This thesis outlines the development of two new techniques that exploit very small structures, on the micro- and nano-scale, to enable innovative electrical transport measurements on a variety of materials of current interest in condensed matter physics. The first technique aims to apply the versatility of electron-beam lithography for micro-fabrication of patterned electronic circuitry to the problem of performing transport experiments on individual crystallites taken from a typical powder sample. We show that these small samples, tens of microns in size, are actually often very high quality single crystals and can be exploited for measurements of electrical transport in materials of which no larger crystals are available. By way of demonstration, we present the results of preliminary transport measurements on a crystallite of the layered oxide chalcogenide Sr₂MnO₂Cu_1.5Se₂. We report a phase transition in the resistivity at 213K which may correspond to the onset of previously reported short-range order in copper and vacancy sites in the Cu_1.5Se₂ planes. The second technique is designed to investigate the topological protection of surface transport in 3-D topological insulators. We decorate the surfaces of single-crystal samples with two different species from a well-characterised family of single-molecule magnets. The two coatings have an electrostatically identical influence on the sample surface, but differ in that one species carries a spin and the other is spinless. The spinless molecule acts as a control, to allow us to cleanly determine the influence of the magnetic component of a scattering potential on transport in the surface. With this technique we investigate proposed topological Kondo insulator SmB₆. We find that the surface state dominates low-temperature transport and demonstrate that the momentum relaxation is very sensitive to a spin degree of freedom in the scatterer, in keeping with expectations of a topological insulator.

This thesis investigates a number of transition-metal coordination polymers and iron-pnictide superconductors through the use of high magnetic fields, low temperatures, and on occasion, high pressures. The thesis will begin by describing my development of the proximity detector dynamic susceptometer, a novel technique that can be used for magnetometery and transport measurements in high magnetic fields. This technique is highly compact and has no moving parts, making it suitable for use in pressure cells, hence opening the way for a variety of new experiments. Through high-field magnetometery and other measurements, I will demonstrate that the pressure can be used to directly control the magnetic properties of the polymeric magnet CuF₂(H₂O)₂(pyrazine). In particular, I observe a transition from quasi-two-dimensional to quasi-one-dimensional antiferromagnetism at 9~kbar, driven by the rotation of the Jahn-Teller axis. I will then present a series of measurements on two coordination polymers, showing how a small chemical difference can lead to drastically different magnetic properties. I show that [Cu(pyrazine)H₂O(glycine)₂]ClO₄ is an excellent spin-chain, while the sister compound [Cu(pyrazine)(glycine)]ClO₄ is a dimerised material that shows a spin-gap and is disordered down to very low temperatures, but then undergoes a field-induced phase transition to an ordered phase. I will also describe a series of pulsed-field measurements of the upper critical field of the iron-based superconductors NaFe_1-xCo_xAs across the whole of the doping phase diagram. It is shown that paramagnetic pair-breaking effects dominate the critical field when the field is parallel to the crystal planes. In the parent compound the paramagnetic limit is equal to that expected from BCS theory, but becomes significantly enhanced above the BCS limit upon doping. It is shown that the multi-band nature of the superconductivity leads to a convex curvature in the evolution of the critical field as the temperature is reduced.

Fermi surface, the locus in momentum space of gapless low-energy excitations, is a concept of fundamental importance in solid state physics. Electronic properties of a material are determined by the long-lived low-energy excitations near the Fermi surface. Conventionally, Fermi surface is understood as a property exclusive to a metallic state, contoured by electronic bands crossed by the Fermi level, although there has been a continuing effort in searching for Fermi surface outside the conventional description. In this thesis, techniques developed to prepare high-quality single crystals of SmB$_6$ and YBa$_2$Cu$_3$O$_{6+x}$ (abbreviated as YBCO$_{6+x}$ hereinafter) are described. By utilising measurement techniques of exceptional sensitivity and exploring a wide range of temperatures, magnetic fields, and electrical currents, we found signatures of unconventional Fermi surfaces beyond the traditional description in these strongly correlated electronic systems. SmB$_6$ is a classic example of Kondo insulators whose insulating behaviour arises due to strong correlation between the itinerant $d$-electrons and localised $f$-electrons. The peculiar resistivity plateau onsets below 4 K has been a decades-long puzzle whose origin has been recently proposed as the manifestation of topological conducting surface states. We found that the insulating behaviour in electrical transport is robust against magnetic fields up to 45 T, while prominent quantum oscillations in magnetisation are observed above 10 T. Angular dependence of the quantum oscillations revealed a three-dimensional characteristics with an absolute amplitude consistent with a bulk origin, and temperature dependence showed a surprising departure from the conventional Lifshitz-Kosevich formalism. Complementary thermodynamic measurements showed results consistent with a Fermi surface originating from neutral itinerant low-energy excitations at low temperatures. Theoretical proposals of the unconventional ground state uncovered by our measurements in SmB$_6$ are discussed. YBCO$_{6+x}$ is a high-temperature superconductor with a maximum $T_{\rm c}$ of 93.5 K and the cleanest member in the family of copper-oxide, or {\it cuprate}, superconductors. The correct description of electronic ground state in the enigmatic pseudogap regime, where the antinodal density of states are suppressed below a characteristic temperature $T^*$ above $T_{\rm c}$, has been a subject of active debates. While the quantum oscillations observed in underdoped YBCO$_{6+x}$ have been predominately interpreted as a property of the normal state where the superconducting parameter is completely suppressed at $\approx$ 23 T, we made the discovery that YBCO$_{6.55}$ exhibits zero resistivity up to 45 T when a low electrical current is used, consistent with the observation of a hysteresis loop in magnetisation. Quantum oscillations in the underdoped YBCO$_{6+x}$ are thus seen to coexist with $d$-wave superconductivity. Characteristics of the quantum oscillations are consistent with an isolated Fermi pocket reconstructed by a charge density wave order parameter and unaccompanied by significant background density of states, suggesting the antinodal density of states is completely gapped out by a strong order parameter involving pairing correlations, potentially in addition to the other order parameters. Transport measurements performed over a wide doping range show signatures consistent with pairing correlations that persist up to the pseudogap temperature $T^*$. The surprising observation of quantum oscillations in insulating SmB$_6$ and superconducting YBCO$_{6+x}$ demonstrates a possible new paradigm of a Fermi surface without a conventional Fermi liquid. A new theoretical framework outside the realm of Fermi liquid theory may be needed to discuss the physics in these strongly correlated materials with enticing electronic properties.

This thesis investigates a number of novel magnetic materials and high temperature superconductors using high-field magnetometry and muon spin spectroscopy (μSR). The main measurement techniques are briefly described and a study of the dimer material [Cu(pyrazine)(glycine)]ClO₄ is presented to demonstrate the use of the proximity detector oscillator as a susceptometer in high magnetic fields. μSR is a highly effective tool for probing magnetic order, spin freezing and spin dynamics. However, in some circumstances its performance may be impaired by the extent to which it perturbs the material under study. Using μSR, density functional theory and crystal field calculations, I identify an experimental situation in the family of candidate quantum spin ices Pr₂B₂O₇ (B = Sn, Zr, and Hf), in which the measured response is dominated by a muon-induced distortion of the local structure. This issue is also addressed in a study of the spin dynamics in the canonical spin ice Ho2Ti2O7. Although computational work indicates a similar muon-induced effect in both Ho₂Ti₂O₇ and Dy₂Ti₂O₇, the μSR data is not dominated by this perturbation. The remainder of this thesis is concerned with studying the superconducting properties of a number of Fe-based materials, including LiFeP which is found to have an enhanced superfluid stiffness in relation to its transition temperature. Also reported is the effect of structural disorder on the superconducting state in recently discovered Sr_0.3(NH₂)_y(NH₃)_1-yFe₂Se₂. Pulsed magnetic field measurements are used to probe the temperature dependence of the upper critical field, giving a maximum value of μ₀H_c2(0)≈33(2) T. I also investigate the effect of intercalating additional ammonia, via reversible adsorption and desorption in the related superconductor Li_x[(NH₂)_y(NH₃)_1-y]_zFe₂Se₂ (z = 1, 2). These reactions were carried out in situ on the muon beamline so that the superfluid stiffness could be measured using transverse-field μSR on a single sample.

Electron spin resonance (ESR) experiments are used to assess the possibilities for processing quantum information in the electronic and nuclear spins of endohedral fullerenes. It is shown that ¹⁵N@C₆₀ can be used for universal two-qubit quantum computing. The first step in this scheme is to initialize the nuclear and electron spins that each store one qubit. This was achieved with a magnetic field of 8.6 T at 3 K, by applying resonant RF and microwave radiation. This dynamic nuclear polarization technique made it possible to show that the nuclear T₁ time of ¹⁵N@C₆₀ is on the order of twelve hours at 4.2 K. The electronic T₂ is the limiting decoherence time for the system. At 3.7 K, this can be extended to 215 μs by using amorphous sulphur as the solvent. Pulse sequences are described that could perform all single-qubit gates to the two qubits independently, as well as CNOT gates. After these manipulations, the value of the qubits should be measured. Two techniques are demonstrated for this, by measuring the nuclear spin. Sc@C₈₂ could also be useful for quantum computation. By comparing ESR measurements with density functional theory calculations, it is shown how the orientation of a Sc@C₈₂ molecule in an applied magnetic field affects the molecule's Zeeman and hyperfine coupling. Hence the g- and A-tensors are written in the coordinate frame of the molecule. Pulsed ESR measurements show that the decoherence time at 20 K is 13 μs, which is 20 times longer than had been previously reported. Carbon nanotubes have been filled with endohedral fullerenes, forming 1D arrays that could lead to a scalable quantum computer. N@C₀₆ and Sc@C₈₂ have been used for this filling in various concentrations. ESR measurements of these samples are consistent with simulations of the dipolar coupling.

Ce travail est dédié à l'étude de l'effet des champs magnétiques sur les propriétés magnétiques et le comportement à la solidification d'alliages à base de Cobalt en surfusion sous champ magnétique intense. Les alliages à base Co sont d'excellents candidats pour obtenir une surfusion en dessous ou proche du point de Curie sous champ intense en raison du faible écart entre ce point de Curie et la température du liquidus. Dans cette étude, un dispositif haute température de surfusion intégrant une mesure magnétique a été construit dans un aimant supraconducteur, et est utilisé pour la mesure in situ de l'aimantation de liquides surfondus et pour l'étude du sur-refroidissement et de l'évolution de la microstructure de solidification en champ intense. Le cobalt liquide en surfusion est fortement magnétique sous champ, et son aimantation est même supérieure à celle du solide au chauffage à la même température. L'aimantation de l'alliage proche eutectique Co-B en surfusion dépend de la température de surchauffe, tandis que le Co-Sn en surfusion est toujours paramagnétique. La surfusion moyenne et l'étendue de la recalescence de différents métaux et alliages est affectée par un champ externe. En champ magnétique uniforme, la surfusion du Cuivre est amplifiée, tandis que la surfusion du Cobalt et de Co-Sn reste identique. Cependant, l'étendue de la recalescence du Cobalt et de Co-Sn est réduite, et l'effet est d'autant plus important pour des teneurs supérieures en Cobalt. Le champ magnétique promeut la précipitation de la phase dendritique a-Co et la formation d'eutectique anormal dans la microstructure des alliages Co-Sn surfondus. Les processus d'évolution de la microstructure sont affectés par le champ magnétique, et dépendent de l'intensité du champ et de la surfusion. Ce travail offre de nouveaux horizons dans l'étude des propriétés magnétiques d'alliages métalliques en forte surfusion et dans l'étude de la solidification hors équilibre sous champ magnétique intense. / This work is devoted to the investigation of the magnetic field effect on the magnetic properties and solidification behavior of undercooled Co based alloys in high magnetic field. Co based alloys are promising candidates to be undercooled below or approaching their Curie point in strong magnetic field due to their small temperature difference between liquid line and Curie point. In this dissertation, a high temperature undercooling facility with magnetization measurement system is built in a superconducting magnet, and is used for in situ measurement of the magnetization of the undercooled melts and study the undercoolability and solidification microstructure evolution in magnetic field. The deep undercooled Co melt is strongly magnetized in magnetic fields, and its magnetization is even larger than the magnetization of heated solid at the same temperature. The magnetization of undercooled Co-B near eutectic alloy is related with overheating temperature while the undercooled Co-Sn melt is always in paramagnetic state. Mean undercooling and recalescence extent of different metals and alloys are affected by external field. In uniform magnetic field, the undercooling of Cu is enhanced while the undercoolings of Co and Co-Sn keep constant. However, the recalescence extents of Co and Co-Sn alloys are reduced, and with the increasing Co content, the effect becomes larger. Magnetic field promotes the precipitation of αCo dendrite phase and the formation of anomalous eutectics in solidified microstructure of undercooled Co-Sn alloys. The microstructure evolution processes are affected by magnetic field depending on the field intensity and undercooling. This work opens a new way to investigate the magnetic properties of deeply undercooled metallic melts and non-equilibrium solidification in strong magnetic fields.

This thesis presents the investigation of magnetism in a selection of low-dimensional systems and its relation to other physical properties, such as superconductivity. The techniques employed are muon spin rotation and pulsed magnetic field magnetisation. The ability of muons to directly probe the local field is used to study SrFeAsF, which is a parent compound of the high-temperature superconducting pnictides. This revealed that the magnetic and structural transitions are separated in this system. I then demon- strate the coexistence of magnetism and superconductivity in NaFeAs for the first time. This discovery is of great interest since the interplay between magnetism and supercon- ductivity is thought to play an important role for high-temperature superconductivity. I further investigate the effect of partially replacing Fe with Co in NaFeAs. I study the ordering and spin reorientation in the Mott insulator Sr₂IrO₄, which has been suggested as a possible high-temperature superconductor. The complex magnetism observed in this system is contrasted to that in related iridates Ca₄IrO₆, Ca₅Ir₃O₁₂ and Sr₃Ir₂O₇. By combining pulsed-field magnetization and low magnetic field experiments with μSR on a series of coordination polymers. I am able to determine the size and direction of the magnetic exchange interaction. I demonstrate how it is possible to adjust the in- teractions by altering the molecular architecture of these Cu-based spin- 1 2 compounds. This is a significant contribution since it will lead to the targeted design of magnetic systems that can be utilized to experimentally test fundamental theories of magnetism.

Les cuprates sont des matériaux que l’on peut faire passer d'un isolant antiferromagnétique à un métal normal en augmentant leur densité de porteurs par dopage chimique. Aux dopages intermédiaires, une riche variété de phases électroniques apparaît aux côtés de la phase supraconductrice, ou même entrelacée avec elle. Le but de cette thèse était de caractériser divers aspects de la compétition entre la supraconductivité et les ordres de charge ou de spin, en utilisant la résonance magnétique nucléaire (RMN). Une première partie du travail a consisté à améliorer la modélisation des spectres RMN des noyaux de 17O dans les deux phases onde de densité de charge (ODC) présentes dans YBa2Cu3Oy: l’ordre à courte portée et l’ordre à longue portée (induit par le champ magnétique). En plus de fournir un cadre d'analyse beaucoup plus précis pour les données de RMN en fonction du champ, du dopage et de la pression (voir ci-après), les résultats indiquent que l’ODC à haut champ est uni-axiale (c'est-à-dire avec un vecteur d'onde unique q), avec une période commensurable avec le réseau de trois cellules unitaires (q = 1/3). Le deuxième aspect de la compétition de phases abordé dans ce travail est l'effet (controversé) de la pression hydrostatique. Nos mesures montrent qu'une pression de 1,9 GPa n'affaiblit que très légèrement l’ODC à courte portée dans l'état normal ainsi que l’ODC à longue portée observée à haut champ. Les résultats soutiennent l’hypothèse selon laquelle l'augmentation continue de Tc lorsque la pression augmente jusqu'à 15 GPa est presque entièrement due à une diminution progressive de la force de l’ODC. Ceci montre que la pression hydrostatique est un paramètre permettant de contrôler la compétition entre l’ODC et la supraconductivité dans les cuprates.Dans la troisième partie de la thèse, des mesures du taux de relaxation spin-réseau (1/T1) des noyaux de 139La ont permis d’étudier l'effet d'un champ magnétique sur la mise en ordre vitreuses des spins dans La2-xSrxCuO4. En utilisant des champs élevés jusqu’à 45 T, nous montrons que le champ est capable d’induire une phase gelée, ou presque gelée, à des niveaux de dopage bien supérieurs à ceux supposés précédemment, à savoir jusqu’au dopage critique attribué à l’extrémité de la phase pseudogap mais pas au-delà de ce point, ou pas loin au-delà. Ce résultat a des implications importantes pour l'interprétation de la phase de pseudogap et de la criticité quantique qui lui est associée. / Cuprates are materials that can be tuned from an antiferromagnetic insulator to a normal metal by increasing the carrier density through chemical doping. At intermediate doping, a rich variety of electronic phases emerges alongside, or intertwined, with the superconducting phase. The aim of this thesis was to characterise various aspects of the competition between superconductivity and charge or spin order, using nuclear magnetic resonance (NMR).A first part of the work consisted in improving the modelling of 17O NMR spectra in the two charge-density wave (CDW) phases present in YBa2Cu3Oy: the short-range order and the (magnetic-field induced) long-range order. Besides providing a much more accurate analysis framework for NMR data as a function of field, doping and pressure (see hereafter), the results indicate that the CDW in high-fields is uniaxial (i.e. single wave vector q) and commensurate with the lattice, with a period of three unit cells (q=1/3).The second aspect of phase competition addressed in this work is the (controversial) effect of hydrostatic pressure. Our measurements show that a pressure of 1.9 GPa weakens the short-range CDW in the normal state and the long-range CDW observed in high fields only slightly. The results support the proposal that the continuous rise in Tc upon increasing pressure up to 15 GPa arises almost entirely from a gradual decrease of the CDW strength. This establishes hydrostatic pressure as a tuning parameter of the competition between CDW order and superconductivity in the cuprates.In the third part of the thesis, 139La spin-lattice relaxation rate (1/T1) measurements were used to study the effect of a magnetic field on glassy spin ordering in La2-xSrxCuO4. Using high fields up to 45 T, we reveal that the field is able to induce a frozen, or nearly so, phase at doping levels much higher than previously assumed, namely up to the putative endpoint of the pseudogap boundary, but not, or not far, beyond that point. This result has important implications for interpreting the pseudogap phase and its associated quantum criticality.

Le présent travail se concentre sur l'étude des niveaux d'énergie et des processus de recombinaison de complexes excitoniques larges - jusqu'à quatre paires électron-trou - considérés au niveau d'une boîte quantique unique remplie optiquement. Les boîtes étudiées dans ces expériences, formées à partir d'une matrice de Ga(Al)As, représentent un système à zéro dimension avec un confinement relativement fort et peuvent en effet avoir plusieurs couches électroniques s, p, comme dans le cas d'atomes. Les boîtes peuvent être facilement sélectionnées à l'état individuel du fait de la très faible densité de surface des structures considérées. Les techniques expérimentales utilisée dans ce travail comprennent : les méthodes de spectroscopie sur boîtes uniques, la détection optique résolue en polarisation, l'utilisation de champs magnétiques intenses et des mesures de corrélation de photons. En ce qui concerne les expériences de photoluminescence, nous avons distingué les excitations en dessous de la barrière de celles se produisant en dessus. Finalement, des expériences de spectroscopie d'excitation de la photoluminescence ont aussi été réalisées en champ magnétique.En fonction des conditions d'excitation, les boîtes étudiées présentent une multitude de raies relativement étroites, chaque boîte révélant un schéma caractéristique de raies groupées en amas distincts, similaires à une série de couches électroniques pour un atome. La présente étude s'est concentrée sur l'intervalle spectral correspondant aux couches électroniques s et p. L'identification des raies spectrales s'est principalement basé sur les résultats obtenus lors d'observations résolues en polarisation ou bien lors de mesures de corrélation de photons. Ces expériences révèlent trois familles distinctes de raies d'émission, chacune étant respectivement reliée à un complexe électron-trou (excitonique) neutre, chargé positivement, ou bien négativement. Une attention particulière a été portée aux raies d'émission observées dans une cascade en quatre étapes partant d'un complexe à quatre excitons, jusqu'au niveau de la recombinaison d'un exciton neutre, ainsi que celles observées dans une cascade en deux étapes partant d'un bi-exciton chargé positivement, jusqu'à la recombinaison d'un état singulet ou triplet d'un exciton chargé positivement. La structure fine induite par les interactions d'échange - et préalablement observée lors des mesures résolues en polarisation à champ magnétique nul - a été étudiée pour différentes raies d'émission. L'évolution de ce dédoublement de raies a été examiné en fonction du champ magnétique. Les résultats sont interprétés en terme d'anisotropie de forme des boîtes et d'une interaction avec les effets spin-orbite, caractéristiques des différents processus de recombinaison. Une partie importante de ce travail a été dévolue à la comparaison entre le spectre d'émission mesuré pour des puissances d'excitations relativement importantes avec les spectres d'excitation de la photoluminescence. De telles expériences ont aussi été conduites sous champ magnétique. Comme attendu, les spectres d'émission des complexes excitoniques d'ordres élevés sont particulièrement affectés par les interactions coulombiennes entre porteurs, et sont par conséquent très différents des spectres d'excitation de la photoluminescence (quasi-absorption) des excitons neutre et chargés. Deux types d'évolution en champ magnétique de raies d'absorption observées (résonance) - reliées aux couches s et p - ont été mesurés. Les résonances de type s sont attribuées à la transition entre un niveau excité de trou de la bande de valence et l'état fondamental de la couche s dans la bande de conduction. Une raie d'émission, observée dans le groupement de la couche p, coïncide cependant avec la raie d'absorption. Nous concluons que cette résonance vient d'un état excitonique excité qui se recombine de manière radiative dû à un blocage efficace de sa relaxation vers l'état fondamental. / The studies of energy levels and of recombination processes of single quantum dots, optically filled with up to four electron-hole pairs are the subject of this work. The dots used in the present experiments, formed out of the Ga(Al)As matrix, represent relatively strongly confined zero-dimensional systems, and display several, atomic-like s-, p-,. . . shells. Single dots can be easily selected in our structures as they exhibit an extremely low surface density. Experimental techniques applied in this work include the methods of single dot spectroscopy, polarization resolved techniques, application of magnetic fields and photon correlation measurements. Distinct, below- and above-dot-barrier laser excitation has been used for photoluminescence experiments. Importantly, the photoluminescence excitations experiments (in magnetic fields) have been carried out, as well.Depending on excitation conditions (power and wavelength of laser), the investigated dots show a multitude of relatively sharp lines, each dot displaying the same, characteristic pattern of lines, grouped into distinct clusters corresponding to subsequent atomic-like shells. Spectral range covering the s- and p-shells region has been explored in the present studies. The assignment of spectral lines has been at large provided by the results of polarization resolved micro-photoluminescence and photon correlation experiments. Those experiments depict three distinct families of emission lines, each related to recombination of, correspondingly, neutral, positively charged and negatively charged electron-hole (excitonic) complexes. The emission lines observed within a four step cascade of a neutral quadexciton down to the recombination of a neutral exciton and two step cascades of positively charged biexcitons down to the recombination of a singlet and triplet state of positively charged excitons have been studied in details. The fine structure, induced by exchange interactions and preliminarily seen in (linear) polarization resolved emission experiment at zero magnetic field, has been studied for various emission lines (related to s- and p- shells). The evolution of this splitting has been then investigated as a function of the magnetic field. The results are interpreted in terms of the shape anisotropy of dots and an interplay between spin- and orbital-mediated effects, characteristic of different recombination processes. A significant portion of this work has aimed to compare the emission spectra measured at a relatively high excitation power (which include the recombination processes of up to quadexciton complexes) with photoluminescence excitation spectra (which probe the excited states of a single exciton). Such experiments have been also carried out as a function of the magnetic field. As expected the emission spectra of high order excitonic complexes are indeed greatly affected by Coulomb interactions between carriers and in consequence are in general very different from the photoluminescence excitation spectra (quasi absorption) of a neutral and charged exciton. Two types of the magnetic field evolution of detected absorption lines (resonant peaks), the s- and p-shell related, have been measured. The s-shell like resonant peaks were attributed to the transition between the excited hole levels in the valence band and the ground s-shell level in the conduction band. Nevertheless, there exists an emission line which is observed within the p-shell cluster, and which coincides with the absorption line. That "coinciding resonance" is concluded to be an excited excitonic state which recombines radiatively due to efficient blocking of its relaxation towards the ground state.

L’IRM quantitative recouvre l’ensemble des méthodes permettant de mesurer des paramètres physiques accessibles en Résonance Magnétique Nucléaire. Elle offre un bénéfice par rapport à l’imagerie en pondération classiquement utilisée, notamment pour la détection, la caractérisation physiopathologique mais aussi pour le suivi thérapeutique des pathologies. Malgré ce potentiel avéré connu de longue date, ces méthodes restent peu utilisées dans la routine clinique. La raison principale est la longueur des acquisitions par rapport à l’approche classique. Les paramètres physiques que nous souhaitons étudier plus particulièrement sont le temps de relaxation longitudinal (T₁), transversal (T₂), le coefficient de diffusion apparent (ADC), et la densité de protons (DP). Malgré la possibilité d’atteindre une meilleure qualité d’images, ces cartographies in vivo sont quasiment inexistantes dans la littérature au-delà de 3T car leur implémentation nécessite de surmonter un certain nombre de limites spécifiques aux IRM ultra-haut champs (UHF). Au travers de ce projet de thèse, une méthode d’imagerie quantitative basée sur les états de configurations (QuICS) a été implémentée, pour déterminer ces paramètres quantitatifs de façon simultanée sous fortes contraintes propres aux UHF. L’approche a été optimisée dans le but d’obtenir des cartographies fiables et rapides. Le potentiel de la méthode a été démontré dans un premier temps in vitro sur un noyau tel que le sodium démontrant des propriétés complexes à cartographier. Puis dans un second temps, des acquisitions ont été réalisées sur proton, in vivo, en un temps d’acquisition compatible avec une utilisation en routine clinique à 7T. L’application d’une telle méthode d’IRM quantitative à UHF sur des populations permettra d’ouvrir de nouvelles voies d’études pour le futur. / Quantitative MRI refers to methods able to measure different physical parameters accessible in Nuclear Magnetic Resonance. It offers benefits compared to weighting imaging commonly used, for the detection, the pathophysiological characterization but also for the therapeutic follow-up of pathologies for example. Despite this long-established potential, these methods remain little used in clinical routine. The main reason is the long acquisition time compared to the classical approach. The physical parameters that we will study more particularly are the longitudinal (T₁), transverse (T₂) relaxation time, the apparent diffusion coefficient (ADC), and the proton density (DP). Despite the possibility to achieve a better image quality, these in vivo mappings are virtually non-existent in the literature beyond 3T because their implementation requires overcom-ing a number of specific ultra-high-field (UHF) MRI limits. Through this thesis project, a Quantitative Imaging method using Configuration States (QuICS) was implemented under strong UHF constraints, to determine these parameters simultaneously. The technique has been optimized to obtain fast and reliable maps. The potential of the method was first demon-strated in vitro on a nucleus such as sodium, exhibiting complex properties. As a second step, acquisitions were performed in proton, in vivo, in an clinically-relevant acquisition time, compatible with a routine use at 7T for population imaging. The application of such a method of quantitative MRI to UHF will open new research possibilities for the future.