A skyrmion is a vortex-like spin pattern which has been observed in so-called B20 magnetic systems such as MnSi, (Mn,Fe)Si and a few other metallic magnets as well as in insulating Cu₂OSeO₃. We conduct a comprehensive study of muon spin relaxation (μSR) on bulk single crystals of MnSi and (Mn,Fe)Si, a MBE thin film of MnSi, and a ceramic specimen of Cu₂OSeO₃ in this work. The generic second-order like phase transition indicated by 1/T₁ peaks at T_c in bulk systems is discussed in light of the Brazovskii-type first-order phase transition due to the presence of the DM interaction. We also discuss the different temperature dependences of μ⁺ spin-lattice relaxation rate 1/T₁ in bulk pure systems MnSi and Cu₂OSeO₃ and their commonalities in the paramagnetic state and the ordered state due to the DM interaction. Furthermore, we highlight the enhanced 1/T₁ in the skyrmion crystal (SkX) phase compared to neighboring conical phases due to an abundance of low-energy magnetic fluctuations/excitations. This abundance is corroborated by the reduced static order parameter in the SkX phase of MnSi compared to neighboring conical phases, deduced by combining μSR experiments and magnetic field simulations. The intermediate (IM) region above T_c, where the modification of magnetic transition by the DM interaction starts to appear in MnSi, exhibit multi-time scale spin fluctuations, topologically non-trivial Hall resistivity and non-Fermi-liquid exponent of longitudinal resistivity in single-crystal Mn₀.₉Fe₀.₁Si and the MnSi MBE thin film, similar to the magnetically disordered phase of pure MnSi under hydrostatic pressure. These three defining features indicate a fluctuating skyrmion liquid in this magnetically ordered state, stabilized by pressure, disorder or reduced dimensionality. Moreover, the magnetic transition is strongly first order in the MnSi MBE thin film sample, different from the Brazovskii-type weakly first order transition in bulk samples, suggesting the importance of reduced dimensionality in modifying the nature of magnetic phase transitions in B20 systems.

Depuis plusieurs années les physiciens de la matière condensée s'intéressent aux propriétés optiques et magnétiques des nanoparticules métalliques. Deux propriétés restent largement étudiées : les résonances plasmon localisées et l'anisotropie magnétique à l'échelle nanométrique. Ces deux effets résultant de propriétés électroniques bien différentes sont habituellement rencontrés dans des nanosystèmes distincts. Depuis les années 2000 des études ont montré qu'il était possible de bénéficier de ces deux caractéristiques dans un seul et même système nanométrique. Dans cette thèse, nous nous intéresserons à la combinaison des propriétés magnétiques et plasmoniques dans des systèmes de taille inférieure à la dizaine de nanomètres: les agrégats bimétalliques de CoAg et de CoAu synthétisés par voie physique sous ultravide encapsulés en matrice (alumine et carbone). Nous nous intéresserons à la structure de ces agrégats bimétalliques de différentes stœchiométries et à l'effet de leur environnement à travers l'étude de leurs propriétés optiques, magnétiques et électroniques (par spectroscopie électronique par perte d'énergie (EELS) sur des particules individuelles). Nous montrerons l'effet de la matrice, carbone ou alumine, sur la structure des agrégats ainsi que sur leurs propriétés magnétiques (moment par agrégat, anisotropie). En optique nous verrons également l'importance de la stœchiométrie entre métal noble et cobalt sur les phénomènes d'amortissement et de décalage de résonance plasmon. Enfin nous montrerons la répartition spatiale des plasmons de surface sur des particules unique par des mesures de STEM-EELS / For several years condensed matter physicists have been interested in the optical and magnetic properties of metallic nanoparticles. Two properties remain largely studied: localized plasmon resonances and magnetic anisotropy at the nanoscale. These two effects resulting from very different electronic properties which are usually encountered in separate nanosystems. Since the 2000's, studies have shown that it is possible to benefit from these two characteristics in a single nanometric system. In this thesis, we will focus on the combination of magnetic and plasmonic properties in systems of size less than ten nanometers: bimetallic clusters of CoAg and CoAu synthesized physically under ultrahigh vacuum and embedded in a matrix (alumina and carbon). We will study the structure of these bimetallic clusters of different stoichiometries and the effect of their environment through the investigation of their optical, magnetic and electronic properties (by electron energy loss spectroscopy (EELS) on individual particles ). We will show the effect of the matrix, carbon or alumina, on the structure of the clusters as well as on their magnetic properties (moment by cluster, anisotropy). In optics we will also see the importance of stoichiometry between noble metal and cobalt on the phenomena of the damping and shifting of the plasmon resonance. Finally we will show the spatial distribution of surface plasmons on single particles by STEM-EELS measurements

Graphene, an atomically thin material consisting of a hexagonal, highly packed carbon lattice, is of great interests in its magnetic properties. These interests can be categorized in several fields: graphene-based magnetic materials and their applications, large diamagnetism of graphene, and the heterostructures of graphene and other two dimensional materials. In the first aspect, magnetic moments can be in theory introduced to graphene by minimizing its size or introducing structural defects, leading to a very light magnetic material. Furthermore, weak spin-orbital interaction, and long spin relaxation length make graphene promising for spintronics. The first part of this thesis addressed our experimental investigation in defect-induced magnetism of graphene. Non-interacted spins of graphene have been observed by intentionally introducing vacancies and adatoms through ion-irradiation and fluorination, respectively. The defect concentration or the magnetic moments introduced in this thesis cannot provide enough interaction for magnetic coupling. Furthermore, the spins induced by vacancies and adatoms can be controlled through shifting the Fermi energy of graphene using molecular doping, where the adatoms were alternatively introduced by annealing in the inert environment. The paramagnetic responses in graphene induced by vacancy-type defects can only be diverted to half of its maximum, while those induced by sp3 defects can be almost completely suppressed. This difference is supposed that vacancy-type defects induced two localized states (pie and sigma). Only the latter states, which is also the only states induced by sp3 defects, involves in the suppression of magnetic moments at the maximum doping achieved in this thesis. The observation through high resolution transmission electron microscope (HR-TEM) provides more information to the hypothesis of the previous magnetic findings. Reconstructed single vacancy is the majority of defects discovered in proton-irradiated graphene. This result verifies the defect-induced magnetic findings in our results, as well as the electronic properties of defected graphene in the literatures. On the other hand, the diamagnetic susceptibility of neutral graphene is suggested to be larger than that of graphite, and vanish rapidly as a delta-like function when graphene is doped. In our result, surprisingly, the diamagnetic susceptibility varies little when the Fermi level is less than 0.3 eV, in contrast with the theory. When the Fermi energy is higher than 0.3 eV, susceptibility then reduces significantly as the trend of graphite. The little variation in susceptibility near the Dirac point is probably attributed to the spatial confinement of graphene nanoflakes, which are the composition of graphene laminates. In the end of this thesis, we discuss the magnetic properties in one of the other two dimensional materials, molybdenum disulfide (MoS2). It is a potential material for graphene-based heterostructure applications. The magnetic moments in MoS2 are shown to be induced by either edges or vacancies, which are introduced by sonication or proton-irradiation, respectively, similar to the suggestions by theories. However, no significant ferromagnetic finding has been found in all of our cases.

This thesis explores the synthesis and characterisation of a range of molecular wheels containing unpaired electron spins. These molecular spin systems are of considerable interest, both for the insight they provide into the physics of such systems and for their potential as quantum bits ("qubits") in a quantum information processing device. In particular, this thesis explores using these wheels to meet criteria 1 and 5 of the DiVincenzo criteria. The synthesis of a novel homometallic and nonametallic ring of CrIII ions is introduced, along with extensive physical characterisation. Inelastic Neutron Scattering measurements suggest that the molecule has an almost degenerate S = 1/2 ground state with only 0.1 meV separation, making this ring a near perfect example of a Type I frustrated spin system. Chemical modification of the heterometallic {Cr7M} family of wheels with both hard and soft Lewis base functionality is also explored. Using a triphenylphosphine derivative, the coordination chemistry of a highly sterically hindered mono-substituted triphenylphosphine derivative with gold is explored, yielding new arrangements of the wheels. Changes in the electronic and steric properties of the system are studied by a combination of 31P NMR spectroscopy and DFT modelling, revealing dramatic changes in the phosphorus donor properties. The effect of this ligand substitution on the anisotropy tensor of CoII contained in a heterometallic {Cr7Co} ring is explored using variable temperature 1H NMR spectroscopy. Using a combination of the experimentally observed 1H NMR dipolar shifts and computational modelling, a significant change in the anisotropy tensor of the cobalt is found. Finally, as part of a g-engineering approach to qubit design the chemistry of the octametallic {Cr7Ni} ring functionalised with triphenylphosphine oxide is introduced. Initial efforts towards developing a hybrid {Cr7Ni}2Ln (Ln = Gd, Eu) qubit system, along with characterisation by EPR and luminescence spectroscopy, suggest that this may be a route to developing a qubit with the capacity for optical control of the communication.

This thesis presents simulation and experimental work on feedback control of the \emph{phase} of non-rotating magnetic islands (locked modes) in the DIII-D tokamak, as well as its application to synchronized modulated current drive, for stability studies and control of the locked mode \emph{amplitude}. A numerical model has been developed to predict mode dynamics under the effect of various electromagnetic torques, due to the interaction with induced currents in the wall, error fields, and applied resonant magnetic perturbations (RMPs). This model was adapted to predict entrainment capabilities on ITER, suggesting that small (5~cm) islands can be entrained in the sub-10~Hz frequency range. Simulations and subsequent experiments on DIII-D demonstrated a novel technique to prevent locked modes. Preemptive entrainment applies a rotating RMP before a neoclassical tearing mode fully decelerates such that it will be entrained by the RMP and mode rotation can be sustained. A feedback control algorithm was designed and implemented on DIII-D to offer the ability to prescribe any toroidal phase to the mode and to allow for smoother entrainment. Experimental results confirmed simulation predictions of successful entrainment, and demonstrated one possible application to electron cyclotron current drive (ECCD). Feedback-controlled mode rotation and pre-programmed ECCD modulation were synchronized at DIII-D. This allowed a fine control of the ECCD deposition relative to the island O-point. Experiments exhibited a modulation of the saturated island width, in agreement with time-dependent modeling of the modified Rutherford equation. This work contributes to control and suppression of locked modes in future devices, including ITER.

The general aim of the work detailed in this thesis is to improve the quality of electroencepholography (EEG) recordings acquired simultaneously with functional magnetic resonance imaging (fMRI) data. Simultaneous EEG-fMRI recordings offer significant advantages over the isolated use of each modality for measuring brain function. The high temporal resolution associated with EEG complements the high spatial resolution provided by fMRI. However, combining the two modalities can have significant effects on the overall data quality. The gradient artefact (GA), which is induced on the EEG cables by the time varying magnetic fields associated with fMRI sequences, can be particularly problematic to correct for in experiments containing any subject movement. In this thesis, two novel, movement-invariant methods are introduced for correcting the GA. The first method is named the gradient model fit (GMF) and relies upon the assumption that the GA can be modelled as a linear combination of basis components, where the relative weighting of each component varies dependent upon subject position. By modelling these underlying components, it is possible to characterise and remove the GA, which is particularly beneficial in the presence of subject movement. The second method named the difference model subtraction (DMS) relies on the assumption that the GA varies linearly for small changes in subject position. By modelling the change in GA for a basis set of likely head movements, it was shown to be possible to combine DMS with standard GA correction methods to improve the attenuation of the GA for data acquired during subject movement. Both methods showed a significant improvement over the existing GA correction techniques, particularly for experiments containing subject movement. These methods are therefore relevant to any experimenter interested in working with subject groups such as children or patients where movement is likely to occur.

Investigamos as propriedades magnéticas e as interações hiperfinas das ligas \'ZR\'\'(\'FE IND. x\'\'CR IND. 1-x\') IND. 2\' (0.3 < x < 0.7) e seus hidretos, através de medidas de magnetização em campos até 9 T, suscetibilidade AC e espectroscopia Mossbauer. A difração de raios X confirmou a estrutura de fase de Laves hexagonal Cl4 para todas as amostras. Os espectros Mossbauer das ligas à temperatura ambiente apresentaram dois dubletos quadrupolares, que podiam ser atribuídos de duas maneiras aos sítios cristalográficos 2a e 6h do Fe. Esta ambigüidade foi resolvida por um cálculo de cargas pontuais, com as posições atômicas determinadas por refinamento Rietveld. Medidas magnéticas e de Mössbauer a baixas temperaturas indicaram que todas as ligas da faixa de concentrações estudada apresentaram comportamento de vidros de spin, sem ordem magnética de longo alcance. As temperaturas de congelamento de spins observadas puderam ser descritas em seu conjunto pela equação de Vogel-Fulcher. A concentração crítica de Fe para a formação de momentos magnéticos foi determinada. O efeito da absorção de hidrogênio foi investigado: a) com o teor máximo de H em todas as ligas, b) com teores intermediários na liga \'ZR\'\'(\'FE IND. 0.5\'\'CR IND. 0.5\') IND. 2\'. Nesta última, os espectros Mössbauer mostraram a coexistência de uma fase pobre e uma fase rica em H. O desdobramento quadrupolar do Fe no sítio 6h apresentou variação com a temperatura, atribuída a efeitos de difusão de hidrogênio. O principal efeito do hidrogênio sobre o magnetismo das ligas foi um incremento notável dos momentos magnéticos, porém sem aparecimento de ordem ferromagnética; ao contrário, observou-se um pequeno abaixamento das temperaturas de congelamento de spins. / We have investigated the magnetic properties and hyperfine interactions in the \'ZR\'\'(\'FE IND. x\'\'CR IND. 1-x\') IND. 2\' (0.3 < x < 0.7) alloys and their hydrides, by means of magnetization measurements in fields up to 9 T, AC susceptibility and Mõssbauer spectroscopy. X-ray diffraction confirmed the Laves phase Cl4 hexagonal structure for all samples. The room temperature Mõssbauer spectra of all samples showed two quadrupolar doublets, which could be assigned in two ways to the Fe crystallographic sites 2a and 6h. This ambiguity was solved by a point-charge calculation using atomic positions determined by Rietveld refinement. Magnetic and Mõssbauer measurements at low temperatures indicated that all investigated samples showed spin-glass behavior, without long-range magnetic order. The set of observed spin freezing temperatures could be described with the Vogel-Fulcher equation. The critical Fe concentration for magnetic moment formation has been determined. The effect of hydrogen absorption was investigated: a) with the maximum H content for all alloys, b) with intermediate H contents for the \'ZR\'\'(\'FE IND. 0.5\'\'CR IND. 0.5\') IND. 2\' alloy. Mõssbauer spectra for the latter samples showed the coexistence of a H-poor and a H-rich phase. The Fe quadrupole splitting at the 6h site exhibited a temperature dependence which was attributed to hydrogen diffusion effects. The main effect of hydrogen on the magnetism was a significant increase in magnetic moments, yet without ferromagnetic ordering; a slight lowering of spin freezing temperatures was actually observed.

Since different parts of the Moon display varying magnetic field strengths, our goal was to determine whether these differences are due to specific geological characteristics. We found that older materials tend to be more magnetic than younger materials. Additional statistical studies found that the mare regions of the Moon are less magnetic than the plains and terra regions. We did not find a simple relationship between lunar magnetism and crustal thickness, and this is inconsistent with the hypothesis that thicker crust is more magnetic since there is additional material. Thus, it is not just a matter of the amount of magnetic material that determines the magnetic field strength of the lunar crust. Our results demonstrate that magnetism and crustal thickness have a complex relationship, with multiple distinct groups corresponding to various regions of the Moon. The lunar maria formed a particularly distinct group, consisting of low magnetism and thin crust, while the lunar highlands consist of thick crust but typical magnetic field values. The ejecta thickness and magnetic field distributions for specific craters showed that larger impact basins have a thicker and more widespread ejecta blanket than smaller craters. We did not find a consistent pattern of magnetic field enhancements near specific craters, but evidence for these strong magnetic signatures was present for Mare Crisium and Mare Nectaris. These results may support the hypothesis that ejecta materials are carriers of magnetism, and this may be the reason for their tendency to have higher magnetic field strengths.

Ionospheric disturbance electric fields of magnetospheric origin play an important role in determining the global morphology and dynamics of the ionosphere of the Earth. In this work, we present a number of numerical simulations of the transient electric fields in the middle and inner magnetosphere and the ionosphere equatorward of the auroral zone caused by idealized changes in the magnetospheric driving parameters. For these studies, we u se the Rice Convection Model (RCM), a large computer code of the magnetosphere-ionosphere coupling which consistently computes the electric fields, currents, and plasma densities in the magnetosphere and the electric field and currents in the ionosphere in the quasi-static slow-flow approximation. We made substantial upgrades to the code, which include a module computing realistic solar EDY-produced ionospheric conductances and a new potential solver. Our upgraded version of the RCM also includes a time - varying magnetospheric magnetic field and a self-consistently estimated auroral zone. We first discuss numerical problems encountered in modeling electrodynamics of convection with a time-varying magnetic field, realistic ionospheric conductances, and a self-consistent auroral zone, and our solutions to those difficulties. We then present a number of "computer experiments" with the new version of the RCM with idealized changes in the magnetospheric parameters such as sudden changes in the cross polar cap potential drop, magnetic field reconfiguration corresponding to the overall changes in the high-latitude convection, as well as rotations of the electric field on the polar cap boundary. Prompt penetration ionospheric electric fields simulated with the upgraded RCM are shown to be consistent with the previous simulations. The new simulations and their results are discussed in the context of (1) possible contribution to the variability of the ionospheric electric fields, and (2) role of time-varying magnetic field on the characteristic lifetimes of prompt penetration electric fields at subauroral, middle, and low latitudes.

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