Thesis advisor: Michael J. Graf / This thesis is concerned with experimentally determining the magnetic and electronic states in a unique class of transition metal oxides known as the pyrochlore iridates, A₂Ir₂O₇ (A = Y or Rare earth). The extended nature of the 5d Ir orbitals in the iridates places these materials in a regime of intermediate electron correlation and large spin-orbit interaction such that this system may host several novel or topological states of matter which may be perturbed by incorporating different A-species. Additionally, the pyrochlore structure is geometrically frustrated and has been long been studied as a potential host of a number of exotic magnetic phenomenon. However, even after years of intense theoretical and experimental interest many fundamental questions still remain about the nature of the magnetic ground sates in this series which are of vital importance in understanding the roles of various interactions and potential of such novel phenomenon. The primary aim of this thesis is therefore to determine how magnetic order develops on the Ir sublattice in this series, particularly how it is perturbed through variation of the crystalline structure, magnetism of the A-site ions, and presence of mobile charges. This thesis is the first comprehensive experimental study of these effects which has utilized several complementary experimental probes of both bulk and local magnetism in a number of compounds. The techniques presented in this work include magnetotransport, bulk magnetization, elastic neutron scattering, and muon spin relaxation (µSR) measurements. All of the three compounds studied in this work (A = Y, Yb, and Nd) are shown to definitively exhibit long-range magnetic order on the Ir sublattice, which has previously only been inferred based on studies of other compounds. The compounds Y₂Ir₂O₇ and Yb₂Ir₂O₇ are correlated insulators at low temperature and are found to have identical configuration of the Ir moments, despite the presence of the large localized Yb³⁺. Numerical investigations presented here have provided the first conclusive evidence that this order is of the `all-in/all-out' type, consistent with recent resonant x-ray studies; additionally, we have shown that this order exists for all insulating compounds regardless of structural parameters or properties of the A-ion. On the other hand, Nd₂Ir₂O₇ is weakly metallic with Kondo-like behavior at low temperature, with long-range order only on the Ir site, in disagreement with previous results from neutron scattering. Measurements of the field dependent magnetization and Hall effect reveal a large anomalous Hall component, suggesting that the Nd<super>3+</super> may exhibit a spin-ice state with very short correlation length, while the Ir sublattice is likely in the `all-in/all-out' state. From this, it is determined that Nd₂Ir₂O₇ lies at an important cross-over point in the series in which correlation energy and conduction bandwidth yield chiral order with features akin to both the metallic unordered Pr₂Ir₂O₇ and those of the magnetically ordered insulators. These results are discussed with regard to recent theoretical models exploring the role of electron correlation, frustration and various exchange interactions in these materials. / Thesis (PhD) — Boston College, 2013. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Thesis advisor: Stephen Wilson / The work performed within this thesis is divided into two parts, each focusing primarily on the study of magnetic phase behavior using neutron scattering techniques. In first part, I present transport, magnetization, and neutron scattering studies of materials within the iridium oxide-based Ruddelsden-Popper series [Srn+1IrnO3n+1] compounds Sr3Ir2O7 (n=2) and Sr2IrO4 (n=1). This includes a comprehensive study of the doped bilayer system Sr3(Ir1-xRux )2O7. In second part, I present my studies of the effect of uniaxial pressure on magnetic and structural phase behavior of the iron-based high temperature superconductor Ba(Fe1-xCox)2As2. Iridium-based 5d transition metal oxides host rather unusual electronic/magnetic ground states due to strong interplay between electronic correlation, lattice structure and spin-orbit effects. Out of the many oxides containing iridium, the Ruddelsden-Popper series [Srn+1IrnO3n+1] oxides are some of the most interesting systems to study both from the point of view of physics as well as from potential applications. My work is focused on two members of this series Sr3Ir2O7 (n=2) and Sr2IrO4 (n=1). In particular, our combined transport, magnetization and neutron scattering studies of Sr3Ir2O7 (n=2) showed that this system exhibits a complex coupling between charge transport and magnetism. The spin magnetic moments form a G-type antiferromagnetic structure with moments oriented along the c-axis, with an ordered moment of 0.35±0.06 µB/Ir. I also performed experiments doping holes in this bilayer Sr3(Ir1-xRux)2O7 system in order to study the role of electronic correlation in these materials. Our results show that the ruthenium-doped holes remain localized within the Jeff=1/2 Mott insulating background of Sr3Ir2O7, suggestive of `Mott blocking' and the presence of strong electronic correlation in these materials. Antiferromagnetic order however survives deep into the metallic regime with the same ordering q-vector, suggesting an intricate interplay between residual AF correlations in the Jeff=1/2 state and metallic nanoscale hole regions. Our results lead us to propose an electronic/magnetic phase diagram for Sr3(Ir1-xRux)2O7 system showing how the system moves from Jeff=1/2 antiferromagnetic Mott insulator (Sr3Ir2O7) to paramagnetic Fermi liquid metal (Sr3Ru2O7). On the other hand, our neutron scattering measurements on Sr2IrO4 (n=1), a prototypical Jeff=1/2 Mott insulator, showed that the spins arranged antiferromagnetically in ab-plane with an ordered moment comparable to that of Sr3Ir2O7. The second part of my work is comprised of a neutron scattering-based study of the Ba(Fe1-xCox)2As2 system, a bilayer family of iron-based high temperature superconductors. Undoped, this system exhibits either simultaneous or nearly simultaneous magnetic and structural phase transitions from a high temperature paramagnetic tetragonal phase to low temperature orthorhombic antiferromagnetic phase. With the gradual suppression of these two temperatures, the superconducting phase appears with the highest TC obtained just beyond their complete suppression. It has been proposed that these coupled magnetostructural transitions are secondary manifestations which arise as a consequence of electronic nematic ordering that occurs at a temperature higher than either of them. My work is mainly focused on probing the spin behaviors coupling to this electronic nematic phase. I devised a small device to apply uniaxial pressure along an in-plane high symmetry axis and studied the magnetic and structural behavior in series of Ba(Fe1-xCox)2As2 compounds via neutron scattering in presence of uniaxial pressure. There is an upward thermal shift in the onset of structural and magnetic transition temperature caused by this uniaxial pressure which is surprisingly insensitive to cobalt concentration in the absolute scale. Furthermore, on the first order side of the phase diagram (below the tricritical point), the structural and magnetic transitions are decoupled with magnetic transition following structural distortion. This study suggests the importance of both spin-lattice and orbital-lattice interactions in these families of compounds. / Thesis (PhD) — Boston College, 2014. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

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

The ability to manipulate colloidal particles with magnetic fields has profound applications both in industry and academic research ranging from automobile shock absorbers to robotic micro-surgery. Many of these applications use field gradients to generate forces on magnetic objects. Such methods are limited by the complexity of the required fields and by the magnitude of the forces generated. Spatially uniform fields only apply torques, but no forces, on magnetic particles. However, by coupling the particles' orientation and location, even static uniform fields can drive particle motion. We demonstrate this idea using particles adsorbed at curved liquid interfaces. We first review the intersection between active colloidal particles and (passive) particles at the fluid-fluid interface (chapter 1), followed by the introduction of magnetism, magnetic manipulation, and magnetic Janus particle fabrication techniques (chapter 2). In chapter 3, we use magnetic Janus particles with amphiphilic surface chemistry adsorbed at the spherical interface of water drop in decane as a model system to study particle response to a uniform field. Owing to capillary constraints, Janus particles adsorbed at curved interfaces will move in a uniform magnetic field to align their magnetic moment parallel to the applied field. This phenomenon is labeled as the magneto-capillary effect in this thesis. As explained quantitatively by a simple model, the effective magnetic force on the particle induced by static uniform field scales linearly with the curvature of the interface. For particles adsorbed on small droplets such as those found in emulsions, these magneto-capillary forces can far exceed those due to magnetic field gradients in both magnitude and range. The time-varying fields induce more complex particle motions that persist as long as the field is applied (chapter 4). Depending on the angle and frequency of a precessing field, particles orbit the drop poles or zig-zag around the drop equator. Magneto-capillary effects are not limited to Janus particles. Similar behaviors are observed in commercially available carbonyl iron particles. Periodic particle motion at the liquid interface can drive fluid flows inside the droplets, which may be useful for enhancing mass transport in droplet micro-reactors. The magneto-capillary effect at curved liquid interfaces offers new capabilities in magnetic manipulation: even static uniform fields can propel magnetic particles and the use of time-varying fields leads to steady particle motions of increasing complexity. These experimental demonstrations and the quantitative models that accompany them should both inspire and enable continued innovations in the use of magnetic fields to drive active processes in colloid and interface science. The final chapter highlights some specific directions for future work in this area.

Nanopartículas de Fe3O4 e Fe2O3 foram sintetizadas a partir da rota de coprecipitação, em atmosfera de N2, mantendo-se o pH entre 9 e 14 na temperatura ambiente e utilizando como precursores o FeCl2 e FeCl3. Após a síntese, as nanopartículas foram tratadas termicamente a 250, 500 e 800 oC. Para obtenção de um sistema híbrido, foram sintetizadas nanopartículas de ouro sobre as nanopartículas de óxido previamente tratadas em diferentes temperaturas. As amostras foram caracterizadas por UV-Vis, difratometria de Raios-X (DRX), microscopia eletrônica de transmissão (MET), microscopia eletrônica de transmissão de alta resolução (MET-AR), espectroscopia no na região do infravermelho (FTIR), magnometria de amostra vibrante (VSM) e espectroscopia Mossbauer (EM), e aplicadas para produção de hidrogênio promovendo a decomposição da hidrazina. Os resultados mostram a síntese de nanopartículas de óxido de ferro com diâmetro médio de cerca de 7 nm. Quando aquecidas a 250 oC o tamanho médio aumentou para ca. de 11 nm e foi observado uma pequena mudança no comportamento óptico e estrutural, mantendo o comportamento superparamagnetico. Quando aquecidas a 500 oC o tamanho médio aumenta para ca. de 50nm e são observadas mudanças significativas nas propriedades ópticas, morfológicas, estruturais. Adicionalmente observa-se transição de comportamento superparamagnetico para paramagnético. Quando aquecidas a 800 oC os efeitos nas propriedades dos materiais são ainda mais significativos; as partículas apresentam tamanho médio de 200 nm, o espectro de absorção no UV-Vis muda significativamente e as partículas passam a ter comportamento pagamagnético. Os resultados obtidos pelas diferentes técnicas e somadas ao Mossbauer sugerem que as amostras sintetizadas são uma mistura de Fe3O4 e -Fe2O3, quando aquecido a 250 e 500 oC uma mistura de -Fe2O3 e -Fe2O3 e a 800 oC somente -Fe2O3. As nanopartículas de ouro sintetizadas sobre as amostras de oxido de ferro apresentaram tamanho médio de 6,0 nm e não afetaram as propriedades magnéticas dos oxidos. As amostras de óxido com nanopartículas de ouro promoveram a decomposição da hidrazina por rota completa, levando a formação de hidrogênio com seletividade de até 33%. / Fe3O4 and Fe2O3 nanoparticles were synthesized by coprecipitation route carried out under N2 atmosphere, maintaining the pH between 9 and 14 at room temperature and using FeCl2 and FeCl3 as precursors. After synthesis the iron oxide nanoparticles were thermally treated at 250, 500 and 800 oC. To obtain a hybrid system, gold nanoparticles were synthesized on the thermally treated oxide nanoparticles. The samples were analyzed by UV-Vis, X-ray diffraction (XRD), transmission electron microscopy (TEM), high resolution transmission electron microscopy (MET-AR), spectroscopy in the region of Infrared (FTIR), vibrating sample magnitude (VSM) and Mossbauer, and applied to produce H2 through hydrazine decomposition. The results show the synthesis of Fe3O4 nanoparticles with average diameter of about 7 nm. When heated to 250 oC the average size increased to about 11 nm and a small change in the optical and structural behavior was observed, while the superparamegnetic behaviour was maintained. When heated to 500 °C, the average particle size increase to ca 51nm, significant changes in the optical, morphological and structural properties are observed, in addition to a transition from superparamegnetic to paramagnetic behaviour. When heated to 800 oC the effects on the properties are even more significant; the nanoparticles increase to ca. 200 nm, the absorption spectrum in UV-Vis changes significantly and the particles present paramagnetic behaviour. The results suggest that when heated to 250 and 500 oC a mixture of -Fe2O3 e -Fe2O3 is obtained, after heating at 800 oC only -Fe2O3 is observed. The gold nanoparticles synthesized on the iron oxides present average size of 6.0 nm, and did not affect the magnetic properties of the oxides. The iron oxides/gold nanoparticle samples were efficiently applied to produce hydrogen, promoting the decomposition of hydrazin. The selectivity to hydrogen reached up to 33%.

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.

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

This thesis presents the application of diffusion weighted imaging (DWI) and relaxometry MRI studies in three organs in the abdomen, the kidney, colon and liver. These methods are first applied at 1.5 Tesla in healthy volunteers and patients with Cardiorenal Syndrome (CRS), a clinical condition in which cardiac and renal dysfunctions (RD) coexist. Diffusion parameters are determined using the mono-exponential ADC, bi-exponential IVIM, stretched-exponential and Kurtosis models, as well as diffusion tensor imaging (DTI) in order to assess pathophysiology and structural changes in the kidney. Complementary longitudinal relaxation time (T1) mapping and PC-MRI flow measures are collected for comparison with diffusion parameters. The assessment of the colonic content using T1 measures as a biomarker of water absorption and health is presented in the second study. The main objective of the study was to determine the robustness of bi-exponential model in the fitting of T1 in the colonic contents of ascending colon. T1 measurements of colonic content are made at different positions in the ascending colon, before and after a liquid challenge. The reliability of T1 measurements of colonic content were compared across observers. The heterogeneity in the measurements was evaluated by considering different ROI sizes and locations. The final study is the assessment of liver function associated with Chronic Liver Disease (CLD) using DWI and T1 measurement at 1.5 T and a comparison with histological measures. the Measurements of T1 and diffusion parameters (ADC and IVIM) to stratify fibrosis stage in liver disease are combined with T2* measurement of iron accumulation. The repeatability and reproducibility of this protocol is then tested in the healthy liver using MRI at 3T for the assessment of structural and haemodynamic changes for future studies, with an evaluation of the choice of b-values to inform fitting of the bi-exponential (IVIM) model provided.

Magnetic nanoparticle (MNP) combined with biomolecules in a microfluidic system can be efficiently used in various applications such as mixing, pre-concentration, separation and detection. They can be either integrated for point-of care applications or used individually in the area of bio-defense, drug delivery, medical diagnostics, and pharmaceutical development. The interaction of magnetic fields with magnetic nanoparticles in microfluidic flows will allow simplifying the complexity of the present generation separation and detection systems. The ability to understand the dynamics of these interactions is a prerequisite for designing and developing more efficient systems. Therefore, in this work proof-of-concept experiments are combined with advanced numerical simulation to design, develop and optimize the magnetic microfluidic systems for mixing, separation and detection. Different strategies to combine magnetism with microfluidic technology are explored; a time-dependent magnetic actuation is used for efficiently mixing low volume of samples whereas tangential microfluidic channels were fabricated to demonstrate a simple low cost magnetic switching for continuous separation of biomolecules. A simple low cost generic microfluidic platform is developed using assembly of readily available permanent magnets and electromagnets. Microfluidic channels were fabricated at much lower cost and with a faster construction time using our in-house developed micromolding technique that does not require a clean room. Residence-time distribution (RTD) analysis obtained using dynamic light scattering data from samples was successfully used for the first time in microfluidic system to characterize the performance. Both advanced multiphysics finite element models and proof of concept experimentation demonstrates that MNPs when tagged with biomolecules can be easily manipulated within the microchannel. They can be precisely captured, separated or detected with high efficiency and ease of operation. Presence of MNPs together with time-dependent magnetic actuation also helps in mixing as well as tagging biomolecules on chip, which is useful for point-of-care applications. The advanced mathematical model that takes into account mass and momentum transport, convection & diffusion, magnetic body forces acting on magnetic nanoparticles further demonstrates that the performance of microfluidic surface-based bio-assay can be increased by incorporating the idea of magnetic actuation. The numerical simulations were helpful in testing and optimizing key design parameters and demonstrated that fluid flow rate, magnetic field strength, and magnetic nanoparticle size had dramatic impact on the performance of microfluidic systems studied. This work will also emphasize the importance of considering magnetic nanoparticles interactions for a complete design of magnetic nanoparticle-based Lab-on-a-chip system where all the laboratory unit operations can be easily integrated. The strategy demonstrated in this work will not only be easy to implement but also allows for versatile biochip design rules and provides a simple approach to integrate external elements for enhancing mixing, separation and detection of biomolecules. The vast applications of this novel concept studied in this work demonstrate its potential of to be applied to other kinds of on-chip immunoassays in future. We think that the possibility of integrating magnetism with microfluidic-based bioassay on a disposable chip is a very promising and versatile approach for point-of care diagnostics especially in resource-limited settings.

This thesis presents the results from investigations of ultrafast magnetisation dynamics driven by pure spin currents. Spin orbit coupling in heavy metal layers - such as tungsten, tantalum or platinum - allows for the generation of pure spin currents, whereby spin up and spin down electrons move in opposite directions. Hence, a flow of angular momentum can be controlled through the manipulation of charge current through a heavy metal layer. When a spin current is injected into a ferromagnet, a torque is exerted on its magnetisation, with the potential to induce a wide variety of ultrafast dynamics. The experimental investigation of these phenomena employed a variety of high-frequency electrical techniques and time-resolved scanning Kerr microscopy (TRSKM) methods. In addition, various simulative and analytical approaches were used to gain insight into the underlying mechanisms. Spin Hall nano-oscillators (SHNOs) have recently been shown to support a tunable GHz spin wave `bullet’ under injection of direct current (DC), making it an exciting candidate for microwave communication applications. This thesis will show how TRKSM can be used to measure the torques within these devices, revealing that radio frequency (RF) current does not possess the same distribution as the DC. The competition between self-inductance and focusing within the device geometry results in a modified distribution of spin current. Further TRSKM measurements show the modified torque landscape to promote the mobility of the `bullet' within the magnetic layer. Devices that exploit spin currents for magnetisation reversal have received interest from academia and industry for their potential use as memory elements. The perpendicular magnetic anisotropy present in Ta/CoFeB/MgO leads to lower write currents and higher thermal stability. However, ultrafast processes have not been previously observed in such devices. TRSKM measurements of Hall bar devices were compared with a macrospin model to understand the underlying torques, and to investigate the conditions required to promote switching. Square elements built from the same stack structure exhibited contrasting static and dynamic behaviour. Pulsed currents drove differing dynamics at the edge and center of the device, while enabling the realignment of magnetic domains. The domains themselves could be driven directly by the spin current leading to domain wall dynamics. Measurements with a bipolar electrical pulse demonstrated that meta-stable switching can be achieved in micron-scale elements.