Transition metal and related compounds have been extensively studied over the past several decades. These investigations revealed a wide range of behavior, encompassing colossal magnetoresistance (CMR), high-TC superconductivity, and magnetic semiconductivity, all of which continue to present fundamental challenges to the understanding of such phenomena. There is, however, a close correlation between such characteristics and the appearance of magnetic order. This correlation underlies the present study, which focuses on the magnetic and transport behavior of various Manganese (Mn), Iron (Fe) and Cobalt (Co) containing materials, with particular emphasis on the nature of the magnetic order they display and the critical exponents that characterize the accompanying phase transition. The magnetic and transport properties of two specific systems will be covered: first various doped manganites from the series (La,Pr)1-x(Ca,Ba)xMnO3, and second the magnetic semiconductors Fe0.8Co0.2Si and Ga0.98Mn0.02As. In the manganites, the influence of doping on; (i) the evolution of the metal-insulator transition (MIT) with composition; (ii) the universality class of the magnetic critical behavior associated with the paramagnetic to ferromagnetic transition, which occurs in the vicinity of a MIT with which CMR is associated; (iii) the mechanisms underlying ferromagnetism across the MIT; (iv) the correlation between the appearance of a Griffiths-like phase and CMR, and (v) the origin of Griffiths-like phase have been investigated. Four different systems have been studied: La1-xCaxMnO3 (0.18 ≤ x ≤ 0.27), La1-xBaxMnO3 (x ≤ 0.33), (La1-yPry)0.7Ca0.3Mn16/18O3 (y ≤ 0.85), and Pr1-xCaxMnO3 (x = 0.27, 0.29). In Fe0.8Co0.2Si and Ga0.98Mn0.02As, the scaling between magnetization and conductivity has been the subject of ongoing debate. In bulk Fe0.8Co0.2Si, a novel scaling between the anomalous Hall effect (AHE) and the magnetization enables the anomalous Hall coefficient to be accurately determined. In turn, this enables the universality class for the transition to ferromagnetism to be established independently from the anomalous Hall conductivity. In an epitaxial (metallic) Ga0.98Mn0.02As microstructure, the magnetization has been indirectly determined from the AHE. Subsequent analysis yields magnetic critical exponents consistent with the Mean-Field model, direct support for which had previously been lacking.

In this thesis the charge dynamics of correlated electron systems, in which a metallic phase lies in close proximity to an ordered phase, are investigated by means of angle resolved photoemission spectroscopy (ARPES). The analysis of the experimental data is complemented by electronic structure calculations within the framework of density functional theory (DFT). First the charge dynamics of the colossal magnetoresistant bilayer manganites are studied. The analysis of the ARPES spectra based on DFT calculations and a Peierls type charge density wave model, suggests that charge, orbital, spin and lattice degrees of freedom conspire to form a fluctuating two dimensional local order that produces a large pseudo gap of about 450 meV in the ferromagnetic metallic phase and that reduces the expected bilayer splitting. Next, the interplay of Kondo physics and (magnetic) order in the heavy fermion superconductor URu2Si2 is investigated. The low energy electronic structure undergoes strong changes at 17.5 K, where a second order phase transition occurs whose phenomenology is well characterized, but whose order parameter could not yet be unambigeously identified. Below THO, non-dispersive quasi particles with a large scattering rate suddenly acquire dispersion and start to hybridize with the conduction band electrons. Simultaniously the scattering rate drops sinificantly and a large portion of the Fermi surface vanishes due to the opening of a gap within the band of heavy quasi particles. The observed behaviour is in stark contrast to conventional heavy fermion systems where the onset of hybridization between localized and itinerant carriers happens in a crossover type transition rather than abruptly. These experimental results suggest that Kondo screening and the hidden order parameter work together to produce the unusual thermodynamic signatures observed in this compound. Finally, the influence of charge doping and impurity scattering on the superconducting porperties of the transition metal substituted iron pnictide superconductor Ba(Fe1-xTMx)2As2 (TM = Co, Ni) is studied. Here, resonant soft X-ray ARPES is applied to see element selective the contribution of the 3d states of the TM substitute to the Fe 3d host bands. The spectroscopic signatures of the substitution are found to be well reproduced by DFT supercell and model impurity calculations. Namely, the hybridization of the dopant with the host decreases with increasing impurity potential and the electronic states of the impurtiy become increasingly localized. Simultaniously, in all simulated cases a shift of the Fermi level due to electron doping is observed. The magnitude of the shift in the chemical potential that accurs in BaFe2As2, however, is in stark contrast to the marginal doping values obtained for the impurity model, where the shift of the chemical potential is largely compensated by the influence of the increasing impurity potential. This suggests that the rigid band behaviour of TM substituded BaFe2As2 is a peculiarity of the compound, which has strong implications for the developement of superconductivity. / In dieser Arbeit wird die Ladungstraegerdynamik in korrelierten Elektronensystemen, in denen eine metallische Phase in direkter Nachbarschaft zu einer geordneten Phase liegt, mit Hilfe von winkelaufgeloester Photoelektronenspektroskopie (ARPES) untersucht. Die Analyse der experimentellen Daten wird ergaenzt durch lektronenstrukturrechnungen im Rahmen der Dichtefunktionaltheorie (DFT). Zuerst wird die Ladungstraegerdynamik in gemischtvalenten zweischichtmanganaten mit kolossalem Magnetiwiderstand studiert. Die Analyse der Photoemissionsspektren basierend auf DFT Rechnungen und einem Peierls artigem Ladungsdichtewellenmodell, legt nahe, dass die Freiheitsgrade von Ladung, Orbitalen, Spin und des Ionengitters konspirieren, um eine fluktuierende zweidimensionale lokale Ordnung zu bilden, die verantwortlich ist fuer die beobachtete Pseudobandluecke von 450 meV, und die zur Reduktion der erwarteten Zweischichtaufspaltung beitraegt. Als naechstes wird das Zusammenspiel von Kondo Physik und (magnetischer) Ordung im Schwerfermionensupraleiter URu2Si2 untersucht. Die iedrigenergetische elektronische Struktur zeigt starke Veraenderungen bei 17.5 K, wo ein Phasenuebergang zweiter Ordnungstattfindet, der phenomenologisch gut charakterisiert ist, aber dessen Ordungsparameter nocht nicht eindeutig identifiziert werden konnte. Unterhalb von THOerlangen nicht dispergierende Quasiteilchen mit gro en Streuraten abrupt Dispersion und hybridisieren mit den Leitungselektronen. Gleichzeitig sinkt die Streurate und ein gro er Teil der Fermiflaeche verschwindet durch das Oeffnen einer Bandluecke innehalb des Bandes schwerer Quasiteilchen. Das beobachtete Verhalten steht in starkem Kontrast zu dem von konventionellen Schwerfermionensystemen, in denen die Hybridisierung zwischen lokalisierten und itineranten Ladungstraegern in einem kontinuierlichen Uebergang ablaeuft, anstatt abrubt. Diese experimentellen Befunde lassen den Schluss zu, dass das zusammenspiel zwischen Kondo Abschirmung und dem unbekannten Ordnungsparameter die ungewoehnlichen thermodynamischen Signaturen in dieser Verbindung hervorruft. Abschliessend wird das Zusammenwirken von Ladungstraegerdotierung und Streuung an Stoeratomen auf die Supraleitung uebergangsmetalldotierter Eisenpniktid Supraleiter Ba(Fe1-xTMx)2As2 (TM = Co, Ni) untersucht. Mit Hilfe von resonantem Weichenroentgen ARPES gelingt es, elementselektiv den Beitrag der 3d Zustaende des TM Substituenten zu den Eisen 3d Wirtsbaendern zu beobachten. Die spektroskopischen Signaturen der Substitution sind mit Hilfe von DFT Rechnungen und Modelrechnungen mit zufaellig verteilten Stoeratomen gut zu reproduzieren. Insbesondere nimmt die Hybridisierung des dotierten Uebergangsmetalls und der Eisenbaender mit zunehmender Kernladungszahl ab und die elektronischen Zustaende der Stoeratome werden zunehmen lokalisiert. Gleichzeitig wird in allen gerechneten Faellen eine Verschiebung des Fermi Niveaus durch Elektronendotierung beobachtet. Der Betrag der Verschiebung des chemischen Potentials in BaFe2As2 steht allerdings in starkem Kontrast zu den Werten, die man im Falle der Modellrechnungen erhaelt, wo die Verschiebung des Fermi Niveaus durch den Einfluss des Potentials der Stoeratome groesstenteils kompensiert wird. Dies legt nahe, dass das beobachtete "rigid band" Verhalten von TM substituiertem BaFe2As2 eine Besonderheit dieser Verbindung ist, welches starke Auswirkungen auf die Ausbildung von Supraleitung hat.

Made available in DSpace on 2014-10-09T12:53:18Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:59:02Z (GMT). No. of bitstreams: 0 / Dissertação (Mestrado) / IPEN/D / Instituto de Pesquisas Energéticas e Nucleares - IPEN-CNEN/SP

Amostras policristalinas de La(5/8-y)Pr(y)Ca(3/8)MnO(3); 0 y 0.625; foram produzidas pelo método da mistura estequiométrica de óxidos e tratadas termicamente ao ar a 1400 oC. As amostras foram caracterizadas através de medidas de difração de raios-X, resistividade elétrica rho(T), susceptibilidade magnética chi(T) e magnetorresistividade rho(T, H = 50 kOe). Os resultados das análises dos diagramas de raios-X indicaram que os materiais são de fase única e que houve uma substituição efetiva de La por Pr no sítio A ao longo da série. Medidas de rho(T) e chi(T) revelaram que a temperatura de transição de fase metal-isolante TMI e temperatura de Curie TC decrescem com o aumento da concentração y e que a resistividade residual rho0 (rho(T = 10 K)) é consideravelmente alta em amostras com y 0.35. Ainda, com o decréscimo de T, as amostras com y 0.35 transitam para uma fase de ordenamento de carga em TOC ~ 194 K e, em seguida, para uma fase metálica em TMI. Essas medidas também sugerem a coexistência de fases ferromagnética-metálica FMM e de ordenamento de carga isolante OCI nesses materiais. Nas propriedades físicas macroscópicas, a fase FMM mostrou ser a dominante para os compostos com pequenas concentrações de Pr (y 0.25) e a fase OCI dominante para os compostos com altas concentrações de Pr (y 0.40). As medidas de rho(T, H = 50 kOe) mostram que a magnitude da resistividade elétrica decresce drasticamente nas vizinhanças de TMI sob a aplicação de um campo magnético externo. A magnitude de MR (MR = (rho(H = 0)-rho(H = 50 kOe))/rho(H = 50 kOe)) entre os extremos da série (y = 0 e 0.625) varia até sete ordens de grandeza, sendo que o máximo valor de MR para amostras com y = 0 é de ~ 0.75 e naquelas com y = 0.625 é ~ 3.4x106 . O diagrama de fases deste composto evidencia uma região crítica (0.30 y 0.40) onde os valores de TMI, TC, MR e 0 variam abruptamente como função de y, sendo que em outras regiões tal variação é mais suave. A variação significativa desses quatro valores indica uma competição mais forte entre as fases coexistentes ocorre na região crítica. Algumas características marcantes podem ser observadas nas amostras da região crítica tais como: a presença de um segundo pico, abaixo de TMI, em ~ 90 K e ~ 72 K na curva de rho(T) de amostras com y = 0.30 e 0.35, histerese térmica mais pronunciada em rho(T) e chi(T), MR torna-se colossal, relaxação significativa da resistividade elétrica com o tempo, entre outras. Assim, as propriedades de transporte e magnéticas nessa região crítica são dominadas pela forte competição entre as fases coexistentes. / Polycrystalline samples of La(5/8-y)Pr(y)Ca(3/8)MnO(3); 0 y 0.625; were synthesized by the solid-state reaction method and sintered in air at 1400 oC. These compounds were studied by measurements of X-ray powder diffraction, magnetic susceptibility chi(T), and electrical resistivity rho(T, H). X-ray powder diffraction measurements indicated single phase materials and an effective substitution of La by Pr. Results from rho(T) and chi(T) revealed that increasing y in this series results in a rapid reduction of both the insulator to metal transition temperature TMI and the Curie temperature TC. Such a rapid decrease in TMI with increasing y is also accompanied by the occurrence of a new transition temperature, termed TCO, which is related to the transition to the charge ordered CO state. Such a temperature, which is essentially independent of y, occurs at TCO ~ 194 K and is mainly observed in samples with y 0.35. The other feature is the presence of a large residual resistivity electrical rho(0 = (10 K)) for large y (y 0.35) at low-T even though rho(T) suggests a metallic behavior below TMI. The temperature for the maximum magnetoresistance effect occurs near TMI, that shifts to higher T with increasing field. The MR is defined here as (rho(H = 0)-rho(H = 50 kOe))/rho(H = 50 kOe) and is enhanced by seven orders of magnitude from ~ 0.75 up to ~ 3.4x106 in samples with y = 0 and y = 0.625, respectively. Some features like the thermal hysteresis observed in both rho(T) and chi(T) curves indicate the coexistence of different phases in a range of y concentration, i. e., the ferromagnetic-metallic FMM and the charge ordered-insulating COI domains. The FMM is stable for y 0.25, but the COI state becomes dominant for y 0.40. There is a critical region in the phase diagram, ranging from y = 0.30 to 0.40, where the magnitude of the TMI, TC, MR, and 0 were found to display abrupt changes with increasing y. Some anomalous features like a second peak in rho(T) below TMI, a two-step increasing in chi(T), a colossal MR effect and others are observed for compositions belonging to this critical region. Our combined data suggest that the general physical properties of these compounds in such a critical region are dominated by the strong competition between coexisting ferromagnetic-metallic and charge ordered-insulating phases.

A ênfase deste trabalho foi a aplicação da técnica de RMN em campo zero para a obtenção do acoplamento quadrupolar e dos espectros de RMN em banda larga dos núcleos de 27Al (amostra GdAl2), 139La e 55Mn (amostras de Manganita), à temperatura de 4,2 K. O objetivo do trabalho foi obter resultados confiáveis tanto em relação aos espectros quanto aos valores dos acoplamentos quadrupolares das amostras de GdAl2 e de Manganitas, e utilizar estes resultados para auxiliar a entender e a determinar algumas das propriedades físicas destes materiais (no caso do GdAl2 o comportamento tipo Spin-Glass e o efeito Magnetocalórico, no caso das Manganitas a Magnetoresistência Colossal). Para se determinar o valor do acoplamento quadrupolar utilizou-se a técnica de medida das oscilações quadrupolares. Os espectros de RMN, em banda larga, foram obtidos utilizando-se a técnica de eco de Hahn variando-se a frequência de excitação em uma faixa de 5 a 500 MHz. A maior sensibilidade do equipamento montado em nosso laboratório permitiu que se observassem os múltiplos ecos devidos à interação quadrupolar. Consequentemente, estes ecos permitiram a obtenção de espectros de RMNz de 27Al com alta resolução e a medida do valor do acoplamento quadrupolar diretamente do espectro. Também foi mostrado que os múltiplos ecos do 27Al são originados em regiões diferentes da amostra. Os resultados dos experimentos de oscilação quadrupolar em amostras de Manganitas permitiram que se observassem vários valores de acoplamento quadrupolar, que podem ser devido à desordem da estrutura cristalina dos octaedros de MnO6 ou à variação angular do ângulo entre o campo magnético hiperfino e a o eixo de fácil magnetização. Também foi mostrado que os valores de acoplamento quadrupolar mudam em função do elemento dopante utilizado. A aplicação do campo externo diminui a distribuição de acoplamento quadrupolar, mostrando que a aplicação do campo magnético externo causa mudanças ou na rede cristalina e/ou na orientação do campo magnético hiperfino e, consequentemente, sobre as propriedades físicas das Manganitas. A comparação entre os espectros de 139La e 55Mn das diferentes amostras de Manganita mostrou a coexistência de fases magnéticas. Estes resultados mostram que a técnica de RMNz é uma excelente ferramenta para se determinar o estado magnético fundamental dos materiais magnéticos e as mudanças na estrutura cristalina através da observação do acoplamento quadrupolar. / The emphasis of this work was the application of zero-field NMR technique to obtain the quadrupole coupling and NMR broadband spectra of the nuclei 27Al (GdAl2 sample), 139La and 55Mn (Manganites samples), at 4.2 K. The objective was to obtain reliable results for both spectra and quadrupole coupling values of samples GdAl2 and Manganites, and use these results to help understand and determine some of the physical properties of these materials (for GdAl2 the Spin-Glass type behavior and the Magnetocaloric effect and for Manganites the Colossal Magnetoresistance effect). To determine the value of the quadrupole coupling it was used the quadrupole oscillations technique. The NMR broadband spectra were obtained using the Hahn echo technique, in the frequency excitation range 5 to 500 MHz. The greater sensitivity of the equipment allowed observing multiples echoes due to quadrupole interaction, these echoes allowed to obtain spectra of zero field RMN of 27Al with high resolution and measure the value of the quadrupole coupling directly from the spectrum. It was also shown that the 27Al multiple echoes are generated in different regions of the sample. The quadrupole oscillation experiments results on Manganite samples have obtaned several values of quadrupole coupling, indicating that the Manganites Ferromagnetic Metallic Phase present high degree of disorder or, in the crystalline structures or in the hyperfine magnetic field direction. It was shown that the change in the value of the quadrupole coupling depends on the dopant used in the Manganites composition. The results of the quadrupole oscillations with application of an external magnetic field showed that the degree of disorder decreases due to the external magnetic field, showing the influence of the external magnetic field on the crystal network and/or in the hyperfine magnetic field direction, and consequently of some Manganites physical properties. The comparison between the spectra of 139La and 55Mn of the different Manganite samples shows the coexistence of different magnetic phases. These results showing that the technique of zero-field NMR is excellent to determine the fundamental magnetic state of the magnetic materials and observe changes in the crystal structure by the observation of the quadrupole coupling.

Made available in DSpace on 2014-10-09T12:53:18Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:59:02Z (GMT). No. of bitstreams: 0 / O compósito de manganito de lantânio dopado com estrôncio (La1-xSrxMnO3 - LSM) e zircônia estabilizada com ítria (ZrO2/Y2O3 - YSZ), tem sido indicado como catodo das células a combustível de óxido sólido, por apresentar melhor desempenho como eletrodo catódico na região da tripla fase reacional (TRIPLE PHASE BOUNDARY - TPB), formada na interface entre eletrólito e eletrodo. Para as temperaturas superiores a 1100 °C, o LSM e o YSZ podem-se reagir formando-se zirconatos de lantânio (La2Zr2O7 - LZO) e zirconatos de estrôncio (SrZrO3 - SZO). Neste sentido, o presente trabalho pretende contribuir no estudo da formação das fases LZO e SZO, estudando diferentes proporções mássicas entre LSM e YSZ e temperaturas de sinterização variando-se entre 1000 °C e 1400 °C. Para obtenção dos pós precursores foram adotadas as rotas de co-precitação para a obtenção do YSZ e mistura convencional de pós para preparação de LSM. Para a preparação do compósito LSM/YSZ, as composições químicas do LSM sintetizado neste trabalho foram para as duas concentrações 30 e 40 % mol de Sr (LSM7 e LSM6) e para YSZ 10 % mol de ítria. Os resultados obtidos por fluorescência de raios X mostraram que as rotas adotadas para síntese de pós foram eficazes na obtenção das composições LSM6, LSM7 e YSZ, com valores próximos aos estequiométricos. As proporções mássicas estudadas foram: 50 % de LSM e 50 % de YSZ (1:1), 25 % de LSM e 75 % de YSZ (1:3) e 75 % de LSM e 25 % de YSZ (3:1). Tais proporções de misturas foram conformadas e submetidas a diferentes condições de temperaturas e tempos de sinterização: 1000 °C, 1200 °C, 1300 °C, 1350 °C e 1400 °C, por 4 e 8 horas. Tanto os valores de tamanho médio das partículas como os valores da área de superfície específica para a mistura de LSM6/YSZ e LSM7/YSZ, após a mistura em um moinho atritor e em diferentes proporções mássicas, são da mesma ordem de grandeza. Na análise por difração de raios X dos compósitos LSM6/YSZ e LSM7/YSZ preparados na proporção 1:1 submetidos às temperaturas a 1200 ºC e a 1400 ºC por 4 horas, verificou-se que não há formação de fases secundárias tais como LZO e SZO. Para estas amostras sinterizadas nestas duas temperaturas, o valor da porosidade obtida é maior para a amostra sinterizada a 1200 ºC. Para as amostras LSM6/YSZ e LSM7/YSZ, com proporções mássicas de 1:3, 1:1 e 3:1, sinterizadas a 1300 °C e 1350 °C, por 8 horas, observou-se o aparecimento de fases de LZO e SZO. Verificou-se também que as intensidades destas fases são maiores para proporção mássica de 3:1, ou seja, para amostras contendo 75 % em massa de LSM no compósito. Para preparação de filmes finos do compósito LSM/YSZ para células a combustível do tipo SOFC, de acordo com as condições adotadas neste trabalho, a temperatura de sinterização adequada é de 1200 °C por tempo abaixo ou igual a 8 horas, sendo que, em tais condições não há formação de fases secundárias como LZO e SZO. / Dissertação (Mestrado) / IPEN/D / Instituto de Pesquisas Energéticas e Nucleares - IPEN-CNEN/SP

Les oxydes sont des matériaux complexes possédant une physique riche et toujours au centre de nombreuses recherches. Parmi ces oxydes, les manganites ont retenu notre attention car ils présentent une transition métal-isolant abrupte en température, générant un très fort coefficient en température en conditions d’environnement standards. L’objectif de ce travail est de démontrer que ce fort coefficient peut être exploité pour l’amélioration des performances des jauges de pression de type Pirani qui subissent un certain essoufflement dans leur développement. La voie menant à l’aboutissement d’une telle jauge à base d’oxydes pose en revanche un certain nombre de limites technologiques à lever et auxquelles nous avons répondu. La première de ces limites concerne l’intégration des oxydes monocristallins sur silicium, que nous avons reproduite et étendue au cas des substrats de type SOI et GaAs. Nos procédés proposent de passer par deux techniques, l’épitaxie par jets moléculaire et l’ablation laser, pour assurer une croissance optimale de nos films sur ces substrats et d’assurer la reproductibilité de leur réponse en température, notamment la position de leur température de transition en accord avec l’état de l’art. L’épitaxie de ces oxydes génère un niveau de contrainte non négligeable qui n’a jamais été mesuré. En concevant divers dispositifs autosupportés, et en s’appuyant sur les considérations théoriques et des modélisations par éléments finis, nous avons pu quantifier la relaxation de cette contrainte importante et assurer près de 100% de reproductibilité des systèmes suspendus. Ces mêmes systèmes nous permettent de caractériser pour la première fois le facteur de jauge des manganites monocristallines par l’application d’une contrainte contrôlée par nanoindentation. Il est également démontré qu’ils constituent des jauges de pression Pirani à la sensibilité accrue de deux ordres de grandeur pour une consommation en puissance réduite. Des solutions permettant d’améliorer l’ensemble des aspects de ces jauges sont étudiées. / Functional perovskite oxides are of great interest for fundamental and applied research thanks to the numerous physical properties and inherent mechanisms they display. With the maturation of thin film deposition techniques, research teams are able to reproduce oxide films and nanostructures of great crystalline quality with some of the most remarkable properties found in physics, a state leading now to upper-level thoughts like their ability to fulfill industrial needs. This thesis work is an answer to some of the problematics that arise when considering the oxide transition from the research to the industrial world, by focusing on their integration for micromechanical devices (MEMS) such as sensors. In order to ease the access to MEMS manufacturing, it is of importance to allow the deposition of thin oxide films on semiconductor substrates. A first study show that these access bridges can be crossed when using appropriate buffer layers such as SrTiO3 deposited on Silicon or gallium arsenide – produced in close collaboration with INL by Molecular Beam Epitaxy - and yttria-stabilized zirconia directly grown on silicon by pulsed laser deposition, which adapts the surface properties of the substrate to perovksite-based materials. Formation of thin epitaxial and monocristalline films of functional oxides is thus allowed on such buffer layers. As an example, characterization of two mixed-valence manganites La0.80Ba0.20MnO3 and La0.67Sr0.33MnO3 demonstrates that both materials are of excellent crystalline quality on these semiconducting substrates and that their physical characteristics match the one found on classical oxide substrates like SrTiO3. Stress evolution in thin films, which has a major effect in epitaxial materials, is then addressed to quantify its impact on oxide microstructure viability. This work gives an identification of the most significant factors favoring stress generation in the case of the films we produced. Then, based on the deformation measurement of free-standing cantilevers made of manganites on pseudo-substrates, and with the support of appropriate analytical models, a new state of equilibrium is established, giving new information about the evolution of static stress from deposition to MEMS device manufacturing. Solutions to manage their reproducibility is then studied. From another perspective, free-standing microstructures made of monocristaline manganites were used to display the effect of dynamical strain on their electrical resistivity (piezoresistivity) and their inherent structures.Finally, a specific example of the capabilities of reproducible free-standing microbridges made of manganites is presented through the conception of a pressure gauge based on Pirani effect. Indeed, it is shown that the abrupt resistivity change this material exhibits near their metal-to-insulating transition creates high temperature coefficients in standard application environments that can be taken as an advantage to improve the sensibility and power consumption of such gauges whose development had significantly slowed down over the past years. A set of improvements on their sensitivity range and their signal acquisition is also presented. Combined to a specific and innovative package, it is also demonstrated that Pirani gauge capabilities can be enhanced and that the complete devices fulfill embedded application requirements.

Electronic structure of transition metal oxides has been a subject of intense research since decades due to the wide spectrum of properties that they exhibit, like high temperature superconductivity, metal-insulator transitions (MIT), phase separation etc. Among these, colossal magnetoresistance (CMR), i.e. a sharp drop in the electrical resistance by the application of an external magnetic field, is a property of fundamental and technological importance. In the present study we investigate several of these interesting properties ranging from colossal magnetoresistance, metal-insulator transitions and phase separation phenomena on a wide range of magnetoresistive systems. All these properties originate in transition metal oxides due to a competition between the strong inter-atomic Coulomb interaction strength within the transition metal d electrons and a large hopping interaction strength between the metal d and oxygen 2p states. In this thesis we report the investigation of the electronic and magnetic structures of some magnetoresistive oxides, including various double perovskites and manganites, using various high energy spectroscopies in conjunction with various theoretical approaches. The samples for the present experimental investigation were prepared by different synthetic routes, such as solid state reaction, nitrate method, d.c arc melting and float zone method, and were characterized by x-ray diffraction, four probe resistivity, magnetic susceptibility, optical absorption and energy dispersive analysis of x-rays while some of the samples were supplied by our collaborators. Various spectroscopic techniques like x-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS) , bremsstrahlung isochromat spectroscopy (BIS), x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism spectroscopy (XMCD) , electron energy loss spectroscopy (EELS), spatially resolved photoelectron spectroscopy and M¨ ossbauer spectroscopy were used to probe the samples. Theoretical methods include configuration interaction cluster approach to fit the XAS and XMCD spectra while ab initio band structure calculations along with the least-square fitting procedure was used to fit some of the valence and conduction bands. Following a general introduction in Chapter 1, the details of various experimental and theoretical techniques are discussed in Chapter 2 of this thesis. Recently, a double perovskite, Sr2FeMoO6, belonging to a general family of halfmetallic ferromagnetic oxides, has shown a spectacularly large magnetoresistance even at the room temperature and at relatively small applied magnetic fields compared to the extensively investigated class of magnetoresistive manganites. Physical properties of this compound is strongly influenced by the Fe -Mo ordering. We hence synthesized Sr2FeMoO6 sample, both with high and low degree of Fe/Mo ordering. Spectroscopic investigations of these samples suggest the presence of Fe rich and Mo rich domains of the type Sr2Fe1+xMo1−xO6 in disordered Sr2FeMoO6 at times. This prompted us to prepare bulk samples of Sr2Fe1+xMo1−xO6. In Chapter 3 we address various issues related to Fe/Mo ordering like saturation magnetization, variation of TC, and CMR as well as oxidation state of Fe and Mo in Sr2FeMoO6using this new series, ”Sr2Fe1+xMo1−xO6” as it offers a better control on the Fe/Mo bonds by controlling x. On the basis of the electron spectroscopic studies in conjunction with a configuration interaction cluster calculation model coupled with the conduction band, we claim that Fe remains in 3+oxidation state throughout the series, where as Mo changes its valency to maintain the charge neutrality. An analysis of the magnetic momentas a function of x suggests that Fe at the ”wrong” crystallographic site is coupled anti-parallel to the Fe moments at the ”correct” site. Additionally, Mo depolarizes to the extend proportional to the number of Mo sites in the near-neighbor co-ordination shell. Continuing with the double perovskites in Chapter 4 we investigate the electronic and magnetic structure of Sr2FeMoO6, Ca2FeMoO6 and Ba2FeMoO6using XAS and XMCD studies. We find that the conventional XAS and XMCD calculations based on configuration interaction of a typical fragment, FeO6in this case, is insufficient to reproduce the experimental spectrum as the compounds considered here are metallic. In order to include the non local charge transfer, we coupled FeO6 octahedra to a conduction band which mimics the Mo band. Within this model we obtained a good fit to the experimental spectrum. Chapter 5 deals with another series of double perovskite (Sr1−yCay)2FeReO6which exhibits a rich phase diagram since it undergoes a metal insulator transition (MIT) with composition at low temperatures. This system becomes more interesting due to the presence of a temperature driven MIT for higher y compositions. We find that the MIT is not related to the change in valency of Fe and Re. Analysis of the near Fermi edge valence band spectra suggests opening up of a soft gap. The main reason for MIT in this system is most likely the presence of strong electron-electron correlation between multiple electrons at the Re site, which is caused by the mismatch of the Re ionic radius and change in the crystal structure across MIT. Another issue which has been extensively investigated in this thesis is phase separation in manganites presented in Chapter 6. We use a spatially resolved, direct spectroscopic probe for electronic structure with an additional unique sensitivity to chemical compositions, to investigate high quality single crystal samples of La1/4Pr3/8Ca3/8MnO3 in the first section. This unique probe establishes the formation of distinct insulating domains embedded in the metallic host at low temperatures, significantly in the absence of any perceptible chemical inhomogeneity, with the domain-size at least an order of magnitude larger than the previous largest estimate. We also provide compelling evidence of memory effects in such domain formation and morphology, suggesting an intimate connection between these electronic domains and long-range strains, often thought to be an important ingredient in the physics of doped manganites. In second part of this chapter we discuss another system namely Eu0.5Y0.5MnO3 which undergoes a chemical phase separation forming alternate stripes of Eu rich (Y deficient) orthorhombic phase and Y rich (Eu deficient) hexagonal phases. These stripes are amazingly straight and run parallel over millimeters. One more system that we investigated is a mixture of ferromagnetic La5/8Sr3/8MnO3and insulating ferroelectric LuMnO3 taken in ratio 3:7, here too the attempt to make a single crystal resulted into a chemical phase separation forming strips of metallic La5/8Sr3/8MnO3and insulating LuMnO3 throughout the sample surface. Preliminary studies suggests that strain between the chemically and crystallographically different species may result into such interesting morphology. In Chapter 7 we study pseudo-one dimensional compounds Sr3CuIrO6 and Sr3ZnIrO6 using photo electron spectroscopy. The experimental results were fitted using band structure calculations with Full Potential Linearized Augmented Plane Wave (FP-LAPW) method.

The aim of the present work was to develop a detailed picture of the electronic and magnetic properties of a number of interesting transition metal compounds. A number of complementary experimental and theoretical techniques have been applied, special emphasis was given to x-ray spectroscopies. The studies led to a number of results, and the following conclusions can be drawn: The influence of the magnetic ground state (high-spin (FeO) vs. low-spin (FeS2)) with respect to the recorded x-ray spectra was investigated. In particular, by performing RXES on the Fe L edge of the two compounds, very different ratios of La / Lβ integrated intensity for excitation energies close to the L2 edge have been observed. This effect has been explained in terms of the magnetic structure of FeO (high spin), which inhibits Coster-Kronig processes. Special attention has been given to the direct investigation of orbital ordering in a three dimensional CMR manganite, namely La7/8Sr1/8MnO3, by means of x-ray linear dichroism (XLD). We obtained, for the first time, rather strong indications that the coherently distorted Jahn-Teller phase in La7/8Sr1/8MnO3 is accompanied by a predominantly cross type (x2-z2) / (y2-z2) orbital ordering. In addition to manganites the double perovskite Sr2FeMoO6 the combined study by means of x-ray spectroscopies, magnetic measurements and theoretical band structure calculations could resolve some points discussed controversially in the literature. Both, paramagnetic measurements as well as core level spectroscopy of the Fe 2p, Fe 3s and the Mo 3d states suggest a mixed iron valence state involving around 30% Fe3+- Mo5+ and 70% Fe2+ - Mo6+ states in highly ordered Sr2FeMoO6. XPS valence band studies reveal that the Fe 3d states are not extremely localized, and we find evidence that charge transfer between Fe 3d and O 2p states plays an essential role.

Transition metal compounds, especially the oxides, containing dn (0 ≤ n ≤ 10) electronic configuration, constitute the backbone of solid state/materials chemistry aimed at realization of novel materials properties of technological importance. Some of the significant materials properties of current interest are spin-polarized metallic ferromagnetism, negative thermal expansion, second harmonic nonlinear optical (NLO) susceptibility, fast ionic and mixed electronic/ionic conductivity for application in solid state batteries, and last but not the least, high-temperature superconductivity. Typical examples for each one of these properties could be found among transition metal oxides. Thus, alkaline-earth metal (A) substituted rare-earth (Ln) manganites, Lnı.xAxMnΟ3, are currently important examples for spin-polarized magnetotransport, ZrV2O7 and ZrW2O8 for negative thermal expansion coefficient, KTiOPO4 and LiNbO3 for second harmonic NLO susceptibility, (Li, La) TiO3 and LiMn2O4 for fast-ionic and mixed electronic/ionic conductivity respectively, and the whole host of cuprates typified by YBa2Cu3O7 for high Tc superconductivity. Solid state chemists constantly endeavour to obtain structure-property relations of solids so as to be able to design better materials towards desired properties. Synthesis coupled with characterization of structure and measurement of relevant properties is a common strategy that chemists adopt for this task. The work described in this thesis is based on such a broad-based chemists' approach towards understanding and realization of novel materials properties among the family of metal oxides. A search for metallic ferro/ferrimagnetism among the transition metal perovskite oxides, metallicity and possibility of superconductivity among transition-metal substituted cuprates and second order NLO susceptibility among metal oxides containing d° cations such as Ti(IV), V(V) and Nb(V) - constitute the main focus of the present thesis. New synthetic strategies that combine the conventional ceramic approach with the chemistry-based 'soft1 methods have been employed wherever possible to prepare the materials. The structures and electronic properties of the new materials have been probed by state-of-the art techniques that include powder X-ray diffraction (XRD) together with Rietveld refinement, electron diffraction, thermogravimetry, measurement of magnetic susceptibility (including magnetoresistance), Mossbauer spectroscopy and SHG response (towards 1064 nm laser radiation), besides conventional analytical techniques for determination of chemical compositions. Some of the highlights of the present thesis are: (i) synthesis of new mixed valent [Mn(III)/Mn(IV)] perovskite-type manganites, ALaMn2O6-y (A = K, Rb) and ALaBMn3O9_y (A = Na, K; B = Ca, Sr) that exhibit ferromagnetism and magnetoresistance; (ii) investigation of a variety of ferrimagnetic double-perovskites that include ALaMnRuO6 (A = Ca, Sr, Ba) and ALaFeVO6 (A = Ca, Sr) and A2FeReO6 (A = Ca, Sr, Ba) providing new insights into the occurrence of metallic and nonmetallic ferrimagnetic behaviour among this family of oxides; (iii) synthesis of new K2NiF4-type oxides, La2-2xSr2XCui.xMxO4 (M = Ti, Mn, Fe, Ru) and investigation of Cu-O-M interaction in two dimension and (iv) identification of the structural rnotif(s) that gives rise to efficient second order NLO optical (SHG) response among d° oxides containing Ti(IV), V(V), Nb(V) etc., and synthesis of a new SHG material, Ba2-xVOSi2O7 having the fresnoite structure. The thesis consists of five chapters and an appendix, describing the results of the investigations carried out by the candidate. A brief introduction to transition metaloxides, perovskite oxides in particular, is presented in Chapter 1. Attention is focused on the structure and properties of these materials. Chapter 2 describes the synthesis and investigation of two series of anion-deficient perovskite oxides, ALaMn2O6-y (A = K, Rb, Cs) and ALaBMn3O9_y (A = Na, K; B = Ca, Sr). ALaMn2O6-y (A = K, Rb, Cs) series of oxides adopt 2 ap x 2 ap superstructure for K and Rb phases and √2 av x √2 ap x 2 ap superstructure (ap = perovskite subcell) for the Cs phase. Among ALaBMn3O9-y phases, the A = Na members adopt a new kind of perovskite superstructure, ap x 3 ap, while the A = K phases do not reveal an obvious superstructure of the perovskite. All these oxides are ferromagnetic (Tc ~ 260-325 K) and metallic exhibiting a giant magnetoresistance behaviour similar to alkaline earth metal substituted lanthanum manganites, Lai_xAxMnO3. However, unlike the latter, the resistivity peak temperature Tp for all the anion-deficient manganites is significantly lower than Tc. In Chapter 3, we have investigated structure and electronic properties of double-perovskite oxides, A2FeReO6 (A = Ca, Sr and Ba). The A = Sr, Ba phases are cubic (Fm3m) and metallic, while the A = Ca phase is monoclinic (P2yn) and nonmetallic. All the three oxides are ferrimagnetic with Tcs 315-385 K as reported earlier. A = Sr, Ba phases show a negative magnetoresistance (MR) (10-25 % at 5 T), while the Ca member does not show an MR effect. 57Fe Mossbauer spectroscopy shows that iron is present in the high-spin Fe3+ (S = 5/2) state in Ca compound, while it occurs in an intermediate state between high-spin Fe2+ and Fe3+ in the Ba compound. Monoclinic distortion and high covalency of Ca-O bonds appear to freeze the oxidation states at Fe+3/Re5+ in Ca2FeRe O6, while the symmetric structure and ionic Ba-O bonds render the FeReO6 array highly covalent and Ba2FeReO6 metallic. Mossbauer data for Sr2FeReO6 shows that the valence state of iron in this compound is intermediate between that in Ba and Ca compounds. It is likely that Sr2FeReO6 which lies at the boundary between metallic and insulating states is metastable, phase-seperating into a percolating mixture of different electronic states at the microscopic level. In an effort to understand the occurrence of metallicity and ferrimagnetism among double perovskites, we have synthesized several new members : ALaMnFeO6 (A = Ca, Sr, Ba), ALaMnRuO6 (A = Ca, Sr, Ba) and ALaVFeO6 (A = Ca, Sr) (Chapter 3). Electron diffraction reveals an ordering of Mn and Ru in ALaMnRuO6 showing a doubling of the primitive cubic perovskite cell, while ALaVFeO6 do not show an ordering. ALaMnRuOs are ferrimagnetic (Tcs ~ 200-250 K) semiconductors, but ALaVFeO6 oxides do not show a long range magnetic ordering . The present work together with the previous work on double perovskites shows that only a very few of them exhibit both metallicity and ferrimagnetism, although several of them are ferrimagnetic. For example, among the series Ba2MReO6 (M = Mn, Fe, Co, Ni), only the M = Fe oxide is both metallic and ferrimagnetic, while M = Mn and Ni oxides are ferrimagnetic semiconductors. Similarly, A2CrMoO6 (A = Ca, Sr), A2CrRe06 (A = Ca, Sr), and ALaMnRuO6 (A = Ca, Sr, Ba) are all ferrimagnetic but not metallic. While ferrimagnetism of double perovskites arise from an antiferromagnetic coupling of B and B' spins through the B-O-B' bridges, the occurrence of metallicity seems to require precise matching of the energies of d-states of B and B' cations and a high covalency in the BB'O6 array that allows a facile electron-transfer between B and B', Bn++B’m+↔B(n+1)++B’(m-1)+ without an energy cost, just as occurs in ReO3 and other metallic ABO3 perovskites. In an effort to understand the Cu-O-M (M = Ti, Mn, Fe, Ru) electronic interaction in two dimension, we have investigated K2N1F4 oxides of the general formula La2-2xSr2XCui.xMxO4 (M = Ti, Mn, Fe or Ru). These investigations are described in Chapter 4. For M = Ti, only the x = 0.5 member could be prepared, while for M = Mn and Fe, the composition range is 0 < x < 1.0, and for M = Ru, the composition range is 0 < x ≤ 0.5. There is no evidence for ordering of Cu(II) and M(IV) in the x = 0.5 members. While the members of the M = Ti, Mn and Ru series are semiconducting/insulating, the members of the M = Fe series are metallic, showing a broad metal-semiconductor transition around 100 K for 0 < x ≤ 0.15 that is possibly related to a Cu(II)-O-Fe(IV) < > Cu(III)-O-Fe(III) valence degeneracy. Increasing the strontium content at the expense of lanthanum in La2-2xSr2XCui.xFexO4 for x ≤ 0.20 renders the samples metallic but not superconducting. In a search for inorganic oxide materials showing second order nonlinear optical (NLO) susceptibility, we have investigated several borates, silicates and phosphates containing /ram-connected MO6 octahedral chains or MO5 square-pyramids, where M = d°: Ti(IV), Nb(V) or Ta(V). Our investigations, which are described in Chapter 5, have identified two new NLO structures: batisite, Na2Ba(TiO)2Si4O12, containing trans-connectd TiO6 octahedral chains, and fresnoite, Ba2TiOSi2O7, containing square-pyramidal T1O5. Investigation of two other materials containing square-pyramidal TiO5, viz., Cs2TiOP2O7 and Na4Ti2Si8O22. 4H2O, revealed that isolated TiO5 square-pyramids alone do not cause a second harmonic generation (SHG) response; rather, the orientation of T1O5 units to produce -Ti-O-Ti-O- chains with alternating long and short Ti-0 distances in the fresnoite structure is most likely the origin of a strong SHG response in fresnoite. Indeed, we have been able to prepare a new fresnoite type oxide, Ba2.xVOSi2O7 (x ~ 0.5) that shows a strong SHG response, confirming this hypothesis. In the Appendix, we have described three synthetic strategies that enabled us to prepare magnetic and NLO materials. We have shown that the reaction CrO3 + 2 NH4X > CrO2 + 2 NH3 + H2O + X2 (X = Br, I), which occurs quantitatively at 120-150 °C, provides a convenient method for the synthesis of CrO2. Unlike conventional methods, the method described here does not require the use of high pressure for the synthesis of this technologically important material. For the synthesis of magnetic double perovskites, we have developed a method that involves reaction of basic alkali metal carbonates with the acidic oxides (e.g. Re2O7) first, followed by reaction of this precursor oxide with the required transition metal/transition metal oxide (e.g. Fe/Fe2O3). By this method we have successfully prepared single-phase perovskite oxides, A2FeReO6, ACrMoO6 and ALaFeVO6. We have prepared the new NLO material Ba2_xV0Si207 from Ba2VOSi2O7 by a soft chemical redox reaction involving the oxidation of V(IV) to V(V) using Br2 in CH3CN/CHCI3. Ba2V0Si207 + 1/2 Br2 > Bai.5V0Si207 + 1/2 BaBr2. The work presented in this thesis was carried out by the candidate as part of the Ph.D. training programme. He hopes that the studies reported here will constitute a worthwhile contribution to the solid state chemistry of transition metal oxides and related materials.