Dans le contexte du « Tout Electrique », les fabricants de l’aéronautique cherchent à augmenter la puissance embarquée tout en limitant la masse de ces dispositifs électriques. Une des solutions envisagées est d’augmenter la densité de flux magnétique des matériaux magnétiques de ces appareils. L’inconvénient de l’emploi de ces matériaux réside dans leurs déformations sous l’effet du champ magnétique. Dans le cas des noyaux magnétiques de transformateurs, ceux-ci sont composés d’un empilement d’une centaine de tôles magnétiques d’épaisseur variant entre 0,2 et 0,5mm. La déformation successive des tôles du transformateur est à l’origine d’un bruit acoustique indésirable. La source principale de ces déformations est la magnétostriction qui provient du réarrangement sous champ magnétique de la structure en domaines du matériau. Dans le cadre de ces travaux de thèse, nous nous intéressons à l’alliage Fe-27%Co produit par la société APERAM Alloys Imphy, commercialement appelé AFK1. Le choix de cet alliage provient du fait qu’il présente une aimantation à saturation la plus élevée de tous les matériaux ferromagnétiques (2,4T). Son emploi permettrait alors un gain certain de densité de puissance. Selon une gamme métallurgique particulière, l’AFK1 présente une basse magnétostriction isotrope, qui s’illustre par une déformation nulle jusqu’à 1,5T puis par une déformation à saturation de l’ordre de 10ppm. L’objectif principal de ces travaux de thèse consiste à déterminer l’origine d’un tel comportement et les mécanismes associés. Les résultats expérimentaux montrent que les conditions de traitements thermiques semblent avoir un effet sur le comportement magnétostrictif. On montre par ailleurs que la magnétostriction est indépendante de l’orientation cristallographique de l’AFK1. Des essais de magnétostriction sous contrainte mécanique ont permis de supposer que l’AFK1 disposait d’une structure en domaines principalement composée de parois à 180°. La mise en place de cette structure a pu être confirmée par microscopie magnéto-optique (effet Kerr). Afin de mieux comprendre l’origine de l’orientation des domaines dans le matériau, l’influence de la géométrie d’échantillon sur le comportement magnétostrictif a également été étudiée au cours de ces travaux de thèse. Une modélisation du comportement faiblement magnétostrictif a finalement été proposée par le biais d’une approche multi-échelle. Le modèle met en évidence la nécessité de considérer une proportion non négligeable de domaines séparés par des parois à 180° pour restituer la basse magnétostriction de l’AFK1. / The main challenge in the aeronautical field concerns the increase of higher power density electrical devices onboard aircrafts. One of the solutions proposed is to increase the magnetic flux density of magnetic materials which compose these devices. The main drawback of this solution leads in the high deformation the materials concerned exhibit under magnetic field. For example, the core of onboard transformers is composed of a stack of about hundred of magnetic steel sheets, with a thickness range between 0.2 and 0.5mm. The deformation of the entire structure leads to an unwanted acoustic noise that originates from the high magnetostriction deformation of the material deriving from the change of magnetic domains configuration under magnetic field. In this thesis work, the magnetostrictive behaviour of the Fe-27$%$Co alloy is studied. This magnetic alloy is produced and marketed by APERAM Alloys Imphy as AFK1. This material leads to a low and isotropic magnetostrictive behaviour after an appropriate metallurgical process. The deformation is null up to 1.5T and the magnetic saturation is reached with a deformation lower than 10ppm. The main goal of this thesis is to understand the origin of the low magnetostrictive behaviour and to model it. The experimental results show that thermal annealing changes significantly the magnetostriction. In addition, we prove that low magnetostriction exhibits no crystallographic orientation dependence. Magnetostriction tests carried out under a mechanical loading show that a microstructure mainly composed magnetic domains separated by 180$^circ$ domain walls can explain the behaviour. The presence of this magnetic configuration was confirmed by magneto optical microscopy observations (Kerr effect) associated with a macroscopic geometry effect and residual magnetic field in the furnaces. A multiscale modeling of the low magnetostriction has been proposed next. This modeling helps us to confirm the requirement of about 80% of grains composed of a bi-domain magnetic structure to simulate low magnetostrictive behaviour in accordance with experiments.

Chapas de aço elétrico (0,7% Si e 0,3% Al) foram laminadas a diferentes graus de deformação e recozidas (760º C, 2h) de modo a obter diferentes tamanhos de grão por recristalização. Um outro conjunto de chapas do mesmo material foi recozido a tempos e temperaturas diferentes de modo a obter tamanho de grão variado por crescimento de grão. Os tamanhos de grão foram medidos pelo método dos interceptos e os parâmetros da distribuições de tamanho de grão determinados por um método semi-analítico que se baseia na medição das áreas individuais de cada grão na micrografia. A partir do ensaio quase-estático de histerese magnética no quadro de Epstein obtiveram-se os campos coercivos de cada amostra, a diferentes induções máximas (0,6 0,8 1,0 1,2 1,4 e 1,5T). Notou-se que o conjunto dos dados se enquadra com menos de 4% de desvio médio absoluto uma lei fenomenológica do tipo: Hc aBmax^c /TG bBmax^d Na qual Hc é o campo coercivo, TG é o tamanho de grão, Bmax é a indução máxima do ensaio de histerese e a, b, c e d são os parâmetros de ajuste. A partir da inserção do efeito do campo desmagnetizante no contorno de grão no modelo físico de Mager obteve-se uma boa estimativa para os valores dos parâmetros independentes da indução a e b, além de propor um significado físico para os mesmos. Com a integração dos modelos Mager e Preisach, determinou-se uma equação diferente para o ajuste dos dados, a qual oferece um valor aproximado de c. Para que o modelo físico explique completamente a relação fenomenológica resta ainda elucidar o parâmetro d. / Electrical steel sheets (0,7% Si and 0,3% Al) were cold-rolled and annealed (760º C, 2h) in order to obtain different grain sizes through recrystallization. Another set of sheets from the same material were annealed at different temperatures and time-frames, so as to produce different grain sizes through grain growth. Grain size was measured through the intercept method and grain size distribution parameters were determined via a semi-analytical method based on measurement of individual grain areas upon micrographs. From quasistatic hysteresis measurements in an Epstein frame, the coercive field of each sample was determined at various maximum induction values (0,6 0,8 1,0 1,2 1,4 e 1,5T). The data set fits with less than 4% average deviation a phenomenological law of the type: Hc aBmax^c /TG bBmax^d In which Hc is the coercive field, TG is the grain size, Bmax is the maximum induction value and a, b, c and d are the fitting parameters. By introducing the grain boundary demagnetizing field effect on Magers physical model, a good estimate of the values of a and b was obtained, as well as a reasonable explanation to their physical meaning. Integrating Mager and Preisach models, a different equation for the data was obtained, which offered an approximate value of c. In order for physical models do fully explain the phenomenological equation, there is still need to understand the parameter d.

Chapas de aço elétrico (0,7% Si e 0,3% Al) foram laminadas a diferentes graus de deformação e recozidas (760º C, 2h) de modo a obter diferentes tamanhos de grão por recristalização. Um outro conjunto de chapas do mesmo material foi recozido a tempos e temperaturas diferentes de modo a obter tamanho de grão variado por crescimento de grão. Os tamanhos de grão foram medidos pelo método dos interceptos e os parâmetros da distribuições de tamanho de grão determinados por um método semi-analítico que se baseia na medição das áreas individuais de cada grão na micrografia. A partir do ensaio quase-estático de histerese magnética no quadro de Epstein obtiveram-se os campos coercivos de cada amostra, a diferentes induções máximas (0,6 0,8 1,0 1,2 1,4 e 1,5T). Notou-se que o conjunto dos dados se enquadra com menos de 4% de desvio médio absoluto uma lei fenomenológica do tipo: Hc aBmax^c /TG bBmax^d Na qual Hc é o campo coercivo, TG é o tamanho de grão, Bmax é a indução máxima do ensaio de histerese e a, b, c e d são os parâmetros de ajuste. A partir da inserção do efeito do campo desmagnetizante no contorno de grão no modelo físico de Mager obteve-se uma boa estimativa para os valores dos parâmetros independentes da indução a e b, além de propor um significado físico para os mesmos. Com a integração dos modelos Mager e Preisach, determinou-se uma equação diferente para o ajuste dos dados, a qual oferece um valor aproximado de c. Para que o modelo físico explique completamente a relação fenomenológica resta ainda elucidar o parâmetro d. / Electrical steel sheets (0,7% Si and 0,3% Al) were cold-rolled and annealed (760º C, 2h) in order to obtain different grain sizes through recrystallization. Another set of sheets from the same material were annealed at different temperatures and time-frames, so as to produce different grain sizes through grain growth. Grain size was measured through the intercept method and grain size distribution parameters were determined via a semi-analytical method based on measurement of individual grain areas upon micrographs. From quasistatic hysteresis measurements in an Epstein frame, the coercive field of each sample was determined at various maximum induction values (0,6 0,8 1,0 1,2 1,4 e 1,5T). The data set fits with less than 4% average deviation a phenomenological law of the type: Hc aBmax^c /TG bBmax^d In which Hc is the coercive field, TG is the grain size, Bmax is the maximum induction value and a, b, c and d are the fitting parameters. By introducing the grain boundary demagnetizing field effect on Magers physical model, a good estimate of the values of a and b was obtained, as well as a reasonable explanation to their physical meaning. Integrating Mager and Preisach models, a different equation for the data was obtained, which offered an approximate value of c. In order for physical models do fully explain the phenomenological equation, there is still need to understand the parameter d.

Amorphous and nanocrystalline transition metal magnetic alloys (TMMAs) have been the subjects of fundamental and applied study due to their unique structure. The lack of long-range order in these materials sets the stage for their soft magnetic properties to be tuned for a variety of technological applications, such as sensitive magnetic field sensors, high frequency transformers, and stress sensors. Fundamental investigation of the magnetic and structural properties of these materials is also motivated by their unique amorphous or nanocrystalline-embedded amorphous matrix morphology, which has consequences on both the magnetism seen from both the atomic and macro-scale. The surfaces of these materials become important to their high frequency applications, where the skin depth of the excitation field is distributed near the surface. In conjunction with high frequency magnetoimpedance measurements, surface sensitive probes of magnetism and structure must be employed to provide a complete picture of the relationship between the surface and dynamic magnetism. This dissertation focuses on the surface impact of chemical composition, annealing conditions, and coatings on TMMAs on their magnetoimpedance response through multiple surface sensitive techniques such as atomic/magnetic force microscopy, magneto-optical Kerr effect, and scanning/transmission electron microscopy. These tools provide a view into the relationship between the nanostructure, microstructure and soft magnetic properties that make these materials highly desired for fundamental study and technological application.

Cette thèse en cotutelle entre l'Université Joseph Fourier à Grenoble et l'Université Nationale du Vietnam à Hanoi traite de l'étude des propriétés magnétiques et électriques de structures basées sur GdCo et sur des multicouches Fe/Cr. GdCo est un alliage ferrimagnétique dont la composition peut être choisie afin d'obtenir la compensation, c'est à dire une aimantation nette nulle, à une température de compensation inférieure à la température de Curie. Les films ont été déposés par la technique de pulvérisation cathodique magnétron. Ils sont amorphes et possèdent une anisotropie perpendiculaire induite par la croissance. Proche de la température de compensation, l'aimantation spontanée est perpendiculaire et l'imagerie par effet Kerr polaire ainsi que l'effet Hall extraordinaire ont été largement utilisés pour étudier les films dans la gamme (4-300 K) et de 0 à 6 Tesla. Un gradient de composition dans le plan peut être induit et a entrainé l'étude d'une paroi de compensation d'aimantation nulle. Les contributions de Gd et Co à l'effet Hall extraordinaire ont été déterminées. Proche de la compensation, l'effet Hall à haut champ permet d'accéder au régime de spin flop. Des multicouches GdCo/Cu/NiFe pour lesquelles GdCo est à anisotropie perpendiculaire et NiFe à anisotropie planaire ont été élaborées. Elles présentent de la magnétorésistance géante (GMR) dont la valeur ne dépend pas de l'angle entre les aimantations nettes desdeux couches magnétiques. <br /> L'étude de la GMR des multicouches Fe/Cr traite de la contribution des interfaces au mécanisme GMR. En variant l'épaisseur du fer et en recuisant les multicouches, il est possible d'accéder aux contributions d'interface.

Magnetic shape memory (MSM) materials are a new class of smart materials which exhibit shape deformation under the influence of an external magnetic field. They are interesting for various types of applications, including actuators, displacement/force sensors, and motion dampers. Due to the huge strain and the magnetic field-driven nature, MSM materials show definite advantages over other smart materials, e.g. conventional thermal shape memory materials, in terms of displacement and speed. The principle behind the magnetic field induced strain (MFIS) is the strong coupling between magnetization and lattice structure. The investigation of both static and dynamic magnetic domain structures in MSM materials is a key step in optimizing the properties for future possible devices. In this work, optical polarization microscopy is applied to investigate the twin boundary and magnetic domain wall motion in bulk NiMnGa single crystals. Surface magnetic domain patterns on adjacent sides of bulk crystals are revealed for the first time providing comprehensive information about the domain arrangement inside the bulk and at the twin boundary. The tilting of the easy axis with respect to the sample surface determines the preferable domain size and leads to spike domain formation on the surface. Out-of-plane surface domains extend into the bulk within a single variant, while a twin boundary mirrors the domain pattern from adjacent variants. Furthermore, magnetic domain evolution during twin boundary motion is observed. The partial absence of domain wall motion throughout the process contradicts currently proposed models. The magnetic state alternates along a moving twin boundary. With the abrupt nucleation of the second variant this leads to the formation of sections of magnetically highly charged head-on domain structures at the twin boundaries. On the other hand, a dynamic actuation experimental setup, which is capable to provide high magnetic fields in a wide range of frequency, was developed in the course of this study. The observation of reversible twin boundary motion up to 600 Hz exhibits the dependence of strain, hysteresis, and twin boundary velocity on the actuation speed. MFIS increases with frequency, while the onset field is similar in all observed cases. Twin boundary mobility enhancement by fast twin boundary motion is proposed to explain the increase in MFIS. The twin boundary velocity is shown to be inversely proportional to the twin boundary density. No limit of twin boundary velocity is observed in the investigated frequency range.

The high degree of modification through chemical substitution afforded by the perovskite crystal structure and its related counterparts allows a systematic study of structure-property relationships critical to understand the wide variety of exotic phenomena observed in these materials where the spin, charge, orbital, and lattice degrees of freedom are highly correlated. From the multiple phenomena observed in these materials, which includes multiferroicity, catalytic activity, and high temperature superconductivity, this study is concerned with a material that displays colossal magnetoresistance (CMR), La₂₋₂xSr₁₊₂xMn₂O₇; this is a naturally bilayered manganite that exhibits CMR at a paramagnetic to ferromagnetic transition that coincides with an insulator to metal transition. The strong correlation between different degrees of freedom in the material leads to considerable variation in its magnetic properties due to doping even in the small range studied of 0.32 [less than or equal to] x [less than or equal to] 0.4, where the easy axis of magnetization changes from the c-axis to the ab plane. Magnetic force microscopy (MFM) was used for this part of the work, to visualize the local variation of the out of plane (c-axis) magnetization or magnetic microstructure of La₂₋₂xSr₁₊₂xMn₂O₇ for 0.32 [less than or equal to] x [less than or equal to] 0.4 at the exposed ab surface and its evolution due to an applied magnetic field at 4 K. For the x = 0.32 composition, which is close to the out of plane to in plane magnetization transition, a strong preferred magnetization direction within the ab plane or magnetocrystalline anisotropy was observed. The stray magnetic field of the MFM tip perturbs the magnetic microstructure of low coercivity materials like diluted magnetic semiconductors, making it unsuitable for the study of such materials. For this reason, as part of this project a scanning Hall probe microscope (SHPM), a magnetic imaging technique complementary to MFM that uses a Hall sensor that provides a magnetically non-invasive calibrated measurement of the stray fields at the surface of a sample with good resolution (~ 1 [micrometer]), was designed. The construction of a compact cryogenic variable-temperature (77 - 300 K) SHPM, highlighting its features, is described. / text

Thin films of terbium-cobalt alloy have been deposited by magnetron sputtering. They exhibit an out-of-plane magnetic anisotropy that may result in elongated domains. The effect of thicknesses and compositions on the domain structure has been studied systematically. The composition and thickness of the films have been verified by Rutherford backscattering spectrometry and x-ray reflectivity. The domain structure was imaged by magnetic force microscopy and hysteresis loops were measured by MOKE (Magneto-optical Kerr effect). Very long and contrasted domains are obtained on the as-grown film with a thickness of 290 Å containing 8.1\% of terbium. Similar very elongated domains are obtained for other thicknesses and compositions if the films are demagnetized.

State-of-the art Fourier transform holography (FTH) techniques use x-ray magnetic circular dichroism (XMCD) as a contrast mechanism for element-specfi c imaging of magnetic domains. With the soft x-ray Nanoscience beamline at Diamond Light Source in the UK, and the Dragon beamline at the European Synchrotron Radiation Facility (ESRF) in France, the possibility of new methods to study nanostructured magnetic systems has been demonstrated. The ability to record images without the use of lenses, in varying magnetic fi elds and with high spatial resolution down to 30 nm has been used to study in-plane magnetism of 50 nm thin permalloy (NiFe alloy) nanoelements. The holographic technique used extended reference objects rather than conventional pinhole references, which allowed a high flexibility on the direction of magnetisation that is probed. The element specific nature of the imaging, with the additional choice in the directions of magnetisation that are probed has been used to study dipolar interactions in a hard/Ta/soft [Co/Pt]30/Ta/Py multi-layered system. Images of the out-of-plane magnetised domains of [Co/Pt]30 were found to bare strong spatial resemblance to the in-plane domains of the permalloy. The domain structure is thought to be magnetostatically imprinted into permalloy during the growth stage of the lm, where stray elds generated by the adjacent Co/Pt multilayer influence the formation of domains in the permalloy. Strong resemblance between the two layers could be found at remanence within a pristine sample, however the similarities disappear after the sample was exposed to a saturating magnetic field. This disagreed with micromagnetic simulations performed in The Object Oriented MicroMagnetic Framework (OOMMF) program, and an explanation for the observations has been sought in the growth process of the multi-layered fi lm, with conditions that are diffi cult to recreate in the model. Optical holography has been used for preliminary insight into implementing a method of FTH in a reflective geometry at soft x-rays wavelength. With scattering chambers at BESSY II in Germany and at the Stanford Synchrotron Radiation Lightsource (SSRL) in California the possibility of reducing scattered noise in a hologram recorded in a reflective geometry has been investigated. Studies into specular and dif use reflections have been performed optically however the use of extended references alone may alleviate the current problem at x-ray wavelengths which lie in the weak signal given by a reflective point-like reference source.

Dans cette thèse, nous nous sommes attachés à mesurer la résistance induite par les parois magnétiques dans différents métaux ferromagnétiques (fer, cobalt, oxydes de manganèse, alliage de Fe0.5Pa0.5). Le premier circuit étudié consistait en un fil de 200 nanomètres de large et 50 nanomètres d'épaisseur, fabriqué par lithographie électronique. Par microscopie à force magnétique et microscopie de Lorentz nous avons observé que les domaines sont en position tête-bêche dans l'état vierge dans des fils de cobalt. La configuration magnétique a été suivie en fonction du champ appliqué. Par lithographie électronique, nous avons pu disposer sur le fil des contacts en or. Dans le cobalt, la résistance d'une paroi magnétique unique a été mesurée et sa valeur est supérieure aux prédictions théoriques. Dans le manganite, sur un circuit équivalent, nous avons pu mesurer la magnétorésistance de joints de grain. Nous avons aussi effectué la mesure de la résistance d'une paroi sur une couche à aimantation perpendiculaire (alliage de fer-palladium), dont les domaines étaient alignés. Nous avons mesuré la résistance perpendiculairement (MR=9%) et parallèlement (MR=2.8%) aux parois, et ainsi corroboré certains modèles. Nous avons, par ailleurs, étudié (expérimentalement et numériquement) des systèmes magnétiques dans lesquels l'anisotropie de forme joue un rôle prépondérant. Dans des plots triangulaires de taille variant entre 0.1 et 2 microns, la frustration géométrique stabilise des états de vortex. Dans des lignes de fer-palladium, nous avons montré que les dimensions des domaines étaient conservées lorsque la taille latérale du système était réduite jusqu'à 300 nanomètres En revanche, l'orientation des domaines devient anisotrope et les parois magnétiques s'orientent perpendiculairement à l'axe de la ligne. Ces études permettent de mieux comprendre les effets de l'anisotropie de forme.