La magnétorésistance anisotrope (AMR) des matériaux ferromagnétiques est largement utilisée comme le phénomène de base pour la mesure ou la détection de champ magnétique. En raison de la relation entre la configuration en domaines magnétiques et la résistivité macroscopique, l'application d'un champ magnétique externe modifie la résistivité des matériaux ferromagnétiques. Bien que cet effet soit largement utilisé dans des applications industrielles, certains aspects fondamentaux du comportement AMR sont encore assez mal compris. Par exemple, le rôle de la texture cristallographique dans le comportement effectif n'est pas décrit avec précision par les outils classiques de modélisation. En raison de ce lien direct entre la microstructure en domaines et l'effet AMR, les modèles de description de l'effet AMR reposent généralement sur des calculs micromagnétiques. Pour ces calculs, le nombre de degrés de liberté et d'interactions peuvent se multiplier rapidement si on recherche à décrire un comportement macroscopique (cas des polycristaux par exemple).La thèse porte sur la modélisation numérique de l'effet de magnétorésistance anisotrope des matériaux ferromagnétiques. Ce nouvel outil de modélisation 3D peut remédier à cet inconvénient majeur des approches micromagnétiques. Un modèle permettant de décrire les effets de couplage magnéto-élastique en utilisant une approche micro-macro est disponible au laboratoire GeePs. Sur la base des mêmes principes de la modélisation micro-macro, un outil de simulation de l'effet AMR en fonction de la contrainte mécanique et de la texture cristallographique des matériaux a été développé.La stratégie de modélisation est la suivante:Trois échelles de description du comportement sont introduites: le Volume Elémentaire Représentatif (VER) polycristallin (échelle macro), le monocristal ou grain, et enfin le domaine magnétique (échelle micro).Une première étape dite de localisation permet de déterminer le chargement magnéto-mécanique (champ magnétique et contrainte mécanique) à l'échelle d'un grain en fonction du chargement extérieur appliqué. L'introduction de variables internes et des lois d'évolution correspondantes permet de décrire de façon statistique l'évolution de la microstructure en domaines magnétiques sous l'influence de ce chargement local. Toujours à cette échelle, l'utilisation du modèle phénoménologique de Doring permet, pour chaque domaine, de calculer la résistivité en fonction de l'orientation relative entre aimantation locale et courant électrique. Une fois cette résistivité locale connue, une étape dite d'homogénéisation s'appuyant sur le modèle de Bruggeman permet de déterminer la résistivité macroscopique du VER polycristallin. Il est ainsi possible de prédire la variation de la résistivité entre un état initial désaimanté et un état sous chargement magnéto-mécanique quelconque.Les résultats obtenus par cette démarche ont été comparés avec succès à des résultats expérimentaux extraits de la littérature portant sur des polycristaux de Nickel, de Fer pur ou encore de Permalloy.Ensuite des simulations reproduisant les conditions de fonctionnement des capteurs AMR ont été effectuées. Ces simulations permettent de conclure qu'il est possible d'améliorer la sensibilité des capteurs AMR en générant une contrainte résiduelle biaxiale. / The anisotropic magnetoresistance (AMR) of ferromagnetic materials is widely used as the basic phenomenon for measuring or detecting magnetic field. Owing to the relationship between magnetic domain configuration and macroscopic resistivity, the application of an external magnetic field changes the resistivity of ferromagnetic materials. Although this effect is widely used in industrial applications, some basic aspects of AMR behavior are still unsufficiently understood. For example, the role of crystallographic texture is not accurately described by conventional modeling tools. As a consequence of the direct relationship between microstructure and AMR, models for AMR effect are generally based on micromagnetic calculations. For these calculations, the number of degrees of freedom and interactions can grow exponentially when investigating macroscopic behavior (case of polycrystals for example).The thesis deals with the numerical modeling of AMR effect in ferromagnetic materials. This new 3D modeling tool can overcome this major drawback of micromagnetic approaches. A model to describe the effects of magneto-elastic coupling using a micro-macro approach is available at the laboratory GeePs. Based on the same principles of micro-macro modeling, an AMR effect simulation tool has been developed including the effect of mechanical stress and the role of crystallographic texture of materials.The modeling strategy is as follows:Three scales of description of the behavior are introduced: the Representative Volume Element (RVE) of polycrystals (macro scale), the single crystal or grain, and finally the magnetic domain (micro scale).A first step, named localization, determines the magneto-mechanical loading (magnetic field and mechanical stress) within a grain depending on the external applied load. The introduction of internal variables and corresponding evolution laws allow describing in a statistical way the evolution of the magnetic domain microstructure under the influence of the local load. Also at this scale, the use of the phenomenological Doring model allows for each area, to calculate the resistivity as a function of the relative orientation between local magnetization and electric current. Once this local resistivity is known, a so-called homogenization step based on the Bruggeman model is used to determine the macroscopic resistivity of the RVE. It is thus possible to predict the variation in resistivity between an initial demagnetized state and a state under any magneto-mechanical loading.The results obtained by this approach were successfully compared to experimental results from literature on polycrystalline nickel, pure iron or Permalloy.Then simulations reproducing AMR sensors operating conditions were carried out. These simulations lead to the conclusion that it is possible to improve the sensitivity of AMR sensors by introducing an appropriate biaxial residual stress.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Anisotropic magnetoresistance (AMR) corresponds to the change of R in an ferromagnetic material with the angle between electric current and magnetization. Sensors using this effect are suited to detect both angular and linear displacements. In this work, structural, magnetic and electric characterization were performed in order to study the exchange interaction between antiferromagnetic IrMn and ferromagnetic NiFe, in a bilayer and a multilayer. Simulations of the AMR measurements were performed and showed good agreement with the experimental data. Different anisotropy field values were observed. The difference between the anisotropy field and the exchange field values is responsible for the different AMR data sets extracted from each sample. The model takes into account the, anisotropy (uniaxial), Zeeman, and exchange-bias (unidirectional) energies was used to explain the observed behavior. / Magnetorresistência anisotrópica (AMR) consiste na variação da resistência de um material ferromagnético em função do ângulo entre a corrente elétrica e a magnetização do material, o que faz com que sensores que utilizam este efeito sejam promissores para medidas de posição tanto angulares quanto lineares. Neste trabalho, caracterização estrutural, magnética e elétrica foram realizadas para estudar a interação de troca entre camadas antiferromagnética de IrMn e ferromagnética de NiFe em uma bicamada e uma multicamada. Simulações das medidas de AMR foram realizadas e boa concordância entre os dados experimentais e os simulados foi obtida. Diferentes valores de campos de anisotropias foram observados. A diferença entre o campo de anisotropia unidirecional e o campo de exchange é responsável pela diferença entre as medidas de AMR obtidas. Um modelo que considera as energias de anisotropia (uniaxial), Zeeman e de exchangebias (unidirecional) foi usado para explicar o comportamento observado.

Železo-rhodium (FeRh) je látka procházející magnetickou fázovou přeměnou prvního druhu z antiferomagnetické (AF) do feromagnetické (FM) fáze, ke které dochází při zahřátí materiálu nad teplotu fázové přeměny nebo působením dostatečně velkého magnetického pole. Tato fázová přeměna je mimo jiné provázena výraznou změnou entropie, magnetizace a elektrického odporu, přičemž její tvar a poloha teploty přeměny je silně závislá na stechiometrii krystalu, na příměsích, tlaku a v případě tenkých vrstev na napjatosti vrstvy způsobené substrátem. Tato práce se zaměřuje na studium magnetotransportních vlastností drátů připravených z tenkých FeRh vrstev rostlých na substrátech indukujících různou napjatost vrstvy. Jedním z hlavních jevů studovaných v této práci je anizotropní magnetorezistance (AMR) projevující se změnou odporu pro různé natočení magnetických momentů v látce vůči směru elektrického proudu. AMR byla studována jak ve FM fázi, tak i v AF fázi FeRh. Byla změřena hodnota AMR ve vysokoteplotní FM fázi a objeveno neočekávané chování AMR ve zbytkové FM fázi v nízkoteplotním stavu. Dále byla pozorována výrazná závislost AMR na orientaci měřených segmentů vůči krystalografickým směrům FeRh.

<p>This research study has been devoted to the study of magnetic properties and magnetic transport of nanocrystalline Ni and Ni-15% Fe alloys consisting of randomly oriented grains with an average size of 23 and 12 (nm), respectively. The structures of the deposits were confirmed by the XRD analysis using Rietveld refinement technique. The as-deposited Ni and Ni-15%Fe sample was comprised exclusively of the γ phase with lattice parameter of 3.5270 (nm) and 3.5424 (nm), respectively. The small increase in lattice parameter was attributed to the replacement of iron solutes in the Ni sites in lattice. Texture analysis of nanocrystalline Ni and Ni-15%Fe revealed that textures components of both materials is qualitatively the same and vary in terms of volume fraction. Both material showed strong <100> fibre texture with some contribution of the <111> component. The calculated volume fraction of the <100> and <111> components were respectively 17.157% and 3.201% for Ni and, 22.032% and 6.160% for Ni-15%Fe and the rest being confined to the random texture.</p> <p>Magnetic measurements show that all samples exhibit low loss hysteresis loops with high permeabilities. The presence of 15%Fe in Ni leads to enhancement of the saturation magnetization (M_s) regardless of the direction of the applied field. M_s shows an increase from 60.169 (emu/gr) in nanocrystalline Ni to 93.67 (emu/gr) in Ni-15%Fe sample at T=2K. No strong temperature–dependence of the magnetization was observed for samples, but the magnetization of the Ni-15%Fe samples at T=2K were slightly higher than that of T=298K. The coercivity values of nanocrystalline Ni-15%Fe were in all cases smaller than that of nanocrystalline Ni samples. Good agreement between random anisotropy model (RAM) theory and experiment for nanocrystalline Ni and Ni-15%Fe samples was observed. The ferromagnetic exchange length (L_ex) was larger than the average grain size (D) for samples at all times. The effective magnetic anisotropy constants (K_eff) of the nanocrystalline Ni and Ni-15%Fe alloys were measured using the law of approach to saturation. At T=2K, the K_eff of Ni-15%Fe samples were measured to be 1.7037´10⁵ (erg/cm³) and 2.71996 ´10⁵ (erg/cm³) at field parallel and perpendicular, respectively. These values were almost half of the values obtained for nanocrystalline Ni samples 4.66091´10⁵ (erg/cm³) and 4.19703´10⁵ (erg/cm³). Temperature dependence measurements showed that K_eff constants decrease with increasing temperature. The angular dependence MR studies on nanocrystalline Ni and Ni-15%Fe resulted in a twofold, and a fourfold symmetric behaviour, respectively. The field dependence MR measured at various sample tilt with respect to the applied field, showed various trends from pure positive MR to pure negative MR, which partially could be explained by magnetocrystalline anisotropy of the samples.</p> / Master of Applied Science (MASc)

This work addresses the relevant errors of the anisotropic magnetoresistance sensor for inertial navigation systems. The manuscript provides resulting guidelines and solution for using the AMR sensors in a robust and appropriate way relative to the applications. New methods also are proposed to improve the performance and, reduce the power requirements and cost design of the magnetometer. The new compensation method is proposed by developing an optimization algorithm. The necessity of the sensor calibration is shown and the source of the errors and compensating model are investigated. Two novel methods of indoor calibration are proposed and examples of operating systems are presented.

Dans le cadre de l’étude des phénomènes magnétiques et de la spintronique qui sont présents aux échelles nanoscopiques nous avons étudié différents aspects des structures asymétriques de Pt/Co/AlOx. L’un des objectifs de cette thèse est le contrôle de l’oxydation et des propriétés magnétiques de ces multicouches. Nous avons combiné les mesures de structures (réflexion de Rayon-X), transports (Effet Hall anormal), et magnétiques (VSM-SQUID) afin de déterminer les rôles des effets magnétiques et d’interfaces. Un objectif était d’analyser le rôle de quelques monocouches (MCs) de CoO (qui peut se former lors de la sur oxydation de l’Al) sur les propriétés de la multicouche. Nous avons utilisé une technique de déposition avec un gradient d’épaisseur pour contrôler l’oxydation à l’échelle nanométrique. Nous avons établis que quelques monocouches (MCs) de CoO a un impact sur l’anisotropie de a multicouche. Pour approfondir l’effet de la couche de CoO, nous avons construit des bicouches ultrafines de Co(0.6nm)/CoO(0.6nm). Nous avons effectué des mesures refroidi sur champ sur ce système et trouvé un fort effet de couplage d'échange. Ces résultats indiquent que la couche CoO garde une forte anisotropie même en dans la limite des monocouches et permet de réfuter certains modèles sur l’effet d’échange bias et indique que les couches, couramment négligé, de CoO doivent être prises en considération dans le bilan énergétiques du système. Nous avons construits un appareil de mesure perpendiculaire de la magnétorésistance tunnel anisotrope (TAMR) à partir de la structure Pt/Co/AlOx. La TAMR est un effet de spintronique relativement récent dans lequel la rotation d’aimantation dans une électrode magnétique (combiné avec un couplage spin-orbite) peut entrainer un changement de la probabilité de l’effet tunnel, ce qui se manifeste comme un effet de magnétorésistance. Nous avons démontré qu’un contrôle précis de l’état d’oxydation est essentiel pour l’effet TAMR. La forte anisotropie magnétique induite nous permet d’atteindre des valeurs de TAMR plus grande comparée à celle des structures Pt/Co/AlOx. / In the context of studying magnetic and spintronics phenomena occurring at the nanoscale, we investigated several aspects of Pt/Co/AlOx asymmetric structures. One of the objectives of this thesis was the control of the oxidation and the tailoring of the magnetic properties of these multilayers. We combined structural (X-Ray Reflectivity), transport (Anomalous Hall Effect) and magnetic measurements (VSM-SQUID), to study the interplay of magnetic and interfacial effects. One objective was to analyze the role that few monolayers (MLs) of CoO (which can form when overoxidizing the Al layer), could have on the properties of the stack. We used a wedge deposition techniques to control the oxidation on a subnanometer scale. We established that few MLs of CoO largely affect the total anisotropy of the stack. To further investigate the impact of the CoO, we engineered ultrathin Co(0.6nm)/CoO(0.6nm) bilayers. We performed field cooled measurements on this system and we found a large exchange bias anisotropy. These results indicate that the CoO keeps a large anisotropy even in the ML regime, help to rule out some of the models proposed to explain the exchange bias effect and imply that the usually neglected CoO presence must be considered in the energy balance of the system. We build perpendicular Tunneling Anisotropic MagnetoResistance (TAMR) devices based on the Pt/Co/AlOx structure. The TAMR is a relatively new spintronics effect in which the rotation of the magnetization in a single magnetic electrode (combined with the Spin-Orbit Coupling) can cause a change of the tunnel probability, which manifests as a magnetoresistance effect. We demonstrated that a careful control of the interface oxidation is crucial for the TAMR effect. The large induced magnetic anisotropy allowed us to achieve enhanced TAMR values compared to similar Pt/Co/AlOx structures.

Title: Relativistic Theory of Electron Transport in Magnetic Layers Author: Rudolf Sýkora Department / Institute: Institude of Theoretical Physics Supervisor of the doctoral thesis: doc. RNDr. Ilja Turek, DrSc., Department of Condensed Matter Physics Abstract: We review the density-functional theory (DFT) in detail using the Levy Lieb ap- proach. The Kohn Sham scheme is discussed, starting from the simplest spinless non- relativistic case, then including spin and considering potential spin magnetism, and finally deriv- ing the full Kohn Sham Dirac relativistic scheme. The Linear Muffin-Tin Orbital (LMTO) method for electronic-structure calculation is presented, together with mentioning the necessary changes to include the spin-orbit (SO) interaction effects to an otherwise scalar-relativistic (SR) theory. Derivation of an electronic-conductance formula for a layered system is given, based on the Landauer scattering picture and using simple non-equilibrium Green functions. The formal- ism is applied to layered metallic systems of light elements Co, Ni, Cu elements, and to layered systems with a tunnelling barrier, Fe/MgO/Ag and Fe/GaAs/Ag. The effects of the SO interac- tion on the Giant Magnetoresistance (GMR) ratio and/or the Tunnelling Anisotropy Magnetore- sistance (TAMR) for these systems are discussed....

In the present thesis, we have explored multiple aspects concerning the stochasticity of magnetic domain wall motion during magnetization reversal, all of which originated from our initial study of magnetic field sensing using planar Hall effect. Magnetic field sensors occupy a very important and indispensable position in modern technology. They can be found everywhere, from cellphones to automobiles, electric motors to computer hard disks. At present there are several emerging areas of technology, including biotechnology, which require magnetic field sensors which are at the same time simple to use, highly sensitive, robust under environmental conditions and sufficiently low cost to be deployed on a large scale. Magnetic field sensing using planar Hall effect is one such feasible technology, which we have explored in the course of the thesis. The work was subsequently expanded to cover some fundamental aspects of the stochasticity of domain wall motion, studied with planar Hall effect, which forms the main body of work in the present study. In Chapter 1, we give an introduction to the phenomenology of planar Hall effect, which is the most important measurement technique used for all the subsequent studies. Some early calculations, which had first led to the understanding of anisotropic magnetoresistance and planar Hall effect as being caused by spin-orbit interaction are discussed. In Chapter 2, we discuss briefly the experimental techniques used in the present study for sample growth and fabrication, structural and magnetic characterization, and measurement. We discuss pulsed laser ablation, which is the main technique used for our sample growth. Particular emphasis is given to the instrumentation that was carried out in-house for MOKE and low field magnetotransport (AMR and PHE) measurement. This includes an attempt at domain wall imaging through MOKE microscopy. Some of the standard equipments used for this work, such as the SQUID magnetometer and the acsusceptometer are also discussed in detail. In Chapter 3 we discuss our work on planar Hall sensors that led to the fabrication of a device with a very simple architecture, having transfer characteristics of 650V/A.T in a range of _2Oe. The sensing material was permalloy (Ni81Fe19), and the value had been obtained without using an exchange biased pinning layer. Field trials showed that the devices were capable of geomagnetic field sensing, as well as vehicle detection by sensing the anomaly in Earth's magnetic field caused by their motion. Its estimated detection threshold of 2.5nT made it well suited for several other applications needing high sensitivity in a small area, the most prominent of them being the detection of macromolecules of bio-medical significance. Chapter 4: The work on Barkhausen noise was prompted by reproducibility problems faced during the sensor construction, both between devices as well as within the same device. Study of the stochastic properties led us to the conclusion that the devices could be grouped into two classes: one where the magnetization reversal occurred in a single step, and the other where it took a 0staircase0 like path with multiple steps. This led us to simulations of Barkhausen noise using nucleation models like the RFIM whence it became apparent that the two different groups of samples could be mapped into two regimes of the RFIM distinguished by their magnetization reversal mode. In the RFIM, the nature of the hysteresis loop depends on the degree of disorder, with a crossover happening from single-step switching to multi-step switching at a critical disorder level. Appropriate changes also appear in the Barkhausen noise statistics due to this disorder-induced crossover. By studying the Barkhausen noise statistics for our permalloy samples and comparing them with simulations of the RFIM, we found nearly exact correspondence between the two experimental groups with the two classes resulting from crossing the critical disorder. What remained was to quantify the 0disorder0 level of our samples, which was done through XRD, residual resistivity and a study of electron-electron interaction effects in the resistivity. All these studies led to the conclusion that the samples reversing in multiple steps were more 0defective0 than the other group, at par with the model predictions. This completed the picture with respect to the modeling of the noise. In experiments, it was found that a high rate of film deposition yielded less 0defective0 samples, which severed as an important input for the sensor construction. These results can be viewed from a somewhat broader perspective if we consider the present scenario in the experimental study of Barkhausen noise, or crackling noise in general. Two classes of models exist for such phenomena: front propagation models and nucleation models. Both appear to be very successful when it comes to experiments with bulk materials, while the comparison with experiments on thin films is rather disappointing. It is still not clear whether the models are at fault or the experiments themselves. Through our study, we could demonstrate that there can be considerable variation in the Barkhausen noise character of the same material deposited in the same way, and what was important was the degree of order at the microscopic level. This may be a relevant factor when experimental papers report non-universality of Barkhausen noise in thin films, which can now be interpreted as either insufficient defects or a sample area too small for the study. Chapter 5: Defects in a sample are not the only cause for stochastic behavior during magnetization. In most cases, random thermal 0events0 are also an important factor determining the path to magnetization reversal, which was also true for our permalloy samples. We studied the distribution of the external fields at which magnetization reversal took place in our samples, and tried to explain it in terms of the popular Neel-Brown model of thermal excitation over the anisotropy barrier. The analysis showed that even though the coercivity behaved 0correctly0 in terms of the model predictions, the behavior of the distribution width was anomalous. Such anomalies were common in the literature on switching field distributions, but there seemed to be no unified explanation, with different authors coming up with their own 0exotic0 explanations. We decided to investigate the simplest situations that could result in such a behavior, and through some model-based calculations, came to the conclusion that one of the causes of the anomalies could be the different magnitudes of barrier heights/anisotropy fields experienced by the magnetic domain wall when the reversal occurs along different paths. Though an exact match for the behavior of the distribution width could not be obtained, the extended Neel-Brown model was able to produce qualitative agreement. Chapter 6 contains a study of some interesting 0geometrical0 effects on Barkhausen noise of iron thin films. By rotating the applied magnetic field out-of plane, we could observe the same single-step to multi-step crossover in hysteresis loop nature that was brought about by varying disorder in Chapter 4. We could explain this through simulations of a random anisotropy Ising model, which, apart from exhibiting the usual disorder induced crossover, showed a transition from sub-critical to critical hysteresis loops when the external field direction was rotated away form the average anisotropy direction. Once again, simulation and experiment showed very good agreement in terms of the qualitative behavior. In the second part of this chapter, a study of exchange biased Fe-FeMn system was carried out, where it was observed that the reversal mode has been changed from domain wall motion to coherent rotation. Barkhausen noise was also suppressed. Though many single-domain models existed for this type of reversal, our system was not found to be strictly compatible with them. The disagreement was with regard to the nature of the hysteresis, which, if present, had to be a single step process for a single domain model. The disagreement was naturally attributed to interaction with the nearby magnetic moments, to verify which, simulations were done with a simplified micromagnetic code, which produced excellent agreement with experiment. In Chapter 7, we have studied the temporal properties of Barkhausen avalanches, to compare the duration distributions with simulation. We had used a permalloy sample that was sub-critical according to avalanche size distributions, and our measurement was based on magneto-optic Kerr effect. We measured duration distributions which showed a similar manifestation of finite-size effects as were shown by the size distributions. The power law exponent was calculated, which was deemed 0reasonable0 upon comparison simulations of the sub-critical RFIM. Appendix A contains a study of high-field magnetoresistance of permalloy, which shows that the dominant contribution to magnetoresistance is the suppression of electron-magnon scattering. An interesting correlation is observed between the magnetization of samples and an exchange stiffness parameter d1, that was extracted from magnetoresistance measurements. Here we also re-visit our earlier observation of permalloy thin films possessing a resistance minimum at low temperature. The origin of this minimum is attributed to electron-electron interaction. Appendix B contains the source codes for most of the important programs used for simulation and data analysis. The programs are written in MATLAB and FORTRAN 95. LabView programs used for data acquisition and analysis are not included due to space requirements to display their graphical source codes. Appendix C discusses the studies on a disordered rare-earth oxide LaMnO3. The re-entrant glassy phase is characterized with ac susceptibility and magnetization measurements to extract information about the nature of interactions between the magnetic 0macrospins0 in the system. Appendix D deals with electron scattering experiments performed with spinpolarized electrons (SPLEED) from clean metal surfaces in UHV. A study of the scattering cross sections as a function of energy and scattering angle provides information about spin-orbit and exchange interactions of the electrons with the surface atoms, and can answer important questions pertaining to the electronic and magnetic structure of surfaces. In the course of this study, planar Hall effect is seen to emerge as a powerful tool to study the magnetic state of a thin film, so that it is interesting to apply it to thin films of other materials such as oxides, where magnetization noise studies are next to nonexistent. What also emerged is that there is still a lot of richness present in the details of supposedly well-understood magnetization phenomena, some of which we have explored in this thesis in the context of stochastic magnetization processes.

Conselho Nacional de Desenvolvimento Científico e Tecnológico / Anisotropic magnetoresistance (AMR) consists in the change of the resistivity of a ferromagnetic metal as a function of the angle between the current and the magnetization, what makes AMR-based sensors promising to measure both angular and linear positions. These devices usually have a structure of Ta/Ni81Fe19/Ta and the thickness of the Ni81Fe19 layer is about 10 nm so as to reduce the demagnetization field parallel to surface. In order to acquire high magnetic field sensitivity (S) and low Barkhausen noise the films should have high AMR values (ΔDR=R) and low coercivity. However, during fabrication, the structures are often exposed to temperatures above 200oC, what changes the characteristics of the interfaces and reduces ΔDR=R. On the other hand, ΔDR=R and S can be remarkably enhanced by insertion of nano-oxide layers that act like difusion barriers on the interfaces. The enhancement is mainly attributed to the large electron specular reflection at the flatter interfaces. In this work we have proposed to verify the possibility of enhance ΔDR=R and S in structures with good thermal stability just by adding an oxidation step after growthing each layer, forming TaOx. We studied the influence of the deposition parameters in the structural and magnetic properties of the samples and otimized the growth of thin films of Ni81Fe19 by magnetron sputtering. We also verified the influence of the annealing in the structural properties of nanostructures of Ta/Ni81Fe19/Ta exposed or not to oxidation on the interfaces. We observed that the TaOx nano-oxide layer can work as expected, however the poor cristalinity of the Ni81Fe19 layers leads to AMR values lower than the literature ones. / Magnetorresistência anisotrópica (AMR) consiste na variação da resistividade de um metal ferromagnético como uma função do ângulo entre a corrente e a magnetização, o que faz com que sensores que utilizam este efeito sejam promissores para medidas de posição tanto angulares quanto lineares. Estes dispositivos normalmente possuem a estrutura Ta/Ni81Fe19/Ta com a espessura da camada de Ni81Fe19 sendo da ordem de 10 nm, para reduzir o campo desmagnetizante paralelo à superfície. Para obter alta sensibilidade (S) e baixos níveis de ruído Barkhausen os filmes devem apresentar alta variação percentual da AMR (ΔDR=R) e baixa coercividade. Entretanto, durante o processo de fabricação as estruturas frequentemente são expostas à temperaturas maiores que 200oC, o que pode alterar as características das interfaces e reduzir ΔDR=R. Por outro lado, ΔDR=R e S podem ser significativamente incrementados através da inserção de nanocamadas de óxidos que atuam como barreiras contra difusão nas interfaces. O aumento é atribuído ao maior nível de reflexões eletrônicas especulares nas interfaces mais lisas. Neste trabalho, propomos verificar se é possível aumentar ΔDR=R e S para estruturas com boa estabilidade térmica simplesmente acrescentando uma etapa de oxidação após o crescimento de cada camada, pela formação de TaOx. Estudamos a influência dos parâmetros de deposição nas propriedades estruturais e magnéticas das amostras e otimizamos o crescimento de filmes finos de Ni81Fe19 pela técnica de magnetron sputtering. Verificamos também a influência do tratamento térmico nas propriedades estruturais de nanoestruturas do tipo Ta/Ni81Fe19/Ta submetidas ou não à oxidação das interfaces. Observamos que a nanocamada de TaOx pode desempenhar o papel esperado, todavia a qualidade cristalina das camadas de Ni81Fe19 acarretou em valores quantitativos de ΔDR=R menores que os encontrados na literatura.

Made available in DSpace on 2015-03-03T15:15:29Z (GMT). No. of bitstreams: 1 CristovaoPNJ_DISSERT.pdf: 20191942 bytes, checksum: ddd5466a16c8a26b044b5c940b443f3c (MD5) Previous issue date: 2013-06-18 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico / The ferromagnetic materials play an important role in the development of various electronic devices and, have great importance insofar as they may determine the efficiency, cost and, size of the devices. For this reason, many scientific researches is currently focused on the study of materials at ever smaller scales, in order to understand and better control the properties of nanoscale systems, i.e. with dimensions of the order of nanometers, such as thin film ferromagnetic. In this work, we analyze the structural and magnetic properties and magnetoresistance effect in Permalloy-ferromagnetic thin films produced by magnetron sputtering. In this case, since the magnetoresistance effect dependent interfaces of thin films, this work is devoted to the study of the magnetoresistance in samples of Permalloy in nominal settings of: Ta[4nm]/Py[16nm]/Ta[4nm], Ta[4nm]/Py[16nm]/O2/Ta[4nm], Ta[4nm]/O2/Py[16nm]/Ta[4nm], Ta[4nm]/O2/Py[16n m]/O2/Ta[4nm], as made and subjected to heat treatment at temperatures of 160?C, 360?C e 460?C, in order to verify the influence of the insertion of the oxygen in the layer structure of samples and thermal treatments carried out after production of the samples. Results are interpreted in terms of the structure of the samples, residual stresses stored during deposition, stresses induced by heat treatments and magnetic anisotropies / Os materiais ferromagn?ticos exercem um papel importante no desenvolvimento de diversos dispositivos eletr?nicos e t?m grande import?ncia na medida que os mesmos podem determinar a efici?ncia, o custo e o tamanho dos dispositivos. Por este motivo, muitas das investiga??es cient?ficas t?m, atualmente, se concentrado no estudo de materiais em escalas cada vez menores, a fim de entender e controlar melhor as propriedades dos sistemas nanosc?pios, ou seja, com dimens?es da ordem de 10?9 m, tais como filmes finos ferromagn?ticos. Nesse trabalho, s?o analisadas as propriedades estruturais e magn?ticas e o efeito da magnetorresist?ncia em filmes finos ferromagn?ticos de Permalloy produzidos por magnetron sputtering. Neste caso, visto que o efeito da magnetorresist?ncia ? dependente das interfaces dos filmes finos, este trabalho ? dedicado ao estudo da magnetorresist?ncia em amostras de Permalloy com configura??es nominais de: Ta[4nm]/Py[16nm]/Ta[4nm], Ta[4nm]/Py[16nm]/O2/Ta[4nm], Ta[4nm]/O2/Py[16nm]/Ta[4nm], Ta[4nm]/O2/Py[16n m]/O2/Ta[4nm], na condi??o de como feitas e submetidas a tratamentos t?rmicos com temperaturas de 160 C, 360 C e 460 C, a fim de verificar a influ?ncia da inser??o das camadas de O2 na estrutura das amostras e de tratamentos t?rmicos realizados ap?s a produ??o das amostras. Os resultados obtidos s?o interpretados em termos da estrutura das amostras, tens?es residuais armazendas durante a deposi??o, tens?es induzidas pelos tratamentos t?rmicos e anisotropias magn?ticas.