Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2007. / Source: Dissertation Abstracts International, Volume: 69-02, Section: B, page: 1068. Adviser: Paul T. Debevec. Includes bibliographical references (leaves 94-100) Available on microfilm from Pro Quest Information and Learning.

In his programmatic article More Is Different (1972), Nobel laureate P. W. Anderson captured the fundamental interest in quantum matter in a nutshell. The central motive in this field is emergence. In the inaugural volume of the homonymous journal, J. Goldstein defined this as "the arising of novel and coherent structures, patterns and properties during the process of self-organization in complex systems". Famously, the idea that the "the whole is greater than the sum of its parts" goes back to Aristotle's metaphysics, and it has served as a stimulating concept in 19th century biology, economics and philosophy. The study of emergence in condensed matter physics is unique in that the underlying complex systems are sufficiently "simple" to be modelled from first principles. Notably, the emergent phenomena discovered in this field, such as high-temperature superconductivity, giant magnetoresistance, and strong permanent magnetism have had an enormous impact on technology, and thus, society. Historically, there has been a distinction between materials with localized, strongly interacting (or correlated) electrons - and non-interacting, itinerant electronic states. In the last decade, several new states of matter have been discovered, which emerge not from correlations, but from peculiar symmetries (or topology) of itinerant electronic states. The term quantum materials has therefore become popular to subsume these two strands of condensed matter physics: Electronic correlations and topology. In this thesis, I report investigations of four quantum materials which each illustrate present key interests in the field: The mechanism of high temperature superconductivity, the search for materials that combine both electronic correlations and non-trivial topology and novel emergent phenomena that arise from the synergy of electronic correlations and a strong coupling of spin- and orbital degrees of freedom. The common factor and potential key to understanding these materials is magnetism. My experimental work is focused on neutron and x-ray scattering techniques, which are able to determine both order and dynamics of magnetic states at the atomic scale. I illustrate the full scope of these methods with experimental studies at neutron and synchrotron radiation facilities. This includes both diffraction and spectroscopy, of either single- or polycrystalline samples. My in-depth analysis of each dataset is aided by structural, magnetic and charge transport experiments. Thus, I provide a quantitative characterization of magnetic fluctuations in an iron-based superconductor and in two Dirac materials, and determine the magnetic order in a Dirac semimetal candidate and a complex oxide. As a whole, these results demonstrate the elegant complementarity of modern scattering techniques. Although such methods have a venerable history, they are presently developing at a rapid pace. Several results of this thesis have only been enabled by very recent instrumental advances.

Hyperpolarised gas Magnetic Resonance Imaging (MRI) is a non-invasive method developed to provide images of the void spaces of the lungs, and evaluate the distal airways via diffusion weighted techniques. The work in this thesis covers the use of both hyperpolarised 3He and hyperpolarised 129Xe gas MRI, for both diffusion measurement studies and ventilation imaging, on healthy, asthmatic and cystic fibrosis volunteers. The first study covered in this thesis aimed to determine the nature of the acinar airway involvement in asthma sufferers via the use of Computed Tomography densitometry and hyperpolarised 3He diffusion MRI at multiple timescales alongside standard lung function tests such as spirometry and multiple breath washout (MBW). It was hypothesised that Asthma patients with an elevated MBW parameter 'Sacin' would manifest altered long-range diffusion, as suggestive of intra-acinar airway disease. It was found that there appeared to be an association between the MBW 'Sacin' parameter and subtle alterations in diffusion within the acinar airways for participants with asthma, suggesting a structural abnormality in the pulmonary acinus. However, further longitudinal studies are required before the long-term prognostic significance of acinar airway disease in asthma can be determined. The second study aimed to determine if a relationship exists between measurements from standard lung function test, spirometry and body plethysmography, and the measurements of hyperpolarised 3He diffusion MRI. 18 Cystic Fibrosis patients were recruited, alongside 27 age-matched healthy subjects. It was found that the short timescale diffusion values were lower in patients with Cystic Fibrosis, although not substantial enough to use an indicator to distinguish between a healthy volunteer and a Cystic Fibrosis patient. No strong correlations were found between the lung function measurements and the hyperpolarised 3He diffusion MRI measurements. The final section of this thesis looks at the preliminary results of a pilot study to develop scan sequences, protocols and gas delivery of hyperpolarised 129Xe MRI, in preparation for future clinical studies at the University of Nottingham. The main focus was on the development of in vivo ventilation images of the human lungs, to a high standard of quality and repeatability. Good progress was demonstrated, with lung images achieved down to 2.5 cm thick slices, with initial images of both dynamic ventilation and dissolved-phase 129Xe lung images. However the pilot study was not yet complete, and more development on the techniques discussed was still required.

Complex lanthanide oxide systems are known to host novel phases of matter, while also providing functionality for practical applications. In this dissertation, the structural, magnetic and magnetocaloric properties of three families of lanthanide oxides have been studied with the dual aims of investigating the magnetic behaviour and identifying promising magnetic refrigerants for cooling to temperatures currently accessible using non-renewable liquid He. The thesis presents a two-part study of the magnetic and magnetocaloric properties of the geometrically frustrated lanthanide garnets, where the magnetic $Ln^{3+}$ form corner-sharing triangles. First, the family of garnets $Ln_3A_2X_3$O$_{12}$, $Ln$ = Gd, Tb, Dy, Ho, $A$ = Ga, Sc, In, Te, $X$ = Ga, Al, Li are investigated. Changes to the single-ion anisotropy of the magnetic ion as well as variations in the chemical pressure radically alters the nature of magnetic ordering, the degree of frustration and the magnetocaloric performance. In the second part, the garnets $Ln_3A$Ga$_4$O$_{12}$, $Ln$ = Gd, Tb, Dy, Ho, $A$ = Cr, Mn, are studied. Introducing additional spins significantly reduces the frustration in the garnet lattice. Low temperature powder neutron diffraction of Ho$_3$MnGa$_4$O$_{12}$ reveals concomitant ordering of Ho$^{3+}$ and Mn$^{3+}$ moments below the ordering temperature, $T_N$ = 5.8 K. The magnetocaloric performance of $Ln$_3CrGa$_4$O$_{12}$, $Ln$ = Gd, Dy, Ho, greatly surpasses that of the parent $Ln_3$Ga$_5$O$_{12}$ at $T$ = 2 K. The final results chapters in the thesis describe the magnetism and magnetocaloric effect in the lanthanide orthoborates, $Ln$BO$_3$ , $Ln$ = Eu, Gd, Tb, Dy, Ho, Er, Yb and the lanthanide metaborates, $Ln$(BO$_2$)$_3$, $Ln$ = Pr, Nd, Gd, Tb. The magnetic $Ln^{3+}$ form slightly distorted edge-sharing triangular layers in $Ln$BO$_3$. Unique magnetic features are observed, including short-range ordering and spin reorientation transitions depending on the single-ion anisotropy of the $Ln^{3+}$. The $Ln$BO$_3$ are also efficient magnetocalorics in the liquid helium temperature range. The lanthanide metaborates contain one-dimensional chains of magnetic lanthanide ions. Bulk magnetic measurements show features consistent with low-dimensional magnetism, such as magnetisation plateaux at one-third of the saturation magnetisation for Nd(BO$_2$)$_3$ and Tb(BO$_2$)$_3$ in a field of 14 T. This thesis provides insight into the fundamental magnetic properties of complex lanthanide oxide systems and also demonstrates strategies for identifying new magnetocaloric materials: both through chemical control of the structure of well-known magnetocalorics and by studying materials that have not been explored previously. The results pave the way for further in-depth investigations and finding new magnetic coolants based on complex lanthanide oxide systems.

Meteorites provide information about the early history of our solar system and the formation and evolution of planetesimals. One of the few direct observations of internal geophysical processes within planetary bodies is the presence or absence of a dynamo-driven magnetic field. These observations provide essential constraints on the degree of differentiation, core solidification timescales and the driving forces for convection. This thesis focusses on the paleomagnetic information recorded by iron and stony-iron meteorites, providing us with a unique view-point for the generation and variability of core dynamo activity. Iron and stony-iron meteorites are primarily comprised of FeNi metal. The Widmanstätten pattern; an intergrowth of taenite and kamacite lamellae. Between these lamellae, a range of microstructures develop, dictated by the ‘M-shaped’ Ni diffusion profile. Among these microstructures is the cloudy zone, a region of tetrataenite islands in a Fe-rich matrix, formed by spinodal decomposition. The tetrataenite islands are extremely reliable paleomagnetic recorders. The direction of magnetisation and composition of FeNi microstructures was imaged using synchrotron X-rays. Magnetic contrast is generated using X-ray magnetic circular dichroism. The dimensions of tetrataenite islands within the cloudy zone directly correlate with cooling rates. Cooling rates vary from ~0.5–10,000°C/Myr and correspond to island sizes of ~500–10nm, respectively. The slowest cooled group of iron meteorites reveal multidomain magnetic behaviour within the cloudy zone, whereas in faster-cooled meteorites islands are vortex state. This demonstrates that cooling rate influences the magnetic properties of the cloudy zone. The subtly different cooling rates between different pallasite meteorites means that each meteorite provides a ‘snapshot’ of the parent body magnetic field at a different point during its thermal evolution. Paleointensity results provide the first observations of a quiescent period in dynamo activity preceding core solidification. This also helps to constrain the paleomagnetic signals associated with core nucleation, which, in turn, constrains the mechanism of solidification. Paleomagnetic studies of meteoritic metal were complemented with measurements of magnetic inclusions in olivines. Alternating-field and thermal demagnetisation experiments were carried out using both 2G SQUID and WSGI small-bore SQUID magnetometers. Results suggest that pallasite silicates are unreliable paleomagnetic recorders, and a planetary-strength paleointensity cannot be recovered. Paleomagnetic fidelity was also investigated for a dusty olivine grain from the Semarkona chondrite. Lorentz microscopy, transmission X-ray microscopy, nanotomography and micromagnetic simulations were used to rigorously test the behaviour of Fe-nanoparticles. The final study in this thesis focusses on the IAB iron meteorites. These meteorites have an unusual and complex history. Paleomagnetic results are accompanied by a detailed microstructural study using X-PEEM and electron backscatter diffraction to constrain the formation of two microstructures: pearlitic and spheroidised plessite. Paleomagnetic results suggest the IAB parent body did not have an active core dynamo. Meteorites represent the oldest material in our solar system, and their complex histories and susceptibility to alteration make them some of the most challenging samples to extract reliable paleointensity estimates from. Advanced electron microscopy and synchrotron techniques are now making it possible to extract reliable paleomagnetic information, with profound implications for the formation and evolution of the solar system.

Over the past years there has been great progression in the field of micro- and nanomechanics with devices with higher and higher Q factors being created. This has been made possible thanks to a combination of advances in fabrication techniques and an increase in understanding as to what causes dissipation in nanometre scale structures. This understanding of dissipation mechanisms is still incomplete however. While lots of work has been done investigating mechanisms such as thermoelastic dissipation and dissipation due to two level systems (TLS) within the standard tunnelling model (STM) a full understanding has not been forthcoming. The increase in the quality of nanomechanical systems has allowed them to be coupled to optical or microwave cavities allowing the position of the mechanical system to be measured with near quantum limited accuracy. This thesis looks at both these streams of research within nanomechanics. It looks at the fabrication of silicon nitride torsional resonators that can have either their flexural or torsional modes preferentially actuated via a piezoelectric drive. It was found for a single paddle resonator that the room temperature Q factor of the flexural mode was 2870±70 and for the torsional mode was 5050±220. It was shown that while thermoelastic damping was reduced in the torsional mode it was still present meaning that we could not use the model for a simple beam to describe thermoelastic damping for a paddle resonator. The properties of an nanomechanical beam fabricated from a single crystal of aluminium were also investigated. It was found that at 1.5 K it had an unloaded Q factor of 36900 which is at least 2 times larger then any other group has reported. We also used our knowledge of high stress silicon nitride membranes to design a system that could couple an aluminium on silicon nitride membrane to a LCR circuit. Calculations show that this would have a coupling constant, g, of over 1000 putting it well within the regime where ground state cooling and quantum limited measurements are possible.

Le sujet de cette thèse est l'observation de parois de domaines ferromagnétiques dans des nanofils cylindriques, ainsi que leur dynamique sous champs magnétiques appliqués. Ces nanostructures ont été électrodéposées par mes collègues dans des membranes d'alumine nanoporeuse servant de gabarits à géométrie adaptable. Les matériaux sont des alliages magnétiques doux de FeNi ou CoNi ; les diamètres vont de 150 nm à 250-300 nm, avec une longueur typique de 30 µm.Mon travail a d'abord englobé des développements instrumentaux de porte-échantillons et d'électronique haute fréquence visant au déplacement de parois sous champ. J'ai étudié cette dernière grâce au dichroïsme magnétique circulaire des rayons X couplé à la microscopie électronique de photoémission (XMCD-PEEM), en géométrie dite de transmission ou d'ombre. Cette technique synchrotron permet le suivi de la configuration interne de paroi avant et après déplacement ; en raison de la forte reproductibilité requise par le XMCD-PEEM résolu en temps, la dynamique en temps réel est pour le moment inaccessible.La réponse des parois de domaines ferromagnétiques à un champ magnétique est notoirement caractérisée par leur mobilité, c'est-à-dire le rapport de la vitesse atteinte sur le champ. Dans les nanofils cylindriques, un ingrédient nouveau apparaît dans le cas d'un type de paroi absent dans les bandes plates : la paroi à point de Bloch (Bloch point wall, BPW). Non seulement cette paroi comporte une singularité micromagnétique, c'est-à-dire un point où l'aimantation disparaît (le point de Bloch), mais elle possède également un degré de liberté discret représentant le sens d'enroulement d'aimantation autour de l'axe du fil. Il a été prédit que le déplacement de BPW sous champ suffisamment intense résulte en la sélection de l'un des deux seuls sens possibles. En d'autres termes, un des deux enroulements devient instable. Dans cette thèse, je rapporte l'observation expérimentale de cette sélection dans une majorité de déplacements de BPW.Il n'a pas été possible de mener des mesures de mobilité, néanmoins, mes expériences ont mis en évidence des transformations jusqu'ici non prévues en simulation entre types de parois. La BPW contenant un défaut topologique (le point de Bloch lui-même), ce comportement inattendu remet en question la protection topologique parfois attribuée aux textures topologiquement non-triviales. Bien que rappelant la conversion entre parois transverse et vortex dans les bandes, ces transformations dans les nanofils cylindriques impliquent des configurations micromagnétiques topologiquement non-équivalentes, par contraste avec les parois des bandes sus-mentionnées. De plus, la toute relative stabilité observée des types de parois suggère la prudence dans l'interprétation de future mesures de mobilité dans de tels systèmes dès lors que la configuration interne de paroi n'est pas résolue.En-dehors de tels échantillons électrodéposés, j'ai également étudié un nanofil cœur-coquille crû verticalement par des collègues. Cette nanostructure cylindrique créée par dépôt induit par faisceau d'électron focalisé (FEBID) possèdait un cœur nanocristallin de cobalt et une coquille de platine. Sa configuration magnétique a été également étudiée par XMCD-PEEM en transmission. Contrairement au fils précédemment mentionnés et posés sur leur substrat sur toute leur longueur, cet échantillon cœur-coquille était vertical et sans modulations de diamètre. En revanche, la géométrie coudée du fil a été conçue pour favoriser le piégeage de parois. Dans cette configuration innovante d'imagerie, le défi a été de remonter autant que possible à l'état magnétique du fil ; il m'a été possible de démontrer la présence d'au moins une paroi de domaine. / The thesis is concerned with the observation of ferromagnetic domain walls in cylindrical nanowires, as well as their dynamics under applied magnetic fields. These nanostructures were electrodeposited by colleagues of mine into nanoporous alumina templates with a tailored pore geometry. The materials are soft FeNi or CoNi alloys; the diameters range from 150 nm to 250-300 nm, with a typical length of 30 µm.My work first comprised experimental developments of sample holders and high-frequency electronics towards field-induced domain wall motion. The latter I investigated with X-ray Magnetic Circular Dichroism coupled to transmission PhotoEmission Electron Microscopy (XMCD-PEEM). This synchrotron-based technique allows to monitor the internal domain wall configuration before and after displacement; due to the stringent requirements of time-resolved XMCD-PEEM experiments in terms of reproducibility, the real-time dynamics is out of reach as of yet.The response of ferromagnetic domain walls to applied magnetic fields is notably characterized by their mobility, i.e. the ratio of attained velocity to field strength. In cylindrical nanowires, a novel ingredient emerges in the case of one domain wall type that is absent in flat strips: the Bloch point domain wall. Not only does this domain wall host a micromagnetic singularity, that is to say a point where magnetization vanishes (the Bloch point), but it also possesses a discrete degree of freedom representing the sense of magnetization winding around the nanowire axis. It has been predicted that Bloch point wall motion under sufficiently high fields leads to this degree of freedom selecting one of its only two possible values. In other words, one winding becomes unstable. I report in this thesis experimental evidence of such a selection in a majority of Bloch point wall motion events.Although mobility measurements could not be carried out, my experiments have furthermore evidenced transformations between domain wall types that had not been predicted in simulations. Since the Bloch point wall contains a topological defect (the Bloch point itself), this unexpected behaviour questions the sometimes argued protection attributed to topologically non-trivial textures. While reminiscent of the well-known conversion between transverse and vortex walls in strips, these transformations in cylindrical nanowires involve topologically non-equivalent micromagnetic configurations, in contrast with the aforementioned transverse and vortex walls. Moreover, the observed only relative stability of domain wall types suggests caution in the interpretation of future mobility measurements in such systems, if the internal wall configuration cannot be resolved.Aside from such electrodeposited samples, I have also studied an upright core-shell nanowire grown by colleagues with Focused-Electron-Beam-Induced Deposition. This nanostructure featured a nanocrystalline cobalt core and a platinum shell. Its magnetic configuration was investigated with transmission XMCD-PEEM as well. Contrary to the aforementioned horizontally-lying wires, the core-shell sample was vertical with no diameter modulations. On the other hand, the geometry featured bends engineered to favour domain wall pinning. In this novel imaging configuration, the challenge was to recover as much of the nanowire's magnetic state as possible. I was able to demonstrate the presence of at least one domain wall.

In this dissertation, we examine quantum ferromagnets and determine various effects of the magnetic Goldstone modes or "magnons'' in these systems. Firstly, we calculate the magnon contribution to the transport relaxation rate of conduction electrons in metallic ferromagnets and find that at asymptotically low temperatures, the contribution behaves as T^2 exp(-T_0/T) and not as T^2 predicted previously. To perform these calculations, we derive and use a very general effective theory for metallic ferromagnets. This activation barrier-like behavior is due to the fact that spin waves only couple electrons from different Stoner subbands that arise from the splitting of the conduction band in presence of a nonzero magnetization. The T^2 behavior is found to be valid only in a pre-asymptotic temperature window. The temperature scale T_0 is the energy of the least energetic ferromagnon that couples electrons of different spins. Second, we discuss magnon-induced long-range correlation functions in quantum magnets. In the ordered phases of both classical ferromagnets and antiferromagnets, the long-range correlations induced by the magnons lead to a singular wavenumber dependence of the longitudinal order-parameter susceptibility in spatial dimensions 2<d<4. We investigate the quantum analog of this singularity using a nonlinear sigma model. In a quantum antiferromagnet at $T=0$, a weaker nonanalytic behavior is obtained, which is consistent with power counting. The analogous result for a quantum ferromagnet is absent if the magnon damping is neglected. This is due to the lack of magnon number fluctuations in the quantum ferromagnetic ground state. Magnon damping due to quenched disorder restores the expected nonanalyticity. Finally, we use an effective field theory for clean, strongly interacting electron systems to calculate the magnon contribution to the density of states, the longitudinal magnetic susceptibility and the conductivity in an itinerant ferromagnet. Utilizing a loop expansion that does not assume the electron-electron interaction to be a small parameter, we obtain the leading nonanalytic corrections to the Stoner saddle-point results for these observables, as functions of the frequency and wavenumber in the hydrodynamic limit. The dissertation includes previously published and unpublished co-authored material.

Neste trabalho, desenvolvemos um gerador de nanopartículas (NPs) como uma adaptação para um sistema de magnetron sputtering. Com ele, somos capazes de produzir NPs de materiais diversos e codepositá-las em matrizes dielétricas ou metálicas. A adaptação consiste em incluir uma região de alta pressão relativa de Ar no caminho do vapor atômico removido do alvo. A aglomeração ocorre termodinamicamente devido a diminuição da energia cinética após colisões com o gás. Desenvolvemos também, uma metodologia para colimar o fluxo de NPs dentro da região de alta pressão. A deposição é feita no substrato na forma de uma mancha com alguns milímetros de diâmetro e o tempo de preparação da amostra é significativamente curto. Desenvolvemos um modelo fenomenológico para explicar a condensação e a colimação do nosso sistema. Este, apesar de não sofisticado, explica bem ambos os fenômenos e consegue prever o diâmetro das nanopartículas para certas condições. Em paralelo ao desenvolvimento, produzimos e caracterizamos nanopartículas de cobalto. Da caracterização morfológica, através de microscopia eletrônica, concluímos que as NPs produzidas tem diâmetros médios de 10 nm com uma dispersão de 13 %. Através de análises de retro espalhamento Rutherford estudamos a distribuição do material sobre o substrato e observamos que este segue uma distribuição Gaussiana de espessuras. Além disso, devido a colimação, observamos que as taxas de deposição são da ordem de 50 vezes maiores que as taxas usuais de um sistema de sputtering. Estudos estruturais através de difração de raios X mostraram que as nanopartículas são nanocristalinas e imagens em alta magnificação de microscopia eletrônica de transmissão comprovaram esta hipótese. Finalmente, estudos magnéticos mostraram que as NPs não possuem eixos preferenciais de magnetização. Desenvolvemos condições padrões de operação e estabilizamos o sistema que atualmente produz amostras confiáveis e reprodutíveis. Além do Co, nanopartículas de Cu e SmCo foram produzidas em condições parecidas. A morfologia destas partículas foi investigadas por microscopia eletrônica e seus tamanhos se mostraram próximos dos das NPs de Co. Estes resultados ilustraram a universalidade do nosso sistema de deposição de nanopartículas. / We have developed a nanoparticle (NP) generator by adapting one of the sputtering guns on a magnetron sputtering system. With it, we are able to produce nanoparticles with different types of material. The adaptation consists of including a high-pressure region in the path of the atomic vapor removed from the sputtering target. The condensation happens thermodynamically through the loss of kinetic energy that the atomic vapor suffers after collisions with the gas. We have also developed a methodology to collimate the flow of nanoparticles inside the high pressure region. The deposition on the substrate is in the form of a stain with a few millimeters in diameter. The sample preparation time is also relatively short. We created a phenomenological model to explain both the condensation and collimation phenomena in our system. Despite being relatively simple, this model explain both quite well. In parallel to the development of the system, we produced and characterized cobalt nanoparticles. From a morphological analysis, carried out using electron microscopy, we determined that the nanoparticles mean diameter is of about 10 nm with a dispersion of 13 %. Through Rutherford back-scattering analysis, we studied the thickness distribution of the sample along the substrate. We observed that it follows a Gaussian distribution. Also, because of the collimation of the material, the deposition rates are about 50 times higher than in a regular sputtering system. Using X ray diffraction we were able to determine that the NPs are nano-crystalline which is corroborated with high resolution transmission electron microscopy images. Finally, magnetic measurements showed that the nanoparticles do not have any preferential magnetization axis. We developed standards of operations and stabilized the system. The samples we produce are trustworthy and reproducible. Besides Co, Cu and SmCo NPs were produced using this system with conditions similar to the ones used on the Co NPs. Through morphological analysis, we determined that their sizes are also similar. These results illustrate the universality of our system.

O objetivo do presente trabalho foi produzir e estudar magnética e estruturalmente filmes finos de Ni IND. 81 Fe IND. 19 Mn IND. 50. Os filmes foram produzidos através da técnica de DC magnetron sputtering. Iniciamos nosso estudo em filmes de Si/buffer/ Ni IND. 81 Fe IND. 19 (30nm)/Fe IND 50 Mn IND. 50 (15nm)/Ta(5nm), utilizando-se substratos de Si(100), Si(111) e buffer de Cu e Ta, a fim de determinar as melhores condições para a obtenção de bom acoplamento de exchange entre as bicamadas magnéticas. Em seguida estudamos a propriedades magnéticas de filmes em função da espessura das camadas magnéticas. As amostras produzidas para este estudo foram filmes de Si(100)/Cu(20nm)/ Ni IND. 81 Fe IND. 19 (t IND. FM nm)/ Fe IND. 50 Mn IND. 50(t IND AFM nm)/Ta(3nm), variando-se t IND. FM entre 5 e 53nm e mantendo fixa t IND. AFM em 10 nm e vice-versa. Finalmente estudamos o efeito das irradiações iônicas de He e Ne no acoplamento de exchange. Os filmes foram caracterizados utilizando-se as técnicas de magnetrometria de amostra vibrante, difração de raios-x, refletometria de raios-x, absorção de raios-x na borda do Mn e microscopia óptica de campo próximo (SNOM). Os resultados da caracterização magnética de todos os filmes de Si/buffer/NiFe/FeMn/Ta mostram que eles apresentam acoplamento de exchange logo após a sua obtenção por sputtering. Porém o acoplamento via troca precisa ser induzido através do procedimento field cooling nos filmes de Si/buffer/FeMn/NiFe//Ta. Os resultados das medidas de difração de raios x mostram que os filmes apresentam estrutura cristalina FCC e texturas cristalográficas (111) e (200) quando as camadas magnéticas são depositadas sobre Cu e uma única textura cristalográfica, a textura (111) quando elas são depositadas sobre Ta. Por outro lado, os resultados da caracterização magnética nos indicam que as amostras com buffer de Cu apresentam as melhores propriedades magnéticas alto valor de H IND. exc e baixa coercividade. A análise de EXAFS dos filmes com diferentes buffers nos indica que as amostras depositadas sobre Ta e diretamente sobre silício apresentam maior desordem na sua estrutura fina em relação às amostras depositadas sobre Cu. A caracterização magnética dos filmes irradiados ionicamente mostrou que o valor de Hexc dos filmes irradiados com diferentes doses de He a temperatura ambiente aumentou em comparação ao valor do campo de exchange das amostras antes das irradiações iônicas. Já nas amostras irradiadas com Ne o campo de exchange diminuiu drasticamente. A partir das medidas realizadas no microscópio óptico de varredura em campo próximo (SNOM), em diferentes regiões de amostras antes e depois de serem irradiadas ionicamente observamos que não há variações no valor do campo de exchange. / The aim of the present work was to produce and to study magnetic and structural properties of Ni81Fe19/Fe50Mn50 thin films. These films were produced by DC magnetron sputtering. We began our study on Si/buffer/Ni81Fe19(30nm)/Fe50Mn50(15nm)/Ta(5nm) films, using Si(100) or Si(111) substrates and Cu or Ta buffer layer, in order to determine the best conditions for obtaining a good exchange coupling between magnetic bilayers. After that, we studied the magnetic properties of these films relative to magnetics layers thickness. The samples studied were Si(100)/Cu (20mn)/Ni81Fe19 (tFM mn)/Fe50Mn50 (tAFM nm)/Ta(3nm), with tFM varying between 5 to 53run, with fixed tAFM of 10mn, and vice versa, namely with fixed tFM and varying tAFM. Finally, we studied the effect of He and Ne ionic irradiation on the exchange-coupling interaction. The thin films were characterized by vibrating sample magnetometry, X-ray diffraction, X-ray reflectometry, X-ray absorption spectroscopy at the Mn K edge, and near field optical microscopy (SNOM). The magnetic characterization of all Si/buffer/NiFe/FeMn/Ta films shows that pristine films present exchange coupling. However, the exchange-coupling interaction needs to be inducing through field cooling procedure for Si/buffer/FeMn/NiFe/Ta films. X ray diffraction measurements show that these films presents FCC crystal structure, plus (111) and (200) crystal textures when the magnetic layers are deposited on Cu buffer layer, and only the (111) crystal texture when the layers are deposited on Ta. On other hand, the magnetic measurement point out that the Cu buffer samples presents the best magnetic properties with high values for Hexc and low values of coercive field. The EXAFS analysis of films with different buffers points out that samples deposited on Ta and directly on silicon present a larger local disorder than samples deposited on Cu. The magnetic measurements of the ionic irradiated films showed an increase of Hexc values for samples irradiated at room temperature with different He rates, compared with the same samples before irradiation. On the other hand, for Ne irradiated samples the exchange field decreased drastically. We observed irradiation no changes on the exchange field values from near field optical microscopy (SNOM) measurements at different areas of samples before and after ionic. These results point out that, even if we are characterizing very small areas of the samples, the exchange field represents the average behavior of the exchange interaction in these areas.