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ispartof: IEEE TRANSACTIONS ON DEVICE AND MATERIALS RELIABILITY vol:21 issue:2 pages:258-266 status: published

This thesis was aimed at studying the generation, propagation and manipulation of spin-waves in nanostructures of thin films of Y₃Fe₅O₁₂ (YIG) and one of its doped variants (BiYIG), in order to enable the design of spin-wave based signal processing schemes and devices operating at radiofrequencies. Two goals have been pursued in parallel: 1.Provide a better understanding of spin-wave radiofrequency devices for analog applications such as filters or delay lines. 2.Explore the physics of spin-orbit-torque (SOT) applied to propagating spin-waves and understand the non-linear phenomena that have so far hindered any practical magnonic amplification scheme. Prior to the present work, spin-orbit effects in YIG|Pt microstructures have been demonstrated to be a powerful new paradigm to control the propagation and emission of coherent spin-waves. An intense research effort has been put in the lab to grow epitaxial nanometer thick YIG films by pulsed laser deposition (PLD). The film thickness was reduced down to a few nanometers while preserving its excellent magnetic properties. More recently, ultra-thin Bi-doped YIG films have been developed, introducing a large and controllable perpendicular magnetic anisotropy term (PMA) while preserving the excellent damping properties of YIG. Additionally, the Bi atoms induce a very large Faraday rotation angle which increases the sensitivity of direct imaging methods like Brillouin Light Scattering microscopy (BLS) by nearly two orders of magnitude. This important step forward on the material aspect opened up new possibilities for the realization of magnonic devices that can have a large impact on the ICT industry. Indeed, microfabrication of YIG is now possible thanks to the advent of high-quality nanometer thick YIG films. In the meantime, BiYIG, with its controllable PMA and large magneto-optical coupling, quickly imposed itself as a material whose properties are particularly well suited to the study of SOT effects. During this thesis, analog devices have been fabricated using standard nano-lithography techniques. They are of two sorts: 1.A delay lines based on a 300 nm thick YIG film have been characterized. The impedance matching problem was analytically solved, and we demonstrated a clear path toward realistic optimized targets in terms of losses and functionalities. All the analysis and modeling tools have been implemented in open source python programs. 2.Additionally, we have studied the efficient implementation of radiofrequency filtering using periodic nanostructurations thanks to the advent of 20 nm thick YIG films with low damping. A width-modulated waveguide was studied, and the experimental results were accurately reproduced using full-scale micromagnetic simulations, paving the way toward the reliable optimization of magnonic crystals’ properties. Using a thickness-modulated waveguide, we strongly suppressed the extra losses induced by defects, demonstrating a very efficient implementation of a 1D magnonic crystal. Finally, active nano-magnonic devices have been fabricated. By engineering the perpendicular magnetic anisotropy of Bi-doped YIG, we showed that we can both achieve a very high temperature stability (50 times larger than YIG) on a wide range of temperatures (260K to 400K) and a suppression of the leading detrimental non-linear coupling term in supercritical SOT systems. The transient regime of the spin-current induced auto-oscillation was fully characterized, and a spin-wave amplification scheme based on the spin-orbit-torque phenomenon was demonstrated. We also revealed a frequency or wavevector dependence of the amplification efficiency in both the subcritical and the supercritical regime.; Cette thèse porte sur l’étude de la génération, la propagation et la détection d’ondes de spins dans des nanostructures élaborées à partir de couches ultra-minces (quelques nanomètres d’épaisseur) de Y₃Fe₅O₁₂ (YIG) et un de ses variant dopé (BiYIG). Le but est de permettre le développement de dispositifs pour le traitement des signaux hyperfréquences, basés sur les ondes de spins. Deux objectifs ont été poursuivis en parallèle : 1.Apporter une meilleure compréhension des dispositifs magnoniques hyperfréquence pour des application analogiques tel que les filtres ou les lignes à retard. 2.Explorer la physique du couple orbital de spin (SOT) appliqué aux ondes de spins progressives et comprendre les effets non-linéaires qui ont jusqu’à présent empêchés la réalisation d’un dispositif efficace d’amplification des ondes de spins. Ce travail repose sur les études démontrant que les effets spin-orbite dans les microstructures YIG|Pt constitue un puissant paradigme pour le control de la propagation et de l'émission d'ondes de spins. Il s’appuie également sur l’expertise développée au laboratoire dans la croissance de couches ultra fine épitaxiées de YIG, élaborées par ablation laser pulsée (PLD). Plus récemment, des couches ultra-minces de YIG dopé au Bismuth (BiYIG) ont été développées. L’introduction du Bi induit une forte anisotropie magnétique perpendiculaire (PMA), tout en préservant les excellentes propriétés dynamiques du YIG. Le Bi augmente également fortement la rotation Faraday et, par conséquent, améliore d’autant la sensibilité des méthodes d'imagerie directe, telle que la microscopie à diffusion Brillouin (BLS). Ces avancées importantes sur l'aspect matériaux ont ouvert de nouvelles possibilités pour la réalisation de dispositifs magnoniques. En effet, la micro-fabrication du YIG est désormais possible. Par ailleurs, le BiYIG, avec son anisotropie accordable et son fort couplage magnéto-optique, s'est rapidement imposé durant ma thèse comme un matériau parfaitement adapté à l’exploration des effets SOT. Au cours de cette thèse, des dispositifs analogiques ont été fabriqués à l'aide de techniques standard de nano-lithographie. Ils sont de deux sortes : 1.Des lignes à retard basées sur un film de YIG de 300 nm d'épaisseur ont été caractérisées. Le problème de l’adaptation d'impédance a été résolu de façon analytique, nous permettant d’établir des objectifs ciblés, basés sur nos paramètres expérimentaux, dans le but d’optimiser les caractéristiques techniques des lignes à retard magnoniques. Les outils d'analyse et de modélisation développés au cours de cette thèse ont été implémentés dans des programmes python et mis en libre accès. 2.Par ailleurs, nous avons démontré l’efficacité de filtres hyperfréquence basés sur la nano-structuration périodique de guides d’ondes de YIG fin (20 nm). Un guide d'ondes modulé en largeur a été étudié et les résultats expérimentaux reproduits avec précision à l'aide de simulations micromagnétiques à l’échelle, ouvrant la voie à une optimisation fiable des propriétés des structures périodiques de YIG (cristaux magnoniques). En utilisant un guide d'ondes modulé en épaisseur, nous avons fortement supprimé les pertes engendrées par les défauts de fabrication, démontrant une réalisation efficace d'un cristal magnonique 1D. Enfin, des dispositifs nano-magnoniques actifs ont été fabriqués. En modulant l'anisotropie du BiYIG, nous avons montré que nous pouvions obtenir une stabilité thermique très élevée sur une large gamme de températures et une suppression du terme de couplage non linéaire préjudiciable dans les systèmes SOT supercritiques. Le régime transitoire de l'auto-oscillation, induite par le courant de spin, a été entièrement caractérisé, et un dispositif d'amplification des ondes de spin, basé sur le phénomène de SOT, a été démontré. Une dépendance en fréquence ou en vecteur d’ondes de l’efficacité de l’amplification a été observée dans les régimes sous-critiques et sur-critiques.

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.

A True Random Number Generator is an essential component in data encryption, hardware security, physical unclonable functions, and statistical analyses. Conventional CMOS devices usually exploit the thermal noise or jitter to generate randomness, which suffers from high energy consumption, slow bit generating rate, large area, and over-complicated circuit. In this mini review, we introduce the novel physical randomness generating mechanism based on the stochastic switching behavior of magnetic tunnel junctions. As compared to CMOS technologies, the random number generator based on spintronic devices can have many inherent advantages, such as simpler structure, compact area, higher throughput, and better energy-efficiency. Here, we review and compare various existing schemes at the device and circuit levels to achieve high performance magnetic tunnel junctions based on a True Random Number Generator. Future research trends and challenges are also discussed to stimulate more works in this area.

This study reports on the synthesis of highly oriented chromium triiodide (CrI3) magnetic inclusions inside nano/microfibres with a polyethylene oxide matrix, prepared by the electrospinning technique. The structural, microstructural and spectroscopic analysis shows uniformly dispersed CrI3 nanosized inclusions inside the fibres, presenting a C2/m monoclinic structure at room temperature, where their c-axis is perpendicular to the fibre mat plane and the ab layers are in-plane. Analysis of the magnetic properties show that the samples have a ferromagnetic-paramagnetic phase transition at ∼55-56 K, lower than that of bulk CrI3. Noticeably, a field-driven metamagnetic transition is observed below ∼45 K, from M versus H curves, when the applied magnetic field is perpendicular to the fibre mat plane, while it is strongly reduced when the field is in-plane. This anisotropic behaviour is attributed to the field-induced changes from antiferromagnetic to ferromagnetic interlayer magnetic moment alignment along the CrI3 c-axis stacked layers. These CrI3 electrospun fibres then show an efficient cost-effective route to synthesize magnetic composite fibres with highly oriented van der Walls inclusions, for spintronic applications, taking advantage of their anisotropic 2D layered materials properties., We are grateful to the Fundacao Para a Ciencia e a Tecnologia (FCT) for the financial support through the Physics Centers of the Universities of Minho and Porto (Ref. UIDB/04650/2020) and projects UTAPEXPL/NTec/0046/2017, NORTE-01-0145-FEDER-028538 and PTDC/FIS-MAC/29454/2017. J H Belo thanks FCT for the Grant SFRH/BD/88440/2012, the project PTDC/FIS-MAC/31302/2017 and his contract DL57/2016 reference SFRH-BPD-87430/2012. J P Araujo and J H Belo thank the funding from the project, with reference POCI-01-0145-FEDER-032527. V B Isfahani acknowledges a Post-Doc grant from the project NORTE-01-0145-FEDER-028538. L Boddapati acknowledges the Nano TRAIN for Growth II program by the European Commission through the Horizon 2020 Marie Sklodowska-Curie COFUND Programme and support provided by the International Iberian Nanotechnology Laboratory. We are gratefull to Professor Michael Belsley, of the Physics Department at Minho University, for the fruitfull discussions on the manuscript.

Achieving multi-level devices is crucial to efficiently emulate key bio-plausible functionalities such as synaptic plasticity and neuronal activity, and has become an important aspect of neuromorphic hardware development. In this review article, we focus on various ferromagnetic (FM) and ferroelectric (FE) devices capable of representing multiple states, and discuss the usage of such multi-level devices for implementing neuromorphic functionalities. We will elaborate that the analog-like resistive states in ferromagnetic or ferroelectric thin films are due to the non-coherent multi-domain switching dynamics, which is fundamentally different from most memristive materials involving electroforming processes or significant ion motion. Both device fundamentals related to the mechanism of introducing multilevel states and exemplary implementations of neural functionalities built on various device structures are highlighted. In light of the non-destructive nature and the relatively simple physical process of multi-domain switching, we envision that ferroic-based multi-state devices provide an alternative pathway toward energy efficient implementation of neuro-inspired computing hardware with potential advantages of high endurance and controllability.

We investigate the structural, electronic and magnetic properties of the full Heusler compounds Mn$_2$YSn (Y = Mo, Nb, Zr) by first-principles density functional theory using the generalized gradient approximation. It is found that the calculated lattice constants are in good agreement with the theoretical values. We observe that the Cu$_2$MnAl-type structure is more stable than the Hg$_2$CuTi type. The calculated total magnetic moments of Mn$_2$NbSn and Mn$_2$ZrSn are 1 $\mu_{\text{B}}$ and 2 $\mu_{\text{B}}$ at the equilibrium lattice constant of 6.18 \AA and 6.31 \AA, respectively, for the Cu$_2$MnAl-type structure. Mn$_2$MoSn have a metallic character in both Hg$_2$CuTi and Cu$_2$MnAl type structures. The total spin magnetic moment obeys the Slater-Pauling rule. Half-metal exhibits 100% spin polarization at the Fermi level. Thus, these alloys are promising magnetic candidates in spintronic devices., Comment: 9 pages, 3 figures, 2 tables

Electric-field-controlled magnetism can boost energy efficiency in widespread applications. However, technologically, this effect is facing important challenges: mechanical failure in strain-mediated piezoelectric/magnetostrictive devices, dearth of room-temperature multiferroics, or stringent thickness limitations in electrically charged metallic films. Voltage-driven ionic motion (magneto-ionics) circumvents most of these drawbacks while exhibiting interesting magnetoelectric phenomena. Nevertheless, magneto-ionics typically requires heat treatments and multicomponent heterostructures. Here we report on the electrolyte-gated and defect-mediated O and Co transport in a Co3O4 single layer which allows for room-temperature voltage-controlled ON-OFF ferromagnetism (magnetic switch) via internal reduction/oxidation processes. Negative voltages partially reduce Co3O4 to Co (ferromagnetism: ON), resulting in graded films including Co- and O-rich areas. Positive bias oxidizes Co back to Co3O4 (paramagnetism: OFF). This electric-field-induced atomic-scale reconfiguration process is compositionally, structurally, and magnetically reversible and self-sustained, since no oxygen source other than the Co3O4 itself is required. This process could lead to electric-field-controlled device concepts for spintronics., Financial support by the European Research Council (SPINPORICS 2014-Consolidator Grant, Agreement No. 648454), the Spanish Government (Projects MAT2017-86357-C3-1-R and associated FEDER), the Generalitat de Catalunya (2017-SGR-292) and the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Grant Agreement No. 665919 is acknowledged. E.P. is grateful to MINECO for the “Ramon y Cajal” contract (RYC-2012-10839). The ICN2 is funded by the CERCA programme/Generalitat de Catalunya. ICN2 also acknowledges the support from the Severo Ochoa Program (MINECO, Grant SEV-2013-0295). Work at UCD is supported by the US NSF (DMR1610060 and ECCS-1611424).

Naresuan University Journal: Science and Technology, 29, 2, 96-105

Here we review various themes of current research within mesoscopic magnetic systems., Comment: APL Special Topic Guest Editorial

With the intention to reveal the effect of the substitution, Ti-doped InSb alloy, we accomplished a first-principles prediction within the FPLAPW+lo method. We used GGA-PBEsol scheme attached with the improved TB-mBJ approach to predict structural, electronic, and magnetic properties of In$_{1-x}$Ti$_{x}$Sb with concentration $x = 0$, $0.125$, $0.25$, $0.50$, $0.75$, $0.875$, and $1$. Our lattice parameters are found in favorable agreement with the available theoretical and experimental data. The calculation shows that all structures are energetically stable. The substitutional doping transforms the ionic character of the InSb compound in half-metallic ferromagnetic comportment for concentration $x = 0$, $0.125$, $0.25$, and $0.50$, with a spin polarization of $100$% at the Fermi level, and metallic nature for In$_{0.25}$Ti$_{0.75}$Sb and In$_{0.125}$Ti$_{0.875}$Sb. The total magnetic moments are also estimated at about 1 $\mu_{\text{B}}$. In$_{0.875}$Ti$_{0.125}$Sb, In$_{0.75}$Ti$_{0.25}$Sb, and In$_{0.50}$Ti$_{0.50}$Sb have half-metallic ferromagnets comportment and they can be upcoming applicants for spintronics applications., Comment: 13 pages, 17 figures, 3 tables

Magnetic anisotropy is a property of great importance both for applications (magnetic storage, ...) and for its fundamental interest. In this manuscript, we are more precisely interested in the computation of the magneto-crystalline anisotropy energy (MCA) that results from spin-orbit coupling. It is a delicate quantity to determine for several reasons : it is generally very small (of the order of meV per atom at most) and depends crucially on many physical and numerical parameters. In particular, it is strongly dependent on the symmetries of the system, the crystal lattice, but also on the nature of the materials, in this case the transition metals 3d. We are more particularly interested in magnetic thin films for which the MCA is calculated as the difference in system energy between a magnetization parallel to the surface of the material and a magnetization orthogonal to it. In particular, we use codes based on density functional theory (DFT, Quantum ESPRESSO and Quantum ATK) and a tight-binding code (TB). The MCA being obtained as an energy difference between two spin orientations using the force theorem. The objective of this PhD thesis is to understand and describe in detail the behavior of the MCA in magnetic thin films and the different ways to tune and modify (controllably or not) its magnitude. Thanks to our computational tools, we have extracted the main trends and highlighted the essential parameters that allow us to control the anisotropy. One of them is of particular interest, the hybridization of orbitals between a 3d transition metal and non-magnetic carbon atom, giving rise to important variations in anisotropy. Such systems are called hybrid-systems, and give us information on the transmission of magnetic properties (polarization, anisotropy) between neighboring atoms., L'anisotropie magnétique est une propriété de grande importance aussi bien pour les applications (stockage magnétiques, ...) que pour son intérêt fondamental. Dans ce manuscrit, nous nous intéressons plus précisément au calcul de l'énergie d'anisotropie magnéto-cristalline (MCA) qui est issue du couplage spin-orbite. C'est une grandeur délicate à déterminer pour plusieurs raisons : elle est en général très faible (de l'ordre du meV par atome au maximum) et elle dépend crucialement de nombreux paramètres physiques et numériques. Elle est notamment fortement dépendante des symétries du système, du réseau cristallin, mais aussi de la nature des matériaux, ici des métaux de transition 3d. Nous nous intéressons plus particulièrement à des couches minces magnétiques pour lesquelles la MCA est calculée comme étant la différence d'énergie du système entre une aimantation parallèle à la surface du matériau et une aimantation orthogonale à celle-ci. Nous utilisons en particulier des codes basés sur théorie de la fonctionnelle densité (DFT, Quantum ESPRESSO et Quantum ATK) et un code de liaisons fortes (TB). La MCA étant obtenue comme une différence d'énergie entre deux orientations de spin en utilisant le théorème de force. L'objectif de cette thèse de doctorat est de comprendre et de décrire en détails le comportement de la MCA dans des films minces magnétiques et les différentes manières de modifier de manière contrôlée (ou non) cette grandeur. Grâce à nos outils de calcul, nous avons extrait des grandes tendances et mis en évidence les paramètres essentiels qui permettent de piloter l'anisotropie. L'une d'entre elles retient particulièrement notre attention, l'hybridation d'orbitales entre un métal de transition 3d et un atome de carbone non-magnétique, donnant lieu d'importantes variations d'anisotropie. De tels systèmes sont dits hybrides, et nous renseignent sur les capacités de transmission du magnétisme (polarisation, anisotropie) entre atomes voisins.

Magnetic anisotropy is a property ofgreat importance both for applications (magnetic storage,...) and for its fundamental interest. In this manuscript,we are more precisely interested in the computationof the magneto-crystalline anisotropy energy(MCA) that results from spin-orbit coupling. It is adelicate quantity to determine for several reasons : itis generally very small (of the order of meV per atomat most) and depends crucially on many physical andnumerical parameters. In particular, it is strongly dependenton the symmetries of the system, the crystallattice, but also on the nature of the materials, in thiscase the transition metals 3d. We are more particularlyinterested in magnetic thin films for which the MCA iscalculated as the difference in system energy betweena magnetization parallel to the surface of the materialand a magnetization orthogonal to it.In particular, we use codes based on density functionaltheory (DFT, Quantum ESPRESSO and Quantum ATK)and a tight-binding code (TB). The MCA being obtainedas an energy difference between two spin orientationsusing the force theorem. The objective of thisPhD thesis is to understand and describe in detail thebehavior of the MCA in magnetic thin films and the differentways to tune and modify (controllably or not) itsmagnitude. Thanks to our computational tools, we haveextracted the main trends and highlighted the essentialparameters that allow us to control the anisotropy. Oneof them is of particular interest, the hybridization of orbitalsbetween a 3d transition metal and non-magneticcarbon atom, giving rise to important variations in anisotropy.Such systems are called hybrid-systems, andgive us information on the transmission of magneticproperties (polarization, anisotropy) between neighboringatoms., L’anisotropie magnétique est une propriétéde grande importance aussi bien pour les applications(stockage magnétique, ...) que pour son intérêtfondamental. Dans ce manuscrit, nous nous intéressonsplus précisément au calcul de l’énergie d’anisotropiemagnéto-cristalline (MCA) qui est issue du couplagespin-orbite. C’est une grandeur délicate à déterminerpour plusieurs raisons : elle est en général très faible (del’ordre du meV par atome au maximum) et elle dépendcrucialement de nombreux paramètres physiques et numériques.Elle est notamment fortement dépendantedes symétries du système, du réseau cristallin, maisaussi de la nature des matériaux, ici des métaux detransition 3d. Nous nous intéressons plus particulièrementà des couches minces magnétiques pour lesquellesla MCA est calculée comme étant la différence d’énergiedu système entre une aimantation parallèle à la surfacedu matériau et une aimantation orthogonale à celle-ci.Nous utilisons en particulier des codes basés sur théoriede la fonctionnelle densité (DFT, Quantum ESPRESSOet Quantum ATK) et un code de liaisons fortes (TB).La MCA étant obtenue comme une différence d’énergieentre deux orientations de spin en utilisant le théorèmede force. L’objectif de cette thèse de doctorat est decomprendre et de décrire en détails le comportementde la MCA dans des films minces magnétiques et lesdifférentes manières de modifier de manière contrôlée(ou non) cette grandeur. Grâce à nos outils de calcul,nous avons extrait des grandes tendances et mis enévidence les paramètres essentiels qui permettent depiloter l’anisotropie. L’une d’entre elles retient particulièrementnotre attention, l’hybridation d’orbitalesentre un métal de transition 3d et un atome de carbonenon-magnétique, donnant lieu d’importantes variationsd’anisotropie. De tels systèmes sont dits hybrides, etnous renseignent sur les capacités de transmission dumagnétisme (polarisation, anisotropie) entre atomesvoisins.

In this work, we investigated the phase transition and electronic structures of some newly designed all-d-metal Heusler compounds, X2MnTi (X = Pd, Pt, Ag, Au, Cu, and Ni), by means of the first principles. The competition between the XA and L21 structures of these materials was studied, and we found that X2MnTi favors to feature the L21-type structure, which is consistent with the well-known site-preference rule (SPR). Under the L21 structure, we have studied the most stable magnetic state of these materials, and we found that the ferromagnetic state is the most stable due to its lower energy. Through tetragonal deformation, we found that the L21 structure is no longer the most stable structure, and a more stable tetragonal L10 structure appeared. That is, under the tetragonal strain, the material enjoys a tetragonal phase transformation (i.e., from cubic L21 to tetragonal L10 structure). This mechanism of L21-L10 structure transition is discussed in detail based on the calculated density of states. Moreover, we found that the energy difference between the most stable phases of L10 and L21, defined as ΔEM (ΔEM = ECubic-ETetragonal), can be adjusted by the uniform strain. Finally, the phonon spectra of all tetragonal X2MnTi (X = Pd, Pt, Ag, Au, Cu, and Ni) phases are exhibited, which provides a powerful evidence for the stability of the tetragonal L10 state. We hope that our research can provide a theoretical guidance for future experimental investigations.