Observamos o comportamento espectral do efeito Kerr transversal, para filmes amorfos de terra rara - cobalto na região do espectro visível e ultravioleta próximo (comprimento de onda, , entre 325 nm e 670 nm), onde o sinal magnetoóptico apresentou um máximo na região azul do espectro. Concluímos que, para esta região espectral, a variação relativa de refletividade é proporcional à magnetização do cobalto e a constante de proporcionalidade depende da composição da liga e do comprimento de onda da luz. Desenvolvemos uma técnica de modulação de fase, própria para medir o efeito Kerr transversal e a comparamos com a técnica de modulação de amplitude. Montamos um magnetômetro a efeito Kerr que opera com a modulação de fase e utiliza um laser de He-Cd ( = 325 nm e 442 nm) ou um diodo laser vermelho ( = 670 nm). Fazendo uma analogia com a figura de mérito para os efeitos Kerr longitudinal e polar (Fe), nós propusemos uma figura de mérito para o efeito Kerr transversal (FJ. Mostramos que sua dependência do ângulo de incidência é semelhante à da variação relativa de refletividade e, comparando medidas da figura de mérito do efeito Kerr longitudinal com a figura de mérito para o efeito transversal, comprovamos que a grandeza proposta é um bom parâmetro para medida de qualidade magnetoóptica de materiais. Observamos os efeitos do sperimagnetismo de ligas com anisotropia local fraca (Gd-Co) e forte (Ho-Co), através do comportamento térmico dos ciclos de histerese magnética e magnetoóptica. Dentre os fenômenos observados estão os efeitos da compensação, o crescimento hiperbólico do campo coercivo e os efeitos da reorientação de spin. Para a liga de Gd20C080a, transição observada foi uma inversão de spin, cujo o campo magnético aplicado, no qual ocorre a transição, cresce exponencialmente com o aumento da temperatura da amostra. / We have observed the spectral behavior of the transverse magnetooptical Kerr effect (TMOKE) in thin films of rare earth - cobalt amorphous alloys for the visible and the near-ultraviolet ranges of spectrurn (wavelength fiom 325 nm up to 670 nm). The relative change of reflectivity has shown a maximum at the blue wavelength. We have concluded that this magneto-optical signal is proportional to the cobalt magnetization and the proportionality constant depends on the alloy composition and on the wavelength. Besides developing an apparatus and a new phase modulation technique for the TMOKE, we have compared it to the more usual amplitude modulation TMOKE technique. This apparatus operates using either a He-Cd laser ( = 325 nm and 442 nm) for a red diode laser ( = 670nm). We have proposed the figure of merit for the transverse Kerr effect (F,) by analogy with the figure of merit for the longitudinal and polar Kerr effect (Fe). We have measured F, as a function of the angle of incidence and we have shown that F, has the same behavior of the magneto-optical signal. A comparison between F, and Fe has shown that F, is as good parameter as well as Fe for the characterization of the materials magneto-optical quality. We have studied the sperimagnetism of thin films of amorphous alloys with weak (Gd-Co) and strong (Ho-Co) random local anisotropy by the thermal behavior of the magnetic and magneto-optical hysteresis loops. The phenomena of compensation, coercivity and spin-reorientation phase transitions were observed. The comparison between magnetic and magneto-optical hysteresis loops for Gd20C~8h0as shown that the transition occurs from a colinear phase to an opposite collinear phase. In this case, the transition magnetic field grows exponentially when the temperature increases.

Diese Arbeit befasst sich mit der Nanostrukturierung von magnetischen Schichtsystemen mit Tunnelmagnetowiderstandseffekt (TMR-Effekt), welche in der Form von Nanosäulen in magnetoresistiven Speichern (MRAM) eingesetzt werden. Solche Nanosäulen können zukünftig ebenfalls als Nanoemitter von Mikrowellensignalen eine Rolle spielen. Dabei wird von der Auswahl eines geeigneten TMR-Schichtsystems mit einer MgO-Tunnelbarriere über die Präparation der Nanosäulen mit Seitenisolierung bis hin zum Aufbringen der elektrischen Zuleitungen eine komplette Prozesskette entwickelt und optimiert. Die Strukturen werden mittels optischer Lithographie und Elektronenstrahllithographie definiert, die anschließende Strukturübertragung erfolgt durch Ionenstrahlätzen (teilweise reaktiv) sowie durch Lift-off. Rückmeldung über Erfolg oder Probleme bei der Strukturierung geben Transmissionselektronenmikroskopie (teilweise mit Zielpräparation per Ionenfeinstrahl, FIB), Rasterelektronenmikroskopie sowie die Lichtmikroskopie. Es können so TMR-Nanosäulen mit minimalen Abmessungen von bis zu 69 nm x 71 nm hergestellt werden, von denen Nanosäulen mit Abmessungen von 65 nm x 87 nm grundlegend magneto-elektrisch charakterisiert worden sind. Dies umfasst die Bestimmung des TMR-Effektes und des Widerstandes der Tunnelbarriere (RA-Produkt). Weiterhin wurde das Verhalten der magnetischen Schichten bei größeren Magnetfeldern bis +-200mT sowie das Umschaltverhalten der magnetisch freien Schicht bei verändertem Winkel zwischen magnetischer Vorzugsachse des TMR-Elementes und dem äußeren Magnetfeld untersucht. Der Nachweis des Spin-Transfer-Torque Effektes an den präparierten TMR-Nanosäulen ist im Rahmen dieser Arbeit nicht gelungen, was mit dem zu hohen elektrischen Widerstand der verwendeten Tunnelbarriere erklärt werden kann. Mit dünneren Barrieren konnte der Widerstand gesenkt werden, allerdings führt ein Stromfluss durch diese Barrieren schnell zur Degradation der Barrieren. Weiterführende Arbeiten sollten das Ziel haben, niederohmige und gleichzeitig elektrisch belastbare Tunnelbarrieren in einem entsprechenden TMR-Schichtsystem abzuscheiden. Eine erste Auswahl an Ansatzpunkten dafür aus der Literatur wird im Ausblick gegeben. / This thesis deals with the fabrication of nanopillars with tunnel magnetoresistance effect (TMR-effect), which are used in magnetoresistive memory (MRAM) and may be used as nanooscillators for future near field communication devices. Starting with the selection of a suitable TMR-layer stack with MgO-tunnel barrier, the whole process chain covering the fabrication of the nanopillars, sidewall isolation and preparation of the supply lines on top is developed and optimised. The structures are defined by optical and electron beam lithography, the subsequent patterning is done by ion beam etching (partially reactive) and lift-off. Techniques providing feedback on the nanofabrication are transmission electron microscopy (partially with target preparation by focused ion beam, FIB), scanning electron microscopy and optical microscopy. In this way nanopillars with minimal dimensions reaching 69 nm x 71 nm could be fabricated, of which nanopillars with a size of 65 nm x 87 nm were characterized fundamentally with respect to their magnetic and electric properties. This covers the determination of the TMR-effect and the resistance of the tunnel barrier (RA-product). In addition, the behaviour of the magnetic layers under higher magnetic fields (up to +-200mT) and the switching behaviour of the free layer at different angles between the easy axis of the TMR-element and the external magnetic field were investigated. The spin transfer torque effect could not be detected in the fabricated nanopillars due to the high electrical resistance of the tunnel barriers which were used. The resistance could be lowered by using thinner barriers, but this led to a quick degradation of the barrier when a current was applied. Continuative work should focus on the preparation of tunnel barriers in an appropriate TMR-stack being low resistive and electrically robust at the same time. A first selection of concepts and ideas from the literature for this task is given in the outlook.

Perovskite rare-earth manganites of the form R1-xAxMnO3 (R – rare earth ion or Bi, A – Ca,Sr) have drawn an overwhelming research interest during the last few years owing to their extraordinary physical properties. Some of the interesting phenomena exhibited by the manganites are (a) colossal magneotresistance (CMR) (b) charge, orbital and spin ordering and (c) phase separation at nano and micron scale. The manganites are strongly correlated systems in which the charge, spin and orbital degrees of freedom are coupled. The properties of these materials are sensitive functions of external stimuli such as the doping, temperature and pressure [1-5] and have been extensively studied both experimentally and theoretically on single crystal, bulk polycrystalline and thin film forms of the samples [6-9]. Recently attention has been drawn towards the properties of nanoscale manganites. The nanoscale materials are expected to behave quite differently from extended solids due to quantum confinement effects and high surface/volume ratio. Nanoscale CMR manganites have been fabricated using diverse methods in the form of particles, wires, tubes and various other forms by different groups. It has been shown that the properties of CMR manganites can be tuned by reducing the particle size down to nanometer range and by changing the morphology [10-14]. The physical properties of antiferromagnetic insulating charge ordered manganites have been well investigated by using numerous experimental techniques on bulk solids. It is known that the charge ordered (CO) phase is ‘melted’ resulting in a ferromagnetic, metallic phase on application of high magnetic fields, electric fields, impurity ion doping, high energetic ion irradiation and by pressure [15-17]. However, no attempts have been made on the fabrication and the physical property investigations on nanoscale charge ordered manganites. Hence, we have undertaken to study the properties of charge ordered manganites prepared at nanoscale using various experimental probes. In this thesis we present the results on magnetization, magnetotransport and Electron Magnetic Resonance (EMR) (electron paramagnetic resonance in the paramagnetic phase and ferromagnetic resonance in the ferromagnetic phase) studies of the following nanoscale compounds and compare the properties with those of their bulk counterparts; (a) highly robust antiferromagnetic insulating CE –type charge ordered manganite Pr0.5Ca0.5MnO3 (PCMO) (b) highly robust antiferromagnetic insulating CE- type charge ordered manganite Nd0.5Ca0.5MnO3 (NCMO) (c) moderately robust A-type charge ordered manganite Pr0.5Sr0.5MnO3 (PSMO) (d) highly robust insulating anti-ferromagnetic charge ordered manganites Bi0.5Ca0.5MnO3 (BCMO) and Bi0.5Sr0.5MnO3 (BSMO) and (e) a CMR manganite Pr0.7Pb0.3MnO3 (PPMO). Chapter 1 of the thesis contains a brief introduction to the general features of manganites describing various interesting phenomena and the interactions underlying them. Further, we have written a detailed review on the properties of nanometric CMR manganites of various sizes and shapes. In this chapter, we have also described the experimental methodology and the analysis procedure adopted in this work Chapter 2 reports the fabrication of nanowires and nanoparticles of Pr0.5Ca0.5MnO3 (PCMO) and the results obtained from magnetization, magnetotransport and electron magnetic resonance measurements performed on nanoscale PCMO along with their comparison with those of the bulk sample. Here, the nanowires of PCMO were prepared by hydrothermal method and the nanoparticles of mean sizes 10, 20 and 40 nm were prepared by polymer assisted sol-gel method. Solid state reaction method was used to prepare the micron sized PCMO bulk material. Different techniques like XRD, TEM, EDAX and ICPAES have been used to characterize the samples. The novel result of the present investigation is the weakening of charge order and switch over from the anti-ferromagnetic phase to ferromagnetic phase in PCMO nanowires [18]. In addition, the charge order is seem to have completely suppressed in 10 nm PCMO nanoparticles as observed from the magnetization measurements. These results are particularly very significant as one needs magnetic fields of ~ 27 T to melt the charge ordered phase in PCMO. Size induced insulator-metal transition TM-I is observed in nanoscale PCMO at low temperatures accompanied by ferromagnetism. CMR of 99.7% is obtained at TM-I and at a field of 11 T. EMR studies have confirmed the presence of ferromagnetic phase at low temperatures. Temperature dependent EMR line width and intensity have shown the presence of CO phase in PCMO10 though static magnetization measurements have shown the absence of CO phase. It is found that the EMR linewidth increases with the decrease of particle size. Chapter 3 reports the fabrication of nanoparticles of Nd0.5Ca0.5MnO3 (NCMO) and the results obtained from magnetization, magnetotransport and electron magnetic resonance measurements performed on nanoscale NCMO along with their comparison with those of bulk NCMO. The nanoparticles of NCMO of mean sizes 5, 20 and 40 nm were prepared by polymer assisted sol-gel method. Solid state reaction method was used to prepare the micron sized NCMO bulk material. Different techniques like XRD, TEM, EDAX and ICPAES have been used to characterize the samples. A striking result of this particular investigation is the complete suppression of charge ordered phase in 5 and 20 nm NCMO nanoparticles as observed from the magnetization measurements [19]. Size induced insulator-metal transition TM-I is observed in nanoscale NCMO at low temperatures accompanied by ferromagnetism in accordance with Zener double exchange meachanism. CMR of 99.7% is obtained at TM-I and at a field of 11 T. EMR studies have confirmed the presence of ferromagnetic phase at low temperatures. Temperature dependent EMR line width and intensity have shown the presence of residual CO fluctuations in NCMO5 though the static magnetization measurements have shown the absence of CO phase. It is found that the EMR linewidth increases with the decrease of particle size. Low temperature X-ray diffraction measurements on NCMO20 indicate the absence of CO phase. But the preliminary results obtained from the optical spectroscopy measurements indicate the evidence for the presence of CO phase. In Chapter 4, we report the investigations on the nanoscale PSMO. PSMO nanoparticles of sizes 20, 40 and 60 nm are prepared by polymer precursor sol-gel method. PSMO nanowires of diameter 50 nm and lengths of a few microns have been prepared by hydrothermal method. The bulk polycrystalline PSMO is obtained by crushing the single crystal of the same prepared by float zone method. Various techniques like XRD, TEM, VSM, transport measurements and EMR spectroscopy have been employed to characterize and to study the size dependent magnetic, transport and electron magnetic resonance properties and to compare them with those of the bulk. Our results show that there is a disappearance of anti-ferromagnetic charge ordering phase and the appearance of a ferromagnetic phase at low temperatures in all PSMO nanoparticles and nanowires. Metal like behaviour is observed in the size induced ferromagnetic phase in nanoparticles. The EMR linewidth increases with the decrease of particle size. A comparison with the properties of the bulk material shows that the ferromagnetic transition at 265 K remains unaffected but the anti-ferromagnetic transition at TN = 150 K disappears in the nanoparticles. Further, the temperature dependence of magnetic anisotropy shows a complex behaviour, being higher in the nanoparticles at high temperatures, lower at lower temperatures in comparison with the bulk [20]. In Chapter 5, we present the fabrication, characterization and the results obtained from the magnetization and EMR measurements carried out on BCMO and BSMO nanoparticles and compare the results with those of the bulk. X-ray diffraction gives evidence for single phasic nature of the materials as well as their structures. Mono-dispersed to a large extent, isolated nanoparticles are seen in the transmission electron micrographs. High resolution electron microscopy shows the crystalline nature of the nanoparticles. Superconducting quantum interferometer based magnetic measurements from 10 K to 300 K show that these nanomanganites retain the charge ordering nature unlike the Pr and Nd based nanomanganites. The CO in Bi based manganites is thus found to be very robust consistent with the observation that magnetic fields of the order of 130 T are necessary to melt the CO in these compounds. These results are supported by electron magnetic resonance measurements [21]. In Chapter 6, we present our results on the effect of particle size on the magnetic properties of Pr0.7Pb0.3MnO3 (PPMO). PPMO nanoparticles of two different sizes (~5 nm and 30 nm) were prepared by the polymeric precursor sol-gel method. The samples are characterized by different techniques like XRD, TEM, SQUID magnetometry, EMR and optical spectroscopic measurements. It is found that the nanoparticles crystallize in the cubic perovskite structure. TEM measurements show that the 5 nm particles are uniform in size. They are also crystalline as seen by HREM and XRD measurements. SQUID magnetometry measurements have shown that the Curie temperature increases (from 220 K to 235 K) with the increase of particle size. Saturation magnetization is higher for the smaller particles studied. We have observed only one EMR signal down to 4 K in both the nanoparticles (5 and 30 nm) in contrast to the two EMR signal behaviour observed in bulk PPMO [22]. It is found that the EMR linewidth increases with the decrease of particle size in the paramagnetic phase. Temperature dependent optical spectroscopy measurements performed on 5 nm PPMO nanoparticles indicate that the insulator-metal transition temperature TM-I = 230 K, is not very different from TM-I = 235 K of the bulk sample [23] The thesis concludes with a brief writeup summarizing the results and pointing out possible future directions of research in the area.

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No description available.

This dissertation summarizes three research projects related to microwave radiation induced electron transport properties in the GaAs/AlGaAs two dimensional electron systems. In chronological order, the projects are: a microwave reflection and electron magneto-transport correlation study, the combined microwave power and polarization dependence on microwave radiation induced magneto-resistance oscillations study, and a comparative study about the effect of circularly polarized and linearly polarized microwaves radiation on magneto-resistance oscillations induced due to the microwave. These three research projects experimentally address many interesting issues in the non-equilibrium low dimensional electron transport under microwave irradiation and provide potential applications of utilizing microwave radiation induced magneto-resistance oscillations in two dimensional electron systems as a method to detect different qualities of microwaves or terahertz waves.

Le procédé de soudage à l'arc sous atmosphère inerte (TIG) est souvent employé pour des assemblages nécessitant une grande qualité du joint soudé. Les propriétés du joint soudé dépendent essentiellement du cycle thermique imposé par l'opération de soudage, de la composition chimique du matériau métallique et des mouvements convectifs du métal fondu dans le bain de fusion. L'écoulement du métal liquide dans le bain de fusion modifie la distribution de température en son sein et à proximité, ainsi que la forme géométrique du joint. Afin d'améliorer l'opération de soudage TIG, par exemple pour accroitre la productivité ou éviter des défauts rédhibitoires, il est nécessaire de bien comprendre les phénomènes physiques mis en jeu dans le bain de fusion ainsi que l'effet des paramètres opératoires (intensité, hauteur d'arc, gaz …) sur ces phénomènes physiques. Dans le but d'appréhender les phénomènes mis en jeu au cours de l'opération TIG et dans le bain de fusion, un modèle multi-physique 2D axisymétrique a été établi et résolu par la méthode des éléments finis (MEF). Les forces telles que Lorentz (électromagnétique), Marangoni (Tension superficielle), Boussinesq et la force de cisaillement du plasma d'arc ont été prises en compte au niveau du bain de fusion. Le modèle TIG établi est utilisé pour prédire la distribution de température et la distribution des vitesses dans le bain de fusion ainsi que la forme géométrique du bain de fusion. Un protocole expérimental a été développé dans le but de valider le modèle proposé. Pour cela, une opération de soudage TIG stationnaire (pas de mouvement de la torche) a été réalisée sur un disque métallique. L'opération a été contrôlée par des mesures de température, par une observation de la formation et de l'évolution de la surface du bain de fusion avec une caméra rapide et un enregistrement des paramètres opératoires (intensité et tension). Toutes les données sont synchronisées entre elles pour permettre une analyse expérimentale pertinente. La confrontation des résultats expérimentaux avec le modèle multi-physique du soudage TIG a fait apparaître une assez bonne adéquation, mais des différences existent, essentiellement liées à la méconnaissance des paramètres décrivant le flux de chaleur utilisé dans la simulation. Le flux de chaleur a été modélisé par une fonction Gaussienne qui nécessite la connaissance du rendement du procédé TIG et la distribution spatiale (ou rayon de la Gaussienne). L'estimation de ces paramètres a été réalisée par une méthode inverse. Cette méthode inverse a consisté à estimer les paramètres inconnus à partir des données expérimentales disponibles. La méthode d'optimisation dite de Levenberg-Marquardt, associée à une technique de régularisation itérative, a été utilisée pour estimer les paramètres. La pertinence et la robustesse de cette méthode ont été validées au travers de plusieurs cas numériques ; soit des cas utilisant des données « exactes » ou des données « bruitées ». Trois types d'erreurs ont été analysés séparément : bruit de mesure, erreur sur la position du capteur et imprécision sur la valeur des propriétés thermophysiques. Les deux dernière erreurs sont celles qui impactent fortement le résultat de l'estimation, essentiellement l'estimation du rendement du procédé TIG. Enfin, une partie des données expérimentales a été utilisée pour résoudre le problème inverse. Les paramètres ont été estimés avec une marge d'erreur inférieure à 10% et ils sont en bon accord avec les valeurs trouvées dans la littérature. / Gas Tungsten Arc Welding (GTAW) process is generally used for assemblies that requires high quality weld joint. The microstructure and the weld joint relies mainly on the thermal cycle due to the welding operation, the chemical composition of the metallic material and the complex flow of molten metal in the weld pool. Moreover the fluid flow in the weld pool play a major role in the temperature distribution and the final weld pool shape. Better understanding of the physical phenomena involved in the welding operation, more exactly in the weld pool, are the fundamental step for improving the GTAW operation, for example increase the productivity or avoid defects. In the present research work, a two dimensional axi-symmetric multiphysics model was established in order to predict the weld pool shape evolution in the frame of a stationary Gas Tungsten Arc Welding using a finite element numerical approach. The weld pool model included various driving forces such as self-induced electromagnetic (Lorentz force), surface tension (Marangoni force), buoyancy and the arc plasma drag force. The stated GTAW model is used for predicting the velocity and temperature distribution in the fusion zone and the final weld pool shape. In order to validate the GTAW model, an experimental set up was defined for synchronizing the acquisition of time dependent data such as temperature, weld pool radius and welding process parameters (current and voltage). Image processing algorithms were developed for the time dependent weld pool size identification from the high speed camera images. Comparison between experimental and calculated data exhibited important discrepancies on the temperature field and weld pool radius. These discrepancies are due to the incoming heat flux from the arc plasma into the work piece. The heat flux was modeled with a Gaussian function itself described with few parameters;two of these required to be estimated: GTAW efficiency and Gaussian distribution.An inverse approach is used for estimating these parameters from the available experimental data: temperature, weld pool radius and macrographs. The Levenberg-Marquardt method is used to solve the inverse heat transfer problem coupled to an iterative process regularization. Afterward the inverse heat transfer problem was investigated through few numerical cases in order to verify its robustness to three sorts of error in the input data (measurement noise, sensor location error and inaccuracies associated with the thermophysical properties). The inverse approach was robust to errors introduced on measurement data. However, errors on the position of sensors or on the knowledge of material thermo-physical properties are problematic on the GTAW efficiency estimation. Finally the inverse problem was solved with experimental measurement. The estimated parameters are in good agreement with the literature. The evaluated error on the estimated parameters is less than 10%.

Made available in DSpace on 2016-12-12T20:15:49Z (GMT). No. of bitstreams: 1 Anne Luise Kruger.pdf: 3331342 bytes, checksum: f90ffb5e34c66b8e268f864d4f6dab4e (MD5) Previous issue date: 2013-02-15 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The magneto-optical traps cool and trap atoms in a given region, forming clouds of cold atoms. As the setting of the trap, clouds can be obtained in different geometries such as spherical or ring-shaped. Traps have a wide range of applications, such as building atomic clocks, to obtain the Bose-Einstein condensate and the study of collisions, wherein the ring geometry is interesting to study in low dimensions. The learning of the trap parameters implies in its characterization and thus the appropriate parameters allow to optimize the trap and improve the performance of it s. The objective of our work is to study the behavior of the motion of atoms and the forces that act upon them by varying some parameters of the trap, such as the magnetic field gradient, the detuning between the laser frequency and the atomic transition, the intensity of the laser beams and also the intensity beams profile of the beams. By using the integration method of Runge-Kutta, we present simulations of the trajectory of a trapped atom and the radius of the orbit varying the same parameters of the trap mentioned above. One can find in literature a description of the force exertd on the trapped atoms with Gaussian profile laser beams in the traditional configuration and also with a small misalignment. From the deduction of the force on the trapped atoms with laser beams of Gaussian profile, we present some considerations to find the force acting on the atoms with Gaussian beam profile and simulations in this new configuration. Thus, the possibility of trapping atoms with other intensity beam profiles can be tested. / As armadilhas magneto-ópticas resfriam e aprisionam átomos em uma determinada região, formando nuvens de átomos frios. Conforme a configuração da armadilha, pode-se obter nuvens em diferentes geometrias, como esféricas ou em forma de anel. As armadilhas tem uma grande gama de aplicações, como a construção de relógios atômicos, a obtenção do condensado de Bose-Einstein e o estudo de colisões, no qual a geometria em anel se torna interessante para o estudo em baixas dimensões. Para conhecer os parâmetros da armadilha é necessário fazer sua caracterização. Parâmetros adequados permitem otimizar a armadilha e melhorar a performance da mesma. O objetivo de nosso trabalho é estudar o comportamento do movimento dos átomos e das forças que atuam sobre eles ao variar alguns parâmetros da armadilha, como por exemplo o gradiente de campo magnético, a dessintonia entre a frequência do laser e da transição atômica, a intensidade dos feixes laser e o perfil de intensidade dos feixes. Utilizando o método de integração de Runge- Kutta, apresentaremos simulações da trajetória de um átomo aprisionado e o raio da órbita variando os mesmos parâmetros da armadilha anteriormente citados. Na literatura encontra-se a descrição da força exercida sobre os átomos aprisionados com feixes laser de perfil gaussiano na configuração tradicional e também com um pequeno desalinhamento. A partir da dedução da força sobre os átomos aprisionados com feixes laser de perfil gaussiano, apresentaremos algumas considerações feitas para encontrar a força que atua sobre os átomos com feixes de perfil não gaussiano e simulações realizadas nesta nova consideração. Desta forma, poderá se verificar se existe a possibilidade de aprisionar átomos com feixes de outros perfis de intensidade.

Avec l’avènement du laser femtoseconde il est devenu possible de mesurer comment la démagnétisation femtoseconde peut permettre de sonder l’interaction d’échange dans les métaux ferromagnétiques. La démagnétisation induite par laser d’un matériau avec un fort couplage magnéto-élastique amène à la relaxation des contraintes mécaniques, générant ainsi des ondes acoustiques longitudinales (L) et transversales (T). Dans ce travail de thèse, la génération d’impulsions acoustiques picosecondes T par le mécanisme de démagnétostriction dans des matériaux fortement magnétostrictifs est traitée analytiquement et montrée expérimentalement dans le cas d’un alliage de Terfenol. En premier lieu, nous avons développé un modèle phénoménologique de magnétostriction directe dans un film monocristallin de Terfenol. Les expériences pompe-sonde linéaire MOKE résolues en temps montrent que la relaxation transitoire des contraintes magnéto-élastiques du film amène à l’excitation d’ondes GHz acoustiques L at T. Ces résultats sont la première observation expérimentale de l’excitation d’ondes acoustiques transversales picoseconde par mécanisme de démagnétostriction induit par laser. En second lieu, nous avons analysé le processus d’interaction d’ondes acoustiques L avec l’aimantation d’un film mince de Terfenol. L’onde acoustique picoseconde produit un changement de magnétisation du film et induit la conversion de modes acoustiques. C’est une autre voie de génération d’ondes acoustiques T que nous avons mis en évidence. La gamme de fréquence des impulsions générées est liée à l’échelle de temps de démagnétisation, qui corresponds à quelques centaines de GHz - 1 THz. / With the advent of femtosecond lasers it became possible to measure how femtosecond optical demagnetization can probe the exchange interaction in ferromagnetic metals. Laser-induced demagnetization of materials with strong magneto-elastic coupling should lead to the release of its build-in strains, thus to the generation of both longitudinal (L) and shear (S) acoustic waves. In this thesis, generation of shear picosecond acoustic pulses in strongly magnetostrictive materials such as Terfenol is processed analytically and shown experimentally. In case of Terfenol with strong magneto-crystalline anisotropy, laser induced demagnetostriction is responsible for S excitation. First, the phenomenological model of direct magnetostriction in a Terfenol monocrystalline film is developed. The shear strain generation efficiency strongly depends on the orientation of the film magnetization. Time-resolved linear MOKE pump-probe experiments show that transient laser-induced release of the magnetoelastic strains lead to the excitation of GHz L and S acoustic waves. These results are the first experimental observation of picosecond shear acoustic wave excitation by laser-induced demagnetostriction mechanism. Second, the interaction of an optically generated L acoustic pulse with the magnetization of a Terfenol thin film is reported. Arrival of the picosecond strain wave alters a change of its magnetization and leads to acoustic mode conversion, which is another pathway of shear acoustic wave generation. The frequency bandwidth of the generated acoustic pulses matches the demagnetization timescale and lies in the range of several hundreds of GHz, close to 1 THz.

Nesta tese apresentamos os estudos de magnetotransporte em poços quânticos largos,\\\\ estreitos e triplos em campos magnéticos baixos. Dependendo dos estudos desejados, me-\\\\dimos a magnetoresistência em regime linear e não linear e sob a aplicação de corrente AC, irradiação de microondas e em gradiente de temperatura ao longo das amostras. Relatamos a observação de efeitos não lineares de corrente alternada em oscilações magneto-inter-sub-bandas de poços quânticos triplos. A oscilação MIS em sistemas de poços quânticos individuais e duplos e também os efeitos não lineares devido à corrente contínua foram estudados antes nestes sistemas. Nossos resultados são explicados de acordo com um modelo generalizado baseado na parte de não equilíbrio da função de distribuição de elétrons. A magnetorresistência não local sob irradiação de microondas é também estudada nesta tese. Os resultados obtidos proporcionam evidências para uma corrente de estado de borda estabilizada por irradiação de microondas, devido às ressonâncias não lineares e foram descritas por um modelo baseado em dinâmica não linear e mapa padrão de Chirikov. Finalmente, observamos uma correlação estreita entre as oscilações de resistência e oscilações de tensão de arraste do fônon induzidas por irradiação de microondas em um sistema bidimensional de eletrons sob campo magnético perpendicular. A influência da resistividade de dissipação modificada por microondas na tensão de arraste do fônon perpendicular ao fluxo de fônons pode explicar nossas observações. Além disso, características nítidas observadas na tensão de arraste do fônon sugerem que os domínios de corrente associados a estes estados podem existir na ausência de condução DC externa. / In this thesis, we present the studies of magneto-transport in narrow , wide and triple quantum wells in low magnetic fields. Depending on the desired studies, we have measured the magneto-resistance both in linear and nonlinear regimes and under the application of AC current, microwave irradiation and temperature gradient along the samples. We have reported the observation of nonlinear effects of AC current on magneto-inter-sub-band oscillations (MIS) of triple quantum wells (TQWs). The MIS oscillations in single and double quantum well system and also nonlinear effects due to DC current have been studied before in these systems. Our results are explained according to a generalized model based on non-equilibrium part of electron distribution function. The nonlocal magneto-resistance under microwave irradiation is also studied within this thesis. The obtained results provide evidence for an edge-state current stabilized by microwave irradiation due to nonlinear resonances and have been described by a model based on the nonlinear dynamics and Chirikov standard map. Finally, we have observed the phonon-drag voltage oscillations correlating with the resistance oscillations under microwave irradiation in a two-dimensional electron gas in perpendicular magnetic field. The influence of dissipative resistivity modified by microwave on phonon-drag voltage perpendicular to the phonon flux can explain our observations. Moreover, sharp features observed in phonon drag voltage suggest the current domains associated with these states can exist in the absence of external DC driving.