetry arguments indicate the existence of two kinds of spin-orbit coupling in the two dimensional lattice, but there is no consensus about the actual strength of these couplings. We calculated the microwave optical conductivity of graphene including both possible spin-orbit interactions. We find the low frequency dependence of the optical conductivity to have a unique imprint of the spin-orbit couplings. This opens a possibility to experimentally determine both couplings separately.

Les nouvelles technologies numériques sont très avides en capacité de stockage, ainsi qu’en efficacité (rapidité et bas coût énergétique) de transport d’information. Les dispositifs d’aujourd’hui atteignant leurs limites, la recherche de nouvelles solutions de stockage est primordiale.L’utilisation de parois magnétiques comme brique élémentaire au codage de l’information a été proposée il y a quelques années, dans l’optique de réaliser un enregistrement tridimensionnel et ainsi d’augmenter considérablement les capacités de stockage.Depuis, un nouvel objet magnétique plus robuste et moins sensible aux perturbations extérieures a été découvert : le skyrmion. Il est récemment devenu un sujet d’étude très actif et a été observé expérimentalement dans deux types de systèmes. Le premier est basé sur les matériaux hélimagnétiques, dont la structure cristalline est non centrosymétrique. Le FeGe est l’un d’eux, avec la température de transition magnétique observée la plus élevée. Le skyrmion a également été observé à l’interface entre un métal lourd à fort couplage spin-orbite et un matériau ferromagnétique. En particulier, les systèmes de multicouches à interfaces non symétriques ont montré leur fort potentiel pour la manipulation de skyrmions à température ambiante.Les deux principales difficultés rencontrées aujourd’hui sont la réalisation de systèmes avec des techniques d’élaboration intégrables à des dispositifs industriels, ainsi que la détection fiable de la présence de skyrmions. Cette thèse est donc séparée en deux problématiques, appliquées aux deux types de systèmes. Un protocole de croissance de couches minces de FeGe hélimagnétique par pulvérisation cathodique a été développé en s’appuyant principalement sur des caractérisations structurales par diffraction de rayons X. L’élaboration de multicouches à interfaces non symétriques du type [métal lourd 1/matériau ferromagnétique/métal lourd 2]n a également été étudiée. Ces deux systèmes ont ensuite été caractérisés magnétiquement par diverses techniques basées sur de l’imagerie magnétique, des mesures de magnéto-transport et de la spectroscopie magnétique. / New technology needs huge storage capacity together with high speed and low-cost transport of information. Current devices meeting their limits, research on new storage solutions is needed.One of them, proposed a few years ago, consists in using magnetic domain walls. Aligning them in nanowires and using the thickness of the layers could enable the realization of a tridimensional recording device and then improve the storage capacity.A new object more robust and less sensitive to perturbations has been discovered since. Skyrmion is now widely studied. It has been experimentally observed in two kinds of systems. The first one is helimagnet, with non centrosymmetric crystal structure. FeGe is one of them, with the highest critical temperature observed yet. Skyrmion has been observed as well at the interface between a heavy metal with strong spin-orbit coupling and a ferromagnet. In particular, multilayers with non-symmetric interfaces are very promising systems for manipulation of skyrmions at room temperature.Nevertheless, the elaboration of systems for industrial devices and reliable detection of skyrmions is still challenging. These two problematics are explored in this thesis, applied to two different systems. A protocol to grow helimagnetic FeGe thin films was first established, thanks to structural characterization mainly based on X-ray diffraction. Growth of multilayers with non symmetrical interfaces [heavy metal 1/ferromagnet/heavy metal 2]n was studied as well. These systems were then magnetically characterized, using numerous techniques such as magnetic imaging, magneto transport measurements and magnetic spectroscopy.