Les mémoires MRAM (Magnetic Random Access Memory) sont l’une des technologies émergentes visant à devenir un dispositif de mémoire «universelle» applicable à une grande variété d’applications. La combinaison du couple de spin-orbite (SOT) résultant de l’effet Hall de spin (SHE) et de l’interaction de Dzyaloshinskii – Moriya (DMI) aux interfaces entre un métal lourd et une couche ferromagnétique s’est révélée être un mécanisme efficace pour induire une propagation de parois magnétiques chirales à des faibles densité de courant. Les dispositifs à parois magnétiques devraient constituer la prochaine génération de supports d’information en raison de leur potentiel pour des densités de stockage très élevées. Cependant, une limitation cruciale est la présence de défauts structuraux qui piègent les parois magnétiques et induisent des courants de seuil élevés ainsi que des effets stochastiques importants. L’origine du piégeage résulte de la présence de défauts structuraux aux interfaces entre la couche magnétique ultra-mince et les autres couches (isolants et/ou métaux lourds) qui induisent une distribution spatiale des propriétés magnétiques comme l’anisotropie magnétique perpendiculaire (PMA) ou le DMI. Comprendre l’influence de la structure des interfaces sur la propagation de parois et sur le DMI en particulier est cruciale pour la conception de futurs dispositifs basse consommation. C’est dans ce contexte très novateur que mon doctorat s’est focalisé sur la manipulation de la structure des interfaces dans des couches ultra-minces à anisotropie magnétique perpendiculaire. Des structures de CoFeB-MgO ont été utilisées afin de mieux comprendre l'impact de la structure des interfaces sur l’anisotropie, le DMI, la propagation de parois et les phénomènes de SOT. L’approche innovante que nous avons utilisée est basée sur l’irradiation par des ions légers pour contrôler le degré de mélange aux interfaces. Sous l’effet du mélange induit par l’irradiation, nous avons observé dans des structures de W-CoFeB-MgO une forte augmentation de la vitesse de parois dans le régime de creep, compatible avec une réduction de la densité des centres de piégeage. Nous avons aussi démontré que l'anisotropie de l'interface Ki et le DMI mesuré par propagation asymétrique de parois se comportent de la même façon en fonction du mélange aux interfaces. Finalement, nous avons fabriqué des barres de Hall afin de mesurer la commutation de l’aimantation induite par SOT. Le centre des croix de Hall a été irradié afin de diminuer localement l’anisotropie. Nous avons observé une réduction de 60% de la densité de courant critique après l’irradiation correspondant au retournement des croix de Hall irradiés par propagation de parois. Notre étude fournit de nouvelles pistes concernant le développement de mémoires magnétiques à faible consommation, de dispositifs logiques et neuromorphiques. / Magnetic Random Access Memory (MRAM), as one of the emerging technologies, aims to be a “universal” memory device for a wide variety of applications. The combination of the spin orbit torque (SOT) resulting from the spin Hall effect (SHE) and the Dzyaloshinskii–Moriya interaction (DMI) at interfaces between heavy metals and ferromagnetic layers has been demonstrated to be a powerful mean to drive efficiently domain-wall (DW) motion, which are expected to be the promising next generation of information carriers owing to ultra-low driving currents and ultra fast DW motion. However, the crucial limitation of SOT induced domain wall motion results from the presence of pinning defects that can induce large threshold currents and stochastic behaviors. Such pinning defects are strongly related to structural inhomogeneities at the interfaces between the ultra-thin ferromagnetic layer and the other materials (insulator and/or heavy metals) that induce a spatial distribution of magnetic properties such as perpendicular magnetic anisotropy (PMA) or DMI. Therefore, understanding the role of the interface structure on DW motion and DMI is crucial for the design of future low power devices.It is under this innovative context that my Ph.D. research has focused on the manipulation of interface structure in ultra-thin magnetic films with perpendicular magnetic anisotropy. CoFeB-MgO structures have been used in order to understand the impact of interface structure on anisotropy, DMI, domain wall motion and SOT phenomena. The innovative approach we have used in this PhD research is based on light ion irradiation to control the degree of intermixing at interfaces. In W-CoFeB-MgO structures with high DMI, we have observed a large increase of the DW velocity in the creep regime upon He⁺ irradiation, which is attributed to the reduction of pinning centres induced by interface intermixing. Asymmetric in-plane field-driven domain expansion experiments show that the DMI value is slightly reduced upon irradiation, and a direct relationship between DMI and interface anisotropy is demonstrated. Using local irradiated Hall bars in SOT devices, we further demonstrate that the current density for SOT induced magnetization switching through DW motion can be significantly reduced by irradiation. Our finding provides novel insights into the development of low power spintronic-memory, logic as well as neuromorphic devices.

Les phases alpha'-Fe8N1-x et alpha''-Fe16N2 ont un fort potentiel d’application, en raison de leur anisotropie magnétocristalline uniaxiale et de leur grande aimantation à saturation. Cependant, les valeurs annoncées pour ces propriétés magnétiques restent sujettes à discussion. Les recherches menées au cours de cette thèse de doctorat ont été initiées dans le but de clarifier cette situation. L’élaboration des échantillons a principalement consisté en l’implantation ionique d’azote dans des couches minces de fer alpha épitaxiées sur ZnSe/GaAs (001). Entre autres, les effets de la température de la cible et de la fluence sur la structure cristalline des échantillons ont été analysés par diffractométrie des rayons X. La présence d’une anisotropie magnétique perpendiculaire a été mise en évidence dans les couches minces contenant les phases alpha'-Fe8N1-x ou alpha''-Fe16N2. La constante d’anisotropie a été évaluée par magnétométrie à échantillon vibrant et résonance ferromagnétique. À l’occasion de ces recherches, des domaines en rubans faibles ont été observés par microscopie à force magnétique dans certaines couches minces en Fe-N. Ceux-ci sont particulièrement rectilignes et des dislocations coin se trouvent au sein de leur structure périodique. Des études ont alors été réalisées dans le but de contrôler avec précision la réorientation des domaines en rubans et le déplacement des dislocations magnétiques, à l’aide d’un champ magnétique. / The alpha'-Fe8N1-x and alpha''-Fe16N2 phases have a high potential of application, because of their uniaxial magnetocrystalline anisotropy and their large saturation magnetization. However, the values announced for these magnetic properties remain a subject of discussion. The research conducted during this PhD thesis was initiated in order to clarify this situation. Sample making consisted mainly of nitrogen ion implantation into alpha-Fe thin films, epitaxially grown on ZnSe/GaAs (001). Among others, the effects of target temperature and fluence on the crystal structure of the samples were analyzed by X-ray diffractometry. The presence of a perpendicular magnetic anisotropy was demonstrated in the thin films containing the alpha'-Fe8N1-x and alpha''-Fe16N2 phases. The anisotropy constant was evaluated by vibrating sample magnetometry and ferromagnetic resonance. In this research, weak stripe domains were observed by magnetic force microscopy in some Fe-N thin films. These are particularly straight and edge dislocations are found within their periodic structure. Studies were then carried out to precisely control the reorientation of the stripe domains and the displacement of the magnetic dislocations, using a magnetic field.

No description available.

The ferromagnetic shape memory alloy, Fe7Pd3, not only offers promising applications, but also reveals a number of unresolved scientific questions, including coupling between a series of martensite and order–disorder transitions, which are in the focus of the present study. To address and understand these aspects, which are of particular importance for controlling phase stability in Fe7Pd3, an ab initio based Monte Carlo simulation code is developed, whose results demonstrate that equilibrated ordered or disordered phases show distinct dependencies coupled to temperature and lattice structure. Moreover, in equiatomic domains emerging from initially randomized disorder, an intermediate, entropy stabilized phase is identified with significantly higher magnetic anisotropy energy, being advantageous for miniaturized applications. This phase, among other observed configurations, is comprehensively characterized by free energy landscapes and magneto-structural coupling derived from vibrational analysis of molecular dynamics trajectories and full relativistic spin polarized density functional theory ground state calculations, respectively.

Выполнено систематическое исследование магнитных свойств аморфных плёнок (РЗМ)xCo100-x, содержащих редкоземельные металлы РЗМ различного типа: La не имеет магнитного момента; Gd обладает сферической электронной оболочкой; Tb характеризуется анизотропией электронной структуры. На основе анализа спонтанной намагниченности плёнок определены концентрационные зависимости средних атомных магнитных моментов кобальта (mCo), гадолиния (mGd) и тербия (mTb). Показано, что в интервале 0 < x < 50 % mCo уменьшается от 1,7 µB до нуля, mGd не меняется и практически совпадает с магнитным моментом свободного атома (7 µB), а mTb монотонно снижается, причем скорость уменьшения зависит от способа получения образцов. Найденные закономерности связываются с концентрационным изменением электронной структуры Со и спецификой магнитной структуры плёнок, которая имеет ферромагнитный, ферримагнитный или сперимагнитный характер для образцов, содержащих La, Gd или Tb соответственно. Установлено, что в пленках систем Gd-Co и Tb-Co может присутствовать одноосная магнитная анизотропия, сильно различающаяся по своей величине. Механизм её формирования может быть связан с упорядочением пар атомов в GdxCo100-x и магнитострикционной деформацией или проявлением локальной анизотропии атомами тербия в TbxCo100-x. / A systematic study of magnetic properties of (RE)xCo100-x amorphous films, containing rare earth metals of various types, was conducted: La has no magnetic moment; Gd has a spherical electron shell; Tb is characterized by anisotropy of the electron structure. Concentration dependences of average atomic magnetic moments of cobalt (mCo), gadolinium (mGd), and terbium (mTb) were determined based on the films spontaneous magnetization analysis. It was shown that mCo decreases in the range of 0 < x < 50 % from 1.7 μB to zero, mGd does not change and it is almost identical to the magnetic moment of a free atom (7 μB), and mTb decreases monotonically, and the reduction rate depends on the method of samples preparation. The observed behavior is associated with the change in the electron shell of Co and specificity of the films magnetic structure, which is ferromagnetic, ferrimagnetic or sperimagnetic for the samples, containing La, Gd, and Tb, respectively. It was found that Gd-Co and Tb-Co systems films can demonstrate uniaxial anisotropy of considerably different magnitude. The mechanism of its formation may be associated with the atoms pairs ordering in GdxCo100-x and magnetostrictive deformation or manifestation of the local anisotropy of terbium atoms in TbxCo100-x.

No description available.

The work presented in this thesis was broadly focussed on the investigation of the magnetic behaviour of different superconducting materials in the form of bulk (singe crystals and pellets) and thin films (nanomagnetic devices like superconducting spin valves etc). Neutrons and muons were extensively used to probe the structural and magnetic behaviour of these systems at the nanoscale along with bulk characterisation techniques like high-sensitive magnetic property measurements, scanning probe microscopy and magneto-transport measurements etc. The nanoscale interplay of Superconductivity and Ferromagnetism was studied in the thin film structures using a combination of Polarised Neutron Reflectivity (PNR) and Low Energy Muon Spin Rotation (LE-µSR) techniques while bulk Muon Spin Rotation (µSR) technique was used for microscopic magnetic investigation in the bulk materials. In the Fe/Pb heterostructure, evidence of the Proximity Effect was observed in the form of an enhancement of the superconducting penetration depth (λs) with an increase in the ferromagnetic layer thickness (dF) in both the bilayered and the trilayered structures. The existence of an Inverted Magnetic Region was also detected at the Ferromagnet-Superconductor (F/S) interface in the normal state possibly originating from the induced spin polarisation within the Pb layer in the presence of the neighbouring Fe layer(s). The spatial size (height and width) of the Inverted Magnetic Region did not change much while cooling the sample below the superconducting transition temperature(Tc)and it also stayed unaffected by an increase in the Fe layer thickness and by a change of the applied magnetic field. In the superconducting spin valve structure containing Permalloy (Py) as ferromagnetic layer and Nb as the superconducting layer, LE-µSR measurements revealed the evidence of the decay of magnetic flux density (as a function of thickness) within the Nb layer symmetrically from the Py/Nb interfaces towards the centre of the Nb layer in the normal state. The thickness dependent magnetisation decay occurred over two characteristic length scales in the normal state that stayed of similar values in the superconducting state also. In the superconducting state, an additional contribution towards the magnetisation was found in the vicinity of the Py/Nb interfaces possibly originating from the spin polarisation of the singlet Cooper pairs in these areas. The nanoscale magnetic investigation on a highly engineered F/S/F structure (where each of the F blocks made of multiple Co/Pd layers with magnetic moments aligned perpendicular to the plane of these layers and neighbouring magnetic blocks separated by Ru layers giving rise to antiferromagnetic alignment) using LE-µSR showed an antisymmetric thickness dependent magnetic flux density profile with two characteristic length scales. In the superconducting state, the magnetic flux density profile got modified within the superconducting Nb₆₇Ti₃₃ layer near the F/S interfaces in a way similar to that of observed in the case of Py/Nb system, most likely because of the spin polarisation of the superconducting electron pairs. The vortex magnetic phase diagram of Bi₂Sr₂Ca₂Cu₃O10-δ was studied using the Muon Spin Rotation (µSR) technique to explore the effects of vortex lattice melting and rearrangements for vortex transitions and crossover as a function of magnetic field and temperatures. At low magnetic fields, the flux vortices undergo a first order melting transition from a vortex lattice to a vortex liquid state with increasing temperature while another transition also occurred with increasing field at fixed temperature to a vortex glass phase at the lowest temperatures. Evidence of a frozen liquid phase was found in the intermediate field region at low temperature in the form of a lagoon in the superconducting vortex state which is in agreement with earlier observations made in BiSCCO-2212. The magnetic behaviour of the unconventional superconductor Sr₂RuO₄ was investigated using µSR to find the evidence of normal state magnetism and the nature of the vortex state. In the normal state, a weak hysteretic magnetic signal was detected over a wide temperature and field range believed to be supporting the evidence of a chiral order parameter. The nature of the vortex lattice structure was obtained in different parts of the magnetic phase diagram and the evidence of magnetic field driven transition in the lattice structure was detected from a Triangular→Square structure while the vortex lattice stayed Triangular over the entire temperature region below Tc at low fields with a disappearance of pinning at higher temperatures.

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.

La miniaturisation du nœud technologique de CMOS en dessous de 90 nm conduit à une forte consommation statique pour les mémoires et les circuits logiques, due aux courants de fuite de plus en plus importants. La spintronique, une technologie émergente, est d’un grand intérêt pour remédier à ce problème grâce à sa non-volatilité, sa grande vitesse d’accès et son intégration facile avec les procédés CMOS. Comparé à la commutation induite par le champ magnétique, le transfert de spin (STT), une approche de commutation induite par le courant, non seulement simplifie le processus de commutation mais aussi permet un fonctionnement sans précédent en termes de consommation et de vitesse. Cette thèse est consacrée à la modélisation compacte et la conception de circuit hybride pour les dispositifs spintroniques basés sur la commutation induite par le courant. La jonction tunnel magnétique (JTM), élément fondamental de la mémoire magnétique (MRAM), et la mémoire racetrack, nouveau concept fondé sur la propagation des parois de domaine induites par le courant, sont particulièrement étudiés. Ces dispositifs et circuits spintroniques sont basés sur les matériaux à anisotropie magnétique perpendiculaire (AMP) qui ouvrent la perspective d’une miniaturisation submicronique tout en conservant une grande stabilité thermique. De nombreux modèles physiques et paramètres réalistes sont intégrés dans la modélisation compacte pour obtenir une bonne cohérence avec les mesures expérimentales. En utilisant ces modèles compacts précis, certaines applications pour la logique et les mémoires magnétiques, tels que l’additionneur complet magnétique (ACM) et la mémoire adressable par contenu (CAM), sont conçues et simulées. Nous analysons et évaluons leur potentiel de performance en termes de surface, vitesse et consommation d’énergie par rapport aux circuits classiques. Enfin, afin de lutter contre la limitation de capacité entravant la large application, nous proposons deux optimisations de conception : la mémoire multivaluée (MLC) pour la STT-MRAM et l’assistance par champ magnétique pour la mémoire racetrack. Ce concept de MLC utilise le comportement stochastique des STT pour atteindre une haute vitesse tout en augmentant la densité de STT-MRAM. La mémoire racetrack assistée par champ magnétique est fondée sur l’observation d’une propagation des parois de domaine en dessous du courant critique, propagation est attribué à l’effet « Walker breakdown ». Ceci ouvre une nouvelle voie pour réduire le courant de propagation et augmenter la capacité des mémoires racetrack au-delà des améliorations des circuits périphériques et des matériaux. / The shrinking of complementary metal oxide semiconductor (CMOS) fabrication node below 90 nm leads to high static power in memories and logic circuits due to the increasing leakage currents. Emerging spintronic technology is of great interest to overcome this issue thanks to its non-volatility, high access speed and easy integration with CMOS process. Spin transfer torque (STT), a current-induced switching approach, not only simplifies the switching process but also provides an unprecedented speed and power performances, compared with the field-induced switching. This thesis is dedicated to the compact modelling and hybrid circuit design for current-induced switching spintronic devices. Magnetic tunnel junction (MTJ), the basic element of magnetic random access memory (MRAM), and racetrack memory, a novel concept based on current-induced domain wall (CIDW) motion, are particularly investigated. These spintronic devices and circuits are based on the materials with perpendicular-magnetic-anisotropy (PMA) that promises the deep submicron miniaturization while keeping a high thermal stability. Numbers of physical models and realistic parameters are integrated in the compact modeling to achieve a good agreement with experimental measurements. By using these accurate compact models of PMA STT MTJ and PMA racetrack memory, some magnetic logic and memory applications, such as magnetic full adder (MFA) and content addressable memory (CAM), are designed and simulated. We analyze and assess their performance potential in terms of speed, area and power consumption compared with the conventional circuits. Finally, in order to tackle the capacity bottleneck hindering the wide application, we propose two design optimizations: MLC for MRAM and magnetic field assistance for racetrack memory. This MLC design benefits from the STT stochastic behavior to achieve an ultra-high speed while increasing the density. The racetrack memory with magnetic field assistance is based on the observation that CIDW motion can be triggered below the critical current due to “Walker breakdown” effect. This opens a new route to reduce the propagation current and increase the capacity of racetrack memory beyond the improvements of peripheral circuits or materials.

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.