Dissertations / Theses on the topic 'THz spintronics'
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Eivarsson, Nils, Malin Bohman, Emil Grosfilley, and Axel Lundberg. "Design and Simulation of Terahertz Antenna for Spintronic Applications." Thesis, Uppsala universitet, Institutionen för materialvetenskap, 2020. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-412982.
Abstract:
Spintronics is a spin-electronic field where the electron spinangular momentum, in conjunction with charge, is used to read andwrite information in magnetic sensors and logic circuits, e.g. hard disk drive (HDD), magnetic random access memory (MRAM) and broadband TeraHertz (THz) emitters. To realize the THz operations of the spin logic circuits THz manipulation of the magnetic state is pivotal. This THz manipulation of the magnetic state in anti-ferromagnetic magnetic materials can be realized by coupling the materials with THz antennas. On the other hand, these antennas enhance the THz amplitude of spin-electronic THz emitters when coupled with its output. Therefore, these THz antennas can not only be coupled with the input of magnetic logics to improve the efficiency of magnetic sate manipulation in logic devices but also with the output of the spintronic THz emitters to enhance the generated THz signal amplitude. In this project, we have examined four types of antennas: h-dipole, spiral, bow-tie, and a sub-THz antenna. All the antennas are placed on top of a MgO substrate material for simplicity. However, a bow-tie antenna is also fabricated on an antiferromagnetic substrate of TmFeO3 to check this antenna’s reliability to manipulate its magnetic state. We have studied the impact of antenna geometries on the generated electric field amplitude. We have optimized each antenna for maximum electric field norm profile, with an increase of 30% for the h-dipole and spiral antennas, and an increase of 100% for the bow-tie antenna. However, in this project we were not able to find any general conclusions about what geometrical parameters can further amplify the generated electric field. None of the antennas generated a large enough peak-to-peak electric field amplitude to manipulate the magnetic state of anti-ferromagnetic materials. However, they did successfully amplify the spintronic THz emitter output and could certainly be useful in that regard.
2
Kaushalya. "Ultrafast manipulation of magnetization using on-chip THz." Electronic Thesis or Diss., Université de Lorraine, 2021. http://www.theses.fr/2021LORR0173.
Abstract:
Le besoin de dispositifs de stockage de mémoire a explosé au cours des dernières décennies, en particulier après le développement d'Internet. Ce besoin a atteint des sommets énormes au cours des deux dernières années, peu après la pandémie due au COVID-19. Les disques durs (HDD) sont connus pour avoir le potentiel de répondre aux demandes de stockage de données haute densité. Cette thèse traite de l'un des défis majeurs rencontrés au sein de la communauté spintronique pour améliorer la vitesse et la consommation d'énergie des dispositifs de mémoire. La vitesse de fonctionnement lors de l'écriture d'un bit magnétique dépend du mécanisme de commutation de magnétisation utilisé. Le mécanisme de commutation est lui-même dépendant des propriétés magnétiques intrinsèques de l'échantillon et de l'excitation induite de l'extérieur qui entraîne l'inversion du trépan magnétique 1. Dans cette thèse, nous nous concentrerons sur l'utilisation des excitations du couple spin-orbite (SOT) pour entraîner l'inversion, qui sont une approche relativement nouvelle mais rapide et économe en énergie par rapport à d'autres méthodes de pointe. La vitesse typique d'inversion de magnétisation à l'aide des SOT est de l'ordre de quelques nanosecondes, bien plus lente que la commutation longue de la picoseconde qui est possible avec les dispositifs de mémoire basés sur la charge 2. En fait, une vitesse d'inversion record avec des impulsions électriques aussi courtes que ~ 200 ps a été signalée par Garello et. al., 3 en 2011 en utilisant des SOT. Cette thèse rapporte des efforts supplémentaires pour accélérer l'inversion de l'aimantation de près de 2 ordres de grandeur en exploitant de tels SOT. Dans ce but, des impulsions électriques THz ont été générées via l'utilisation de commutateurs photoconducteurs Auston. Nous démontrons qu'une seule impulsion électrique de 6ps de large peut induire un SOT sur une couche ferromagnétique de Co d'une épaisseur de 1 nm et entraîner une inversion complète de l'aimantation. Une étude systématique pour comprendre les SOT dans le régime temporel picoseconde est également entreprise via l'utilisation de différentes nanostructures magnétiques. Dans les dispositifs à mémoire magnétique, une "tête de lecture" est utilisée pour lire les informations stockées dans le dispositif. Typiquement, dans les dispositifs spintroniques, des têtes de lecture à magnétorésistance géante (GMR) ou à magnétorésistance tunnel (TMR) sont utilisées pour de telles opérations. Dans cette thèse, nous rapportons également les tentatives de développement d'un capteur GMR fonctionnant en régime THz. Pour entreprendre les études susmentionnées, un montage expérimental optique et optoélectrique pompe-sonde a également été construit et un rapport détaillé de celui-ci est également fourni dans la thèseThe need for memory storage devices has skyrocketed over the last few decades especially after the development of the internet. This need has reached enormous heights in the past two years, soon after the pandemic due to COVID-19. Hard disk drives (HDDs) are known to have the potential to meet up with the high-density data storage demands. This thesis deals with one of the major challenges faced within the spintronic community to improve the speed and the energy consumption of memory devices.The speed of operation during the writing of a magnetic bit depends on the magnetization switching mechanism employed. The switching mechanism is itself dependent on the intrinsic magnetic properties of the sample and the externally induced excitation that drives the reversal of the magnetic bit 1. In this thesis, we will focus on the use of spin-orbit torque (SOT) excitations to drive the reversal, which is a relatively new but fast and energy-efficient approach in comparison with other state-of-the-art methods.The typical speed of magnetization reversal using SOTs is in the range of few nanoseconds, far slower than the picosecond-long switching that is possible with charge-based memory devices2. In fact, a record reversal speed with electrical pulses as short as ~200ps was reported by Garello et. al., 3 in 2011 using SOTs. This thesis reports further efforts to speed up the magnetization reversal by almost 2 orders of magnitude by exploiting such SOTs. To this aim, THz electrical pulses were generated via the use Auston photoconductive switches. We demonstrate that a single 6ps wide electrical pulse can induce a SOT to a 1nm thin Co ferromagnetic layer and result in a full magnetization reversal. A systematic study to understand SOTs in the picosecond time regime is also undertaken via using different magnetic nanostructures.In magnetic memory devices, a “read-head” is used to read the stored information in the device. Typically, in spintronic devices, giant magnetoresistance (GMR) or tunnel magnetoresistance (TMR) based read heads are used for such operations. In this thesis, we also report on the attempts of developing a GMR sensor working in the THz regime.To undertake the aforementioned studies, a pump-probe optical and optoelectrical experimental setup has also been built and a detailed report of the same is also provided in the thesis
3
Hawecker, Jacques. "Terahertz time resolved spectroscopy of Intersubband Polaritons and Spintronic Emitters." Electronic Thesis or Diss., Sorbonne université, 2021. http://www.theses.fr/2021SORUS101.
Abstract:
Le domaine térahertz (THz) inclut une riche variété d’applications pratiques et fondamentales où la basse énergie des photons permettent l’investigation de nouveaux phénomènes d’interaction lumière-matière. Ces travaux se focalisent sur deux thématiques émergentes où la spectroscopie THz ultra-rapide représente un outil particulièrement adapté à l’étude de phénomènes fondamentaux ainsi qu’à la conception de nouvelles sources THz. La première thématique concerne la spintronique THz capable de générer des ondes THz équivalentes à celles obtenues au sein des cristaux non-linéaires. Cette technologie est basée sur des hétérostructures constituées de matériaux ferromagnétiques – métaux de transitions possédant des épaisseurs nanométriques permettant une émission THz par excitation ultrarapide de la structure, induisant un courant de spin et via l’effet Hall de spin, une conversion de courant de spin en courant de charge. Au-delà de ces structures métalliques, des matériaux « quantiques » faisant intervenir des phénomènes tel que l’effet inverse Edelstein au sein des isolants topologiques sont également étudiés. La seconde thématique abordée est celle des polaritons intersous-bandes dans le domaine THz. Les polaritons sont des quasi-particules résultant d’un couplage lumière-matière fort provenant d’un mode de cavité couplé à une transition intersous-bande. Leur nature bosonique représente une perspective pertinente à long terme pour développer de nouvelles sources THz lasers basées sur le principe de condensat de Bose-Einstein. Dans ces travaux, nous étudions le pompage optique monofréquence et résonant d’une branche polaritonique par une sonde large bande constituée d’impulsions THz. Cette investigation révèle de fortes indications d’effets non-linéaires et potentiellement des signatures de diffusions polaritoniques. Enfin, nous présentons également des optimisations technologiques de sources THz existantes utiles à l’études des deux thématiques abordées. Notamment de sources THz de type antennes photoconductrices haute puissance en cavités, qui ont permis la première démonstration d’imagerie THz en temps réel obtenue avec de tel dispositifsThe terahertz (THz) domain provides a rich playground for many practical and fundamental applications, where the low energy of THz photons permits to probe novel light-matter interactions. This work investigates two recent and emerging scientific areas where ultrafast THz spectroscopy can be used as a probe of fundamental phenomena, as well as potentially enabling the conception of new THz sources. In the first case, ultrafast THz spintronics are studied where ultrafast excitations of spintronic heterojunctions result in efficient pulse generation. These structures consist of nanometer thick ferromagnetic - heavy metal junctions, where an optically generated spin-charge in the former is converted to a charge-current in the latter via the Inverse Spin Hall Effect. Beyond these metal-based junctions, ultrafast THz spintronics based on “quantum” materials is also investigated, where THz pulses are generated using quantum phenomena such as the Inverse Edelstein Effect in Topological Insulators, shown to be a promising research direction. The second subject area is focused on THz intersubband polaritons, quasi-particles that emerge from the strong light-matter coupling of a THz photonic cavity and an intersubband transition. Here we are interested in the bosonic nature of the intersubband polaritons, as a long-term aim of realizing a novel THz laser based on Bose-Einstein condensation. In this work, we investigate resonant narrowband pumping of a polariton branch and probe using spectrally broad THz pulses. This shows strong indications of nonlinear effects and potential signatures of scattering processes that could eventually lead to the demonstration of THz polaritonic gain. Finally, to support our work in the above subject areas, technological developments were made in existing THz sources. This included high power THz photoconductive switches using cavities, which permitted the first demonstrations of real time THz imaging with such devices, and high power THz quantum cascade lasers as narrowband laser pumps
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LONGO, EMANUELE MARIA. "HETEROSTRUCTURES BASED ON THE LARGE-AREA Sb2Te3 TOPOLOGICAL INSULATOR FOR SPIN-CHARGE CONVERSION." Doctoral thesis, Università degli Studi di Milano-Bicocca, 2021. http://hdl.handle.net/10281/311358.
Abstract:
I dispositivi elettronici che sfruttano proprietà legate allo spin elettronico costituiscono un settore molto promettente per lo sviluppo della nanoelettronica del futuro. Recentemente, gli isolanti topologici tridimensionali (IT-3D), quando posti a contatto con materiali ferromagnetici (FM), giocano un ruolo centrale nel contesto del miglioramento dell’efficienza di conversione tra spin e carica elettronici in eterostrutture di tipo FM/TI. L’oggetto principale di questa tesi è lo studio delle interazioni chimico-fisiche tra l’IT-3D Sb2Te3, nelle sue forme granulare ed epitassiale, con film di Fe e Co attraverso l’uso di tecniche di Diffrazione/Riflettività di raggi-X, spettroscopia di risonanza ferromagnetica (FMR) e pompaggio di spin in risonanza ferromagnetica (SP-FMR). In concomitanza con l’ottimizzazione delle proprietà dei materiali, un particolare interesse è stato rivolto verso l’impatto industriale della ricerca presentata. Per questo motivo, per la produzione di Sb2Te3 e di alcuni dei FM impiegati, sono state impiegate tecniche di deposizione di materiali su larga scala ( 4 pollici), quali la Metal Organic Chemical Vapor Deposition (MOCVD) e l’Atomic Layer Deposition (ALD) rispettivamente.
Una approfondita caratterizzazione chimica, strutturale e magnetica dell’interfaccia Fe/ Sb2Te3-granulare ha evidenziato un marcato intermixing tra i materiali e una generale tendenza degli atomi di Fe nel legare con l’elemento calcogenuro quando presente in un IT. Attraverso trattamenti termici rapidi e a bassa temperatura sottoposti sui film di Sb2Te3 granulare prima della crescita del Fe, l’interfaccia Fe/Sb2Te3-granulare è risultata morfologicamente più netta e chimicamente stabile. Lo studio di film sottili di Co cresciuti attraverso ALD su Sb2Te3 granulare ha permesso la produzione di interfacce Co/Sb2Te3-granulare di alta qualità, con la possibilità inoltre di modificare le proprietà magneto-strutturali dei film di Co attraverso una selezione appropriata di substrati.
Con l’obbiettivo di migliorare le proprietà dei film di Sb2Te3, dei trattamenti termici specifici sono stati condotti su Sb2Te3 granulare appena cresciuto, ottenendo film di Sb2Te3 altamente orientati con una qualità cristallina vicina al cristallo singolo di tipo epitassiale. Questi substrati di Sb2Te3 sono stati utilizzati per produrre eterostrutture di Au/Co/Sb2Te3-epitassiale e Au/Co/Au/Sb2Te3-epitassiale per studiare la loro risposta di FMR. I dati di FMR per il campione Au/Co/Sb2Te3-epitassiale sono stati interpretati considerando un contributo di Two Magnon Scattering (TMS) dominante, verosimilmente a causa della presenza di rugosità magnetica all’interfaccia Co/Sb2Te3-epitassiale. L’introduzione di un interlayer di Au per evitare il contatto diretto tra Co e Sb2Te3 si è dimostrato vantaggioso per la totale eliminazione del contributo di TMS.
Misure di SP-FMR sono state condotte sulla struttura ottimizzata Au/Co/Au/Sb2Te3-epitassiale, sottolineando il ruolo giocato dallo strato di Sb2Te3-epitassiale nel processo di SP. I segnali di SP ricavati da campioni di Au/Co/Au/Si(111) e Co/Au/Si(111) sono stati utilizzati per determinare l’efficienza di conversione spin-carica ottenuta dall’introduzione dello strato di Sb2Te3. L’efficienza estratta è stata calcolata interpretando i dati di SP-FMR attraverso i modelli di effetto Edelstein inverso ed effetto di Spin-Hall inverso, i quali hanno dimostrato che l’IT-3D Sb2Te3 è un candidato promettente per essere impiegato nella prossima generazione di dispositivi spintronici.Spin-based electronic devices constitute an intriguing area in the development of the future nanoelectronics. Recently, 3D topological insulators (TI), when in contact with ferromagnets (FM), play a central role in the context of enhancing the spin-to-charge conversion efficiency in FM/TI heterostructures. The main subject of this thesis is the study of the chemical-physical interactions between the granular and epitaxial Sb2Te3 3D-TI with Fe and Co thin films by means of X-ray Diffraction/Reflectivity, Ferromagnetic Resonance spectroscopy (FMR) and Spin Pumping-FMR. Beside the optimization of the materials properties, particular care was taken on the industrial impact of the presented results, thus large-scale deposition processes such as Metal Organic Chemical Vapor Deposition (MOCVD) and Atomic Layer Deposition (ALD) were adopted for the growth of the Sb2Te3 3D-TI and part of the FM thin films respectively. A thorough chemical, structural and magnetic characterization of the Fe/granular Sb2Te3 interface evidenced a marked intermixing between the materials and a general bonding mechanism between Fe atoms and the chalcogen element in chalcogenide-based TIs. Through rapid and mild thermal treatments performed on the granular Sb2Te3 substrate prior to Fe deposition, the Fe/granular-Sb2Te3 interface turned out to be sharper and chemically stable. The study of ALD-grown Co thin films deposited on top of the granular-Sb2Te3 allowed the production of high-quality Co/granular-Sb2Te3interfaces, with also the possibility to tune the magneto-structural properties of the Co layer through a proper substrate selection. In order to improve the structural properties of the Sb2Te3, specific thermal treatments were performed on the as deposited granular Sb2Te3, achieving highly oriented films with a nearly epitaxial fashion. The latter substrates were used to produce Au/Co/epitaxial-Sb2Te3 and Au/Co/Au/epitaxial-Sb2Te3 and the dynamic of the magnetization in these structures was investigated studying their FMR response. The FMR data for the Au/Co/Sb2Te3 samples were interpreted considering the presence of a dominant contribution attributed to the Two Magnon Scattering (TMS), likely due to the presence of an unwanted magnetic roughness at the Co/epitaxial-Sb2Te3 interface. The introduction of a Au interlayer to avoid the direct contact between Co and Sb2Te3 layers was shown to be beneficial for the total suppression of the TMS effect. SP-FMR measurements were conducted on the optimized Au/Co/Au/epitaxial-Sb2Te3 structure, highlighting the role played by the epitaxial Sb2Te3substrate in the SP process. The SP signals for the Au/Co/Au/Si(111) and Co/Au/Si(111) reference samples were measured and used to determine the effective spin-to-charge conversion efficiency achieved with the introduction of the epitaxial Sb2Te3 layer. The extracted SCC efficiency was calculated interpreting the SP-FMR data using the Inverse Edelstein effect and Inverse Spin-Hall effect models, which demonstrated that the Sb2Te3 3D-TI is a promising candidate to be employed in the next generation of spintronic devices.
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Kane, Matthew Hartmann. "Investigaton of the Suitability of Wide Bandgap Dilute Magnetic Semiconductors for Spintronics." Diss., Georgia Institute of Technology, 2007. http://hdl.handle.net/1853/16166.
Abstract:
New semiconductor materials may enable next-generation â spintronicâ devices which exploit both the spin and charge of an electron for data processing, storage, and transfer. The realization of such devices would benefit greatly from room temperature ferromagnetic dilute magnetic semiconductors. Theoretical predictions have suggested that room temperature ferromagnetism may be possible in the wide bandgap semiconductors GaMnN and ZnMnO, though the existing models require input from the growth of high-quality materials. This work focuses on an experimental effort to develop high-quality materials in both of these wide bandgap materials systems.
ZnMnO and ZnCoO single crystals have been grown by a modified melt growth technique. X-ray diffraction was used to examine the structural quality and demonstrate the single crystal character of these devices. Substitutional transition metal incorporation has been verified by optical transmission and electron paramagnetic resonance measurements. No indications of ferromagnetic hysteresis are observed from the bulk single crystal samples, and temperature dependent magnetization studies demonstrate a dominant antiferromagnetic exchange interaction. Efforts to introduce ferromagnetic ordering were only successful through processing techniques which significantly degraded the material quality.
GaMnN thin films were grown by metalorganic chemical vapor deposition. Good crystalline quality and a consistent growth mode with Mn incorporation were verified by several independent characterization techniques. Substitutional incorporation of Mn on the Ga lattice site was confirmed by electron paramagnetic resonance. Mn acted as a deep acceptor in GaN. Nevertheless, ferromagnetic hysteresis was observed in the GaMnN films. The apparent strength of the magnetization correlated with the relative ratio of trivalent to divalent Mn. Valence state control through codoping with additional donors such as silicon was observed. Additional studies on GaFeN also showed a magnetic hysteresis. A comparison with implanted samples showed that the common origin to the apparent strong ferromagnetic hysteresis related to contribution from Mn substitutional ions. The observed magnetic hysteresis is due to the formation of Mn-rich regions during the growth process. This work demonstrated that the original intrinsic models for room temperature ferromagnetism in the wide bandgap semiconductors do not hold and the room temperature ferromagnetism in these materials results from extrinsic contributions.
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Davesne, Vincent. "Organic spintronics : an investigation on spin-crossover complexes from isolated molecules to the device." Phd thesis, Université de Strasbourg, 2013. http://tel.archives-ouvertes.fr/tel-01062266.
Abstract:
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.
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Tseng, Hsiang-Han. "Towards controlling the coercivity in molecular thin films for spintronic applications." Thesis, Imperial College London, 2015. http://hdl.handle.net/10044/1/33845.
Abstract:
Organic semiconductors have attracted worldwide interest for the past two decades. The properties of these molecules can be easily manipulated and exploited, and furthermore benefit from chemical versatility, mechanical flexibility and low cost. This has led to a remarkable success in the field of plastic electronics and molecules have found numerous device applications such as photovoltaic cells (PV), organic light emitting diode (OLED), organic field effect transistor (OFET) and sensors. Organic semiconductors have recently become of considerable interest for spintronic applications, due to the long spin relaxation times and magneto-resistive effects observed in these systems. In order to fully exploit the advantages of these molecules for spintronic applications, it is essential to explore molecular routes towards all organic spin valves and search for molecule-based magnets as alternatives to conventional spin injector/detector such as La0.67Sr0.33 MnO3 (LMSO) and Co. The scope of this thesis is to investigate the way to control the functional properties and in particular the magnetic interactions and coercivities in molecular thin films, with an emphasis on the charge-transfer salt, [MnTPP][TCNQ], and a ferromagnetic system, FePc (including mixed H2Pc:FePc), respectively, fabricated by organic molecular beam deposition (OMBD). Although the magnetic couplings are currently limited to cryogenic temperature, it is shown that it is possible to engineer exotic physical properties in these mixed films, where the magnetism seen as an intrinsic property to the functional molecules shows a strong dependence on the local chemical structure and spatial displacement for the magnetic ions, which can be manipulated by addition of electron acceptor and non-magnetic substituent. Compared to conventional magnetic semiconductors, this approach is a molecular route towards tuneable magnetic properties, allowing one to directly control the magnetic interactions by varying the film composition via co-deposition, a desirable property that is obtained in the film form and readily exploited in all organic spintronic applications.
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Bruneel, Pierre. "Electronic and spintronic properties of the interfaces between transition metal oxides." Thesis, université Paris-Saclay, 2020. http://www.theses.fr/2020UPASP047.
Abstract:
Cette thèse porte sur les propriétés de transport anormal des oxydes de métaux de transition, en particulier de la surface de SrTiO₃ ou de l’interface entre SrTiO₃ et LaAlO₃. Dans ces systèmes on observe l’apparition de gaz d’électrons bidimensionnels. Des mesures d’Effet Hall non linéaire indiquent que ces gaz sont constitués de plusieurs sortes de porteurs de charge, et que leurs populations varient de manière non monotone sous l’effet du dopage électrostatique. L’effet des propriétés électrostatiques et des corrélations électroniques sur ces variations sont discutées. Celles-ci sont à l’origine de réponses remarquables en ce qui concerne la conversion du spin en charge dans ces systèmes à l’aide d’un modèle de liaisons fortes et de la théorie de la réponse linéaire. Les effets conjoints du spin-orbite atomique et de la brisure de symétrie d’inversion à l’interface verrouille les nombres quantiques de spin, de caractère orbital et d’impulsion des électrons, et induit des textures de spin complexe dans l’espace réciproque. Ces textures sont responsables de l’apparition des effets Edelstein et Hall de spin dans ces hétérostructures et sont caractéristiques de la nature multi-orbitale de ces systèmes électroniques. Enfin nous conduirons une étude ab initio des hétérostructures STO/LAO/STO pour expliquer les observations expérimentales de nouvelles manières de former un gaz d’électrons à ces interfaces d’oxydes. Nous discuterons des rôles respectifs de la chimie, de l’électrostatique et des défauts dans l’apparition de ce gazThe anomalous transport properties of transition metal oxides, in particular the surface of SrTiO₃ or at the interface between SrTiO₃ and LaAlO₃ is investigated in this thesis. These systems host two-dimensional electron gases. Nonlinear Hall Effect measurements suggest that several species of carriers are present in these systems, and that their population is varying on a nontrivial manner upon electrostatic doping. The role of the electrostatics properties of the electron gas and of the electronic correlations are discussed in this light. Next we discuss the spin to charge conversion of these systems thanks to tight-binding modeling and linear response theory. The complex interplay between atomic spin-orbit coupling and the inversion symmetry breaking at the interface leads to a complex spin-orbital-momentum locking of the electrons, inducing spin textures. These spin textures are responsible for the appearance of the Edelstein and Spin Hall Effect in these heterostructures and are characteristic of the multi-orbital character of these electronic systems. Finally an ab initio study of STO/LAO/STO heterostructures is performed to explain experimental evidence of new ways to produce an electron gas at this interface. The respective roles of the chemistry, electrostatics and defects are discussed
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Chaluvadi, Sandeep kumar. "Influence of the epitaxial strain on magnetic anisotropy in LSMO thin films for spintronics applications." Thesis, Normandie, 2017. http://www.theses.fr/2017NORMC248/document.
Abstract:
Nous présentons une étude des effets de contrainte induits par l’épitaxie dans des couches minces La1-xSrxMnO3 (LSMO) (001) (x = 0.33) pour 3 épaisseurs de films (50, 25 et 12 nm) déposés par Ablation Laser Pulsée (PLD) sur différents substrats tels que SrTiO3 (STO) (001), STO buffered MgO (001), NdGaO3 (NGO) (110) et (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) (001). L’étude est complétée par l’effet de la composition sur les propriétés magnétiques de couches minces de La1-xSrxMnO3 avec x=0,33 et 0,38 déposées par Epitaxie à Jets Moléculaires (MBE). Des caractérisations par diffraction de rayons X (XRD), et microscopie à force atomique (AFM), des mesures de résistivité électrique en quatre points en fonction de la température, d’aimantation par magnetometrie à SQUID (Superconducting Quantum Interference Device) et d’anisotropie magnétique par magnétométrie magnéto-optique Kerr vectorielle (MOKE) sont présentées. Les évolutions angulaires de l’anisotropie magnétique, de l’aimantation à rémanence, du champ coercitif et du champ de renversement d’aimantation ont ainsi pu être analysées pour des films épitaxiés LSMO de différentes épaisseurs. Des études en fonction de la température complètent les données. L’origine de l’anisotropie (magnétique, magnétocristalline, magnétostrictive ou liée aux effets de marches et d’angle de désorientation du substrat) est finalement discutéeWe report a quantitative analysis of thickness dependent epitaxial strain-induced effects in La1-xSrxMnO3 (LSMO) (001) (x = 0.33) thin films of thicknesses (50, 25 and 12 nm) grown on various single crystal substrates such as SrTiO3 (STO) (001), STO buffered MgO (001), NdGaO3 (NGO) (110) and (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) (001) by Pulsed Laser Deposition (PLD) technique. We also report the composition dependent magnetic properties of LSMO thin films with x = 0.33 and 0.38 in particular grown onto LSAT (001) substrate by Molecular Beam Epitaxy (MBE). The study mainly includes measurements such as X-ray Diffraction (XRD), Atomic Force Microscopy (AFM), temperature dependent four-probe resistivity, magnetization properties by Superconducting Quantum Interference Device (SQUID), magnetic anisotropy by Magneto-Optical Kerr Magnetometry (MOKE). Our results highlight the detailed study of angular evolution and thickness dependent magnetic anisotropy, remanence, coercivity and switching field in epitaxial LSMO thin films. Temperature-dependent studies are also performed on few selected films. We will also discuss the cause of magnetic anisotropy in LSMO films i.e., magneto-crystalline and magnetostriction anisotropy and the effects of steps or substrate mis-cut induced anisotropy
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Owen, Man Hon Samuel. "Electrical gating effects on the magnetic properties of (Ga,Mn)As diluted magnetic semiconductors." Thesis, University of Cambridge, 2010. https://www.repository.cam.ac.uk/handle/1810/228705.
Abstract:
The aim of the research project presented in this thesis is to investigate the effects of electrostatic gating on the magnetic properties of carrier-mediated ferromagnetic Ga1-xMnxAs diluted magnetic semiconductors. (Ga,Mn)As can be regarded as a prototype material because of its strong spin-orbit coupling and its crystalline properties which can be described within a simple band structure model. Compressively strained (Ga,Mn)As epilayer with more complex in-plane competing cubic and uniaxial magnetic anisotropies is of particular interest since a small variation of these competing anisotropy fields provide a means for the manipulation of its magnetization via external electric field. An all-semiconductor epitaxial p-n junction field-effect transistor (FET) based on low-doped Ga0.975Mn0.025As was fabricated. It has an in-built n-GaAs back-gate, which, in addition to being a normal gate, enhances the gating effects, especially in the depletion of the epilayer, by decreasing the effective channel thickness by means of a depletion region. A shift in the Curie temperature of ~2 K and enhanced anisotropic magnetoresistance (AMR) (which at saturation reaches ~30%) is achieved with a depletion of a few volts. Persistent magnetization switchings with short electric field pulses are also observed. The magnitude of the switching field is found to decrease with increasing depletion of the (Ga,Mn)As layer. By employing the k . p semiconductor theory approach (performed by our collaborators in Institute of Physics, ASCR, Prague), including strong spin-orbit coupling effects in the host semiconductor valence band, a change in sign of Kc at hole density of approximately 1.5x1020 cm-3 is observed. Below this density, the [110]/[1⁻10] magnetization directions are favoured, consistent with experimental data. A double-gated FET, with an ionic-gel top-gate coupled with a p-n junction back-gate based on the same material, was also employed in an attempt to achieve larger effects through gating. It reaffirms the results obtained and demonstrates enhanced gating effects on the magnetic properties of (Ga,Mn)As.
11
Yang, Chunlei. "Studies of the spintronic systems of ferromagnetic GaMnAs and non-magnetic InGaAs/InAlAs two dimensional electron gas /." View abstract or full-text, 2005. http://library.ust.hk/cgi/db/thesis.pl?PHYS%202005%20YANG.
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Battiato, Marco. "Superdiffusive Spin Transport and Ultrafast Magnetization Dynamics : Femtosecond spin transport as the route to ultrafast spintronics." Doctoral thesis, Uppsala universitet, Materialteori, 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-205265.
Abstract:
The debate over the origin of the ultrafast demagnetization has been intensively active for the past 16 years. Several microscopic mechanisms have been proposed but none has managed so far to provide direct and incontrovertible evidences of their validity. In this context I have proposed an approach based on spin dependent electron superdiffusion as the driver of the ultrafast demagnetization. Excited electrons and holes in the ferromagnetic metal start diffusing after the absorption of the laser photons. Being the material ferromagnetic, the majority and minority spin channels occupy very different bands. It is then not surprising that transport properties are strongly spin dependent. In most of the ferromagnetic metals, majority spin excited electrons have better transport properties than minority ones. The effect is that majority carriers are more efficient in leaving the area irradiated by the laser, triggering a net spin transport. Recent experimental findings are revolutionising the field by being incompatible with previously proposed models and showing uncontrovertibly the sign of spin superdiffusion. We have shown that spin diffusing away from a layer undergoing ultrafast demagnetization can be used to create an ultrafast increase of magnetization in a neighboring magnetic layer. We have also shown that optical excitation is not a prerequisite for the ultrafast demagnetization and that excited electrons superdiffusing from a non-magnetic substrate can trigger the demagnetization. Finally we have shown that it is possible to control the time shape of the spin currents created and developed a technique to detect directly spin currents in a contact-less way. The impact of these new discoveries goes beyond the solution of the mystery of ultrafast demagnetization. It shows how spin information can be, not only manipulated, as shown 16 years ago, but most importantly transferred at unprecedented speeds. This new discovery lays the basis for a full femtosecond spintronics.
13
Boehnke, Alexander [Verfasser], and Günter [Akademischer Betreuer] Reiss. "Tunnel magneto-Seebeck effect: improving the effect size ; spintronics and spincaloritronics / Alexander Boehnke ; Betreuer: Günter Reiss." Bielefeld : Universitätsbibliothek Bielefeld, 2016. http://d-nb.info/1115271709/34.
14
Sharma, Amit. "Spintronics with metals current perpendicular-to-the-plane magneto-transport studies in metallic multilayers and nanopillars /." Diss., Connect to online resource - MSU authorized users, 2008.
15
Lacoste, Bertrand. "Mastering the influence of thermal fluctuations on the magnetization switching dynamics of spintronic devices." Thesis, Grenoble, 2013. http://www.theses.fr/2013GRENY039/document.
Abstract:
Les mémoires magnétiques à couple de transfert de spin (STTRAM) sont des mémoires vives non-volatiles et endurantes très prometteuses pour remplacer les mémoires à base de condensateurs. Cependant, pour les technologies actuelles de STTRAM à aimantation planaire ou hors-du-plan, le temps de commutation est limité à 10~ns car le processus de renversement de l'aimantation est stochastique, déclenché par les fluctuations thermiques. Dans l'optique de rendre la commutation déterministe et plus rapide, une approche consiste à ajouter à la jonction tunnel magnétique une autre couche polarisante en spin, avec une aimantation orthogonale à celle de la couche de référence. Nous nous sommes intéressé plus particulièrement aux jonctions tunnels magnétiques planaires avec un polariseur perpendiculaire (à aimantation hors du plan). Le STT du polariseur perpendiculaire amorce le retournement d'aimantation, mais il provoque aussi des oscillations de la résistance de la jonction entre ses valeurs extrêmes. Cette particularité est mise à profit pour la réalisation de nano-oscillateurs (STO). Dans cette thèse, la dynamique d'aimantation du système comprenant une couche libre planaire, une couche de référence planaire et un polariseur perpendiculaire est étudiée, aussi bien expérimentalement que théoriquement (analytiquement et en simulations), dans l'approximation de macrospin. Dans le cas d'une couche libre oscillante sous l'action du STT du polariseur perpendiculaire, une description précise de ces oscillations est présentée, dans laquelle le champ d'anisotropie, le champ appliqué et le STT de la couche de référence planaire sont traités en perturbations. Dans le cas d'une couche libre ferrimagnétique synthétique (SyF), les expressions analytiques des courants critiques et des équations du mouvement sont calculées et comparées aux simulations. Ces résultats sont ensuite utilisés pour réaliser le diagramme de phase du système complet. L'anisotropie uniaxiale joue un role important, ce qui est confirmé par des mesures de retournement en temps réel réalisées sur des échantillons de nano-piliers à base de MgO. L'influence relative des STT provenant de la couche de référence et du polariseur perpendiculaire peut être ajsutée en jouant sur le rapport d'aspect des cellules, ce qui permet d'obtenir un retournement controlé en moins d'une nanoseconde avec une STTRAMSpin-transfer torque magnetic random-access memory (STTRAM) are very promising non-volatile and enduring memories to replace charged-based RAM. However, in conventional in-plane or out-of-plane STTRAM technologies, the switching time is limited to about 10~ns because the reversal process is stochastic i.e. it is triggered by thermal fluctuations. In order to render the reversal deterministic and faster, an approach consists in adding to the magnetic tunnel junction (MTJ) stack another spin-polarizing layer whose magnetization is orthogonal to that of the MTJ reference layer. We particularly investigated the case where a perpendicular polarizer is added to an in-plane magnetized tunnel junction. The STT from the perpendicular polarizer initiates the reversal, but it also creates oscillations of the resistance between its two extremal values. This behavior is usually interesting to realize STT nano-oscillators (STO). In this thesis, the dynamics of the system comprising an in-plane free layer, an in-plane reference layer and a perpendicular polarizer is studied both experimentally and theoretically (analytically and by simulations) in the framework of the macrospin approximation. For a single layer free layer oscillating due to the STT of the perpendicular polarizer, an accurate description of the oscillations is presented, in which the anisotropy field, the applied field and the in-plane STT are treated as perturbations. In the particular case of a synthetic ferrimagnetic (SyF) free layer, analytical expressions of the critical currents and of the oscillations equation of motion are computed and compared to simulations. These results are used to determine the phase diagram of the complete system. The in-plane anisotropy field is found to play a dramatic role, which is confirmed by experimental data from real-time measurements on MgO-based nano-pillars. It is shown that the cell aspect ratio can be used to tune the relative influence of the STT from the in-plane reference layer and from the out-of-plane polarizer. This allows achieving well controlled sub-nanosecond switching in STTRAM
16
Yamashita, Naoto. "Study on the Physics of Metal/Si Interfaces in Si-based Spin Devices." Doctoral thesis, Kyoto University, 2021. http://hdl.handle.net/2433/265203.
Abstract:
付記する学位プログラム名: 京都大学卓越大学院プログラム「先端光・電子デバイス創成学」京都大学
新制・課程博士
博士(工学)
甲第23431号
工博第4886号
新制||工||1764(附属図書館)
京都大学大学院工学研究科電子工学専攻
(主査)教授 白石 誠司, 教授 木本 恒暢, 教授 引原 隆士
学位規則第4条第1項該当
Doctor of Philosophy (Engineering)
Kyoto University
DGAM
17
Scheid, Matthias. "Tailoring semiconductor spintronics devices : tools for the creation and control of spins in two-dimensional electron gases." Regensburg Univ.-Verl. Regensburg, 2010. http://epub.uni-regensburg.de/14048/.
18
Scheid, Matthias [Verfasser]. "Tailoring semiconductor spintronics devices : tools for the creation and control of spins in two-dimensional electron gases / Matthias Scheid." Regensburg : Univ.-Verl. Regensburg, 2010. http://d-nb.info/1003371213/34.
19
Davesne, Vincent [Verfasser], and W. [Akademischer Betreuer] Wulfhekel. "Organic spintronics: an investigation on spin-crossover complexes from isolated molecules to the device / Vincent Davesne. Betreuer: W. Wulfhekel." Karlsruhe : KIT-Bibliothek, 2013. http://d-nb.info/1047383411/34.
20
Shary, Stephen. "Java Simulator of Qubits and Quantum-Mechanical Gates Using the Bloch Sphere Representation." University of Cincinnati / OhioLINK, 2011. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1298044339.
21
Hope, B. T. "The electronic structure and spintronic potential of carbon nanotubes and transition metal nanowires : a theoretical investigation." Thesis, University of Cambridge, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.604218.
Abstract:
The new technology of spintronics seeks to employ the quantum mechanical spin of electrons to encode and process information. For the construction of spintronic devices, a wide range of possible materials is available. Two highly promising candidates are carbon nanotubes (CNTs) and transition metals (TMs). The former are famous for their long coherence-lengths and novel electronic properties; the latter bring strong exchange forces and ferromagnetism. In this thesis I investigate, theoretically, a number of CNT and TM-nanowire systems to characterize their potential for use in spintronics. First the idea of using applied magnetic fields to influence spin in CNTs is studied to see if they could act either as polarizers or to rotate the polarization angel. Then the spin-dependent electronic structure of free-standing and CNT-encapsulated Co nanowires is examined for signs of spin-polarization under the density of states (DOS), ballistic and diffusive definitions. A variety of methods are used in the investigation. Tight-binding, elementary energy-scale analysis and free-electron rectangular-barrier models are applied to study CNTs in magnetic fields. To compute magnetic moments, energy bands and DOS of the Co nanowires, density-functional theory is used, implemented by the Vienna ab initio Simulation Package (VASP). It is shown that magnetic fields of feasible strength, applied to CNTs, do have potential in certain specialized schemes, most relevant to the quantum computation (QC) regime, but in general are too weak to have an impact on the spin ensembles relevant to classical computation (CC) applications. Unless acting upon isolated resonances, the system is inefficient both as a polarizer and as a spin processor. In contrast, very significant spin-polarization, with CC potential, is found to arise spontaneously in CO nanowires, albeit in a highly definition-dependent manner. For all but the monatomic wire, there is stark disagreement between the DOS, ballistic and diffusive degree of spin-polarization (DSPs). This is shown to result from the intrinsic nature of d and sp bands together with hybridization effects. Magnetism is also a central theme in nanoscience, in particular for high-density information storage. Magnetic moments tend to increase at reduced dimensionality but long-range order is more sensitive to temperature effects which can result in superparamagnetism unless it is supported by magnetic anisotropy. With relevance to this, the analysis of Co nanowires encompasses the mechanisms of ferromagnetism and the degree of magnetic apisotropy. An inverse correlation between magnetic moment and coordination number is found and discussed with reference to the Stoner model, the second moment theorem, and sp-d hybridization. The magnetocrystalline anisotropy is found to be much larger than in bulk Co and the easy axis depends sensitivity on wire diameter. Encapsulating Co wires inside CNTs provides a way of protecting the Co against oxidation. It also has the potential to unify the desirable properties of the component systems in a novel approach to inducing spin-polarization in CNTs. By repeating the VASP simulations on these hybrid nanostructures, I show that a high DSP can be associated with a CNT in this way, but it is argued that the transport length-scales of pure CNTs are no longer applicable.
22
Harmon, Nicholas Johann. "Topics on the theory of electron spins in semiconductors." The Ohio State University, 2010. http://rave.ohiolink.edu/etdc/view?acc_num=osu1284995889.
23
Mueller, Steve, Thomas Waechtler, Lutz Hofmann, Andre Tuchscherer, Robert Mothes, Ovidiu Gordan, Daniel Lehmann, et al. "Thermal ALD of Cu via Reduction of CuxO films for the Advanced Metallization in Spintronic and ULSI Interconnect Systems." Universitätsbibliothek Chemnitz, 2012. http://nbn-resolving.de/urn:nbn:de:bsz:ch1-qucosa-84003.
Abstract:
In this work, an approach for copper atomic layer deposition (ALD) via reduction of CuxO films was investigated regarding applications in ULSI interconnects, like Cu seed layers directly grown on diffusion barriers (e. g. TaN) or possible liner materials (e. g. Ru or Ni) as well as non-ferromagnetic spacer layers between ferromagnetic films in GMR sensor elements, like Ni or Co. The thermal CuxO ALD process is based on the Cu (I) β-diketonate precursor [(nBu3P)2Cu(acac)] and a mixture of water vapor and oxygen ("wet O2") as co-reactant at temperatures between 100 and 130 °C. Highly efficient conversions of the CuxO to metallic Cu films are realized by a vapor phase treatment with formic acid (HCOOH), especially on Ru substrates. Electrochemical deposition (ECD) experiments on Cu ALD seed / Ru liner stacks in typical interconnect patterns are showing nearly perfectly filling behavior. For improving the HCOOH reduction on arbitrary substrates, a catalytic amount of Ru was successful introduced into the CuxO films during the ALD with a precursor mixture of the Cu (I) β-diketonate and an organometallic Ru precursor. Furthermore, molecular and atomic hydrogen were studied as promising alternative reducing agents.
24
Mueller, Steve, Thomas Waechtler, Lutz Hofmann, Andre Tuchscherer, Robert Mothes, Ovidiu Gordan, Daniel Lehmann, et al. "Thermal ALD of Cu via Reduction of CuxO films for the Advanced Metallization in Spintronic and ULSI Interconnect Systems." Technische Universität Chemnitz, 2011. https://monarch.qucosa.de/id/qucosa%3A19675.
Abstract:
In this work, an approach for copper atomic layer deposition (ALD) via reduction of CuxO films was investigated regarding applications in ULSI interconnects, like Cu seed layers directly grown on diffusion barriers (e. g. TaN) or possible liner materials (e. g. Ru or Ni) as well as non-ferromagnetic spacer layers between ferromagnetic films in GMR sensor elements, like Ni or Co. The thermal CuxO ALD process is based on the Cu (I) β-diketonate precursor [(nBu3P)2Cu(acac)] and a mixture of water vapor and oxygen ("wet O2") as co-reactant at temperatures between 100 and 130 °C. Highly efficient conversions of the CuxO to metallic Cu films are realized by a vapor phase treatment with formic acid (HCOOH), especially on Ru substrates. Electrochemical deposition (ECD) experiments on Cu ALD seed / Ru liner stacks in typical interconnect patterns are showing nearly perfectly filling behavior. For improving the HCOOH reduction on arbitrary substrates, a catalytic amount of Ru was successful introduced into the CuxO films during the ALD with a precursor mixture of the Cu (I) β-diketonate and an organometallic Ru precursor. Furthermore, molecular and atomic hydrogen were studied as promising alternative reducing agents.
25
Hui, I. Pui. "Construction of the preparation, growth and characterization chamber of molecular beam epitaxy system and some studies of the iron-gallium nitride system with a view to spintronics applications." Click to view the E-thesis via HKUTO, 2007. http://sunzi.lib.hku.hk/HKUTO/record/B39558435.
26
Miah, Mohammad Idrish. "Photo-Induced Anomalous Hall Effect and the Electrical Detection of Spin Current in Nonmagnetic Semiconductors." Thesis, Griffith University, 2010. http://hdl.handle.net/10072/366548.
Abstract:
Spintronics is a rapidly growing research field aimed at realizing new high performance (spintronic) devices that takes advantage of the electron spin as well as of its charge. One of the important requirements necessary in developing semiconductor spintronic devices is the efficient generation of spin-polarized charge carriers (or spin current) in a semiconductor, transporting them reliably over reasonable distances and then detecting them. An electrical means of detecting spin current in semiconductors is very desirable for fully exploring the possibility of utilizing spin degree of freedom and spintronic device applications. A more general drift-diffusion equation that takes into account electric-field effects and both nondegenerate (NDG) and degenerate (DG) electron statistics was derived. Using this drift-diffusion model, it was shown that the extension of the spin diffusion length by a strong electric-field does not result in a significant increase in spin current in semiconductors owing to the competing effect of the electric field on diffusion. It was found that there is a spin drift-diffusion crossover field (E ~) for a process in which the drift and diffusion contribute equally to the spin current. Expressions were derived that relate E ~ to the intrinsic spin diffusion length (D0) of a semiconductor for all electron statistical regimes. The anomalous Hall effect (AHE) arising from the spin current was also derived on the basis of the drift-diffusion model and a useful way was demonstrated for the electrical detection of spin current and spin diffusion length in a semiconductor. For the possible enhancement of the diffusive part, DG and NDG cases were considered. It was found that, owing to an increase in the diffusion coefficient, the spin current and hence the AHE increase in a DG semiconductor. Devices for the electrical detection of spin current, based on the AHE, were fabricated on undoped as well as on Si-doped GaAs using nonmagnetic contact materials, producing a successful grecipe h for growth and metallization. They were characterized as to their suitability for applications and were found to contain transparent contacts. The spin polarized electrons generated in GaAs by circularly polarized light were dragged by an electric field and the AHE was observed without an external magnetic field. As no magnetic field was applied, the observed photo-induced AHE was the pure AHE. The experiments also detected photogenerated spin current and spin relaxation electrically, based on the measurements of the photo-induced anomalous Hall voltage (VAH) at room temperature as well as at low temperatures. It was found that the effect strongly depends on the applied electric field and excited photon energy. The AHE was also found to be enhanced by moderately increasing the doping density or decreasing the temperature. The results are discussed in comparison with a quantitative evaluation of the Dyakonov and Perel spin relaxation frequencies of the photogenerated electrons in GaAs. A good agreement between theory and experiment was obtained. Three spin transport regimes were considered in relation to photo-induced AHE measurements with moderately-doped samples, namely, diffusive, drift-diffusion crossover and drift. Of them, spin transport in the diffusive and drift-diffusion crossover regimes were studied in more details and E ~ and D0 were estimated in the NDG regime. The electrically obtained value of D0 was found to be 1.74 ƒÊm, a factor of 0.87 lower than that reported in the literature of optical measurements. A time-resolved pump-probe polarization investigation was performed to measure the spin polarization for the samples. The spin polarization of conduction band electrons, as measured using probe pulses with the same and opposite circular polarizations, was studied via the dependences of pump-probe delay, temperature, doping density as well as of the excitation photon energy. For a comparison with the experimental data, a calculation for the AHE was performed using the measured degree of spin polarization. A band energy structure calculation for Si-doped GaAs was performed by norm-conserving pseudopotential and Green function methods. The results showed that simultaneously accounting for the external biased electric-field and delocalization of the spin-polarized wave-functions by anharmonic electron-phonon interactions can explain the electric field- and doping-dependent spin current and hence the observed VAH. The theoretically obtained results generally agreed with the experimental resultsThesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Biomolecular and Physical Sciences
Science, Environment, Engineering and Technology
Full Text
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Hui, I. Pui, and 許貽培. "Construction of the preparation, growth and characterization chamber of molecular beam epitaxy system and some studies of the iron-galliumnitride system with a view to spintronics applications." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2007. http://hub.hku.hk/bib/B39558435.
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Riou, Mathieu. "Brain-inspired computing leveraging the transient non-linear dynamics of magnetic nano-oscillators." Thesis, Université Paris-Saclay (ComUE), 2019. http://www.theses.fr/2019SACLS033/document.
Abstract:
L’objectif de cette thèse est la réalisation expérimentale de calcul bio-inspiré en utilisant la dynamique transitoire d’oscillateurs magnétique nanométriques.Pour bien des tâches telle que la reconnaissance vocale, le cerveau fonctionne bien plus efficacement en terme d’énergie qu’un ordinateur classique. Le développement de puces neuro-inspirées offre donc la perspective de surmonter les limitations des processeurs actuels et de gagner plusieurs ordres de grandeurs sur la consommation énergétique du traitement de données. L’efficacité du cerveau à traiter des données est due à son architecture, qui est particulièrement adaptée à la reconnaissance de motifs. Les briques de base de cette architecture sont les neurones biologiques. Ceux-ci peuvent être vus comme des oscillateurs non linéaires qui interagissent et génèrent des cascades spatiales d’activations en réponse à une excitation. Cependant le cerveau comprend cent milliards de neurones et le développement d’une puce neuro-inspiré requerrait des oscillateurs de très petite dimension. Les oscillateurs à transfert de spin (STNO) sont de taille nanométrique, ont une réponse rapide (de l’ordre de la nanoseconde), sont fortement non-linéaires et leur réponse dépendante du couple de transfert de spin est aisément ajustable (par exemple par l’application d’un courant continu ou d’un champ magnétique). Ils fonctionnent à température ambiante, ont un très faible bruit thermique, et sont compatible avec les technologies CMOS. Ces caractéristiques en font d’excellents candidats pour la réalisation de réseaux artificiels de neurones compatibles avec un ordinateur classique.Dans cette thèse, nous avons utilisé un unique STNO pour générer le comportement d’un réseau de neurones. Ainsi l’oscillateur joue à tour de rôle chaque neurone. Une cascade temporelle remplace donc la cascade spatiale d’un réseau de neurones biologiques. En particulier nous avons utilisé la relaxation et la dépendance non-linéaire de l’amplitude des oscillations afin de réaliser du calcul neuromorphique. L’un des résultats principaux de cette thèse est la réalisation de reconnaissance vocale (reconnaissance de chiffres dits par 5 locuteurs différents) en obtenant un taux de reconnaissance à l’état de l’art de 99.6%. Nous avons pu montrer que les performances de la reconnaissance sont étroitement dépendantes des propriétés physiques du STNO tel que l’évolution de la largeur de raie, la puissance d’émission, ou la fréquence d’émission. Nous avons donc optimisé les conditions expérimentales (champs magnétiques et courant continu appliqués, fréquence du signal à traiter) afin de pouvoir utiliser au mieux les propriétés physiques du STNO pour la reconnaissance. Les signaux vocaux requièrent d’être transformés du domaine temporel au domaine fréquentiel, avant de pouvoir être traités, et cette étape est réalisée numériquement en amont de l’expérience. Nous avons étudié l’influence de différents prétraitements sur la reconnaissance et mis en évidence le rôle majeur de la non-linéarité de ces derniers. Enfin, afin de pouvoir traiter des problèmes requérant de la mémoire, tel que par exemple des signaux sous forme de séquences temporelles, nous avons mesuré la mémoire que possède intrinsèquement un STNO, du fait de sa relaxation. Nous avons aussi augmenté cette mémoire à l’aide d’une boucle à retard. Ce dispositif a permis d’accroître la plage de mémoire de quelques centaines de nanosecondes à plus d’une dizaine de microsecondes. L’ajout de cette mémoire extrinsèque a permis de supprimer jusqu’à 99% des erreurs sur une tâche de reconnaissance de motifs temporels (reconnaissance de signaux sinusoïdaux et carrés)This thesis studies experimentally the transient dynamics of magnetic nano-oscillators for brain-inspired computing.For pattern recognition tasks such as speech or visual recognition, the brain is much more energy efficient than classical computers. Developing brain-inspired chips opens the path to overcome the limitations of present processors and to win several orders of magnitude in the energy consumption of data processing. The efficiency of the brain originates from its architecture particularly well adapted for pattern recognition. The building blocks of this architecture are the biological neurons, which can be seen as interacting non-linear oscillators generating spatial chain reactions of activations. Nevertheless, the brain has one hundred billion neurons and a brain-inspired chip would require extremely small dimension oscillators. The spin-transfer torque oscillators (STNO) have nanometric size, they are fast (nanosecond time-scales), highly non-linear and their spin-torque dependent response is easily tunable (for instance by applying an external magnetic field or a d.c. current). They work at room temperature, they have a low thermal noise and they are compatible with CMOS technologies. Because of these features, they are excellent candidates for building hardware neural networks, which are compatible with the standard computers.In this thesis, we used a single STNO to emulate the behavior of a whole neural network. In this time multiplexed approach, the oscillator emulates sequentially each neuron and a temporal chain reaction replace the spatial chain reaction of a biological neural network. In particular, we used the relaxation and the non-linear dependence of the oscillation amplitude with the applied current to perform neuromorphic computing. One of the main results of this thesis is the demonstration of speech recognition (digits said by different speakers) with a state-of-the-art recognition rate of 99.6%. We show that the recognition performance is highly dependent on the physical properties of the STNO, such as the linewidth, the emission power or the frequency. We thus optimized the experimental bias conditions (external applied magnetic field, d.c. current and rate of the input) in order to leverage adequately the physical properties of the STNO for recognition. Voice waveforms require a time-to-frequency transformation before being processed, and this step is performed numerically before the experiment. We studied the influence of different time-to-frequency transformations on the final recognition rate, shading light on the critical role of their non-linear behavior. Finally, in order to solve problems requiring memory, such as temporal sequence analysis, we measured the intrinsic memory of a STNO, which comes from the relaxation of the oscillation amplitude. We also increased this memory, using a delayed feedback loop. This feedback improved the range of memory from a few hundreds of nanoseconds to more than ten microseconds. This feedback memory allows suppressing up to 99% of the errors on a temporal pattern recognition task (discrimination of sine and square waveforms)
29
Wan, Junjun. "Towards the realization of an all electrically controlled Spin Field Effect Transistor." University of Cincinnati / OhioLINK, 2011. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1292519781.
30
Valkass, Robert Alexander James. "Exploration of the sub-nanosecond magnetisation dynamics of partially built hard disk drive write-head transducers and other topical magnetic and spintronic materials and devices." Thesis, University of Exeter, 2017. http://hdl.handle.net/10871/31175.
Abstract:
In this thesis both the static and dynamic magnetic behaviour of complex three-dimensional nanoscale commercial hard disk drive write heads and thin film structures of interest to emerging spintronic devices have been investigated using a plurality of experimental techniques. The magneto-optical Kerr effect (MOKE) provides the basis for an optical microscopy technique sensitive to the magnetisation of a sample, detectable as a change in polarisation of light reflected from the sample surface. With a modelocked laser light source, synchronised electrical pulse generator and lock-in amplifier (LIA), a stroboscopic technique has been used to observe the magnetisation dynamics of hard disk drive write heads at 600 nm spatial resolution and 10 ps time resolution in response to a driving electrical pulse. The equilibrium magnetic state of these devices has been directly imaged by x-ray photo-emission electron microscopy (XPEEM), as well the stability of the equilibrium state in response to the application of an external bias field. Direct images of the equilibrium state obtained by XPEEM were found to agree with inferences made from MOKE images. Time-resolved scanning Kerr microscopy (TRSKM) images of magnetisation dynamics showed that flux does not form in ‘beams’ as commonly believed, but instead nucleates in separate sites across the writer. Static and time-resolved x-ray techniques have also been used to investigate a number of thin films of interest to spintronics. Spin pumping and spin transfer torque in Co2MnGe / Ag / Ni81Fe19 spin valves were explored using time-resolved x-ray ferromagnetic resonance (XFMR) carried out at Diamond Light Source (DLS), a as well as static x-ray magnetic circular dichroism (XMCD) for sample characterisation. This has provided element-specific measurements of the spin state in the source and sink layers of the spin valve, revealing a clear sign of spin transfer torque, while also investigating the role of sink layer thickness in spin pumping and damping. Ferrimagnetic yttrium iron garnet (Y3Fe2(FeO4)3) (YIG), a material of great interest in spintronics, has been studied by static and dynamic XMCD in comparison with ferromagnetic Co. While static and dynamic spectra for Co were identical, those for YIG differed markedly. While this may hint at a phase difference between the precession of Fe moments on different lattice sites, the true source of this difference has not been identified. Comparisons between vector network analyser ferromagnetic resonance (VNA-FMR) and XFMR measurements further suggest the presence of long-range inhomogeneities in the YIG. The spin dynamics of an antiferromagnet being driven by a ferromagnet have also been investigated using XMCD and x-ray magnetic linear dichroism (XMLD). A CoO / Fe / Ni81Fe19 trilayer wherein the thickness of the CoO layer varies across the sample has been thoroughly characterised by static XMCD and XMLD, providing information necessary to fully interpret time-resolved MOKE measurements on these samples. Measurements have shown that even small amounts of ordered CoO significantly modify the resonant field and linewidth of the adjacent ferromagnetic layers. Phase-resolved measurements of CoO spins have shown these spins to precess in phase with those of the adjacent Fe. The viability of dynamic XMLD measurements has also been confirmed. Finally, potential directions for future work in each project are discussed.
31
Dutta, Maitreya. "Hysteresis in the Conductance of Quantum Point Contacts with In-Plane Side Gates." University of Cincinnati / OhioLINK, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1396530257.
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Taylor, James Mark [Verfasser], Stuart S. P. [Gutachter] Parkin, Ingrid [Gutachter] Mertig, and Günter [Gutachter] Reiss. "Epitaxial thin films of the noncollinear antiferromagnets Mn3Ir and Mn3Sn for topological spintronic applications / James Mark Taylor ; Gutachter: Stuart S. P. Parkin, Ingrid Mertig, Günter Reiss." Halle (Saale) : Universitäts- und Landesbibliothek Sachsen-Anhalt, 2020. http://d-nb.info/1219508276/34.
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Harberts, Megan Marie. "Materials engineering, characterization, and applications of the organic-based magnet, V[TCNE]." The Ohio State University, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=osu1440096659.
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Bersweiler, Mathias. "From Sm1-xGdxAl2 electronic properties to magnetic tunnel junctions based on Sm1-xGdxAl2 and/or [Co/Pt] electrodes : Towards the integration of Zero Magnetization ferromagnets in spintronic devices." Thesis, Université de Lorraine, 2014. http://www.theses.fr/2014LORR0146/document.
Abstract:
Le contexte général de ce travail est le développement et l'intégration de nouveaux matériaux magnétiques ayant des propriétés originales et d'intérêt potentiel pour la spintronique. En tant que matériau ferromagnétique d’aimantation nulle, le composé Sm1-xGdxAl2 (SGA) suscite un intérêt particulier, puisqu’il est capable, dans son état magnétique compensé, de polariser en spin un courant d’électrons. Dans un premier temps, des expériences de photoémission résolues en angle et en spin sur synchrotron ont permis d’effectuer une analyse précise de la structure électronique selon diverses directions de la zone de Brillouin et d’estimer de manière directe la polarisation de spin au niveau de Fermi du composé SGA. Dans un second temps, une attention particulière a été portée aux multicouches [Co/Pt] et aux JTMs à base de [Co/Pt]. Les multicouches [Co/Pt] constituent la seconde électrode des JTMs à base de SGA. Leurs propriétés magnétiques (en particulier l'anisotropie perpendiculaire et l'aimantation à saturation) ont été soigneusement étudiées en fonction de l'épaisseur de Pt et de la nature de la couche tampon (Pt, MgO ou Al2O3), et en liaison avec leurs caractéristiques structurales. Leur intégration dans des JTMs à base de [Co/Pt] a permis ensuite de remonter d’une part à la polarisation tunnel effective des multicouches [Co/Pt] et d’autre part aux configurations magnétiques des différentes électrodes, configurations parfaitement expliquées et reproduites par des simulations micro-magnétiques. Dans un troisième temps, les résultats de magnéto-transport au sein des JTMs SGA/MgO/[Co/Pt] sont présentés et discutésThe general context of this work is the development and integration of new magnetic materials with original properties of potential interest for spintronic applications. In this field, the Sm1-xGdxAl2 (SGA) compound drives a particular attention, as a zero-magnetization ferromagnet that can exhibit a spin polarization in its magnetic compensated state. In a first step, synchrotron-based angle and spin resolved photoemission spectroscopy experiments have permitted to perform an accurate analysis of the electronic structure along various directions of the Brillouin Zone and to get a direct estimation of the spin polarization at the Fermi level. In a second step, a special attention has been the paid to [Co/Pt] multilayers and to [Co/Pt]-based MTJs. The [Co/Pt] multilayers would constitute the second electrode in SGA-based MTJs. Their magnetic properties (especially the perpendicular anisotropy and the saturation magnetization) have been carefully investigated as a function of Pt thickness and nature of the buffer layer (Pt, MgO or Al2O3), and in close connection with structural characteristics. Their integration in [Co/Pt]-based MTJs has permitted to determine the [Co/Pt] effective tunnel polarization and to unravel the magnetic configurations of both electrodes which are perfectly explained and reproduced by micromagnetic simulations. In a third step, the results concerning the magneto-transport experiments in SGA/MgO/[Co/Pt] MTJs are presented and discussed
35
Talatchian, Philippe. "Bio-inspired computing leveraging the synchronization of magnetic nano-oscillators." Thesis, Université Paris-Saclay (ComUE), 2019. http://www.theses.fr/2019SACLS008/document.
Abstract:
Les nano-oscillateurs à transfert de spin sont des composants radiofréquences magnétiques non-linéaires, nanométrique, de faible consommation en énergie et accordables en fréquence. Ce sont aussi potentiellement des candidats prometteurs pour l’élaboration de larges réseaux d’oscillateurs couplés. Ces derniers peuvent être utilisés dans les architectures neuromorphiques qui nécessitent des assemblées très denses d’unités de calcul complexes imitant les neurones biologiques et comportant des connexions ajustables entre elles. L’approche neuromorphique permet de pallier aux limitations des ordinateurs actuels et de diminuer leur consommation en énergie. En effet pour résoudre des tâches cognitives telles que la reconnaissance vocale, le cerveau fonctionne bien plus efficacement en terme d’énergie qu’un ordinateur classique. Au vu du grand nombre de neurone dans le cerveau (100 milliards) une puce neuro-inspirée requière des oscillateurs de très petite taille tels que les nano-oscillateurs à transfert de spin. Récemment, une première démonstration de calcul neuromorphique avec un unique nano-oscillateur à transfert de spin a été établie. Cependant, pour aller au-delà, il faut démontrer le calcul neuromorphique avec plusieurs nano-oscillateurs et pouvoir réaliser l’apprentissage. Une difficulté majeure dans l’apprentissage des réseaux de nano-oscillateurs est qu’il faut ajuster le couplage entre eux. Dans cette thèse, en exploitant l'accordabilité en fréquence des nano-oscillateurs magnétiques, nous avons démontré expérimentalement l'apprentissage des nano-oscillateurs couplés pour classifier des voyelles prononcées avec un taux de reconnaissance de 88%. Afin de réaliser cette tache de classification, nous nous sommes inspirés de la synchronisation des taux d’activation des neurones biologiques et nous avons exploité la synchronisation des nano-oscillateurs magnétiques à des stimuli micro-ondes extérieurs. Les taux de reconnaissances observés sont dus aux fortes accordabilités et couplage intermédiaire des nano-oscillateurs utilisés. Enfin, afin de réaliser des taches plus difficiles nécessitant de larges réseaux de neurones, nous avons démontré numériquement qu’un réseau d’une centaine de nano-oscillateurs magnétiques peut être conçu avec les contraintes standards de nano-fabricationSpin-torque nano-oscillators are non-linear, nano-scale, low power consumption, tunable magnetic microwave oscillators which are promising candidates for building large networks of coupled oscillators. Those can be used as building blocks for neuromorphic hardware which requires high-density networks of neuron-like complex processing units coupled by tunable connections. The neuromorphic approach allows to overcome the limitation of nowadays computers and to reduce their energy consumption. Indeed, in order to perform cognitive tasks as voice recognition or image recognition, the brain is much more efficient in terms of energy consumption. Due to the large number of required neurons (100 billions), a neuromorphic chip requires very small oscillators such as spin-torque nano-oscillators to emulate neurons. Recently a first demonstration of neuromorphic computing with a single spin-torque nano-oscillator was established, allowing spoken digit recognition with state of the art performance. However, to realize more complex cognitive tasks, it is still necessary to demonstrate a very important property of neural networks: learning an iterative process through which a neural network can be trained using an initial fraction of the inputs and then adjusting internal parameters to improve its recognition or classification performance. One difficulty is that training networks of coupled nano-oscillators requires tuning the coupling between them. Here, through the high frequency tunability of spin-torque nano-oscillators, we demonstrate experimentally the learning ability of coupled nano-oscillators to classify spoken vowels with a recognition rate of 88%. To realize this classification task, we took inspiration from the synchronization of rhythmic activity of biological neurons and we leveraged the synchronization of spin-torque nano-oscillators to external microwave stimuli. The high experimental recognition rates stem from the weak-coupling regime and the high tunability of spin-torque nano-oscillators. Finally, in order to realize more difficult cognitive tasks requiring large neural networks, we show numerically that arrays of hundreds of spin-torque nano-oscillators can be designed with the constraints of standard nano-fabrication techniques
36
Halisdemir, Ufuk. "Probing the impact of structural defects on spin dependent tunneling using photons." Thesis, Strasbourg, 2016. http://www.theses.fr/2016STRAE018/document.
Abstract:
L’étude de l’impact des défauts sur les propriétés électriques des semi-conducteurs a joué un rôle crucial dans la révolution des technologies de l’information dans le milieu du 20ème siècle. Jusqu’ici, la course à la miniaturisation a permis de répondre à la demande croissante en termes de puissance de calcul. Cependant, cette stratégie est vouée à rencontrer des limites physiques qu’il ne sera pas possible de surpasser, c’est pourquoi de nouvelles approches sont nécessaire. Dans ce nouveau paradigme de recherche, les dispositifs électroniques à base d’oxydes sont des candidats prometteurs afin de réaliser de nouveaux dispositifs multifonctionnels. L’importance des défauts sur les propriétés nominale des oxydes n’est pas autant reconnue que dans le domaine des semi-conducteurs. Notre projet de recherche tourne autour de deux objectifs principaux, le premier a pour but d’identifier explicitement l’impact de défauts spécifiques sur les propriétés électrique de dispositifs à base d’oxydes. Le second a pour but de tirer avantage des propriétés induites pour les défauts pour des applications optoélectroniquesThe study of the impact of defects on the electrical properties of semiconductors played a crucial role in the revolution of information technologies in the middle of the 20th century. Up to now, the race to miniaturization allowed to meet the increasing demand in terms of processing power. However, this strategy is predicted to encounter physical limits impossible to overcome and new approaches are necessary. Within this new research paradigm, oxide based electronic devices are promising candidates to fabricate new multifunctional devices. The importance of defects on the nominal properties of oxides is not acknowledged as much as it is in the field of semiconductors. Our research project revolved around two primary objectives, the first one aimed to explicitly identify the impact of specific defects on the properties of oxide-based electronic devices. The second one aimed to actually take advantage of properties induced by defects for optoelectronic applications
37
Jasper, Evan. "Development of Techniques in Time Domain Terahertz Spectroscopy for the Study of Chiral and Topological Materials." The Ohio State University, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=osu1597048083501651.
38
Ferraro, Filippo Jacopo. "Magnetic anisotropies and exchange bias in ultrathin cobalt layers for the tunnel anisotropic magnetoresistance." Thesis, Université Grenoble Alpes (ComUE), 2015. http://www.theses.fr/2015GREAY086/document.
Abstract:
Dans le cadre de l’étude des phénomènes magnétiques et de la spintronique qui sont présents aux échelles nanoscopiques nous avons étudié différents aspects des structures asymétriques de Pt/Co/AlOx. L’un des objectifs de cette thèse est le contrôle de l’oxydation et des propriétés magnétiques de ces multicouches. Nous avons combiné les mesures de structures (réflexion de Rayon-X), transports (Effet Hall anormal), et magnétiques (VSM-SQUID) afin de déterminer les rôles des effets magnétiques et d’interfaces. Un objectif était d’analyser le rôle de quelques monocouches (MCs) de CoO (qui peut se former lors de la sur oxydation de l’Al) sur les propriétés de la multicouche. Nous avons utilisé une technique de déposition avec un gradient d’épaisseur pour contrôler l’oxydation à l’échelle nanométrique. Nous avons établis que quelques monocouches (MCs) de CoO a un impact sur l’anisotropie de a multicouche. Pour approfondir l’effet de la couche de CoO, nous avons construit des bicouches ultrafines de Co(0.6nm)/CoO(0.6nm). Nous avons effectué des mesures refroidi sur champ sur ce système et trouvé un fort effet de couplage d'échange. Ces résultats indiquent que la couche CoO garde une forte anisotropie même en dans la limite des monocouches et permet de réfuter certains modèles sur l’effet d’échange bias et indique que les couches, couramment négligé, de CoO doivent être prises en considération dans le bilan énergétiques du système. Nous avons construits un appareil de mesure perpendiculaire de la magnétorésistance tunnel anisotrope (TAMR) à partir de la structure Pt/Co/AlOx. La TAMR est un effet de spintronique relativement récent dans lequel la rotation d’aimantation dans une électrode magnétique (combiné avec un couplage spin-orbite) peut entrainer un changement de la probabilité de l’effet tunnel, ce qui se manifeste comme un effet de magnétorésistance. Nous avons démontré qu’un contrôle précis de l’état d’oxydation est essentiel pour l’effet TAMR. La forte anisotropie magnétique induite nous permet d’atteindre des valeurs de TAMR plus grande comparée à celle des structures Pt/Co/AlOxIn the context of studying magnetic and spintronics phenomena occurring at the nanoscale, we investigated several aspects of Pt/Co/AlOx asymmetric structures. One of the objectives of this thesis was the control of the oxidation and the tailoring of the magnetic properties of these multilayers. We combined structural (X-Ray Reflectivity), transport (Anomalous Hall Effect) and magnetic measurements (VSM-SQUID), to study the interplay of magnetic and interfacial effects. One objective was to analyze the role that few monolayers (MLs) of CoO (which can form when overoxidizing the Al layer), could have on the properties of the stack. We used a wedge deposition techniques to control the oxidation on a subnanometer scale. We established that few MLs of CoO largely affect the total anisotropy of the stack. To further investigate the impact of the CoO, we engineered ultrathin Co(0.6nm)/CoO(0.6nm) bilayers. We performed field cooled measurements on this system and we found a large exchange bias anisotropy. These results indicate that the CoO keeps a large anisotropy even in the ML regime, help to rule out some of the models proposed to explain the exchange bias effect and imply that the usually neglected CoO presence must be considered in the energy balance of the system. We build perpendicular Tunneling Anisotropic MagnetoResistance (TAMR) devices based on the Pt/Co/AlOx structure. The TAMR is a relatively new spintronics effect in which the rotation of the magnetization in a single magnetic electrode (combined with the Spin-Orbit Coupling) can cause a change of the tunnel probability, which manifests as a magnetoresistance effect. We demonstrated that a careful control of the interface oxidation is crucial for the TAMR effect. The large induced magnetic anisotropy allowed us to achieve enhanced TAMR values compared to similar Pt/Co/AlOx structures
39
Possanner, Stefan. "Modeling and simulation of spin-polarized transport at the kinetic and diffusive level." Toulouse 3, 2012. http://thesesups.ups-tlse.fr/1735/.
Abstract:
L'objectif de cette thèse est de contribuer à la compréhension des phénomènes de mouvement de l'électron induits par le spin. Ces phénomènes aparaissent lorsqu'un électron se déplace à travers un environnement (partiellement) magnétique, de telle sorte que son moment magnétique (spin) peut interagir avec l'environnement. La nature quantique pure du spin nécessite des modèles de transport qui traitent des effets comme la cohérence quantique, l'intrication (corrélation) et la dissipation quantique. Sur le niveau méso- et macroscopique, il n'est pas encore clair dans quelles circonstances ces effets quantiques du spin peut transparaitre. Le but de ce travail est, d'une part, de dériver des nouveaux modèles de transport de spin à partir des principes de base et, d'autre part, de développer des algorithmes numériques qui permettent de trouver une solution de ces modèles. Cette thèse se compose de quatre parties. La première partie introductive contient un aperçu des concepts fondamentaux liés au transport polarisé en spin, tels que la magnéto-résistance géante (GMR), le couple de transfert de spin dans les multi-couches magnétiques et le caractère matriciel des équations de transport qui prennent en compte la cohérence de spin. L'accent est mis sur la modélisation du couple de transfert de spin, qui représente l'intersection de ces concepts. En particulier, nous considérons pour sa description le modèle diffusif de Zhang-Levy-Fert (ZLF) qui se compose de l'équation de Landau-Lifshitz et d'une équation de diffusion matricielle pour le spin. Un schéma de différences finies est développé pour résoudre numériquement ce système non-linéaire dans des structures multi-couches. Le modèle est testé par comparaison des résultats obtenus aux données expérimentales récentes. Les parties deux et trois forment le noyau thématique de cette thèse. Dans la deuxième partie nous proposons une équation de Boltzmann matricielle qui permet la description de la cohérence de spin sur le niveau cinétique. La nouveauté est un opérateur de collision dans lequel les taux de transition de la quantité de mouvement sont modélisés par une matrice 2x2 hermitienne; par conséquent, les libre parcours moyens des électrons spin-up et spin-down sont représentés par les valeurs propres de cette matrice de scattering. Après une dérivation formelle de l'équation de Vlasov matricielle à partir de l'équation de Wigner, l'équation cinétique qui suit est étudiée en ce qui concerne l'existence, l'unicité et la positivé d'une solution. En outre, le nouveau opérateur de collision est étudié rigoureusement et la limite de diffusion tc -> 0, correspondant à l'annulation de la moyenne de temps de scattering, est effectué. Les équations de drift-diffusion matricielle qui sont obtenues représentent une amélioration par rapport au modèle traité dans la première partie. Ce dernier est obtenu dans la limite ou la différence entre les deux valeurs propres de la matrice de scattering va disparaître. La troisième partie est consacrée à l'obtention de l'opérateur de collision matricielle introduit auparavant, à partir des principes quantiques. Pour cela, nous augmentons l'équation de von Neumann d'un système composite par un terme dissipatif qui fait tendre l'opérateur de densité totale vers l'approximation de Born. En vertu de la prémisse que la relaxation est le processus dominant, on obtient une hiérarchie d'équations non-Markoviennes. Celles-ci découlent d'une expansion de l'opérateur de densité en termes de tr, le temps de relaxation. Dans la limite de Born-Markov, tr -> 0, l'équation de Lindblad est récupérée. Elle a la même structure que l'opérateur de collision proposé dans la deuxième partie. Cependant, l'équation de Lindblad est encore une équation microscopique; donc la prochaine étape serait de procéder à la limite semi-classique du résultat obtenu. Dans la quatrième partie nous procédons à une étude numérique d'un modèle quantique-diffusif de spin qui décrit le transport dans un gaz d'électrons bidimensionnel avec un couplage spin-orbite de Rashba. Ce modèle suppose que les électrons sont dans un état d'équilibre quantique sous la forme d'un opérateur de Maxwell. Nous présentons deux discrétisations espace-temps du modèle couplé par l'équation de Poisson. Dans une première étape on applique une discrétisation en temps et on montre que les systèmes sont bien définis. Ceux-ci sont basés sur un formalisme fonctionnel pour traiter les relations non-locales entre les densités de spin. Nous utilisons ensuite des discrétisations espace-temps pour simuler la dynamique dans une géométrie typique d'un transistor. Les approximations différences finies sont du premier ordre en temps et du second ordre en espace. Les fonctionnelles discrètes sont minimisée à l'aide d'un algorithme du gradient conjugué et la méthode de Newton est appliquée afin de trouver les minima dans la direction désiréeThe aim of this thesis is to contribute to the understanding of spin-induced phenomena in electron motion. These phenomena arise when electrons move through a (partially) magnetic environment, in such a way that its magnetic moment (spin) may interact with the surroundings. The pure quantum nature of the spin requires transport models that deal with effects like quantum coherence, entanglement (correlation) and quantum dissipation. On the meso- and macroscopic level it is not yet clear under which circumstances these quantum effects may transpire. The purpose of this work is, on the one hand, to derive novel spin transport models from basic principles and, on the other hand, to develop numerical algorithms that allow for a solution of these new and other existing model equations. The thesis consists of four parts. The first part has introductory character; it comprises an overview of fundamental spin-related concepts in electronic transport such as the giant-magneto-resistance (GMR) effect, the spin-transfer torque in metallic magnetic multilayers and the matrix-character of transport equations that take spin-coherent electron states into account. Special emphasis is placed on the modeling of the spin-transfer torque which represents the intersection of these concepts. In particular, we consider the diffusive Zhang-Levy-Fert (ZLF) model, an exchange-torque model that consists of the Landau-Lifshitz equation and a heuristic matrix spin-diffusion equation. A finite difference scheme based on Strang operator splitting is developed that enables a numerical, self-consistent solution of this non-linear system within multilayer structures. Finally, the model is tested by comparison of numerical results to recent experimental data. Parts two and three are the thematic core of this thesis. In part two we propose a matrix-Boltzmann equation that allows for the description of spin-coherent electron transport on a kinetic level. The novelty here is a linear collision operator in which the transition rates from momentum k to momentum k' are modeled by a 2x2 Hermitian matrix; hence the mean-free paths of spin-up and spin-down electrons are represented by the eigenvalues of this scattering matrix. After a formal derivation of the matrix-Vlasov equation as the semi-classical limit of the one-electron Wigner equation, the ensuing kinetic equation is studied with regard to existence, uniqueness and positive semi-definiteness of a solution. Furthermore, the new collision operator is investigated rigorously and the diffusion limit tc -> 0 of the mean scattering time is performed. The obtained matrix drift-diffusion equations are an improvement over the heuristic spin-diffusive model treated in part one. The latter is obtained in the limit of identical eigenvalues of the scattering matrix. Part three is dedicated to a first step towards the derivation of the matrix collision operator, introduced in part two, from first principles. For this, we augment the von Neumann equation of a composite quantum system by a dissipative term that relaxes the total state operator towards the Born approximation. Under the premise that the relaxation is the dominant process we obtain a hierarchy of non-Markovian master equations. The latter arises from an expansion of the total state operator in powers of the relaxation time tr. In the Born-Markov limit tr -> 0 the Lindblad master equation is recovered. It has the same structure as the collision operator proposed in part two heuristically. However, the Lindblad equation is still a microscopic equation; thus the next step would be to carry out the semi-classical limit of the result obtained. In part four we perform a numerical study of a quantum-diffusive, two-component spin model of the transport in a two-dimensional electron gas with Rashba spin-orbit coupling. This model assumes the electrons to be in a quantum equilibrium state in the form of a Maxwellian operator. We present two space-time discretizations of the model which also comprise the Poisson equation. In a first step pure time discretization is applied in order to prove the well-posedness of the two schemes, both of which are based on a functional formalism to treat the non-local relations between spin densities via the chemical potentials. We then use fully space-time discrete schemes to simulate the dynamics in a typical transistor geometry. Finite difference approximations applied in these schemes are first order in time and second order in space. The discrete functionals introduced are minimized with the help of a conjugate gradient-based algorithm in which the Newton method is applied to find the desired line minima
40
Bouquin, Paul. "The switching paths of spin transfer torque magnetic random access memories." Thesis, université Paris-Saclay, 2020. http://www.theses.fr/2020UPAST009.
Abstract:
Dans les mémoires magnétiques à transfert de spin, l’aimantation d’une couche mince ferromagnétique est retournée sous l’effet d’un courant polarisé. Au cours de ce manuscrit est étudiée la façon dont ce retournement s’opère, appelée chemin de retournement. Après avoir posé les concepts théoriques de base nécessaires et effectué un état de l’art du chemin de retournement, je présente les résultats de nos simulations micromagnétiques. Nous avons étudié le chemin de retournement en fonction du diamètre du dispositif. Ces calculs numériques prédisent un retournement composé d’une phase cohérente suivie de la nucléation et de la propagation d’une paroi de domaine. Ce chemin de retournement est attendu pour les dispositifs the 20 à 100 nm à température ambiante, donc dans nos mesures à venir. La propagation de paroi de domaine observée dans les simulations présente de complexes oscillations de Walker qui ne sont pas expliquées par les modèles de l’état de l’art. Aussi je présente un modèle de dynamique de paroi plus complet, où la géométrie exacte du système est prise en compte. Dans cette géométrie l’élasticité de la paroi donne naissance à un nouveau champ que nous appelons champ d’étirement. Ce champ d’étirement joue un rôle capital dans la dynamique de paroi et va nous permettre de comprendre et de prédire les oscillations de Walker complexes. Nos mesures sont effectuées pour des dispositifs de mémoires magnétiques à transfert de spin dernière génération, basé sur une jonction tunnel magnétique à anisotropie perpendiculaire. Le diamètre de nos dispositifs varie entre 26 et 200 nm. Nous effectuons des mesures de magnétométrie, de résonance ferromagnétique et des mesures électriques résolues en temps de la commutation. Le chemin de retournement mesuré dans ces dernières présente les signatures d’une phase initiale cohérente suivie d’un déplacement de paroi de domaine, comme calculé dans nos simulations. Les fortes oscillations de Walker prédites par nos modèles sont observées pour des échantillons spécifiques où la couche libre présente peu de défauts, mais pas dans nos échantillons les plus standards. Ceci met en lumière l’intérêt de nos travaux analytiques dans la compréhension du retournement dans des dispositifs destinés aux applications industriellesIn spin transfer torque random access memories (STTMRAM), the magnetization of a thin ferromagnetic layer is reversed under the action of a polarized spin current. Along this manuscript we study the switching path that the STTMRAM undergo. First I present the basic theoretical concepts necessary for our forthcoming calculations. Then comes a state of the art of the switching path. The first results I present are micromagnetic simulations of the switching. We study the impact of the diameter of the device on the switching path. From these numerical calculations we predict for devices between 20 and 100 nm at room temperature a switching path composed of a coherent phased followed by a domain wall nucleation and motion. It is the switching path expected in our forthcoming measurements. The domain wall dynamics observed in the micromagnetic simulations present complex Walker oscillations that are not understood from the domain wall models of the state of the art. Therefore, I present a more complete model for the domain wall dynamics within a STTMRAM which takes into account the exact geometry of the system. In this geometry the elasticity terms act as a new effective field called the stretch field. The stretch field plays a key role in the wall dynamics and explains the complex Walker oscillations. The conditions under which these effects can be measured are also predicted by our new model. Our measurements are performed on state-of-the-art STTMRAM based on perpendicular magnetic tunnel junction. The diameter of the devices varies between 26 and 200 nm. We characterize our devices by magnetometry, ferromagnetic resonance and electrical time-resolved measurements of the switching path. The switching path in our time-resolved measurements presents the signatures of an initial coherent phase and of a domain wall motion. This is in agreement with the simulated switching path. The complex Walker oscillations predicted by our models are measured in specific devices with an ultrasoft free layer, but not in our most standard stack. This highlight the interest of our analytical models for understanding the behavior of application-oriented devices
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Endichi, Asmaa. "Thin films based on Gadolinium applied to the magnetic refrigeration." Electronic Thesis or Diss., Université de Lorraine, 2020. http://www.theses.fr/2020LORR0150.
Abstract:
La recherche en matière de caractérisation de matériaux à effet magnétocalorique géant à l’état massif et à une température proche de la température ambiante est d'un grand intérêt pour l’application de la réfrigération magnétique. Il est admis que la transition de premier ordre dans ces matériaux présente une hystérésis thermique considérable, les rendant ainsi difficiles à manipuler dans les applications pour les réfrigérateurs fonctionnant de manière cyclique. Beaucoup d'efforts ont été accomplis au cours de ces dernières années pour réduire cette hystérésis, mais les performances obtenues avec ces matériaux massifs ne répondent pas aux exigences d’une réfrigération magnétique efficace. Si les matériaux magnétocaloriques à l’état massif ont été largement étudiés ; l'échelle nanométrique correspondante reste cependant insuffisamment explorée. À cet effet, la nanostructuration, une approche largement bien connue et utilisée pour la mise au point et l’optimisation des relations structure-propriété des matériaux en questions, permet des nouvelles perspectives en matière d’amélioration de leurs caractéristiques magnétiques et magnétocaloriques en modifiant leur taille et leur forme. Pour ce faire, l’étude des propriétés magnétocaloriques des matériaux sous forme de couches minces est centrale pour pouvoir réduire au maximum l’hystérésis thermique, sachant que l’effet magnétocalorique dans les couches minces magnétiques est particulièrement intéressant pour la micro-réfrigération. Dans ce sens, peu d’études ont été menées pour montrer le potentiel des matériaux sous forme de couches minces pour la réfrigération magnétique. De même, les propriétés magnétiques (aimantation de saturation, la variation de l’entropie magnétique et du rapport de refroidissement relatif…) mesurées restent limitées. C’est dans ce cadre que le pèsent travail a été mené en étudiant le gadolinium métallique, en tant que matériau réfrigérant magnétique de référence pour la plupart des prototypes de régénérateur magnétique actif (AMR) sous forme de couche mince. Les propriétés magnétocaloriques (MCE) et électrocaloriques (ECE) des films de gadolinium fabriqués à cette fin (Si/Ta/Gd(100nm)/Pt(3nm)) sont alors mesurées dans le but d'obtenir plus d'informations sur la physique derrière ses intéressantes propriétés électroniques et magnétique en démontrant notamment l'effet magnéto-calorique du film mince Gd par la mesure du transport électrique de la résistance. Ainsi, au cours de cette thèse, les comportements électriques et surtout magnétiques de LaCr2Si2C et de multiferroïques TbMn2O5 sont décrits en utilisant la méthode ab-initio dans le but d'élargir notre compréhension des caractéristiques électroniques, magnétiques et par conséquent magnétocaloriques de ces composés à base de terre rare. L’élaboration et la caractérisation des couches minces pour la réfrigération magnétique, le traitement des données correspondantes ont été effectués conjointement au sein du laboratoire de recherche en science des matériaux avec l’équipe nanomagnétisme et électronique de spin à l’institut Jean Lamour à Nancy et au laboratoire de matière condensée et sciences interdisciplinaires à la faculté des sciences de RabatThe search for materials with a giant magnetocaloric effect in a massive state and at a temperature close to ambient temperature is of great interest and is mainly obtained by varying the composition of the materials. However, the first-order transition in these materials exhibits considerable thermal hysteresis, making them difficult to handle in applications for refrigerators operating cyclically. Much effort has been made in recent years to reduce this hysteresis, but the performance obtained with these massive materials does not meet the requirements of efficient magnetic refrigeration. Magnetocaloric materials have been largely unexplored on the nanoscale. However, nanostructuring is a well-known and used approach to disrupt the developed structure-property relationships, hence the interest in manufacturing new nanoscale materials. This will improve their magnetic and magnetocaloric characteristics by varying the size and shape. On the other hand, the magnetocaloric effect in magnetic thin layers is particularly interesting for micro-refrigeration. It is therefore important to study the magnetocaloric properties of materials in the form of thin layers in order to eliminate thermal hysteresis. In this sense, few studies have been done to show the potential of thin film materials for magnetic refrigeration and magnetic properties (saturation magnetization, variation of magnetic entropy and relative cooling ratio ...) measured so far limited remains. In this thesis project, we studied metallic gadolinium, which is the preferred choice as a magnetic refrigerant for most prototypes of active magnetic regenerator (AMR) in the form of a thin layer. The magnetocaloric (MCE) and electrocaloric (ECE) properties of the manufactured gadolinium films (Si / Ta / Gd (100 nm) / Pt (3nm)) are measured, in order to obtain more information on the physics behind the interesting electronic and magnetic properties of this material we demonstrate the magneto-caloric effect of the thin film Gd by measuring the electrical transport of the resistance. Thus, during this thesis, the electrical and especially magnetic behaviors of LaCr2Si2C and multiferroics TbMn2O5 are described using the ab-initio method, in order to broaden our understanding of the electronic, magnetic and therefore magnetocaloric characteristics of these compounds based on rare earth. The development of thin layers for magnetic refrigeration was carried out in the materials science research laboratory with the nanomagnetism and spin electronics team at the Jean Lamour Institute in Nancy and the theoretical calculations are made in the material laboratory condensed and interdisciplinary sciences at the Faculty of Sciences of Rabat
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Peters, Brian. "Tuning the structural, magnetic and transport properties of full Heusler Co2FeAlxSi1-x compounds." The Ohio State University, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=osu1408984221.
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Verlhac, Benjamin. "Atomic-scale spin-sensing with a single molecule at the apex of a scanning tunneling microscope." Thesis, Strasbourg, 2019. http://www.theses.fr/2019STRAE007/document.
Abstract:
L’étude présentée dans ce manuscrit s’inscrit dans le domaine du magnétisme de surface, qui a connu de grands développements ces dernières années grâce au microscope à effet tunnel (STM). Elle a pour but de montrer qu’une molécule simple, le nickelocène [Ni(C5H5)2], peut être attachée au sommet d’une pointe STM afin de produire une pointe-sonde magnétique, qui, dans le cadre de l’imagerie magnétique, présente des avantages indéniables comparés à des pointes conventionnelles. À la différence d’autres systèmes moléculaires étudiés avec le STM, nous montrons que les propriétés magnétiques du nickelocène en phase gazeuse sont préservées en présence d’un métal, même lorsque la molécule est attachée au sommet d’une pointe STM. Nous présentons trois résultats marquants avec cette pointe-sonde moléculaire: 1) Nous montrons que l’on peut contrôler le spin du nickelocène, activant à souhait un effet Kondo ; 2) Nous produisons à l’aide du courant tunnel des excitations entre les états de spin du nickelocène, que nous pouvons aisément identifier au travers de la conductance moléculaire. Ces états sont sensibles à toute perturbation magnétique extérieure au nickelocène; 3) Au travers de ces excitations, nous sondons alors le magnétisme de surface. Nous montrons qu’en couplant magnétiquement la pointe-sonde moléculaire avec des atomes, soit isolés soit dans une surface ferromagnétique, nous pouvons mesurer leur polarisation de spin, ainsi que le couplage d’échange nickelocène-atome. Ce dernier permet d’obtenir un contraste magnétique en imagerie STM à l’échelle atomiqueThe study presented in this manuscript is part of the field of surface magnetism, which has undergone major developments in recent years thanks to the scanning tunneling microscope (STM). It aims to show that a single molecule, nickelocene [Ni(C5H5)2], can be attached to the tip of a STM to produce a magnetic probe-tip, which, in the context of magnetic imaging, has undeniable advantages compared to conventional tips. Unlike other molecular systems studied with STM, we show that the magnetic properties of nickelocene in the gas phase are preserved in the presence of a metal, even when the molecule is attached to the tip of a STM. We present three remarkable results with this molecular probe-tip: 1) We show that we can control the spin of nickelocene, activating at will a Kondo effect; 2) We monitor the spin states of nickelocene by producing electrically-driven excitations, which we can easily identify through the molecular conductance. These states are sensitive to the magnetic environment surrounding nickelocene; 3) We use these states to probe surface magnetism. We show that by magnetically coupling the molecular probe tip with single atoms, either isolated or in a ferromagnetic surface, we can measure their spin polarization, as well as the nickelocene-atom exchange coupling. By monitoring this coupling it is possible to obtain a magnetic contrast in the STM images with atomic-scale resolution
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VASCONCELOS, Thiago Conrado de. "Flutuações universais da condutância de Spin-Hall em uma cavidade caótica de Dirac." Universidade Federal Rural de Pernambuco, 2016. http://www.tede2.ufrpe.br:8080/tede2/handle/tede2/6214.
Abstract:
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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES
Throughout the latest years, the interest on Spintronics has increased. The principal purposes of the eld are to detect, manipulate, create and polarize spin currents. Within this topic, it is possible to emphasize the Spin Hall E ect(SEH) and the Inverse Spin Hall E ect(ISEH). In this dissertation, we analytically investigate the universal fluctuation of the conductance of the spin in a chaotic quantum point with chiral symmetry at low temperatures. We used random matrices theory and the expansion of the diagrammatic method for that purpose. We showed that when the chirality is broken, the universal fluctuation of the conductance dispersion is in the order of rms hGf sHi 0:18e=4 and that when there is the preservation of the chiral symmetry, the universal fluctuation of the conductance dispersion occurs in the order of rms [GqsH] 0:283e=4 which coincides with the literature. We also worked on ISEH, through the analytical analysis with the semi-classic expansion of the conductance and showed that in the semi-classic limit the relation rms [GqsH] = p2 rms hGf sHi is valid.
Ao longo dos últimos anos tem aumentado o interesse pelo estudo da spintrônica. O objetivo principal deste campo é detectar, manipular, criar e polarizar correntes de spin. Dentro deste tópico, se destaca o Efeito Hall (SHE) de Spin e Efeito Hall de Spin Inverso (ISHE). Neste trabalho investigamos analiticamente a flutuação universal da condutância de spin num ponto quântico caótico com simetria quiral a baixas temperaturas. Para isso, utilizamos a teoria de matrizes aleatória e a expansão do método diagramático. Mostramos que, quando a simetria de quiralidade é quebrada, a flutuação universal da condutância tem uma dispersão na ordem de na ordem de rms[GfsH] p2 0:18 e/4 e que, quando a simetria de quiralidade é preservada, a flutuação universal da condutância ocorre na ordem de rms[GqsH] 0.283 e/4 , o que está de acordo com a literatura. Em nosso trabalho também investigamos o (ISHE), por meio de uma análise analítica utilizamos a expansão semi-clássica da condutância e mostramos que no limite semi-clássico vale a relação rms[GqIsH] = p2 rms[GfIsH].
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Guerra, Gabriel Andrés Fonseca. "Study of the longitudinal spin Seebeck effect in hybrid structures with yttrium iron garnet and various metallic materials." Universidade Federal de Pernambuco, 2014. https://repositorio.ufpe.br/handle/123456789/12719.
Abstract:
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Conselho Nacional de Desenvolvimento Científi co e Tecnol ógico; Coordenação de Aperfeiçoamento de Pessoal de Ní vel Superior; Financiadora de Estudos e Projetos; Fundação de Amparo a Ciência e Tecnologia do Estado de Pernambuco.
In this master thesis we study experimentally the longitudinal spin Seebeck effect (LSSE) in bilayers made of a ferromagnetic insulator (FMI) and a metallic layer (M). We also present a theoretical model based on the spin current density ⃗ Js carried by a non-equilibrium magnon distribution, generated by a thermal gradient ∇T across the thickness of the FMI. When ⃗ Js reach the FMI/M interface it is pumped towards the M layer due to conservation of the angular momentum, so, the M layer is essential for the LSSE existence. Here the FMI consists of a Yttrium Iron Garnet (YIG) lm, grown over a Gadolinium Gallium Garnet (GGG) substrate. Different metallic materials were used as the M layer i.e. Pt and Ta that have normal behavior and Py that is a ferromagnetic metal (FMM). The experimental procedure consists of systematic measurements of the electric voltage VISHE, produced by ⃗ Js through the Inverse Spin Hall Effect (ISHE) in the normal metal or (FMM) layer. In YIG/Pt measurements were done in the temperature range from 20 to 300 K. The experimental data are tted to the proposed model for the LSSE and good agreement is obtained. The results shows that the Py and Ta can be used to detect the LSSE with the ISHE. The results of this master thesis have strong interest in the area of spin caloritronics helping to the development of the eld and to raise possibilities of new spintronic devices. ----- Nesta diserta ção e estudado experimentalmente o Efeito Seebeck de Spin Longi- tudinal (LSSE), em bicamadas formadas por um isolante ferromagn etico (FMI) e um lme metalico (M). Tamb em foi desenvolvido um modelo te orico baseado na den- sidade de corrente de spin ⃗ Js que existe quando uma distribui c~ao de m agnons fora do equil brio e gerada por um gradiente t ermico ∇T aplicado na sec ção transversal do FMI. Quando ⃗ Js chega na interface FMI/M e bombeada para a camada M satis- fazendo a conserva ção do momentum angular, assim que a camada NM e essencial para ter um LSSE. Como camada FMI foi utilizada a granada de trio e ferro (YIG) crescida num substrato de (GGG). Diferentes materiais metalicos foram utilizados como camada M, sendo Pt e Ta paramagn eticos e o Py ferromagnetico. O proced- imento experimental consiste na medi c~ao sistem atica da voltagem el etrica VISHE, que e produzida por ⃗ Js por meio do efeito Hall de spin inverso (ISHE) que ocorre na camada M. As medidas em YIG/Pt foram feitas numa faixa ampla de temperatura de 20 a 300 K. Os dados experimentais são fi tados com a teoria proposta para o LSSE encontrando-se boa concordância. Nossos resultados mostram que o Py e o Ta s~ao bons candidatos para detec ção do LSSE. Esta disserta ção e de grande interesse na area da caloritrônica de spin, ajudando no desenvolvimento deste campo e na concep ção de novos dispositivos tecnol ogicos baseados na spintrônica.
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Huang, Tianxun. "A study about the behavior and mechanism of all-optical switching." Electronic Thesis or Diss., Université de Lorraine, 2023. http://www.theses.fr/2023LORR0054.
Abstract:
Afin de répondre aux besoins des futures technologies de stockage magnétique à haute densité, à faible consommation d’énergie et à haut débit, le développement d’une nouvelle méthode de manipulation de l’aimantation avec des temps d’inversion d’aimantation plus courts et une consommation d’énergie plus faible est l’une des tâches urgentes dans le domaine de la spintronique. La technologie laser à impulsions ultracourtes offre une nouvelle façon de manipuler le spin sur une échelle de temps femtoseconde, suscitant un grand intérêt de recherche dans les universités et l’industrie. Deux méthodes de contrôle de l’aimantation par laser, l’interrupteur à corrélation d’hélicoïdalité totale (AO-HDS) et l’interrupteur à corrélation d’hélicoïdalité totale (AO-HIS), ont récemment été découvertes et leurs mécanismes, comportements et applications ont fait l’objet de nombreuses discussions. Cependant, l’origine de ces deux phénomènes reste très controversée et ce sera la tâche principale de cet article. Le mécanisme de l’AO-HDS a été étudié à l’aide d’un empilement multicouche Co/Pt présentant le phénomène AO-HDS. La membrane a été réalisée sur un barreau de Hall en un carré magnétique de 10x10 um^2 et son comportement de commutation a été observé à différentes échelles de temps. La commutation de cette cellule magnétique peut être démontrée par dix impulsions laser successives polarisées circulairement. La dynamique de spin de AO-HDS peut être comprise par thermonucléation de domaine magnétique induite par Gradient thermique et propagation de paroi de domaine. Au cours des dernières années, l’AO-HIS n’a jamais été observé dans d’autres alliages de métaux de transition de terres rares, à l’exception du fait que la terre rare est Gd. Pour étudier les caractéristiques de GD, on a cultivé et étudié une série d’alliages GdRCo (R pour Tb, Dy ou Ho), l’AO-HIS peut être observé lorsque la composition de R est aussi faible que 1,5% au voisinage du point de compensation du ferromagnétique. Les diagrammes d’état décrivant les paramètres clés qui dépendent de la concentration de l’élément et de la dynamique de spin dans divers échantillons ont été étudiés, ce qui donne quelques suggestions sur l’origine de l’AO-HIS et ses applications futures en ingénierieTo meet the future needs of high density, low power consumption, and fast rate of magnetic storage technology, it is one of the urgent tasks in the field of spintronics to develop a new method of magnetization manipulation with shorter magnetization reversal time and lower energy consumption. Ultrashort pulsed laser technology offers a new way to manipulate spins in femtosecond timescale, sparking great research interest in both academia and industry. Two methods of controlling magnetization by laser, all-optical helicity-dependent switching (AO-HDS) and all-optical helicity-independent switching (AO-HIS), are discovered recently and raise numerous discussion on their mechanisms, behaviors and applications. However, the origin of two phenomena is still largely debated, which will be the main task of this thesis. A Co/Pt multilayered stack exhibiting AO-HDS phenomenon is employed to study the mechanism of AO-HDS. The film is fabricated to a 10x10 um^2 magnetic square on a Hall bar and its switching behavior is observed optically and electrically at different timescale. The switching of this magnetic unit can be demonstrated with ten consecutive circularly polarized laser pulses. The spin dynamics of AO-HDS can be understood in terms of the magnetic domain thermal nucleation and domain wall propagation driven bythermal gradient. For the past years, AO-HIS has never been observed in other rare-earth transition-metal alloys except when the rare-earth is Gd. To study the speciality of Gd, a complete series of GdRCo (R represents Tb, Dy or Ho) alloys is grown and investigated, it is demonstrated that AO-HIS can be observed when the composition of R is as low as 1.5% near the compensation point of ferrimagnet. State diagrams describing the key parameters depending on the element concentrations and spin dynamics in various samples are studied, providing some suggestion on the origin of AO-HIS and its engineering application in the future
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Pincelli, T. "PROBING ELECTRON CORRELATION DYNAMICS: A MULTI-TECHNIQUE STUDY APPLIED TO THE HALF-METALLIC OXIDE LA1-XSRXMNO3." Doctoral thesis, Università degli Studi di Milano, 2017. http://hdl.handle.net/2434/543731.
Abstract:
This thesis completes my work as doctoral student of the Scuola di Dottorato in Fisica, Astrofisica e Fisica Applicata at the Università degli Studi di Milano that has been carried out, starting in November 2014, mostly at the Laboratorio TASC of IOM-CNR in the premises of the Elettra - Sincrotrone Trieste and FERMI@Elettra infrastructures, in the framework of the NFFA and APE-beamline facilites, as well as by accessing international large scale infrastructures and laboratories.
The activity has addressed the development of experimental methodologies and novel instrumentation oriented to the study of the dynamical properties of highly correlated materials after high energy excitation. The science programme has been carried out by exploiting ultrafast femtosecond probes from the optical regime (Ti-Sa lasers, fibre laser oscillators) to the extreme UV-soft X rays at FERMI, to the picosecond hard X-rays from the SPring-8 and Diamond synchrotron radiation source. The sample synthesis of correlated oxides and its characterization has been performed within the NFFA facility and APE-group collaboration in Trieste as well as the design and construction of the all new laser High Harmonic Generation beam line NFFA-SPRINT and its end station for time resolved vectorial electron spin polarimetry.
This report concentrates on the main scientific concern of my work that has been the relaxation of external perturbations in a correlated electron material both in the time and space domain.
I have employed Photoelectron Spectroscopy (PES) mostly in the Hard X-ray regime (HAXPES), pushing the boundaries of its application to achieve a coherent perspective. The material I have mainly focused on is La0.67Sr0.33MnO3 (LSMO), of high interest for spintronics. This system is prototypical, yielding the highest simplicity in the class of transition metal oxides.
In the spatial investigation, I have controlled with high precision the PES probing depth and I have observed the evolution of one spectral feature. I have identified it as probe of electronic hybridization and long-range ordering. I have studied LSMO films of 40 nm in three substrate-induced strain states (1% tensile in-plane, relaxed, 1% compressive in-plane) and a 18 nm film of (Ga,Mn)As (GMA), a well-studied diluted magnetic semiconductor. I have found that the electronic properties to be modified at significant distances from the surface, 4 nm for LSMO and 1.2 nm for GMA, while strain had no detectable effects.
In the temporal study, I have employed HAXPES in pump-probe mode (TR-HAXPES) to observe the evolution of the electronic structure after intense optical excitation. A detailed dynamical characterization with optical techniques has allowed me to identify the characteristic time of the collapse of long-range magnetic order to be significantly longer than the one of elemental transition metals. I have ascribed this effect to the half-metallic character of LSMO. With TR-HAXPES I have observed that the whole electronic band-structure evolution is bottlenecked by the slow response of the magnetization, proceeding on hundreds of picoseconds timescales.
Finally, I have described the techniques and the instrumentation that can be used to push these investigations to shorter spatial and temporal scales. This has been realized in the form of the NFFA-SPRINT laboratory, a facility open to users, which I participated in designing and developing.
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Yan, Bin. "Towards the development of organic magnetoconducting materials: structural, magnetic and conducting properties of diaryl-1,2,4-benzotriazinyl radicals and polyradicals." Thesis, 2009. http://hdl.handle.net/1828/3331.
Abstract:
Considerable interest exists in the development of purely organic magneto-conducting materials for spintronics. Towards this end, an investigation of the effect of electronic structure, magnetic exchange, and electrical conductivity on the properties of open-shell delocalized organic conjugated polymers was carried out. The design, synthesis and properties of radical-based multifunctional magnetic materials have been developed in which (i) a series of functionalized benzotriazinyl radicals have been synthesized and characterized, (ii) a series of benzotriazinyl alternating copolymers with vinyl and heteroaromatic bridges have been prepared and characterized, and (iii) the magnetic properties of the resulting polymers have been investigated as a function of doping state in order to probe relationships between magnetic and electrical properties of these
systems. New synthetic methodology -has been developed which allows the syntheses of 1,3-diphenyl-l,2,4-benzotriazinyl (DPBT) radicals and derivatives under mild reaction conditions. The condensation of arylamines with chlorohydrazones followed by reductive or oxidative ring closure allows generation of the desired radicals. The series of DPBT radicals prepared were found to be thermally and chemically stable in both the solid and
solution states. The radicals were spectroscopically characterized by mass spectrometry, EPR, electrochemistry, UV-Vis, electronic absorption spectroscopy, X-ray crystallography, and magnetometry. Single crystal X-ray diffraction studies indicate that these radicals exhibit [pi]-stacking interactions in the solid state which lead to unusually strong anti ferro- or ferromagnetic exchange interactions. Cyclic voltammetry
experiments reveal that these radicals exhibit unusually low oxidation potentials, leading to an important class of spin-polarized donors for multifunctional magnetic materials. A new class of DPBT containing alternating copolymers with varying connectivity, bridge structure, and redox activity has been prepared by metal-catalyzed cross coupling. All of the polymers were spectroscopically characterized by EPR, cyclic voltammetry.
electronic absorption spectroscopy, and magnetometry. The relationship between magnetic exchange and electrical conductivity was studied as a function of redox state (chemical doping). Preliminary results suggest that there is a correlation between magnetic property and electrical conductivity. As a result, a purely organic ferromagnetic semiconductor critical for investigations in organic spintronic materials was designed and
synthesized. Lastly, a new triazinyl donor-acceptor dyad was designed and synthesized. Spectroscopic investigation of electronic and magnetic properties reveal that this radical exhibits an intramolecular - charge-transfer band and strong intermolecular antiferromagnetic interactions.Graduate
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Jalili, Helia. "Materials physics of half-metallic magnetic oxide films by Pulsed Laser Deposition: Controlling the crystal structure and near-surface properties of Sr2FeMoO6 and CrO2 films." Thesis, 2009. http://hdl.handle.net/10012/4243.
Abstract:
The idea of half-metallic ferromagnets was first introduced by de Groot et al. in 1983 based on their calculations. The density of state at the Fermi level for half-metallic ferromagnet is completely polarized, meaning that only one of the spin up or spin down channel exists and has metallic behaviour while the other spin channel behaves as a semiconductor or insulator. This unusual electronic structure can be seen in different materials including Sr2FeMoO6, CrO2 and Mn-based Heusler alloys. The high spin polarization degree of the half-metallic ferromagnets makes them a perfect candidate to be used as a spin-injector/detector in spin-based electronics device (spintronics). However, the degree of spin polarization of these materials, particularly in the multilayered structure spintronic devices, strongly depends on the surface/interface quality and the presence of defects, which was the subject of the present study. Pulsed laser deposition (PLD) has been used to grow two examples of the half-metallic ferromagnets, namely, Sr2FeMoO6 and CrO2. The effects of the growth conditions (deposition temperature, gas pressure, laser power, target-to-substrate distance, post-annealing) and of the substrate lattice mismatch and thickness evolution have been studied. By optimizing the growth conditions, nanocrystalline Sr2FeMoO6 films have been grown on a Si(100) substrate for the first time. This single-phase Sr2FeMoO6 film was obtained at a temperature as low as 600°C, and it exhibits a high saturation magnetic moment of 3.4 μB per formula unit at 77 K. By using glancing-incidence X-ray diffraction with different incident beam angles, the crystal structure of the film was sampled as a function of depth. Despite the lack of good lattice matching with the Si substrate, a preferential orientation of the nanocrystals in the film was observed for the as-grown Sr2FeMoO6 films thicker than 60 nm. Furthermore, effects of the deposition temperature on the epitaxial growth of the Sr2FeMoO6 films on MgO(001) have been studied by means of high-resolution X-ray diffraction. The film grown at 800°C was post-annealed in oxygen, producing epitaxial films of SrMoO4 on top of the Sr2FeMoO6 film. The corresponding magnetization data showed that the post-annealing treatment lowered the saturation magnetic moment from 3.4 µB per formula unit (or /f.u.) for the as-grown Sr2FeMoO6 film to 1.4 µB/f.u. after annealing. X-ray photoemission measurements as a function of sputtering time further revealed the presence of SrMoO4 on both the as-grown and annealed films, and their corresponding depth profiles indicated a thicker SrMoO4 overlayer on the annealed film. The intensity ratios of the 3d features of Mo4+, Mo5+, and Mo6+ for Sr2FeMoO6 remained unchanged with sputtering depth (after 160 s of sputtering), supporting the conclusion that the observed secondary phase (SrMoO4) was formed predominantly on the surface and not in the sub-grain boundaries of the as-grown Sr2FeMoO6 film. The epitaxial growth evolution of Sr2FeMoO6 films of different thickness on substrates of MgO(001), SrTiO3(100) and LaAlO3(100) have also been studied. For each thickness, surface morphology, grain size, film epitaxy, and crystal quality were determined by atomic force microscopy and X-ray diffraction (-2θ scan and reciprocal space mapping). For thicker films (~120 nm), high resolution X-ray diffraction studies revealed that SrMoO4 and other parasitic phases tend to forms on SrTiO3 and LaAlO3 substrates, but not on those grown on MgO substrates.
As a second part of the project, single-phase CrO2 nanostructured thin films have been grown for the first time directly on MgO(001) by PLD from a metallic Cr target in an O2 environment. X-ray diffraction shows that these films are strained and consist of CrO2 crystallites with two possible epitaxial relationships to the substrate: either CrO2(110) or CrO2(200) is parallel to MgO(001). X-ray photoemission further confirms that the films are primarily CrO2 covered with a thin CrO3 overlayer, and indicates its complete synthesis without any residual metallic Cr.
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Alshehri, Nisreen. "The Umklapp Scattering and Spin Mixing Conductance in Collinear Antiferromagnets." Thesis, 2020. http://hdl.handle.net/10754/664989.
Abstract:
Antiferromagnetic spintronics is a new promising field in applied magnetism. Antiferromagnetic materials display a staggered arrangement of magnetic moments so that they exhibit no overall magnetization while possessing a local magnetic order. Unlike ferromagnets that possess a homogeneous magnetic order, the spin-dependent phenomena occur locally upon the interaction between the itinerant electron and the localized magnetic moments. In fact, unique spin transport properties such as anisotropic magnetoresistance, anomalous Hall effect, magnetooptical Kerr effect, spin transfer torque and spin pumping have been predicted and observed, proving that antiferromagnetic materials stand out as promising candidates for spin information control and manipulation, and could potentially replace ferromagnets as the active part of spintronic devices. As a matter of fact, owing to their vanishing net magnetization, they produce no parasite stray fields, hence, they are mostly insensitive to external magnetic fields perturbations and displaying ultrafast magnetic dynamics.
When a spin current is sent into an antiferromagnet, it experiences spin-dependent scattering, a mechanism that controls the spin transfer torque as well as the spin transmission across the antiferromagnet. The fully compensated antiferromagnetic interfaces are full of intriguing properties. For example, itinerant electron impinging on such an interface experiences a spin-flip associated with the sub-lattices interchange. This process, associated with Umklapp scattering, gives rise to a non-vanishing spin mixing conductance that governs spin transfer torque, spin pumping, and spin transmission.
The thesis explores the mechanism of Umklapp scattering at a staggered antiferromagnetic interface and its associated spin mixing conductance. In this project we consider two systems of bilayer and trilayer antiferromagnetic (L-type, G-type) heterostructures. We first study the scattering coeffcients at the interface implemented by adopting the tight-binding model and proper boundary conditions. Then, in the trilayer case, we study the spin mixing conductance and the dephasing length associated with the transition from ferromagnetic order to antiferromagnetic order.