Electronic inhomogeneities play an important role in the definition of physical properties of correlated systems. To study these inhomogeneities one has to use local probe techniques which can distinguish electronic, magnetic and structural variations at the nanoscale. In the present work the high-field electron spin resonance technique (HF-ESR) is used to probe electronic and magnetic inhomogeneities in two transition-metal element based systems with very different properties. The first system is an iron based hightemperature superconductor, namely a member of a so called 1111-family, the (La,Gd)O1−xFxFeAs compound. Our HF-ESR spectroscopy study on Gd3+ ion has revealed that this material exhibits anisotropic interaction between Gd and Fe layers, which is frustrated in the absence of an external magnetic field. Moreover, the study of the superconducting samples has shown a coexistence of a static short range magnetic order with superconductivity up to high doping levels. The second system is a lightly hole doped cubic perovskite LaCoO3. Here, our HF-ESR investigation, complemented with static magnetometry and nuclear magnetic resonance techniques, has established that the hole doping induces a strong interaction between electrons on neighboring Co ions which leads to a collective high-spin state, called a spin-state polaron. These polarons are inhomogeneously distributed in the nonmagnetic matrix. This thesis is organized in three chapters. The first chapter gives basic ideas of magnetism in solids, focusing on the localized picture. The aim of the second chapter is to introduce the method of ESR. The third chapter is dedicated to the study of 1111-type iron arsenide superconductors. In the first part X-band (9.5 GHz) ESR measurements on 2% and 5% Gd-doped LaO1−xFxFeAs are presented. In the second part a combined investigation of the properties of GdO1−xFxFeAs samples by means of thermodynamic, transport and high-field electron spin resonance methods is presented. The last, fourth chapter presents the investigation of the unexpected magnetic properties of lightly hole-doped LaCoO3 cobaltite by means of the electron spin resonance technique complemented by magnetization and nuclear magnetic resonance measurements.

Electronic inhomogeneities play an important role in the definition of physical properties of correlated systems. To study these inhomogeneities one has to use local probe techniques which can distinguish electronic, magnetic and structural variations at the nanoscale. In the present work the high-field electron spin resonance technique (HF-ESR) is used to probe electronic and magnetic inhomogeneities in two transition-metal element based systems with very different properties. The first system is an iron based hightemperature superconductor, namely a member of a so called 1111-family, the (La,Gd)O1−xFxFeAs compound. Our HF-ESR spectroscopy study on Gd3+ ion has revealed that this material exhibits anisotropic interaction between Gd and Fe layers, which is frustrated in the absence of an external magnetic field. Moreover, the study of the superconducting samples has shown a coexistence of a static short range magnetic order with superconductivity up to high doping levels. The second system is a lightly hole doped cubic perovskite LaCoO3. Here, our HF-ESR investigation, complemented with static magnetometry and nuclear magnetic resonance techniques, has established that the hole doping induces a strong interaction between electrons on neighboring Co ions which leads to a collective high-spin state, called a spin-state polaron. These polarons are inhomogeneously distributed in the nonmagnetic matrix. This thesis is organized in three chapters. The first chapter gives basic ideas of magnetism in solids, focusing on the localized picture. The aim of the second chapter is to introduce the method of ESR. The third chapter is dedicated to the study of 1111-type iron arsenide superconductors. In the first part X-band (9.5 GHz) ESR measurements on 2% and 5% Gd-doped LaO1−xFxFeAs are presented. In the second part a combined investigation of the properties of GdO1−xFxFeAs samples by means of thermodynamic, transport and high-field electron spin resonance methods is presented. The last, fourth chapter presents the investigation of the unexpected magnetic properties of lightly hole-doped LaCoO3 cobaltite by means of the electron spin resonance technique complemented by magnetization and nuclear magnetic resonance measurements.

Made available in DSpace on 2014-12-17T15:42:27Z (GMT). No. of bitstreams: 1 WashingtonCMG_TESE.pdf: 1906631 bytes, checksum: 6b7dc12db13ffeb89c11288d55cea5a8 (MD5) Previous issue date: 2013-09-09 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / In this work, the structures of LaCoO3, La0,8Ba0,2CoO3 and La0,8Ca0,2CoO3 perovskites were characterized as a function of temperature (LaCoO3 structure being analyzed only at room temperature). The characterization of these materials were made by X-Ray Absorption Spectroscopy (XAS), in the cobalt K-edge, taking into account the correlated Einstein model X-ray absorption fine structure (EXAFS). The first part of the absorption spectrum corresponded the X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). These materials were prepared by the combustion method. The combustion products were calcinated at 900 0C, for 6 hours in air. Noted that the sample LaCoO3 at room temperature and samples doped with Calcium and Barium in the temperature range of 50 K to 298 K showed greater distortion to monoclinic symmetry with space group I2/a. However, the sample doped with barium at the temperatures 50 K, 220 K, and 260 K showed a slight distortion to rhombohedral symmetry with space group R-3c. The La0,8Ca0, 2CoO3 structure was few sensitive to temperature variation, showing a higher local distortion in the octahedron and a higher local thermal disorder. These interpretations were in agreement with the information electronic structural on the XANES region and geometric in the EXAFS region / Neste trabalho foram investigados estruturas de perovskitas LaCoO3, La0,8Ba0,2CoO3 e La0,8Ca0,2CoO3 em fun??o de temperatura, sendo que a estrutura LaCoO3 foi analisada apenas em temperatura ambiente. As caracteriza??es destes materiais foram realizadas pela espectroscopia de absor??o de raios X (XAS) na borda K do cobalto levando em considera??o modelo de Einstein correlacionado na estrutura fina de absor??o (EXAFS). A primeira parte do espectro de absor??o de raios X corresponde absor??o de raios X pr?xima ? borda de absor??o (XANES) e a outra estende ? espectroscopia da estrutura fina de absor??o (EXAFS). Estes materiais foram preparados pelo m?todo de combust?o. Os produtos obtidos da combust?o foram tratados termicamente por 9000C por 6 horas nesses materiais. Observou que a amostra LaCoO3 em temperatura ambiente e as amostras dopadas com C?lcio e B?rio na faixa de temperatura entre 50 K a 298 K apresentaram uma maior distor??o com simetria monocl?nica com grupo espacial I2/a. No entanto, a amostra dopada com B?rio nas temperaturas 50 K, 220 K e 260 K mostrou uma leve distor??o com simetria rombo?drica com grupo espacial R-3c. A estrutura La0,8Ca0,2CoO3 foi pouco sens?vel com a varia??o de temperatura, apresentando uma maior distor??o local no octaedro e uma maior desordem t?rmica local. Estas interpreta??es est?o de acordo com as informa??es estruturais eletr?nica na regi?o XANES e geom?trica na regi?o EXAFS

Perovskite based materials have great potentials for various energy applications and the search for new materials for uses in SOFCs has largely been concentrated to this class of compounds. In this search, we have studied perovskite phases in the system La2Co1+z(Ti1-xMgx)1-zO6, with 0  x 0.9 and z = 0.0, 0.2, 0.4, 0.6. Crystal structures were characterized by XRD and, for selected compositions, also by NPD and SAED. They exhibit with increasing x, as well as increasing z, a progressive increase in symmetry from monoclinic to orthorhombic to rhombohedral. The main focus in this work has been on the investigation of structure-property relations for compositions with 0.0 x 0.5 and z = 0. The nominal oxidation state of Co increases for these with increasing x, from Co2+ for x = 0 to Co3+ for x = 0.5. Magnetic measurements and XANES studies showed that the average spin state of Co changes linearly with increasing x, up to x = 0.5, in accordance with varying proportions of Co with two fixed oxidation states, i.e. Co2+ and Co3+. The data suggests that the Co3+ ions have an IS spin state or a mixture of LS and HS spin states for all compositions with nominally only Co2+ and Co3+ ions, possibly with the exception of the composition with x = 0.1, 0.2 and z = 0, for which the data indicate that the spin state might be HS. The XANES data indicate furthermore that for the perovskite phases with z = 0 and x > 0.5, which in the absence of O atom vacancies contain formally Co4+, the highest oxidation state of Co is Co3+, implying that the substitution of Ti4+ by Mg2+ for x ³ 0.5 effects an oxidation of O2- ions rather than an oxidation of Co3+ ions. The thermal expansion was found to increase nearly linearly with increasing oxidation state of Co. This agrees well with findings in previous studies and is attributable to an increase in the ionic radius of Co3+ ions with increasing temperature, due to a thermal excitation from a LS to IS or LS/HS spin states. High temperature electronic conductivity measurements indicate that the electronic conductivity increases with an increase of both relative and absolute amount of Co3+. The latter can be attributed to an increase in the number of Co-O-Co connections. Additional high temperature magnetic measurements for selected samples, whose susceptibilities did not follow a Curie law behaviour up to room temperature, showed effective magnetic moments that did approach plateaus even at high temperatures (900 K). Interpretations of these data are, however, hindered by the samples losing oxygen during the applied heating-cooling cycle. The present study has shown that the investigated system is suitable for further studies, of more fundamental character, which could provide further insight of the structure-property relationships that depend on the oxidation state of Co. / Studies of cobalt based perovskites for cathode materials in solid oxide fuel cells.

Transition metal oxides have proven to be a fertile research area for condensed matter physicists due to the fascinating array of superconducting, magnetic and electronic properties they exhibit. A particular resurgence of intense activity in investigating the properties of these systems followed the discovery of high temperature superconductivity in the cuprates, colossal magnetoresistance in the manganites, ferroelectricity in the cobaltites and simultaneous ferroelectric and ferromagnetic ordering in the manganites. These diverse properties of transition metal compounds arise due to the presence of strong electron-electron interactions within the transition element 3d states. Indeed, it is the competition between the localizing effects of such interactions and the comparable hopping strengths driving the system towards delocalization, that is responsible for these wide spectrum of interesting properties. In terms of theoretical and fundamental issues, electronic structure of transition metal oxides play a most important role, providing a testing ground for new many-body theoretical approaches treating the correlation problem at various levels of approximations. In addition to this rich spectrum of properties, metal-insulator transitions often occur and can even be coincident with structural or magnetic changes due to the strong coupling between charge, magnetic and lattice degrees of freedom. However, in spite of the immense activities in this area, the underlying phenomena is not yet completely understood. A careful investigation of the electronic structure of these systems will help in the microscopic understanding of these and photoelectron spectroscopy has been established as the most powerful tool for investigating the electronic structures of these systems. In this thesis we investigate the electronic structures of some of these transition metal oxides and the metal-insulator transition as a function of electron correlation strength and doping of charge carriers by means of photoelectron spectroscopy; we analyze the experimental results using various theoretical approaches, in order to obtain detailed and quantitative understandings. This thesis is organized into seven chapters. Chapter 1 is a general introduction to the various concepts discussed in this thesis. Here we briefly describe the various mechanisms and theoretical formalisms used for understanding the metal-insulator transitions in strongly correlated systems and the evolution of the electronic structure across the transition. The experimental and the calculational techniques used in this thesis is described in Chapter 2. This includes different sample synthesis techniques and the characterization tools used in the present study. Photoelectron spectroscopic techniques used for probing the electronic structure of various systems are also discussed in this chapter. In Chapter 3, we discuss the coexistence of ferromagnetism and superconductivity in ruthenocuprates by looking at the electronic structures of RuSr2Eu1.5Ce0.5Cu2O10 which is a ferromagnetic superconductor having the ferromagnetic TC ~ 100 K and a superconducting transition of ~ 30 K compared with RuSr2EuCeCu2O10 which is a ferromagnetic (TC ~ 150 K) insulator in conjunction with two reference systems, RuSr2GdO6and Sr2RuO4. The coexistence of ferromagnetic order with superconductivity below the superconducting temperature is an interesting issue since the pair-breaking due to magnetic interactions is not significant in these cases. Extensive photoelectron spectroscopic measurements were performed on these systems and our results show that Eu and Ce in both the ruthenocuprates exists in 3+ and 4+ states, respectively. Also the analysis of the Ru 3d and 3p core levels suggests that Ru remains in the pentavalent state in both the cases. The constancy of Ru valency with doping of charge carriers that bring about an insulator to metal transition and the superconducting state suggests that the electronic structure and transport properties of these compounds are not governed by the Ru-O plane, but by the Cu-O plane, much as in the case of other high TC cuprates. Analysis of the Cu 2p core level spectra in terms of a cluster model, including configuration interaction and multiplet interactions between Cu 3d and 2p as well as that within the Cu 3d states, establish a close similarity of the basic electronic structure of these ruthenocuprates to those of other high TC cuprates. Here the charge transfer energy, Δ << Udd,Cu 3d multiplet-averaged Coulomb repulsion energy, establishing the compounds to be deep in the charge transfer regime. Continuing with the ruthenocuprate systems in Chapter 4, we look at the electronic structure of hole doped La2CuRuO6systems using various photoemission techniques. It was expected that since the substitution of La3+by Sr2+changes the d electron count, the system will undergo a metal to insulator transition, but the transport properties show that all of them remain semiconducting through out the lowest temperature of measurement. A careful analysis of the Ru 3d and 3p core level spectra shows that Ru exists in Ru 4+state in La2CuRuO6and goes towards Ru 5+state with hole doping. This suggests that the doped holes affects the electronic structure of the Ru levels in these systems. A spectral decomposition of the Ru 3d core level suggests the existence of a spin orbit split doublet having two peaks, a main core level peak and a satellite peak at the higher binding energy side of the main peak and the intensity ratio of the satellite peak to the main peak increases with the insulating nature of the compounds as reported for other Ru 4d strongly correlated systems. This observation is also consistent with the transport properties. Cu 2p core level spectra also shows variations in the satellite-to-main peak Cu 2p intensities suggesting that the electronic structure of the Cu levels are also getting affected with Sr doping. Valence band spectral features near the Fermi level shows that the spectral weight is highest for La2CuRuO6and depletes slowly with Sr doping consistent with the expected d electron count as suggested by the Ru valencies. In Chapter 5 and Chapter 6 we discuss the electronic structure investigations of two early transition metal oxide series, namely Ca1−xSrxVO3and Ce1−xSrxTiO3. Surface sensitivity dependence of photoemission experiments has been explored to show that the surface and the bulk electronic structures of Ca1−xSrxVO3system is different. Photoemission spectra of this system using synchrotron radiation reveal a hither to unnoticed polarization dependence of the photoemission matrix elements for the surface component leading to substantial underestimation. Extracted bulk spectra from experimentally determined electron escape depth and underestimation of surface contributions resolve the puzzling issues that arose due to the recent diverse interpretations of the electronic structure in Ca1−xSrxVO3. Keeping in mind the above-mentioned caveat, the present results still clearly establish that the linear polarization of synchrotron radiation plays a key role in determining the spectral lineshape in these systems. The experimentally-determined bulk spectra provide an understanding of the electronic structure in Ca1−xSrxVO3, consistent with experimental γ values, calculated change in the d-bandwidth and the geometrical/structural trends across the series, thereby resolving the puzzle concerning the structure-property relationship in this interesting class of compounds. In Chapter 6 we discuss the issues of metal-insulator transition close to the d0limit as well as the evolution of the electronic structure of a strongly correlated system as a function of electron occupancy, by investigating the family of Ce1−xSrxTiO3compounds by recording core level as well as valence band photoemission spectra using lab source as well as synchrotron radiations. Core level Ce 3d spectra from Ce1−xSrxTiO3samples establish a trivalent state of Ce in these compounds for all values of x confirming that charge doping in the present system does not alter the electronic structure of Ce. Hence the change in valency due to Sr substitution and thus, the carrier number, takes place only in the Ti 3d-O 2p manifold. We also carried out extensive VUV photoemission experiments on these samples with the photon energy varying between 26-122 eV. From the difference spectrum obtained by subtracting the off-resonance spectrum from the on-resonance one, we obtain the Ce 4f spectral signature; thus obtained Ce 4f spectrum which has a peak at about 3 eV binding energy and shows no intensity at EF even for the metallic samples, consistent with a Ce3+state. In order to study the states near EF responsible for the metal-insulator transition in these compounds, we recorded the valence band spectra at the Ce 4f off-resonance condition so that the coherent and the incoherent spectral features arising from the Ti 3d states could be clearly resolved, allowing us to investigate the metal insulator transition in the Ce1−xSrxTiO3system as a function of Sr or hole doping. The experimental spectra of the metallic compounds exhibit an intensity of the incoherent feature considerably larger than that predicted by theory. This discrepancy is possibly due to a difference in the surface and the bulk electronic structures of these compounds. Chapter 7 is divided into two parts. In the first part we discuss the extended x-ray absorption fine structure (EXAFS) studies performed on two transition metal oxide series, La1−xSrxCoO3and La1−xSrxFeO3to look at the local structure distortions happening around the transition metal ions and its role in bringing out metal to insulator transitions in transition metal oxide systems. Here we chose to investigate these two systems since La1−xSrxCoO3undergoes an insulator to metal transition for x ∼ 0.15 and La1−xSrxFeO3remains insulating for the entire range of doping. The static mean square relative displacement, which we believe to be a representation of the disorder present in the system, extracted by fitting the experimental data by a correlated Einstein model, as a function of composition in La1−xSrxCoO3saturates beyond the critical composition where as the disorder parameter continues to increase through out the entire doping range in the case of La1−xSrxFeO3where metal-insulator transition is absent. In the second part of Chapter 7 we discuss the x-ray absorption near edge structure (XANES) studies performed on the above mentioned series of systems. Co K-edge XANES spectra of La1−xSrxCoO3show that there is a systematic shift of the main absorption peak with hole doping suggesting that the Co valency changes systematically with Sr doping. Also, the pre-edge feature of LaCoO3shows the transitions to t2g level clearly showing that Co3+in LaCoO3is not in a pure low spin (t6 2g) state. The Fe K-edge XANES spectra of La1−xSrxFeO3also exhibit a systematic shift to the higher energy side with increase in Sr content, indicating an increase in the Fe valence. Also from the La L3edge analysis, it can be concluded that the oxygen environment around La and the electronic configuration of La are systematically changing with Sr doping.