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.

Depuis deux dernières décennies, la séparation magnétique revient sur le tapis grâce aux applications biomédicales émergentes à la séparation de cellules ou de protéines et aux tests immunologiques. Cette thèse porte sur l’exploration détaillée de la séparation magnétique de nanoparticules à l’échelle microfluidique, amplifiée par la séparation de phase induite par le champ. Dans ce but, nous synthétisons des nanoclusters superparamagnétiques d’oxyde de fer de taille 40-70 nm, composés de nanoparticules de taille 7-9 nm. Nous faisons une étude détaillée de la cinétique de la séparation de phase de ces nanoclusters induite par le champ ainsi que de leur séparation magnétique dans des canaux microfluidiques munis de réseaux ordonnés de micropiliers magnétisables. Le taux d’agrégation de nanoclusters est principalement régi par le paramètre du couplage dipolaire et par la fraction volumique de nanoclusters, tandis que l’efficacité de capture – par le nombre de Mason. Les couches de molécules adsorbées sur la surface de nanoclusters d’habitude affaiblissent les interactions magnétiques et diminuent l’efficacité de capture, cependant, dans certains cas, elles peuvent induire des interactions colloïdales attractives et augmenter l’efficacité de capture. Les résultats de ce travail peuvent être utiles pour le développement des tests immunologiques magnéto-microfluidiques. / Magnetic separation has been gaining a new interest during two last decades thanks to emerging biomedical applications to cell or protein separation and immunoassays. This thesis is aimed at detailed exploration of magnetic nanoparticle separation in microfluidic scale enhanced by field-induce phase separation of nanoparticles. To this purpose, we synthesize superparamagnetic iron oxide nanoclusters of a size of 40-70 nm composed of numerous nanoparticles of a size 7-9 nm. We perform a detailed study of the kinetics of the field-induced phase separation of these nanoclusters as well as of their magnetic separation in microfluidic channels equipped with ordered arrays of magnetizable micropillars. The nanocluster aggregation rate is mostly governed by the dipolar coupling parameter and the nanocluster volume fraction, while the capture efficiency – by the Mason number. Molecular layers adsorbed on the nanocluster surface usually weaken magnetic interactions and decrease the capture efficiency, however, in some casesthey may induce attractive colloidal interactions and enhance the capture efficiency. The results of this work could be useful for development of magnetomicrofluidic immunoassays.

Measurement of elastic properties (thermal expansion and magnetostriction) under (multi)extreme conditions is a difficult task. In the vicinity of the room temperature or above it an abundance of methods is available, with decreasing temperature and adding magnetic field and/or hydrostatic pressure their number is limited. Dilatometric cells (either planparaller or tilted plate design) provide superior sensitivity in low temperatures and applied magnetic fields, however, cannot be used in hydrostatic cell. Common choice for the measurement of thermal expansion under hydrostatic pressure are methods based on strain-gauges, with mediocre sensitivity and more importantly a difficult or even impossible usage at very low temperatures (T ≲ 3 K). Measurement of magnetic properties (especially magnetization) under (multi)extreme conditions is also a difficult task. In the vicinity of temperature 2 K and above it an abundance of methods is available, with decreasing temperature and adding magnetic field and/or hydrostatic pressure their number is limited. VSM system provides great sensitivity, but can not be used under 2 K and hydrostatic pressure. MPMS aparature provides pressure up to 9 GPa (diamond pressure cell), but still we can not apply lower temperatures than 2 K. Our aim is to develop a simple yet sensitive...

Graphene, the two-dimensional lattice of sp2-hybridized carbon atoms, has a great potential for future electronics, in particular for opto-electronic devices. The carrier relaxation dynamics, which is of key importance for such applications, is in the main focus of this thesis. Besides a short introduction into the most prominent material properties of graphene and the experimental techniques, this thesis is divided into three main parts. The investigation of the carrier relaxation dynamics in the absence of a magnetic field is presented in Chapter 3. In the first experiment, the anisotropy of the carrier excitation and relaxation in momentum space was investigated by pump-probe measurements in the near-infrared range. While this anisotropy was not considered in all previous experiments, our measurements with a temporal resolution of less than 50 fs revealed the polarization dependence of the carrier excitation and the subsequent relaxation. About 150 fs after the electrons are excited, the carrier distribution in momentum space gets isotropic, caused by electron-phonon scattering. In a second set of two-color pump-probe experiments, the temperature of the hot carrier distribution, which was obtained within the duration of the pump pulse (about 200 fs), could be estimated. Furthermore, a change in sign of the pump-probe signal can be used as an indicator for the Fermi energy of different graphene layers. Pump-probe experiments in the far-infrared range in reflection and transmission geometry were performed at high pump power. A strong saturation of the pump-induced transmission was found in previous experiments, which was attributed to the pump-induced change in absorption. Our investigation shows the strong influence of pump-induced reflection at long wavelengths, as well as a lot smaller influence of the saturation of the pump-induced change in absorption. At a high pump power, the increase of the reflection exceeds the change in absorption strongly, which leads to negative pump-probe signals in transmission geometry. In Chapter 4, investigations of the carrier dynamics of graphene in magnetic fields of up to 7T are presented. Even though the optical properties of Landau-quantized graphene are very interesting, the carrier dynamics were nearly unexplored. A low photon energy of 14meV allows the investigation of the intraband Landau-level (LL) transitions. These experiments revealed two main findings: Firstly, the Landau quantization strongly suppresses the carrier relaxation via optical-phonon scattering, resulting in an increased relaxation time. Secondly, a change in sign of the pump-probe signal can be observed when the magnetic field is varied. This change in sign indicates a hot carrier distribution shortly after the pump pulse, which means that carrier-carrier scattering remains very strong in magnetic fields. In a second set of pump-probe measurements, carried out at a photon energy of 75meV, the relaxation dynamics of interband LL transitions was investigated. In particular, experiments on the two energetically degenerate LL transitions LL(−1)->LL(0) and LL(0)->LL(1) showed the influence of extremely strong Auger processes. An ultrafast and extremely broadband terahertz detector, based on a graphene flake, is presented in the last chapter of this thesis. To couple the radiation efficiently to the small flake, the inner part of a logarithmic periodic antenna is connected to it. With a rise time of about 50 ps in a wavelength range of 9 μm to 500 μm, this detector is very interesting to obtain the temporal overlap in two-color pump-probe experiments with the free-electron laser FELBE. Furthermore, the importance of the substrate material, in particular for the high-speed performance, is discussed.

Title: Modelling of global ocean circulation and ocean-induced magnetic field Author: Libor Šachl Department: Department of geophysics Supervisor: prof. RNDr. Zdeněk Martinec, DrSc., Department of geophysics Abstract: The ocean modelling community commonly use several renown ocean general circulation models (OGCMs) such as NEMO, MOM and FESOM. These models have been developed by research groups for many years, which resulted in complex mathematical and numerical algorithms. There are geophysically rele- vant problems, such as the glacial isostatic adjustment, in which the global ocean plays an important role. Ocean circulation does not need to be modeled extremely complex, but other phenomena such as time changing geometry of ocean domain needs to be considered. Geophysical applications motivated us to develop a new OGCM called LSOMG. The LSOMG model is not meant to substitute the ex- isting OGCMs but to provide a modelling framework for geophysical rather than purely oceanographic applications. LSOMG is a 3-D baroclinic ocean model fully parallelized using the MPI standard. It is forced by atmospheric fluxes (wind stresses, heat fluxes, etc.) but also by tides. The model can be run in a simplified 2-D barotropic version if 3-D effects can be neglected. LSOMG was tested in a series of simplified...

Graphene, the two-dimensional lattice of sp2-hybridized carbon atoms, has a great potential for future electronics, in particular for opto-electronic devices. The carrier relaxation dynamics, which is of key importance for such applications, is in the main focus of this thesis. Besides a short introduction into the most prominent material properties of graphene and the experimental techniques, this thesis is divided into three main parts. The investigation of the carrier relaxation dynamics in the absence of a magnetic field is presented in Chapter 3. In the first experiment, the anisotropy of the carrier excitation and relaxation in momentum space was investigated by pump-probe measurements in the near-infrared range. While this anisotropy was not considered in all previous experiments, our measurements with a temporal resolution of less than 50 fs revealed the polarization dependence of the carrier excitation and the subsequent relaxation. About 150 fs after the electrons are excited, the carrier distribution in momentum space gets isotropic, caused by electron-phonon scattering. In a second set of two-color pump-probe experiments, the temperature of the hot carrier distribution, which was obtained within the duration of the pump pulse (about 200 fs), could be estimated. Furthermore, a change in sign of the pump-probe signal can be used as an indicator for the Fermi energy of different graphene layers. Pump-probe experiments in the far-infrared range in reflection and transmission geometry were performed at high pump power. A strong saturation of the pump-induced transmission was found in previous experiments, which was attributed to the pump-induced change in absorption. Our investigation shows the strong influence of pump-induced reflection at long wavelengths, as well as a lot smaller influence of the saturation of the pump-induced change in absorption. At a high pump power, the increase of the reflection exceeds the change in absorption strongly, which leads to negative pump-probe signals in transmission geometry. In Chapter 4, investigations of the carrier dynamics of graphene in magnetic fields of up to 7T are presented. Even though the optical properties of Landau-quantized graphene are very interesting, the carrier dynamics were nearly unexplored. A low photon energy of 14meV allows the investigation of the intraband Landau-level (LL) transitions. These experiments revealed two main findings: Firstly, the Landau quantization strongly suppresses the carrier relaxation via optical-phonon scattering, resulting in an increased relaxation time. Secondly, a change in sign of the pump-probe signal can be observed when the magnetic field is varied. This change in sign indicates a hot carrier distribution shortly after the pump pulse, which means that carrier-carrier scattering remains very strong in magnetic fields. In a second set of pump-probe measurements, carried out at a photon energy of 75meV, the relaxation dynamics of interband LL transitions was investigated. In particular, experiments on the two energetically degenerate LL transitions LL(−1)->LL(0) and LL(0)->LL(1) showed the influence of extremely strong Auger processes. An ultrafast and extremely broadband terahertz detector, based on a graphene flake, is presented in the last chapter of this thesis. To couple the radiation efficiently to the small flake, the inner part of a logarithmic periodic antenna is connected to it. With a rise time of about 50 ps in a wavelength range of 9 μm to 500 μm, this detector is very interesting to obtain the temporal overlap in two-color pump-probe experiments with the free-electron laser FELBE. Furthermore, the importance of the substrate material, in particular for the high-speed performance, is discussed.

The heliosphere and solar magnetic field play an important role in protecting the solar system from harmful, high-energy Galactic radiation. Until recently, the magnetic field had been assumed to be passive, carried outwards by the solar wind. The influence of the solar magnetic field on the plasma has just begun to be understood. Among the consequences, the magnetic field could cause the heliotail to be short, collimating the flow into two lobes instead of the classical long, comet-like tail. In this dissertation, I investigate the role certain aspects of the magnetic field have on the heliosphere and detail how interstellar neutral particles alter its effect on the environment. From the observation by Voyager 1 (V1) and Voyager 2 (V2), it is clear that the plasma environment in the outer heliosphere is not fully understood. I present the first time-dependent model of the outer heliosphere that includes solar-cycle variations of the magnetic field strength. I find that the model can accurately predict the plasma environment at V2 but cannot describe all features observed at V1, suggesting additional processes are present. The effect of including the heliospheric current sheet (HCS) on large-scale modeling of the heliosphere is also studied. The inherent numerical dissipation in the HCS reduces the magnetic field strength in the heliosheath; however, the two-lobe structure of the heliotail remains. Neutral hydrogen has also been shown to greatly affect the location of the heliospheric boundaries. The large mean free path of these neutrals requires them to be described kinetically. To understand how the neutrals affect the influence of the solar magnetic field, I developed the Solar-wind with Hydrogen Ion Exchange and Large-scale Dynamics (SHIELD) model, a kinetic-magnetohydrodynamic model of the outer heliosphere. The model couples a 3D Monte-Carlo model to the magnetohydrodynamic solver. SHIELD reproduces the results of similar models, namely a higher filtration of neutrals into the heliosphere when compared to a fluid description of the atoms. When SHIELD is applied to the heliotail, the two-lobe structure persists even with kinetic neutrals. These results show that the solar magnetic field plays a crucial role in determining the heliospheric structure.

Magnetic reconnection is a fundamental process that changes magnetic field configuration and converts a magnetic energy to flow energy and plasma heating. It can be found in a plasma with frozen magnetic field lines at boundaries where different magnetic field topologies encounter each other and thin current sheets are created as it is typical in the solar wind. In the thesis, we have used spacecraft measurements of solar wind plasma and magnetic field to found magnetic reconnection exhausts. We analyze and compare them with theoretical predictions. The results of the statistical analysis oriented on re-distribution of the magnetic energy in reconnection showed that both a portion of the energy deposited into heat as well as the energy spent on an acceleration of the exhaust plasma increase with the magnetic shear angle in accord with the increase of the magnetic flux available for reconnection. Moreover, we identify unusual events in the solar wind; we found magnetic reconnection exhausts accompanied by one or two side jets and explained their possible causes.

The presence of a magnetic field is shown to affect the evolution and properties of stars. Hence, it is necessary to observe different types of stars to explore these effects. The detached eclipsing binary UV Piscium is the object of interest in this study, where a first step of analyzing its global magnetic field is done. The observational data was collected during 2016, at the 3.6-m Canada-France-Hawaii Telescope at Mauna Kea, Hawaii. The analysis of the magnetic field is based on the line-addition technique least-squares deconvolution (LDS) of the polarisation signatures, and the aim is to search for circular polarisation signals produced by the Zeeman effect. The result shows a strong circular polarisation signature for the primary star of the binary, which is a direct evidence for the presence of a magnetic field. In contrast to this, the secondary star only shows a weak signal of circular polarisation in one of the analysed observations and further analysis of its magnetic field is needed. The secondary goal of the project was to calculate the stellar masses of the binary. This is done by measuring the radial velocities of the two stars via the line profiles, and preforming an orbital fit. The results gave: M1= 1.0211 ± 0.0040 Msol and M2 = 0.7728 ± 0.0028 Msol.

The production and control of antihydrogen at very low temperatures provided a key tool to test the validity for the antimaterial of the fundamental principles of the interactions of nature such as the weak principle of equivalence (WEP), and CPT symmetry (Charge, Parity, and Time reversal). The work presented in this dissertation studies the collisions of electrons and positrons in strong magnetic fields that generate magnetobound positronium (positron-electron system temporarily bound due to the presence of a magnetic field) and its possible role in the generation of antihydrogen.