This diploma thesis is focused on investigation of metallic thin films and nanostructures using scanning probe microscopy. Magnetic properties of these objects are studied by magnetic force microscopy, which is modification of scanning probe microscopy. In the theoretical part basic principles of scanning probe microscopy and magnetic force microscopy are summarized, and also principle of creation of magnetic domains and some special properties of ferromagnetic and antiferromagnetic materials. Further, two techniques of fabricating nanostructures are described. Experimental part is focused on imaging and simulating of magnetic domains. Further, exchange bias is revealed. This phenomenon is present in systems composed from ferromagnetic and antiferromagnetic materials. One part of this diploma thesis is also focused on discussion of problems with magnetic force microscopy.

Title: Investigation of spin structure and dynamics in magnetically ordered thin films Author: Tomáš Janda Abstract: This thesis is devoted to the development of methods for imaging and ultrafast manipulation of magnetic textures, such as magnetic domains and domain walls (DWs), and to the investigation of the corresponding magnetiza- tion dynamics. We focused on techniques that are, besides ferromagnets (FMs), applicable also to antiferromagnets (AFs), in particular, AF thin films. The employed excitation techniques were mostly based on direct or indirect effects of ultrashort laser pulses. We studied the DW motion induced by a transfer of angular momentum from circularly polarized femtosecond laser pulses in a FM GaMnAsP and we found that the observed macroscopic DW displacement is only possible due to its inertia. In a distinct experiment, picosecond current pulses were generated by an absorption of the ultrashort laser pulses in a verti- cal PIN diode-based photoconductive switch and used to excite a magnetic film deposited directly on top of the diode. For imaging of AF domain patterns, we developed a simple table-top laser-scanning technique, which is based on a magneto-thermoelectric response of the AF textures in the presence of a local laser-induced heating. We first used the method to image the...

À la fois semi-conducteur et ferromagnétique, le Ga1-xMnxAs offre des potentialités intéressantes pour l'électronique de spin. Cette double propriété est due à l'interaction d'échange entre les spins localisés des atomes de manganèse Mn et les spins des porteurs de charge. Le travail présenté dans cette thèse est centré sur le contrôle de l'aimantation de ces structures magnétiques. Une étude expérimentale, comparative et détaillée, de l'anisotropie magnétique a été menée sur deux séries d'échantillons. Par ailleurs, une méthode d'analyse basée sur l'étude de la densité d'énergie libre magnétocristalline des systèmes observés a été développée en vue de confronter les résultats aux prédictions théoriques. Les mesures d'effet Hall et d'aimantation par SQUID sur des monocouches à aimantation planaire ont permis de mettre en évidence deux types d'anisotropie : une anisotropie cubique pour T < TC/2 avec un retournement d'aimantation par sauts de 90°, et une anisotropie uniaxiale pour des températures TC/2 < T < TC avec un renversement d'aimantation à 180°. La technique du recuit post-croissance réduit cependant l'anisotropie cubique au profit de l'anisotropie uniaxiale. Les structures à aimantation perpendiculaire présentent, quant à elles, un retournement d'aimantation à 180° pour toutes les températures T < TC. Par conséquent, dans ces dispositifs, l'anisotropie magnétique est fortement uniaxiale. En dernier lieu, cette étude porte sur la dynamique des domaines magnétiques et la détermination des vitesses de propagation de parois de domaines, induites par un champ magnétique. Les résultats révèlent une anisotropie de propagation de parois suivant les axes cristallographiques <110> avec deux régimes de vitesses distincts, dont l'un est fortement contrôlé par des défauts de structure. / At the same time semiconductor and ferromagnetic, the Ga1-xMnxAs offers interesting potentialities for spintronic. This double property is due to the exchange interaction between localized spin of Mn atoms and the spin charge carrier. The work presented in this thesis is centred on the magnetization control of these magnetic structures. A comparative and detailed experimental study of the magnetic anisotropy is carried out on two series of samples. An analysis method based on the free energy density study of the observed systems was also been developed to confront the results with theoretical predictions. The Hall effect and SQUID measurements on the structures with planar magnetization allowed us to bring to light two types of anisotropy: a cubic anisotropy for T < TC / 2 with a magnetization reversal by jumps of 90 ° and an uniaxial anisotropy for temperatures TC / 2 < T < TC with a reversal of magnetization in 180°. The structure with perpendicular magnetization present a magnetization switch at 180° for all temperatures T < TC. Consequently in these compounds, the magnetic anisotropy is strongly uniaxiale. Lastly this study deals the magnetic domains structures and the determination of domain wall propagation velocity. The results reveal an anisotropic domain wall propagation along crystallographic axes <110> with two distinct velocity regimes, including one strongly controlled by structural defects.

Dans cette thèse, nous avons étudié la dynamique des parois de domaine sous courant dans des couches ferromagnétiques ultra-minces de type métal lourd/ métal ferromagnétique/ oxyde présentant un fort couplage spin-orbite. Dans ces systèmes, deux éléments liés au fort couplage spin-orbite et l'asymétrie structurelle d'inversion jouent un rôle clé sur la dynamique des parois : d'une part, l'amplitude des couples de spin-orbite (SOT) exercés sur la paroi lors de l'injection de courant; d'autre part, l'amplitude de l'interaction Dzyaloshinskii-Moriya qui stabilise la structure Néel interne de la paroi. L'objectif de ce travail a été de caractériser les couples agissant sur la paroi induits par le courant ainsi que l'amplitude de l'interaction DMI.Pour y parvenir, j'ai mis au point une nouvelle technique de mesure basée sur la mesure des déplacements nanométriques induits par le courant d'une paroi piégée dans un nanoplot découpé dans le matériau magnétique. Cette mesure quasi-statique permet de s'affranchir des difficultés liées à la modélisation de la dynamique des parois magnétique sous courant en présence de défauts.Par ailleurs, le dispositif a été conçu de façon à ce que le courant et le champ magnétique externe statique puissent être appliqués dans différentes directions orthogonales, ce qui permet séparer clairement les contributions des couples de transfert de spin (NA-STT) et de spin-orbite (DL-SOT).Les mesures ont permis de caractériser le couple exercé sur la paroi par le courant en fonction d'un champ magnétique planaire pour un empilement Pt/Co/AlOx et ceci pour quatre orientations champ/courant différentes. Les résultats permettent d'écarter l'hypothèse d'une structure de type Bloch bi-stable.Dans le cas où le courant est injecté au travers de la paroi, la comparaison des résultats avec le modèle aboutit à une valeur du couple NA-STT très faible. Nos mesures faites avec le champ magnétique planaire permettent de conclure à un champ SOT de 7,5+/-0,5 Oe pour 10 MA/m² en accord avec les résultats de couple publiés précédemment dans le cas d'une paroi de Néel. Si les deux configurations donnent de mesures de couple SOT similaire, elles ne permettent pas de conclure sur la valeur de DMI dans ce système. L'origine de ces valeurs contradictoires reste à expliquer. / In this thesis, we studied the current induced domain walls (DWs) dynamics in ultra-thin ferromagnetic films of heavy metal/ ferromagnetic metal/ oxide type with a high spin-orbit coupling. In these systems, two ingredients linked to the high spin-orbit coupling and the structural inversion asymmetry play a key role on the DWs dynamics: the amplitude of the spin-orbit torques (SOT) acting on the domain when a current is injected; and the amplitude of the Dzyaloshinskii-Moriya interaction (DMi) which stabilizes the Néel structure of the DW. The purpose of this work was to characterize the current induced torques acting on the DW and the amplitude of the DMi.For that purpose, I developed a new measurement technique relying on the measurement of current induced nanometer size motion of a DW, trapped inside a nanodot patterned in the magnetic material. This quasi-static measurement enables to avoid the difficulties related to the modelling of the DW dynamics in the presence of defects.Besides that, the device has been designed to enable different perpendicular directions for the current and the external magnetic field, which enable a clear measurement of spin transfer (NA-STT) and spin-orbit (DL-SOT) torques contributions.The measurements allowed the characterization of the torque exerted by the current on the DW with respect to a planar magnetic field for a Pt/Co/AlOx stack in 4 different couples of field/current directions. The results allow to exclude the hypothesis of a Bloch structure for the DW.In the case where the current is injected through the DW, the comparison between the results and the model leads to a very weak value for the NA-STT. Our measurements made with the planar magnetic field leads to a value of 7,5+/-0,5 Oe per 10 MA/m² for the DL-SOT, which is in agreement with previously published results in the case of a Néel DW. If both configurations lead to similar measurements for the SOT, they don't permit to conclude on the exact value of the DMi in this system. The origin of these contradictories values is still to be understood.

L'étude du déplacement des parois de domaines magnétiques au moyen d'un courant électrique, par couple de transfert de spin, a généré beaucoup d'intérêt ces dernières années, notamment depuis que de nouveaux dispositifs de mémoires magnétiques utilisant cet effet ont été proposés. Récemment, un nouveau mécanisme capable de propager les parois sous courant avec une grande efficacité a été mis en évidence dans les matériaux tri-couches à anisotropie perpendiculaire et à fort couplage spin-orbite. La compréhension de ce mécanisme, appelé couple de spin-orbite, reste néanmoins loin d'être acquise, tout comme son effet sur la propagation des parois de domaines.L'objectif de ce travail de thèse était d'étudier l'influence de ce couple de spin-orbite sur la dynamique des parois. Pour cela, j'ai étudié expérimentalement le déplacement de paroi sous l'action d'un courant et d'un champ magnétique dans une tri-couche de Pt/Co/AlOx en présence d'un champ magnétique planaire, utilisé pour modifier la structure interne de la paroi et ainsi moduler l'action du couple de spin-orbite sur la dynamique de celle-ci. Ce travail a permis de mettre en évidence l'existence d'un effet asymétrique dans la dynamique de la paroi pour ce type de système.Pour expliquer ce résultat, nous avons proposé une nouvelle structure de paroi dans les matériaux ultra-minces à anisotropie perpendiculaire, résultant de l'interaction Dzyaloshinskii-Moriya. En combinant des calculs analytiques et des simulations micro-magnétiques, la dynamique d'une telle paroi a été étudiée et comparée aux résultats expérimentaux. Enfin, toujours dans le but d'expliquer l'effet asymétrique observé expérimentalement, une seconde interprétation basée sur la présence d'un mécanisme d'amortissement anisotrope a également été proposée. / The study of current-induced magnetic domain wall motion through spin transfer torque has attracted a lot of attention in recent years, especially since new magnetic memories devices based on this effect have been proposed. Recently, a new mechanism allowing for highly efficient current-induced domain wall motion has been discovered in ultrathin asymmetric materials with perpendicular magnetic anisotropy and high spin-orbit coupling. However this mechanism, named spin-orbit torque, and its effect on domain wall motion are not yet well understood.The objective of this work was to study the influence of this spin-orbit torque on domain wall motion. For that, I have studied field- and current-induced domain wall motion in Pt/Co/AlOx trilayer, in the presence of an in-plane magnetic field. This work allowed highlighting the existence of an asymmetric effect in the domain-wall dynamics of this system.In order to explain this result, we have proposed a new kind of domain wall structure, resulting from Dzyaloshinskii-Moriya interaction in materials with perpendicular magnetic anisotropy and high spin-orbit coupling. Using analytic calculations and micro-magnetic simulations, this domain wall dynamics has been studied and compared to the experimental results. Finally, a second approach based on the presence of an anisotropic damping mechanism has also been proposed to explain the asymmetric effect observed experimentally.

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.

Para o desenvolvimento da nanociência atual há forte demanda por equipamentos capazes de caracterizar sistemas em escalas da ordem nanométrica. Este contexto impulsionou o desenvolvimento de microscópios ópticos de varredura em campopróximo (Scanning Near-field Optical Microscope SNOM). Diferentemente da microscopia óptica tradicional, os SNOMs detectam a radiação eletromagnética evanescente e, conseqüentemente, a resolução não é limitada pelo critério de Rayleigh. No Laboratório de Materiais Magnéticos IFUSP desenvolvemos um SNOM sensível a efeitos Kerr magnetoópticos (MO-SNOM). Dessa maneira, associamos a alta resolução da técnica à alta sensibilidade dos efeitos magnetoópticos. Trata-se se uma área relativamente pouco explorada e carente de resultados sistemáticos na literatura. Utilizando o MO-SNOM, caracterizamos partículas microestruturadas de Co70.4Fe4.6Si15B10 amorfo com dimensões de 16x16x0.08 microm3 e 4x4x0.08 microm3. Os resultados compreendem dezenas de imagens de susceptibilidade magnetoóptica diferencial com resolução melhor que 200 nm e curvas de histerese local. Em primeira análise, a demonstração de resultados sistemáticos ajuda a estabelecer a técnica. O comportamento magnético das partículas, estudadas sob várias condições de campo aplicado, se mostrou determinado basicamente pela anisotropia de forma. As curvas de histerese local mostraram comportamentos intrinsecamente locais e motivaram uma interessante discussão sobre os parâmetros de caracterização magnética convencionais. As medidas realizadas indicam que o efeito Kerr magnetoótico transversal em campopróximo é similar ao campo-distante. Os resultados são fortemente sustentados por medidas de microscopia magnetoóptica de campo-distante, simulações micromagnéticas e medidas de microscopia de força magnética. Medidas complementares revelam o potencial do MO-SNOM para caracterizações de objetos extensos quanto a potenciais de pinning. Além disso, medidas em filmes finos de NiFe/FeMn acoplados por exchange-bias evidenciam a alta sensibilidade do MO-SNOM, estimada de DM ~ 2 x 10-12 emu. / To support nanosciences evolution, there is a strong demand for developing new instrumentation devoted to nano-scale characterization. In this context, the development of the Scanning Near-field Optical Microscope (SNOM) took place. In contrast to traditional optical microscopes, SNOM deals with evanescent electromagnetic radiation and, consequently, the resolution is no longer limited by the Rayleigh criterion. At Laboratório de Materiais Magnéticos (LMM) IFUSP a SNOM devoted to magneto-optical Kerr effect measurements (MO-SNOM) has been developed. The MOSNOM associates the high resolution of the near-field technique to the high sensibility of the magneto-optical Kerr effect. Near-field magneto-optical microscopy is not yet wellestablished and there is a lack of systematic results in the literature. Using the MO-SNOM, amorphous Co70.4Fe4.6Si15B10 particles with 16x16x0.08 microm3 and 4x4x0.08 microm3 dimensions were studied. With resolution better than 200 nm, several magneto-optical differential susceptibility images and local hysteresis loops were obtained. The systematic results uphold the establishment of this new technique. Under the different applied field conditions, the magnetic behavior of the particles was found to be determined by shape anisotropy. Local hysteresis loops presented shapes intrinsic of local field induced process. The unusual hystesesis loops motivated interesting discussion about the conventional magnetic parameters. The MO-SNOM measurements indicate that the near-field transverse magneto-optical Kerr effect is similar to the far-field case. The results are highly supported by far-field magneto-optical microscopy, micromagnetic simulations and magnetic force microscopy measurements. Complementary measurements indicate the MO-SNOM potential to extensive magnetic surface characterization related to pinning potential distribution. Furthermore, measurements on the exchange-bias coupled NiFe/FeMn thin films make evident the MO-SNOM high sensitivity, estimated to be DeltaM ~ 2 x 10-12 emu.

The concept of percolated perpendicular media (PPM) for magnetic data storage is expected to surpass the areal storage density of 1 Tbit in -², which is regarded as the fundamental limit of conventional granular CoCrPt:oxide based recording media. PPM consist of a continuous ferromagnetic thin film with densely distributed defects acting as pinning sites for magnetic domain walls. In this study, practical realizations of PPM were fabricated by the deposition of [Co/Pt]8 multilayers with perpendicular magnetic anisotropy onto nanoperforated templates with various perforation diameters and periods. The structural defects given by the templates serve as pinning sites for the magnetic domain walls within the [Co/Pt]8 multilayers. Magnetometry at both the integral and the local level was employed to investigate the influence of the template on the magnetization reversal and the domain wall pinning. It was found, that magnetic domains can be pinned at the ultimate limit, between three adjacent pinning sites. The coercivity and the depinning field, which both are a measure for the strength of the magnetic domain wall pinning, were found to increase with increasing perforation diameter. The size of magnetic domains within the magnetic film appeared not to depend solely on the diameter of the nanoperforations or on the period of the template, but on the ration between diameter and period. By means of micromagnetic simulations it was found, that the presence of ferromagnetic material within the pinning site given supports the pinning of magnetic domain walls, compared to a pinning site that is solely given by a hole in the magnetic thin film. Investigation of the evolution of the magnetization in magnetic fields smaller than the coercive field revealed, that the energy barrier against thermally induced magnetization reversal is sufficiently large to provide long-term (> 10 years) stability of an arbitrary magnetization state. This could also be qualitatively supported by micromagnetic simulations. Static read/write tests with conventional hard disk recording heads revealed the possibility of imprinting bit patterns into the PPM under study. The minimum bit pitch that could be read back thereby depended on the period of the nanoperforated template.

La dynamique des parois de domaine magnétiques (DW) soulève actuellement un très fort intérêt à la fois du point de vue fondamental mais aussi en lien avec ses applications dans des dispositifs logique et mémoire. Des dispositifs nouveaux basés sur les DW ont déjà été proposés, par exemple présentant des très fortes densités de stockage et des taux de transfert élevés pour un remplacement des disques durs. De plus dans les Mémoires Magnétiques à Accès Aléatoire (MRAM), identifiées comme l'une des solutions les plus prometteuses pour le remplacement des DRAM et SRAM, le retournement de l'aimantation implique une propagation des DW. Le contrôle de la dynamique des DW sous courant est longtemps resté un challenge, principalement à cause d'imperfections dans les matériaux utilisés. Des déplacements rapides et contrôlé des DW au moyen d'un courant ont été reportés il y a quelques années seulement dans des multicouches présentant une asymétrie d'inversion (SIA). Plus récemment un mécanisme a été proposé basé sur la présence de couple de spin orbite (SOT) et de l'interaction Dzyaloshinskii-Moriya (DMI), tout deux trouvant leur origine dans l'interaction spin-orbite et nécessitant une SIA.Mon objectif initial était de tester ce modèle dans deux systèmes présentant différents SIA. Dans des multicouches Pt/Co/Pt à faible SIA, j'ai étudié la propagation des DW sous courant et sous champ et j'ai mis en évidence l'existence d'un amortissement chiral. Ce phénomène nouveau, pendant de DMI pour les mécanismes dissipatifs, influence à la fois la dynamique sous courant et sous champ et doit être pris en compte pour avoir une description complète des mécanismes. Dans des multicouches Pt/Co/AlOx à fort SIA, j'ai étudié de nouvelles géométries pour lesquelles le mouvement de la paroi de domaine et la direction du courant ne sont pas colinéaires. J'ai mis en évidence un déplacement asymétrique des DW en fonction de cette non-colinéarité qui ne peut pas être expliquée avec un modèle simple DMI+SOT. En se basant sur ces résultats expérimentaux, j'ai introduit un nouveau concept de dispositifs, appelé « origami magnétique » : la forme du dispositif gouverne le mécanisme de retournement. Ce concept apporte une grande flexibilité dans la construction de mémoires magnétiques non volatiles, rapides et peu gourmandes en énergie : des fonctionnalités différentes peuvent être obtenues sur un même wafer simplement par la maîtrise de la forme des différents éléments. Je montre la preuve de concept de deux dispositifs. / Magnetic domain wall (DW) dynamics is currently attracting tremendous interest both from a fundamental point of view as well as in relation with emerging magnetic memory and logic devices. New DW-based devices were recently proposed, for example to replace hard drive disks with higher density and faster date transfer. Moreover, in Magnetic Random Access Memory (MRAM), identified as one of the most promising candidate for DRAM and SRAM replacement, switching occurs through DW propagation. Control of current induced DW dynamics has long been a challenge mainly due to material imperfections. Only some years ago, fast and controllable motions were reported in multilayers presenting structural inversion asymmetry (SIA). More recently, a mechanism was proposed based on the presence of spin orbit torques and Dzyaloshinskii-Moriya interaction (DMI), both phenomena originating from the spin orbit interaction and needing (SIA).My initial objective was to test this model in two systems presenting different SIA. In Pt/Co/Pt multilayers with weak SIA, I studied both current and field induced DW motion and evidenced a chiral damping. This new phenomena, counterpart of the DMI for the dissipative aspects, influences both current and field induced dynamics and has to be taken into account for a complete picture of the mechanism. In Pt/Co/AlOx multilayers with strong SIA, I studied new geometries where the DW motion the and current flow are not collinear. I evidenced asymmetric DW motion as a function of this non-collinearity that cannot be explained with a simple SOT+DMI model. Based on these experimental results I introduce a new device concept named “magnetic origami”: the shape of the device governs the switching mechanism. This concept provides large flexibility to construct fast, low power non-volatile magnetic memory: different functionalities can be achieved on a wafer by simply mastering the shape of the different elements. I show the proof of concept of two such devices.

Para o desenvolvimento da nanociência atual há forte demanda por equipamentos capazes de caracterizar sistemas em escalas da ordem nanométrica. Este contexto impulsionou o desenvolvimento de microscópios ópticos de varredura em campopróximo (Scanning Near-field Optical Microscope SNOM). Diferentemente da microscopia óptica tradicional, os SNOMs detectam a radiação eletromagnética evanescente e, conseqüentemente, a resolução não é limitada pelo critério de Rayleigh. No Laboratório de Materiais Magnéticos IFUSP desenvolvemos um SNOM sensível a efeitos Kerr magnetoópticos (MO-SNOM). Dessa maneira, associamos a alta resolução da técnica à alta sensibilidade dos efeitos magnetoópticos. Trata-se se uma área relativamente pouco explorada e carente de resultados sistemáticos na literatura. Utilizando o MO-SNOM, caracterizamos partículas microestruturadas de Co70.4Fe4.6Si15B10 amorfo com dimensões de 16x16x0.08 microm3 e 4x4x0.08 microm3. Os resultados compreendem dezenas de imagens de susceptibilidade magnetoóptica diferencial com resolução melhor que 200 nm e curvas de histerese local. Em primeira análise, a demonstração de resultados sistemáticos ajuda a estabelecer a técnica. O comportamento magnético das partículas, estudadas sob várias condições de campo aplicado, se mostrou determinado basicamente pela anisotropia de forma. As curvas de histerese local mostraram comportamentos intrinsecamente locais e motivaram uma interessante discussão sobre os parâmetros de caracterização magnética convencionais. As medidas realizadas indicam que o efeito Kerr magnetoótico transversal em campopróximo é similar ao campo-distante. Os resultados são fortemente sustentados por medidas de microscopia magnetoóptica de campo-distante, simulações micromagnéticas e medidas de microscopia de força magnética. Medidas complementares revelam o potencial do MO-SNOM para caracterizações de objetos extensos quanto a potenciais de pinning. Além disso, medidas em filmes finos de NiFe/FeMn acoplados por exchange-bias evidenciam a alta sensibilidade do MO-SNOM, estimada de DM ~ 2 x 10-12 emu. / To support nanosciences evolution, there is a strong demand for developing new instrumentation devoted to nano-scale characterization. In this context, the development of the Scanning Near-field Optical Microscope (SNOM) took place. In contrast to traditional optical microscopes, SNOM deals with evanescent electromagnetic radiation and, consequently, the resolution is no longer limited by the Rayleigh criterion. At Laboratório de Materiais Magnéticos (LMM) IFUSP a SNOM devoted to magneto-optical Kerr effect measurements (MO-SNOM) has been developed. The MOSNOM associates the high resolution of the near-field technique to the high sensibility of the magneto-optical Kerr effect. Near-field magneto-optical microscopy is not yet wellestablished and there is a lack of systematic results in the literature. Using the MO-SNOM, amorphous Co70.4Fe4.6Si15B10 particles with 16x16x0.08 microm3 and 4x4x0.08 microm3 dimensions were studied. With resolution better than 200 nm, several magneto-optical differential susceptibility images and local hysteresis loops were obtained. The systematic results uphold the establishment of this new technique. Under the different applied field conditions, the magnetic behavior of the particles was found to be determined by shape anisotropy. Local hysteresis loops presented shapes intrinsic of local field induced process. The unusual hystesesis loops motivated interesting discussion about the conventional magnetic parameters. The MO-SNOM measurements indicate that the near-field transverse magneto-optical Kerr effect is similar to the far-field case. The results are highly supported by far-field magneto-optical microscopy, micromagnetic simulations and magnetic force microscopy measurements. Complementary measurements indicate the MO-SNOM potential to extensive magnetic surface characterization related to pinning potential distribution. Furthermore, measurements on the exchange-bias coupled NiFe/FeMn thin films make evident the MO-SNOM high sensitivity, estimated to be DeltaM ~ 2 x 10-12 emu.