Thèses sur le sujet « Multiferroics - Spintronics »

The primary obstacle to continued downscaling of charge-based electronic devices in accordance with Moore's law is the excessive energy dissipation that takes place in the device during switching of bits. Unlike charge-based devices, spin-based devices are switched by flipping spins without moving charge in space. Although some energy is still dissipated in flipping spins, it can be considerably less than the energy associated with current flow in charge-based devices. Unfortunately, this advantage will be squandered if the method adopted to switch the spin is so energy-inefficient that the energy dissipated in the switching circuit far exceeds the energy dissipated inside the system. Regrettably, this is often the case, e.g., switching spins with a magnetic field or with spin-transfer-torque mechanism. In this dissertation, it is shown theoretically that the magnetization of two-phase multiferroic single-domain nanomagnets can be switched very energy-efficiently, more so than any device currently extant, leading possibly to new magnetic logic and memory systems which might be an important contributor to Beyond-Moore's-Law technology. A multiferroic composite structure consists of a layer of piezoelectric material in intimate contact with a magnetostrictive layer. When a tiny voltage of few millivolts is applied across the structure, it generates strain in the piezoelectric layer and the strain is transferred to the magnetostrictive nanomagnet. This strain generates magnetostrictive anisotropy in the nanomagnet and thus rotates its direction of magnetization, resulting in magnetization reversal or 'bit-flip'. It is shown after detailed analysis that full 180 degree switching of magnetization can occur in the "symmetric" potential landscape of the magnetostrictive nanomagnet, even in the presence of room-temperature thermal fluctuations, which differs from the general perception on binary switching. With proper choice of materials, the energy dissipated in the bit-flip can be made as low as one attoJoule at room-temperature. Also, sub-nanosecond switching delay can be achieved so that the device is adequately fast for general-purpose computing. The above idea, explored in this dissertation, has the potential to produce an extremely low-power, yet high-density and high-speed, non-volatile magnetic logic and memory system. Such processors would be well suited for embedded applications, e.g., implantable medical devices that could run on energy harvested from the patient's body motion.

L'avenir de la spintronique repose sur le développement de matériaux ayant des propriétés magnétiques remarquables. L'objectif de cette thèse est de comprendre la physique des deux matériaux fonctionnels proposés pour des applications spintroniques qui utilisent des simulations de la densité fonctionnelle.Nous nous sommes intéressés dans une première partie au ferrite de gallium pour lequel il a été montré que les propriétés dépendaient de la concentration de fer.Les spectres optiques ont été calculés et comparés aux spectres expérimentaux suggérant des niveaux élevés de désordre. Dans la deuxième partie, nous avons montré une polarisation de spin à l’interface hybride formée entre la phthalocyanine de manganèse et la surface de cobalt,en accord avec les expériences de photoémission.La formation de la spinterface a été expliquée par différents mécanismes d'hybridation dans chaque canal de spin.Cette polarisation de spin est coordonnée avec des moments magnétiques induits sur les sites moléculaires
The future of the spintronics technology requires developing functional materials with remarkable magnetic properties. The aim of this thesis is to understand the physics of functional materials proposed for spintronic applications using ab-initio density functional simulations. We investigated the properties of two different functional materials. We first studied the magnetoelectric gallium ferrite GFO. The dependence of the different properties on the iron concentration has been demonstrated and discussed. The optical spectra were calculated and compared to the experimental once suggesting high levels of iron disorder. In the second part, we demonstrated a highly spin polarized hybrid interface formed between manganese phthalocyanine and cobalt surface in agreement with photoemission experiments. The formation of this spinterface was described by different hybridization mechanisms in each spin channel. This high spin polarization is coordinated with induced magnetic moments on the molecular sites

This project was a step forward in discovering new two-dimensional (2D) structures for electronic and spintronic applications. This work comprehensively investigates seven intriguing 2D structures with novel electronic, optical and magnetic properties on the basis of the global structural search and first-principles calculations. These findings not only highlight the promising materials platforms for advanced nanodevices but also provide the theoretical guides for designing multifunctional 2D materials.

Les mémoires magnétiques actuelles commencent à atteindre leurs limites physiques en terme de stabilité, vitesse et consommation énergétique, alors que la course à la miniaturisation s'intensifie. Le champ émergeant de la spintronique étudie le comportement collectif des spins dans la matière ainsi que leurs interactions aux interfaces, afin de trouver une solution en termes de matériaux, architectures et sources excitatrices. En particulier, les matériaux antiferromagnétiques sont particulièrement prometteurs. Ces matériaux ordonnées sont abondants, naturellement stables, robustes, ultra rapides et compatibles avec l'électronique des isolants. En effet, la plupart des oxydes à base de métaux de transition sont des isolants antiferromagnétiques ayant leur fréquence de résonance dans le terahertz et un champ de flop de quelques dizaines de teslas. Ils peuvent aussi être semi-métalliques, métalliques, semiconducteurs, supraconducteurs ou multiferroïques. Cette thèse s'intéresse aux deux antiferromagnétiques: oxyde de nickel (NiO) et ferrite de bismuth (BiFeO₃). NiO est un antiferromagnétique type à température ambiante, avec une structure cristalline simple. Une étude basée sur des simulations dynamiques atomiques montre que des courants de spin atteignables peuvent réaliser une mémoire à trois états avec ce composé, avec un temps de réponse de l'ordre de la picoseconde. La simulation explique aussi la formation de structures chirales dans BiFeO₃, un antiferromagnétique également ferroélectrique, présentant un couplage magnétoélectrique entre ses deux ordres. Dans une deuxième partie, les domaines antiferromagnétiques dans BiFeO₃ sont observés expérimentalement par génération de seconde harmonique optique, avec une résolution spatiale de un micron. Les domaines antiferromagnétiques de BiFeO₃ sont ensuite excités par une impulsion laser intense, et la dynamique des deux ordres couplés (antiferromagnétisme et ferroélectricité) est étudiée dans le régime picoseconde. Enfin, l'injection d'impulsions de spins dans dans un antiferromagnétique, tel que BiFeO₃ ou NiO est envisagée en utilisant la génération de courant de spin induite par la désaimantation ultrarapide de couches adjacentes magnétiques par des impulsions laser
Current magnetic memory devices are reaching their physical limits in terms of stability, speed and power consumption as the race to miniaturization intensifies. The emergent research field of spintronics studies the collective behavior of spins in matter and their interplay at interfaces, to find new avenues in terms of materials, architectures and stimulation sources. A particularly promising group of materials are the antiferromagnets. These abundant magnetically ordered materials are naturally stable, robust, ultra-fast and compatible with insulator electronics. Indeed, most transition metal oxide compounds are antiferromagnetic insulators, have resonance in the terahertz range and flop fields of tens of teslas. They can also be semi-metals, metals, semiconductors, superconductors or multiferroics. This thesis focuses on two antiferromagnets: nickel oxide (NiO) and bismuth ferrite (BiFeO₃). NiO is the archetypical antiferromagnet at ambient temperature with a simple crystalline structure. Using dynamical atomistic simulations, I show that this compound can be the elemental brick of a three state memory device controlled by currently available pulses of spin currents, with a picosecond response time. The simulations also explain the formation of chiral structures in BiFeO₃, a ferroelectric antiferromagnet with magnetoelectric coupling between the two orders. In a second part, antiferromagnetic domains in BiFeO₃ are experimentally observed using second harmonic generation of light, with a sub-micron spatial resolution. Antiferromagnetic domains of BiFeO₃ are then excited by an intense femtosecond laser pulse, and the dynamics of the two coupled orders (antiferromagnetism and ferroelectricity) is studied with a sub-picosecond time resolution. Finally, the injection of spin current in an antiferromagnet such as BiFeO₃ or NiO is envisioned by characterizing the spin bursts generated by ultrafast laser-induced demagnetization of adjacent ferromagnetic layers

Nanomagnetic logic, incorporating logic bits in the magnetization orientations of single-domain nanomagnets, has garnered attention as an alternative to transistor-based logic due to its non-volatility and unprecedented energy-efficiency. The energy efficiency of this scheme is determined by the method used to flip the magnetization orientations of the nanomagnets in response to one or more inputs and produce the desired output. Unfortunately, the large dissipative losses that occur when nanomagnets are switched with a magnetic field or spin-transfer-torque inhibit the promised energy-efficiency. Another technique offering superior energy efficiency, “straintronics”, involves the application of a voltage to a piezoelectric layer to generate a strain which is transferred to an elastically coupled magnetrostrictive layer, causing magnetization rotation. The functionality of this scheme can be enhanced further by introducing magnetocrystalline anisotropy in the magnetostrictive layer, thereby generating four stable magnetization states (instead of the two stable directions produced by shape anisotropy in ellipsoidal nanomagnets). Numerical simulations were performed to implement a low-power universal logic gate (NOR) using such 4-state magnetostrictive/piezoelectric nanomagnets (Ni/PZT) by clocking the piezoelectric layer with a small electrostatic potential (~0.2 V) to switch the magnetization of the magnetic layer. Unidirectional and reliable logic propagation in this system was also demonstrated theoretically. Besides doubling the logic density (4-state versus 2-state) for logic applications, these four-state nanomagnets can be exploited for higher order applications such as image reconstruction and recognition in the presence of noise, associative memory and neuromorphic computing. Experimental work in strain-based switching has been limited to magnets that are multi-domain or magnets where strain moves domain walls. In this work, we also demonstrate strain-based switching in 2-state single-domain ellipsoidal magnetostrictive nanomagnets of lateral dimensions ~200 nm fabricated on a piezoelectric substrate (PMN-PT) and studied using Magnetic Force Microscopy (MFM). A nanomagnetic Boolean NOT gate and unidirectional bit information propagation through a finite chain of dipole-coupled nanomagnets are also shown through strain-based "clocking". This is the first experimental demonstration of strain-based switching in nanomagnets and clocking of nanomagnetic logic (Boolean NOT gate), as well as logic propagation in an array of nanomagnets.

This work targeted the growth of gadolinium (Gd)-doped gallium nitride (GaN) thin films (Ga₁₋ₓGdₓN) by metal organic chemical vapor deposition (MOCVD). Characterization and evaluation of these Ga₁₋ₓGdₓN thin films for application in spintronics/optoelectronics devices also formed part of this work. This work presents: (1) the first report of stable, reproducible n- and p-type Ga₁₋ₓGdₓN thin films by MOCVD; (2) the first Ga₁₋ₓGdₓN p-n diode structure; and (3) the first report of a room temperature spin-polarized LED using a Ga₁₋ₓGdₓN spin injection layer. The Ga₁₋ₓGdₓN thin films grown in this work were electrically conductive, and co-doping them with Silicon (Si) or Magnesium (Mg) resulted in n-type and p-type materials, respectively. All the materials and structures grown in this work, including the Ga₁₋ₓGdₓN-based p-n diode and spin polarized LED, were characterized for their structural, optical, electrical and magnetic properties. The spin-polarized LED gave spin polarization ratio of 22% and systematic variation of this ratio at room temperature with external magnetic field was observed.

Excessive energy dissipation in CMOS devices during switching is the primary threat to continued downscaling of computing devices in accordance with Moore’s law. In the quest for alternatives to traditional transistor based electronics, nanomagnet-based computing [1, 2] is emerging as an attractive alternative since: (i) nanomagnets are intrinsically more energy-efficient than transistors due to the correlated switching of spins [3], and (ii) unlike transistors, magnets have no leakage and hence have no standby power dissipation. However, large energy dissipation in the clocking circuit appears to be a barrier to the realization of ultra low power logic devices with such nanomagnets. To alleviate this issue, we propose the use of a hybrid spintronics-straintronics or straintronic nanomagnetic logic (SML) paradigm. This uses a piezoelectric layer elastically coupled to an elliptically shaped magnetostrictive nanomagnetic layer for both logic [4-6] and memory [7-8] and other information processing [9-10] applications that could potentially be 2-3 orders of magnitude more energy efficient than current CMOS based devices. This dissertation focuses on studying the feasibility, performance and reliability of such nanomagnetic logic circuits by simulating the nanoscale magnetization dynamics of dipole coupled nanomagnets clocked by stress. Specifically, the topics addressed are: 1. Theoretical study of multiferroic nanomagnetic arrays laid out in specific geometric patterns to implement a “logic wire” for unidirectional information propagation and a universal logic gate [4-6]. 2. Monte Carlo simulations of the magnetization trajectories in a simple system of dipole coupled nanomagnets and NAND gate described by the Landau-Lifshitz-Gilbert (LLG) equations simulated in the presence of random thermal noise to understand the dynamics switching error [11, 12] in such devices. 3. Arriving at a lower bound for energy dissipation as a function of switching error [13] for a practical nanomagnetic logic scheme. 4. Clocking of nanomagnetic logic with surface acoustic waves (SAW) to drastically decrease the lithographic burden needed to contact each multiferroic nanomagnet while maintaining pipelined information processing. 5. Nanomagnets with four (or higher states) implemented with shape engineering. Two types of magnet that encode four states: (i) diamond, and (ii) concave nanomagnets are studied for coherence of the switching process.

The ability to control the bi-stable magnetization states of shape anisotropic single domain nanomagnets has enormous potential for spawning non-volatile and energy-efficient computing and signal processing systems. One of the most energy efficient switching methods is to adopt a system of a 2-phase multiferroic nanomagnet, where a voltage applied on the piezoelectric layer generates a strain in it and the strain is elastically transferred to the magnetostrictive nanomagnet which rotates the magnetization states of the nanomagnet at room temperature via the converse magnet-electric effect. Recently, it has been demonstrated that the magnetization of a Co nanomagnet can be switched between two stable orientations by this technique. The switching probability, however, is low due to the relatively small magnetostriction of Co. One possible way to improve the statistics is to use a better magnetostrictive material like Galfenol which has much higher magnetostriction and is therefore desirable, but it also presents unique material challenges owing to the existence of many phases. Nonetheless, there is a need to step beyond elemental ferromagnets and examine compound or alloyed ferromagnets with much higher magnetostriction to advance this field. There has not been much work in nanoscale FeGa magnets which are important for nanomagnetic logic and memory applications. Here, we have experimentally demonstrated switching of magnetization of Galfenol nanomagnets and proposed a core component of ultra-energy-efficient memory cell. We also demonstrated a bit writing scheme which completely reverses the magnetization with only strain, thus overcoming the fundamental obstacle of strain induced switching of magnetizations of nanomagnets.

Les matériaux antiferromagnétiques suscitent un intérêt croissant pour la spintronique de par leur insensibilité aux champs magnétiques parasites et leur dynamique magnétique ultrarapide. Cependant, la lecture et le contrôle de l’ordre antiferromagnétique restent des verrous pour le développement des dispositifs. Dans les matériaux multiferroïques, le couplage magnétoélectrique entre les ordres électrique et magnétique pourrait permettre de contrôler l’antiferromagnétisme avec un champ électrique. Dans cette thèse, nous imageons une grande variété de textures antiferromagnétiques que nous contrôlons par l’ingénierie des contraintes et le champ électrique pour l’archétype des matériaux multiferroïques, BiFeO₃. Nous élaborons des films minces sous différentes contraintes d’épitaxie, maîtrisant ainsi la texture de domaines ferroélectriques, telle qu’imagée par microscopie à force piézoélectrique. De plus, nous montrons qu’une transition de phase inverse peut être utilisée pour accroître l’ordre électrique global, d’une configuration labyrinthique de domaines vers un réseau périodique en bandes rectilignes. La magnétométrie à centre NV nous permet de corréler les textures antiferromagnétiques et ferroélectriques. Nous démontrons que les contraintes stabilisent différents types de cycloïdes ainsi qu’un ordre antiferromagnétique colinéaire. La diffraction X élastique résonante permet de confirmer macroscopiquement l’existence de deux types de cycloïdes. Enfin, nous contrôlons électriquement ces textures antiferromagnétiques, passant d’une cycloïde à une autre ou transformant un ordre colinéaire en cycloïde. Sur la base d’un substrat imposant une contrainte anisotrope, nous stabilisons des films ne présentant qu’un seul domaine ferroélectrique associé à un unique domaine antiferromagnétique. Ceci ouvre de larges perspectives pour explorer le couplage entre l’antiferromagnétisme non-colinéaire et le transport de spin
Antiferromagnetic materials are generating a growing interest for spintronics due to important assets such as their insensitivity to spurious magnetic fields and fast magnetization dynamics. A major bottleneck for functional devices is the readout and electric control of the antiferromagnetic order. In multiferroics, the magnetoelectric coupling between ferroelectric and antiferromagnetic orders may represent an efficient way to control antiferromagnetism with an electric field. In this thesis, we observe a wide variety of antiferromagnetic textures that we control by strain engineering and electric field in the archetypical multiferroic, BiFeO₃. We elaborate epitaxial BiFeO₃ thin films, harbouring various ferroelectric domain landscapes, as imaged by piezoresponse force microscopy. Furthermore, we resort on an inverse phase transition to improve the global electrical order from maze to perfect array of striped ferroelectric domains. Using scanning NV magnetometry, we correlate the antiferromagnetic landscapes to the ferroelectric ones. We demonstrate that strain stabilizes bulk or exotic spin cycloids, as well as collinear antiferromagnetic order. With resonant X-ray elastic scattering, we macroscopically confirm the existence of two types of cycloid. Furthermore, we electrically design antiferromagnetic landscapes on demand, changing one type of cycloid to another or turning collinear states into non-collinear ones. Finally, resorting on anisotropic strain, we stabilize a single domain ferroelectric state, in which a single spin cycloid propagates. This opens a fantastic avenue to investigate the coupling between non-collinear antiferromagnetism and spin transport

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

Avec le développement des nouvelles technologies de l'information et de la communication (NTIC), la consommation énergétique des systèmes de traitement et de stockage de données est devenue un problème majeur. Les limites des systèmes actuels à cet égard impliquent le besoin de technologies de rupture ultra-basse consommation.Cette thèse propose une approche originale de cette problématique, basée sur l'utilisation d'un élément magnétoélectrique composite (piézoélectrique/magnétostrictif) bistable et commandable de façon univoque, baptisé MELRAM.L'étude énergétique statique montre que la combinaison d'une anisotropie uni-axiale et d'un champ de polarisation magnétique statique définit deux positions d'équilibre stables perpendiculaires pour l'aimantation dans la partie magnétostrictive. L'application de contraintes piézoélectriques sur celle-ci permet de contrôler électriquement la position de l'aimantation. L'étude énergétique du système permet également de montrer la stabilité du système à long terme (10 ans), dans une large gamme de températures autour de l'ambiante, avec une barrière énergétique de 60kBT. L'étude dynamique, utilisant le modèle du macrospin, permet quant à elle d'exhiber un temps de réponse inférieur à 1ns. L'énergie dissipée lors de l'écriture, d'origine électrique et magnétique, est évaluée à 261kBT (1,1aJ), soit quatre ordres de grandeur en dessous de l'état de l'art.Plusieurs stratégies de lecture par vanne de spin et jonction tunnel magnétique sont proposées et commentées. Les premières réalisations d'éléments nanométriques magnétostrictifs sont présentées ainsi qu'une solution de polarisation magnétique intégrée par aimant permanent.

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 Rabat
The 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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O desenvolvimento da spintrônica tem motivado a busca por novos materiais multiferróicos devido à multifuncionalidade desses compostos associada ao acoplamento entre diferentes ordens ferróicas em uma estrutura cristalina. No presente estudo, propomos a investigação teórica, baseada na Teoria do Funcional de Densidade, dos materiais ATiO3 (A = Mn, Fe, Ni) na estrutura R3c com objetivo de esclarecer o efeito da substituição do cátion A sobre as propriedades estruturais, magnéticas e eletrônicas, bem como descrever diferentes mecanismos de controle das propriedades multiferróicas baseados em arquiteturas de filmes-finos, morfologia e controle de defeitos intrínsecos. Para uma maior compreensão dos efeitos envolvidos nos materiais ATiO3, diferentes funcionais de troca e correlação foram investigados e o funcional PBE0 apresentou os menores desvios, consequentemente, a melhor representação comparado aos resultados experimentais. Com objetivo de investigar as propriedades conectadas a filmes-finos dos materiais ATiO3, propomos uma metodologia inovadora que permite descrever as deformações uni- e biaxial que se originam na região de interface entre o filme e o substrato. Nesse caso, os resultados obtidos indicam que as distorções estruturais induzem uma transição magnética para o NiTiO3, originando ordenamento ferromagnético a partir de um critério magneto-estrutural associado a deformação dos clusters [MO6] que reproduz satisfatoriamente os resultados experimentais reportados na literatura. De modo análogo, para elucidar a relação entre o magnetismo e a morfologia dos materiais ATiO3, combinamos cálculos de Energia de Superfície, Construção de Wulff e um formalismo avançado para descrever o magnetismo superficial considerando a existência de spins não compensados ao longo dos planos polares (100), (001), (101), (012), (111) e apolares (110). Os resultados indicam que a redução do número de coordenação dos metais A e Ti para os planos (001) e (111) resulta na transferência de carga entre os cátions A2+ e Ti4+, originando espécies Ti3+ magnéticas que aumentam o magnetismo superficial ao longo desses planos. Além disso, esse efeito é capaz de induzir uma alteração do caráter eletrônico para esses materiais, permitindo indicar que a clivagem das superfícies contribui para o controle das propriedades eletrônicas, reduzindo o valor de band-gap ou gerando comportamento meio-metálico. Os mapas morfológicos obtidos indicam que o controle da exposição majoritária do plano (001) para obtenção de discos hexagonais induz um aumento do magnetismo superficial para os materiais ATiO3 em acordo com resultados experimentais, além de predizer diferentes morfologias acessíveis com interessantes propriedades magnéticas. Ademais, o efeito de defeitos intrínsecos como vacâncias de oxigênio no bulk e superfície apolar (110) dos materiais ATiO3 foi investigado indicando que a redução do número de coordenação na região do defeito induz que os elétrons remanescentes sejam localizados, principalmente, nos orbitais 3d vazios dos cátions Ti vizinhos, gerando espécies [TiO5]ꞌ e [TiO4]ꞌ (3d1 ) que possibilitam uma interação ferromagnética nos materiais MnTiO3 e FeTiO3. A combinação entre os diferentes mecanismos investigados permitiu estabelecer um guia científico para o estudo teórico de materiais multiferróicos, contribuindo para descrever as potencialidades dos diferentes materiais bem como predizer novos candidatos.
The development of spintronic has motivated the search for new multiferroic materials due to the multifunctionality of these materials that are associated with the coupling of different ferroic orders into a single crystalline structure. In the present study, we propose a theoretical investigation, based on Density Functional Theory, of ATiO3 (A = Mn, Fe, Ni) materials in the R3c structure in order to clarify the effect of A-site cation replacement on the structural, magnetic and electronic properties, as well as to describe a different mechanism to control the multiferroic properties based on thin-film architectures, morphology and point defects. For a more comprehensive overview of the main effects involved on the ATiO3 materials several exchange-correlation functionals were investigated, being the PBE0 the functional with smallest deviations and, consequently, the best representation in comparison to the experimental results. Aiming to describe the main fingerprints related with the creation of ATiO3 thin-films, we propose an innovative methodology that allows to describe the uniaxial and biaxial deformations originated in the interface region between the film and the substrate. In this case, the results indicate that structural distortions induce a magnetic transition for the NiTiO3, originating ferromagnetic ordering from magneto-structural criteria, which is associated to the deformation of the [MO6] clusters that reproduces satisfactorily the experimental results reported in the literature. Similarly, in order to elucidate the relationship between the magnetism and the morphology of the ATiO3 materials, we combined Surface Energy, Wulff Construction, and an advanced formalism to describe surface magnetism by considering the existence of uncompensated spins along the polar planes (100), (001), (101), (012), (111) and non-polar (110). The results indicate that the reduction of the coordination for both A and Ti metals along the (001) and (111) planes induces a charge transfer between the A 2+ and Ti4+ cations, resulting in magnetic Ti3+ species that increase the superficial magnetism along such planes. Moreover, this effect allowed a change in the electronic structure for these materials, allowing to point out that the cleavage of the surfaces contribute to the control of the electronic properties reducing the band-gap value or generating half-metallic behavior. The morphological maps indicated that the control of the major exposure for the (001) surface to obtain hexagonal discsinduces an increase of the superficial magnetism for the ATiO3 materials according to experimental results, besides predicting different accessible morphologies with interesting magnetic properties. In addition, the effect of intrinsic defects such as oxygen vacancies on the bulk and non-polar (110) surface of the ATiO3 materials were investigated, indicating that the reduction of coordination in the defect region induces the localization of the remaining electrons in the empty 3d orbitals of neighboring Ti cations, generating [TiO5]'and [TiO4]' (3d1 ) species that allow a ferromagnetic interaction for MnTiO3 and FeTiO3 materials. The combination of the different mechanisms investigated has allowed to stablish a scientific guide for the theoretical study of multiferroic materials, contributing to describe the potentialities of the different materials as well as to predict new candidates.

Indiana University-Purdue University Indianapolis (IUPUI)
Thermal constraints and the quantum limit will soon put a boundary on the scale of new micro and nano magnetoelectronic devices. This necessitates a push into the limits of harnessable natural phenomena to facilitate a post-Moore’s era of design. Requirements for thermodynamic stability at room temperature, fast (Ghz) switching, and low energy cost narrow the list of candidates. Here we show voltage controllable, room temperature, stable locking of the spin state, and the corresponding conductivity change, when molecular spin crossover thin films are deposited on a ferroelectric substrate. This opens the door to the creation of a non-volatile, room temperature, molecular multiferroic gated voltage controlled device.

Indiana University-Purdue University Indianapolis (IUPUI)
Thermal constraints and the quantum limit will soon put a boundary on the scale of new micro and nano magnetoelectronic devices. This necessitates a push into the limits of harnessable natural phenomena to facilitate a post-Moore’s era of design. Requirements for thermodynamic stability at room temperature, fast (Ghz) switching, and low energy cost narrow the list of candidates. Here we show voltage controllable, room temperature, stable locking of the spin state, and the corresponding conductivity change, when molecular spin crossover thin films are deposited on a ferroelectric substrate. This opens the door to the creation of a non-volatile, room temperature, molecular multiferroic gated voltage controlled device.

Die vorliegende Arbeit befasst sich mit Züchtung und Untersuchung von dünnen Kompositschichten aus BaTiO3 und BiFeO3 sowie Sr2FeMoO6-Dünnschichten. Diese Materialien haben gemeinsam, dass sie Bausteine in neuartigen elektronischen Bauelemente z. B. in Computerspeicher-Modulen werden könnten. BiFeO3 ist ein magnetoelektrisches Multiferroikum bei Raumtemperatur, was bedeutet, dass es sowohl eine spontane magnetische als auch eine spontane ferroelektrische Ordnung besitzt. Zusätzlich sind diese aneinander gekoppelt (magnetoelektrisch). Das macht BiFeO3 zu einem sehr begehrten Forschungsobjekt. Da die magnetoelektrische Kopplung im BiFeO3 zu schwach ist, um anwendungsrelevant zu sein, werden im Rahmen dieser Arbeit Kompositmaterialien unter Zuhilfenahme des Ferroelektrikums BaTiO3 hergestellt und untersucht, mit welchen eine Steigerung der magnetoelektrischen opplungskonstante von 4,2 V/cmOe auf über 35 V/cmOe bei Raumtemperatur erreicht wird. Zudem wird das entdeckte Sauerstoffvakanz-Übergitter untersucht und die daraus erwachsende elektrische Polarisation beschrieben. Im zweiten Teil der Arbeit wird zunächst die Herrstellung von Sr2FeMoO6 per gepulster Laserplasmaabscheidung als Dünnfilm beschrieben und die Untersuchung des magnetfeldabhängigen elektrischen Widerstandes gezeigt. Dies geschieht in Abhängigkeit von verschiedenen Parametern wie Temperatur und Substratmaterial. Als Untersuchungsmethodenkommen unter anderem Raster- und Tunnelelektronenmikroskopie sowie Rasterkraftmikroskopie, Rutherford- ückstreuung und Röntgediffraktometrie für strukturelle Charakterisierungen und Strom-, Magnetisierungsund temperatur- und magnetfeldabhängige Spannungsmessungen für die elektrische und magnetische Charakterisierung der Dünnschichten zum Einsatz. Nach einer Einleitung mit der Motivation der Arbeit werden im zweiten Kapitel die teilweise übergreifenden experimentellen Methoden inklusive der Probenherstellung beschrieben. Das dritte und vierte Kapitel beinhaltet jeweils einen Überblick über den Stand der Forschung zu den einzelnen Materialien und die experimentellen Ergebnisse samt Diskussion. Das darauf folgende Kapitel liefert die Zusammenfassung und einen Ausblick.