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Thermoelectricity can directly convert a temperature difference into a voltage or charge current. Recently, the development of spin caloritronics has introduced spin as another degree of freedom in traditional thermoelectrics. This discovery bodes a new generation of magnetic random access memories (MRAMs), where thermal spin-transfer torque (TSTT) rather than voltage driven spin-transfer torque (STT) is used to switch the magnetization in magnetic tunnel junctions (MTJs). To advance the rising trend of spin caloritronics, the coupling of charge, spin, and heat flow during electron transport in MTJs was systematically studied in this thesis. To begin with, the static transport properties of MTJs were studied by observing current dependent tunnel magnetic resistance (TMR). The observed decrease of TMR with a biased current is attributed to the change in spin polarization of the free ferromagnetic layer. A phenomenological model has been built based on the current dependent polarization, which agrees with our experimental results. Next, the Seebeck rectification effect in MTJs was studied. By applying microwave currents to MTJs, an intrinsic thermoelectric coupling effect in the linear response regime of MTJs was discovered. This intrinsic thermoelectric coupling contributes a nonlinear correction to Ohm's law. In addition, this effect can be controlled magnetically since the Seebeck coefficient is related to magnetization configuration. Finally, TSTT in MTJs was systematically studied. A laser heating technique was employed to apply a temperature difference across the tunnel barrier and ferromagnetic resonance (FMR) spectra were measured electrically through spin rectification. By analyzing the FMR spectra, TSTT in MTJs was observed and the angular dependence of TSTT was found to be different from dc-biased STT. By solving the Landau-Lifshitz-Gilbert equation including STT, the experimental observations were well explained. The discovery of Seebeck rectification refines the previous understanding of magneto-transport and microwave rectification in MTJs and provides a new possibility for utilizing spin caloritronics in high-frequency applications. The study of TSTT in MTJs shows clear experimental evidence of TSTT in MTJs. Further optimization of the design of MTJs may succeed in decreasing the necessary switching fields strength or even achieve a switching by only TSTT in MTJs. / February 2017

The design, fabrication and performance of an Extraordinary Magnetoresistance (EMR) and a Vertical Mesa Hall Sensor (VMHS) are studied. EMR devices have been fabricated from a 2DEG InAs/GaSb structures which exhibit a low carrier density and high mobility that achieve the best performance. The general electrical magneto-transport properties are given. The experiments investigate mainly different metallic patterns, which are Rectangular, Triangular and Tip pattern between 4-300 K. Probe configurations and the enhancement of relative size of metallic patterns are described. EMR effect is due to current deflection around the metal-semiconductor interface. The results are metallic pattern dependent. Using finite element analysis, good agreement between experimental and theoretical results was found. The best performance sensor is a symmetrical metallic Tip pattern. It is enhanced by the length of the Tip’s point and the large metallic area. This pattern when combines with an asymmetrical probe configuration, exhibits the highest EMR of 900% at –0.275T measured by inner probes and the best sensitivity of 54Ω/T at room temperature. The second study presents in-plane Hall effect sensors made from InSb. A simple device geometry has been used in which current flows in a plane perpendicular to the device surface. Device sensitivity depends on its geometry and a series of different contacts are used to investigate the geometry of the current flow distribution. The structures produced are only sensitive to the presence of one in-plane field component, and they also demonstrate good angular selectivity. Multi-electrodes were used to investigate biasing current from both mesa and substrate condition. We are able to examine the Hall voltage as a function of contact positions and also to create multiple VMHS. Offset reduction of devices has been achieved by moving the ground contacts to re-balance the current distribution under the mesa surface.

Fin des années 1980, la découverte de l'effet magnétorésistif géant, qui se caractérise par une variation importante de la résistance électrique d'un matériau lorsqu'on le soumet à un champ magnétique, a eu un impact très important tant au niveau des études fondamentales qu'en vue d'applications industrielles telles que la réduction de la taille des disques durs des ordinateurs (Prix Nobel d'A. Fert en 2007). L'engouement ainsi suscité a permis de mettre en évidence cet effet, au début des années 1990, dans les couches minces d'oxyde de type pérovskite ABO3 et plus particulièrement dans les manganites de terres rares (Ln1-xAx)MnO3. Le but de ce travail s'inscrit dans ce cadre et concerne l'élaboration et l'étude des propriétés physiques (structurales, magnétiques, de transport et magnétocaloriques) de nouveaux manganites qui pourraient avoir des applications dans un domaine connexe qui est la réfrigération magnétique. En effet, cette dernière décennie, a vu les découvertes de nouveaux composés présentant des effets magnétocaloriques géants qui ont conduit aux premiers essais de laboratoire de la réfrigération magnétique. Celle-ci semble être l'une des alternatives très sérieuses pour le remplacement des systèmes de réfrigération classique basés sur la compression-détente des gaz. Cette nouvelle technique, comparée aux techniques traditionnelles, présente plusieurs avantages, elle est plus efficace sur le plan énergétique, plus compacte et surtout moins nuisible à l'environnement. La première partie de ce travail porte sur l'élaboration et la caractérisation des composés de formule La1-xCexMnO3. Nous avons étudié l'effet du recuit sur les propriétés morphologique, structurale, magnétique et magnétocalorique de ces composés. L'application du modèle de Landau, en bon accord avec les résultats expérimentaux de la mesure l'entropie magnétique SM, a montré que la nature de transition de phase dépend aussi de la température de recuit. La composition x=0.4 de ce composé présente la valeur la plus élevée du facteur de mérite RCP, ce qui en fait un bon candidat pour les applications à la réfrigération magnétique. Dans une deuxième partie une étude des propriétés morphologique, structurale, magnétique et magnétocalorique des manganites de formule La0,7Ca0,15Sr0,15Mn1-xFexO3 a été réalisée. Le fer n'influe pas sur les propriétés structurales mais entraîne une diminution de la température de Curie TC. Afin d'approfondir ces études, nous avons proposé un matériau composite basé sur deux composés La0,7Ca0,15Sr0,15Mn1-xFexO3 (x = 0,025 et 0,75). La variation d'entropie du composite reste approximativement constante entre 260 et 300 K. En conséquence, ce matériau composite peut être un très bon candidat pour la réfrigération magnétique au voisinage de l'ambiante. Dans une dernière partie, nous avons étudié l'effet du double échange, de la méthode de préparation, le rayons du site A et la nature magnétique du dopant au site B sur les propriétés magnétocaloriques en caractérisant la famille des composés La0,6A0,4Mn1-xFexO3 (A= Ca, Sr et 0≤x≤0,2) par diffraction des rayons X et par mesures magnétiques. D'une part, l'entropie magnétique maximale augmente avec le rayon du site A et est peu affecté par le rayon du site B et d'autre part, la méthode de préparation solide-solide est à privilégier puisqu'elle permet d'obtenir les plus grandes valeurs d'entropie magnétique maximale. / Since the discovery of the giant magnetoresistance effect (end of 1980s), which is characterized by a large change in the electrical resistance of a material under the effect of a magnetic field, a major impact has been motivated both on fundamental and practical aspects (Nobel Prize of A. Fert in 2007). The intensive research activities in this field have leaded in the end of 1990 to point out the giant magnetoresitance in thin films of perovskite family, in particular the manganites (Ln1-xAx)MnO3. The aim of this work concern the study of the structural, magnetic, electrical and magnetocaloric properties of new manganites based materials in view of their application in the magnetic cooling. It is worth noting that in recent years, a giant magnetocaloric effect has been reported in several materials leading to the implementation of new efficient magnetic cooling systems. This technology is considered actually as the most alternative to replace the classical systems based on the compression-relaxation process. Compared with conventional refrigeration, magnetic cooling presents relevant advantages such as a decrease of energy consumption (high efficiency) and reduction of the acoustic and environmental pollution (elimination of the standard coolants: CFC, HCFC). The first part of this work concerns the elaboration as well as the characterization of the compound with La1-xCexMnO3 formula. We have studied the role of the annealing on the morphological, structural, magnetic and magnetocaloric properties of these materials. Using the Landau theory, we have calculated the magnetic entropy change ΔSM, which is found in good agreement with the measurements, and we have shown that the nature of the magnetic transition depends also on the annealing temperature. The compound with the composition x = 0.4, presents a large value of the figure of merit RCP, which make this material a good candidate for magnetic cooling application. In the second part, a detailed study of the morphological, structural, magnetic and magnetocaloric properties of the compounds with La0,7Ca0,15Sr0,15Mn1-xFexO3 formula has been performed. The iron Fe don't affect the structural properties, but induces a decrease of the Curie temperature. Based on the La0,7Ca0,15Sr0,15Mn1-xFexO3 (x = 0, 025 et 0,075) compositions, a composite material was proposed. The entropy change of the composite remains approximately constant in the temperature range between 260 and 300 K. Consequently, the proposed composite can be a good refrigerant for room temperature applications, in particular the magnetic cooling systems that use AMR or Ericsson thermodynamic cycles. In the last part, we have investigated the effect of the double exchange, preparation method and, ionic radius in A site and the magnetic nature on the doping in B site on the physical properties of La0,6A0,4Mn1-xFexO3 (A= Ca, Sr et 0≤x≤0,2) by using X-rays diffraction and magnetic measurements. The results demonstrate that the maximum entropy change increases with the ray of A site while it is slightly affected by the B site ray. On the other hand, it seems that the solid-solid preparation technique allows to obtain compounds with high magnetocaloric performances.

Este trabalho teve como finalidade determinar e otimizar as condições para a preparação e a caracterização de monocristais de alguns óxidos que apresentam considerável dependência de suas propriedades físico-químicas finais e magnitude da pressão da atmosfera de crescimento. Experimentos que envolvem diversas adaptações de um sistema de fusão a laser de pedestais foram projetados para investigar o melhor crescimento de monocristais de molibdatos alcalinos isolados e mistos (CaMoO4, cristal gradiente de CaMoO4-SrMoO4, Ca2</subFeMo06) e de um óxido titanato alcalino (Ba0,77Ca0,23TiO3). Também, os aspectos teóricos das condições experimentais foram estudados nos termos de um modelo termodinâmico existente. Adicionalmente, é mostrado que técnicas de caracterização estruturais (curvas de rocking com raios-X, topografia de raios X, método de Rietveld), térmicas (análise térmica diferencial) e ópticas (absorção ótica) foram aplicadas com sucesso além de outros métodos de caracterização bem estabelecidos (fotografia de retro-reflexão de raios-X, análise por raios-X de elétrons retro-espalhados, microscopia eletrônica de varredura). Os experimentos e os dados das caracterizações revelam que CaMoO4, SrMoO4 e Ba0,77Ca0,23 podem ser crescidos ao ar sem alterações de suas propriedades quando tratamento térmico cuidadoso é realizado nos reagentes de partida e aos produtos, enquanto que o Ca2FeMoO6 somente pode ser crescido em atmosfera isostática de N2 e em uma faixa pequena de pressões (0,25-0,75 bar). Em qualquer caso, a estabilidade do processo de crescimento é alcançada quando gradientes térmicos específicos e relações geométricas na zona fundida são estabelecidos após a imposição de uma atmosfera adequada de crescimento. As medidas estruturais, térmicas, ópticas e de composição convencionais confirmam a formação de fases e estequiometrias desejadas. Os dados estruturais refinados e de alta resolução demonstram o controle da composição e conseqüentemente do espaçamento interplanar de rede no cristal gradiente CaMoO4-SrMoO4, um primeiro exemplo de cristal gradiente de um óxido crescido por uma técnica de zona flutuante, e abrindo um panorama largo de possibilidades para a preparação de outros cristais gradientes de óxidos. / This thesis examines the conditions for the optimized single crystal preparation and characterization of some oxides compounds that present high dependency of the nature and pressure magnitude of the growth atmosphere on their final physico-chemical properties. Experiments involving several adaptations to a laser-heated pedestal growth system were designed to investigate the best crystal growth of a mixed alkaline titanate oxide (Ba0,77Ca0,23TiO3) and single and mixed alkaline molybdates oxides (CaMoO4, cristal gradiente de CaMoO4-SrMoO4, Ca2</subFeMo06). AIso, theoretical aspects of the experimental conditions were studied in terms of an existent thermodynamical model. In addition, it is shown that non-standard to techniques to characterize crystal\' s structural features (X-ray rocking, X-ray topography, Rietveld\'s method), thermo-physical (Differential Thermal Analysis) and optical properties (Optical Absorption) were succesfully applied besides others well established characterization methods. (X-ray Laue backreflection photography, Electron Dispersive X-Ray Analysis, Scanning Electron Microscopy). Experiments and characterizations data reveal that CaMoO4, SrMoO4 e Ba0,77Ca0,23succesfully grow in air at room conditions when careful thermal treatment of the starting materiaIs and products, while Ca2FeMo06 only can be grown in isostatic atmosphere of N2/H2 (95&#8240 vol./5&#8240 voI.) and at a short pressure range (0.25-0.75 bar). In any case, the stability of the growth process is reached when specific thermal gradients and geometric relations in the meU are established after the imposition of an adequate growth atmosphere. Conventional structure, thermal, optical and composition measurements confirm the formation of desired phases and stoichiometries with excellent results for both single and mixed compounds. Refined and high resolution structural data demonstrate large control of composition and therefore interplanar lattice spacing in the gradient crystal CaMoO4-SrMoO4, a first example of an oxide gradient crystal grown by a floating zone technique, and open a wide panorama of possibilities for the preparation of novel gradient oxides crystals.

by Chiah Man Fat. / Thesis submitted in: September 1999. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2000. / Includes bibliographical references (leaves 94-97). / Abstracts in English and Chinese. / Acknowledgements --- p.2 / Abstract --- p.3 / Table of Contents --- p.5 / List of Figures --- p.7 / List of Tables --- p.13 / Chapter Chapter 1 --- Introduction --- p.14 / Chapter 1.1. --- Overview --- p.14 / Chapter 1.2. --- Giant Magnetoresistance (GMR) --- p.15 / Chapter 1.3. --- Application of GMR Materials --- p.20 / Chapter 1.4. --- Preparation Methods --- p.22 / Chapter 1.5. --- This Thesis --- p.23 / Chapter Chapter 2 --- Sample Preparation and Experimental Methods --- p.24 / Chapter 2.1. --- MEVVA Ion Source Implanter --- p.24 / Chapter 2.2. --- The Pulsed Filtered Cathodic Arc Co-deposition System --- p.26 / Chapter 2.3. --- Sample Preparation --- p.29 / Chapter 2.3.1 --- Implantation Condition --- p.29 / Chapter 2.3.2 --- Co-deposition Conditions --- p.31 / Chapter 2.4. --- Characterization methods --- p.32 / Chapter 2.4.1 --- Magnetoresistance Measurement --- p.32 / Chapter 2.4.2 --- Atomic Force Microscopy and Magnetic Force Microscopy --- p.34 / Chapter 2.4.3 --- Rutherford Backscattering Spectroscopy (RBS) --- p.37 / Chapter 2.4.4 --- SQUID Magnetometer --- p.38 / Chapter Chapter 3 --- Characterization of Implanted Samples --- p.39 / Chapter 3.1. --- Introduction --- p.39 / Chapter 3.2. --- Results and Discussion --- p.39 / Chapter 3.2.1 --- Ag Film Thickness Dependence --- p.39 / Chapter 3.2.2 --- Dose Dependence --- p.44 / Chapter 3.2.3 --- Extraction Voltage Dependence --- p.46 / Chapter 3.2.4 --- Annealing Temperature Dependence --- p.49 / Chapter 3.2.5 --- Thicker Layer Formation --- p.56 / Chapter 3.2.6 --- AFM and MFM Measurements --- p.58 / Chapter 3.3. --- Summary --- p.64 / Chapter Chapter 4 --- Characterization of Co-deposited Samples --- p.65 / Chapter 4.1. --- Introduction --- p.65 / Chapter 4.2. --- Results and discussion --- p.65 / Chapter 4.2.1 --- RBS Measurement --- p.65 / Chapter 4.2.2 --- Magnetoresistance Measurement --- p.66 / Chapter 4.2.3 --- AFM Measurement --- p.69 / Chapter 4.2.4 --- MFM Measurement --- p.76 / Chapter 4.3. --- Summary --- p.84 / Chapter Chapter 5 --- Conclusion --- p.85 / Chapter 5.1. --- Main Results of This Work --- p.85 / Chapter 5.2. --- Suggestions on Future Works --- p.87 / Appendix --- p.89 / Reference --- p.94 / Publications --- p.97

This thesis uses ferromagnetic resonance to explore the current-induced torques (CITs) in two different systems, namely YIG/heavy metal bilayers and bulk NiMnSb, at room temperature. We apply a microwave current to the sample while sweeping the external magnetic field, and measure the longitudinal DC voltage. From a symmetry analysis of the ferromagnetic resonance lineshape, the amplitudes and directions of the CITs parameterised by an effective magnetic field are accurately estimated. In Chapter 4, YIG samples of different thickness, capped by either Pt or Ta, are studied. The resonance is driven by both spin-transfer torque and Oersted field, and the DC voltage is attributed to both spin rectification and spin pumping. The CITs can be well analysed from the lineshape of the voltage and its dependence on YIG thickness, from which we deduce that the Oersted field dominates over the spin-transfer torque in driving magnetization dynamics. In Chapter 5, we characterise the CITs in bulk NiMnSb induced by the relativistic spin-orbit coupling effect. Both field-like and antidamping-like spin-orbit torques are observed and analysed in detail. At the end of this chapter, we study the spin-wave resonance driven by the CITs, from which the exchange stiffness of NiMnSb is determined. In Chapter 6, we extrapolate a new form of magnetoresistance in NiMnSb: unidirectional spin-orbit magnetoresistance (USOMR). USOMR scales linearly with the current and has opposite sign when the magnetization is reversed. Similar to the giant magnetoresistance in magnetic multilayers, USOMR can be used to distinguish between two opposite magnetization directions directly in the bulk of the ferromagnet.