Comme de nombreux autres oxydes de métaux de transition lamellaires, les cobaltates dopés au sodium, NaxCoO2, présentent un riche diagramme de phase. Les nombreuses instabilités (magnétiques, de charge) qui les caractérisent seraient notamment le fruit des corrélations électroniques. Dans cette thèse, nous nous intéressons au cas fortement dopé x=2/3 (proche de la limite de l'isolant de bande). Expérimentalement, ce composé est sujet à une disproportion de charge locale sur les atomes de cobalt, ce qui en fait un terrain d'étude privilégié pour le calcul des fonctions de corrélation de charge. Le traitement théorique de ce système est difficile a bien des égards. D'abord, la corrélation électronique demande de recourir à des approximations avancées. Ensuite, le système est sensible aux détails microscopiques, tels que transcrits dans la structure électronique réelle. Dans cette thèse, nous abordons ces deux aspects, à travers un approche sur modèle et une approche ab initio.Nous examinons d'abord l'effet des corrélations au niveau d'un modèle sur réseau triangulaire, pertinent pour les cobaltates. La compétition entre les interactions de Coulomb locale et non-locale sur un modèle de Hubbard étendu donne lieu à des fluctuations de charge, que nous traitons grâce à la théorie du champ moyen dynamique étendue. Nous dressons le diagramme de phase de notre modèle en fonction des interactions locales et non-locales. Celui-ci présente une transition du second ordre entre un état métallique homogène et une phase ordonnée de charge. Nous calculons les observables à une et deux particules dans la phase homogène, et nous déterminons l'effet des corrélations sur ces deux types d'observables. Nous interprétons ces résultats comme des conséquences du fort dopage. Nous identifions une région du diagramme de phase où la partie statique de l'interaction de Coulomb écrantée devient négative. Enfin, nous montrons comment incorporer un terme de Fock non-local à ces calculs, et nous détaillons son effet sur le diagramme de phase et sur les observables physiques.En complément de notre approche sur modèle, nous étudions les détails microscopiques du matériau réel grâce à la théorie de la fonctionnelle de la densité. Nous analysons le rôle de l'hybridation avec l’oxygène et des processus de saut électronique sur la structure de bandes pour les plans de cobalt-oxygène. Nous clarifions l'effet de certains paramètres physiques, tels que le dopage au sodium, l'arrangement cristallin ou le magnétisme, sur la structure électronique. Puis nous calculons la susceptibilité de charge selon les premiers principes, selon des approximations suggérées par notre étude de modèle sur réseau.La comparaison entre les calculs sur modèle et ab initio montre que ces approches se complètent l'une l'autre. D'une part, le modèle permet de dégager les approximations pertinentes pour le calcul des fonctions de corrélation de charge. D'autre part, les calculs ab initio et la compréhension des processus microscopiques sont le préalable à la construction d'un modèle réaliste et prédictif. / As many other layered transition-metal oxides, sodium-doped cobaltates, NaxCoO2, present a rich phase diagram. They display numerous instabilities (magnetic, charge-order), originating most likely from electronic correlations. In this thesis, we focus on the case x=2/3, in the strong-doping limit (close to the band-insulating limit). An experimentally-observed charge disproportionation on the cobalt atoms makes this particular composition an interesting playground to study charge-correlation functions. The theoretical treatment of this system is difficult in several aspects. In order to capture electronic correlations, one needs to resort to advanced non-perturbative approaches. Also, the system is sensitive to its microscopic details, as encoded in the real electronic structure. This is why, in this thesis, we adopt model as well as ab initio approaches to address both these issues.We first study the effect of correlations at the model level, on a triangular lattice, specifically designed for the cobaltates. The interplay between local and non-local Coulomb interactions gives rise to charge fluctuations, which we capture using the Extended Dynamical Mean-Field Theory. We establish the phase diagram of our model as a function of local and non-local interactions. It displays a second-order phase transition between a homogeneous metallic phase, and a charge-ordered phase. We compute one and two-particle observables in the homogeneous phase, and we analyse how they are influenced by correlations. We show that our findings can be understood as a consequence of the strong doping. We find a region of the phase diagram where the static screened Coulomb interaction becomes negative. Finally, we show how to incorporate a non-local Fock term in the calculation of the self-energy, and how it influences the phase diagram and the physical observables.Next, in order to complement the model approach, we study the microscopic details of the real sodium-cobaltates by using Density-Functional Theory. We analyse the effect of oxygen hybridisation and electronic hopping processes on the band structure of cobalt-oxygen planes. We study the effects of sodium doping, crystal structure and magnetism, on the band structure of this material. Then, we compute the charge susceptibility from first principles in the independent-particle approximation and in the random-phase approximation. We use the insight gained from the model calculation to discuss these results.The comparison between the model and the ab initio calculations shows that these approaches are complementary. The model enables us to define the best approximations for the calculation of charge correlation functions. The ab initio calculations, and the detailed understanding of microscopic electronic processes are the prerequisite for a model that is both realistic and predictive.

Strong electron-electron correlations and lattice frustration are two physical interactions that pose serious challenges to condensed matter physics. A variety of exotic physical phenomena, for example, charge ordering, spin liquid, and unconventional superconductivity, are believed to arise from the interplay of the two interactions. In this dissertation, I examine two families of systems which exhibit both electron-electron correlations and lattice frustration – charge transfer solids and layered cobaltates. The half-filled band Hubbard model on the triangular lattice has been proposed by mean-field theories as the minimal model for the superconductivity in the charge transfer solids. In the first part of this dissertation, by using exact calculations, I prove the absence of superconductivity in this model. This result calls for a new theoretical approach to describe the rich physics in charge transfer solids. In the second part of this dissertation, I study charge transfer solids by focusing on its real bandfilling ¼. I show that a new kind of insulating phase, paired electron crystal, emerges from antiferromagnetism as the frustration is increased. The paired electron crystal state can explain the various insulating states adjacent to the superconducting state, thus provides a new avenue towards the understanding of the unconventional superconductivity in charge transfer solids and other ¼ filled systems. In the third part of this dissertation, I investigate the carrier concentration-dependent electronic behavior in layered cobaltates. I provide a natural yet simple explanation for this behavior. I show that it can be described within correlated-electron Hamiltonians with finite on-site and significant nearest neighbor hole-hole Coulomb repulsions. I also point out the similarities between organic charge transfer solids and layered cobaltates, which may involve superconductivity.

The synthesis of NaxCoO2 by Pechini method (with initial sodium stoichiometry x = 1.0, 0.9, 0.8, and 0.7) was studied for the samples heat-treated in the temperature range from 550 řC to 750 řC. The structure characterization and phase composition was carried out by the powder X-ray diffraction (XRD) analysis. The elementary analysis was done using atomic absorption spectroscopy. From the thermoelectric properties, the Seebeck coefficient was measured for both α- and γ- phase. The Weiss constant was determined for α- and γ- phase by linear regression of 1/χ. The real sodium content in sodium cobaltate phase was found to be independent on initial sodium precursor weight and measurement by PXRD and Seebeck coefficient showed that the sodium content in NaxCoO2 falls in the range from x = 0.65 to 0.75. The magnetization measurements showed presence of spinel Co3O4 impurities in the samples and the analysis of magnetic susceptibility showed unusual high values of the Weiss constant for α- phase in comparison with structurally similar γ- NaxCoO2. Thin film of NaxCoO2 with initial sodium stoichiometry x = 1.0, 0.52, 0.32, and 0.175 was deposited by chemical solution deposition method using spin-coating deposition on a 00l-oriented α-Al2O3 single crystal substrate. The formation and structure characterization of...

Šiame darbe vienfazis perovskito struktūros gadolinio aliuminatas GdAlO3 zolių-gelių metodu buvo gautas 1000 oC temperatūroje, gadolinio jonų šaltiniu naudojant Gd2O3, kompleksus sudarančiu reagentu – 1,2 etandiolį, natūralią vyno rūgštį, citrinų rūgštį arba citrinų rūgšties ir etanolamino mišinį. Sintetinant GdAlO3, gautų tyrimų rezultatai parodė, kaip yra svarbu zolių gelių procese pasirinkti tinkamas pradines medžiagas bei kompleksus su metalais sudarančius reagentus: gadolinio jonų šaltiniu naudojant Gd2O3 buvo gauti geresni rezultatai nei naudojant Gd(NO3)3•6H2O, vienfazis GdAlO3 gali būti sėkmingai susintetintas kompleksus sudarančiu reagentu naudojant tiek natūralią vyno rūgštį, tiek 1,2 etandiolį, tiek citrinų rūgštį, tiek citrinų rūgšties ir etanolamino mišinį. Nustatyta, kad sintetinant stronciu pakeistus gadolinio aliuminatus Gd1–xSrxAlO3– (x = 0,10, 0,25, 0,50, 0,75), tinkamesnis gadolinio jonų šaltinis yra gadolinio oksidas nei gadolinio nitratas heksahidratas, kompleksus sudarantis reagentas – EDTA nei natūrali vyno rūgštis. Vienfazis perovskito struktūros stronciu pakeistas gadolinio aliuminatas Gd0,90Sr0,10AlO3– buvo susintetintas zolių-gelių metodu 1000 oC temperatūroje, gadolinio jonų šaltiniu naudojant Gd2O3, stroncio jonų šaltiniu – Sr(NO3)2, kompleksus sudarančiu reagentu – EDTA. Be to, 800 ºC iškaitinimo temperatūra yra per maža šiems junginiams sintetinti. Pirmą kartą pasiūlytas vandeninis zolių gelių metodas lantano ir gadolinio kobaltatams... [toliau žr. visą tekstą] / In the present thesis the reported results let us to conclude that the developed aqueous sol–gel procedure could be successfully used for the low-temperature synthesis of monophasic perovskite gadolinium aluminate ceramics. It was also demonstrated that the selection of raw materials for the sol–gel processing should be done with care. Monophasic perovskite structure gadolinium aluminate GdAlO3 has been synthesized by sol gel synthesis method at 1000 oC, using gadolinium oxide as gadolinium raw material and 1.2 ethanediol, natural tartaric acid, citric acid or mixture of citric acid and ethanolamine as complexing agent. According to the XRD results, the Gd2O3 instead of Gd(NO3)36H2O preferably should be used as starting material for the preparation of GdAlO3. Monophasic perovskite structure strontium substituted gadolinium aluminate Gd0.90Sr0.10AlO3– was synthesized by the same sol gel synthesis method at 1000 oC, using gadolinium oxide as gadolinium raw material, strontium nitrate as strontium ion source and EDTA as complexing agent. It was proved that better gadolinium ion source is gadolinium oxide than gadolinium nitate hexahydrate, better complexing agent – EDTA than natural tartaric acid in the synthesis of GdAlO3 and strontium substituted Gd1–xSrxAlO3– (x = 0.10, 0.25, 0.50, 0.75) samples. According to the XRD results, increasing amount of strontium reduces amount of peaks of perovskite structure gadolinium aluminate. Besides, 800 ºC temperature is too low for the... [to full text]

In the present thesis the reported results let us to conclude that the developed aqueous sol–gel procedure could be successfully used for the low-temperature synthesis of monophasic perovskite gadolinium aluminate ceramics. It was also demonstrated that the selection of raw materials for the sol–gel processing should be done with care. Monophasic perovskite structure gadolinium aluminate GdAlO3 has been synthesized by sol gel synthesis method at 1000 oC, using gadolinium oxide as gadolinium raw material and 1.2 ethanediol, natural tartaric acid, citric acid or mixture of citric acid and ethanolamine as complexing agent. According to the XRD results, the Gd2O3 instead of Gd(NO3)36H2O preferably should be used as starting material for the preparation of GdAlO3. Monophasic perovskite structure strontium substituted gadolinium aluminate Gd0.90Sr0.10AlO3– was synthesized by the same sol gel synthesis method at 1000 oC, using gadolinium oxide as gadolinium raw material, strontium nitrate as strontium ion source and EDTA as complexing agent. It was proved that better gadolinium ion source is gadolinium oxide than gadolinium nitate hexahydrate, better complexing agent – EDTA than natural tartaric acid in the synthesis of GdAlO3 and strontium substituted Gd1–xSrxAlO3– (x = 0.10, 0.25, 0.50, 0.75) samples. According to the XRD results, increasing amount of strontium reduces amount of peaks of perovskite structure gadolinium aluminate. Besides, 800 ºC temperature is too low for the... [to full text] / Šiame darbe vienfazis perovskito struktūros gadolinio aliuminatas GdAlO3 zolių-gelių metodu buvo gautas 1000 oC temperatūroje, gadolinio jonų šaltiniu naudojant Gd2O3, kompleksus sudarančiu reagentu – 1,2 etandiolį, natūralią vyno rūgštį, citrinų rūgštį arba citrinų rūgšties ir etanolamino mišinį. Sintetinant GdAlO3, gautų tyrimų rezultatai parodė, kaip yra svarbu zolių gelių procese pasirinkti tinkamas pradines medžiagas bei kompleksus su metalais sudarančius reagentus: gadolinio jonų šaltiniu naudojant Gd2O3 buvo gauti geresni rezultatai nei naudojant Gd(NO3)3•6H2O, vienfazis GdAlO3 gali būti sėkmingai susintetintas kompleksus sudarančiu reagentu naudojant tiek natūralią vyno rūgštį, tiek 1,2 etandiolį, tiek citrinų rūgštį, tiek citrinų rūgšties ir etanolamino mišinį. Nustatyta, kad sintetinant stronciu pakeistus gadolinio aliuminatus Gd1–xSrxAlO3– (x = 0,10, 0,25, 0,50, 0,75), tinkamesnis gadolinio jonų šaltinis yra gadolinio oksidas nei gadolinio nitratas heksahidratas, kompleksus sudarantis reagentas – EDTA nei natūrali vyno rūgštis. Vienfazis perovskito struktūros stronciu pakeistas gadolinio aliuminatas Gd0,90Sr0,10AlO3– buvo susintetintas zolių-gelių metodu 1000 oC temperatūroje, gadolinio jonų šaltiniu naudojant Gd2O3, stroncio jonų šaltiniu – Sr(NO3)2, kompleksus sudarančiu reagentu – EDTA. Be to, 800 ºC iškaitinimo temperatūra yra per maža šiems junginiams sintetinti. Pirmą kartą pasiūlytas vandeninis zolių gelių metodas lantano ir gadolinio kobaltatams... [toliau žr. visą tekstą]

Orientador: Iris Concepcion Linares de Torriani / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-08T13:36:14Z (GMT). No. of bitstreams: 1 Aguero_OscarEnrique_D.pdf: 9568417 bytes, checksum: db4b9632a94f1d1cb2691c4eeb1404ec (MD5) Previous issue date: 2007 / Resumo: Uma serie de compostos ineditos ou pouco estudados foram sintetizados e as propriedades físicas foram estudadas usando difração de raios X e medidas magneticas em função da temperatura. Neste tipo de compostos as propriedades estruturais estão fortemente correlacionadas com as propriedades magnéticas e de transporte. Neste trabalho apresentaremos a síntese, caracterização estrutural e magnética de ligas metálicas nanoestruturadas de Cu-Fe, compostos intermetálicos R3M4Sn13(R= terra rara, M= Co e Ir), e cobaltitas (Nd1-xC axCoO3 e GdBaCo2O5,5). Parâmetros de cela, tamanho de domínios e tensões de ligas nanoestruturadas Cu1-xFex (0<x<0,5) preparadas por ball-milling foram determinados usando difração de pó. Todos as amostras apresentam a estrutura FCC do Cu. Os parâmetros de cela aumentam com x. Usando métodos de análise do perfil de difração (largura integrada e Warren-Averbach), determinamos o tamanho dos domínios cristalinos e tensões na rede. O tamanho dos domínios decresce e as tensões aumentam com x. As estruturas cristalinas dos compostos R3M4Sn13(R= terra rara, M= Co e Ir) foram determinadas por difração de monocristais à temperatura ambiente. Todos os compostos R3C o4Sn13 e Eu3I r4Sn13 cristalizam com uma cela cúbica Pm-3n, com parâmetros de cela a~ 9,5- 9,8 Å. Os compostos R3I r4Sn13 (R= La, Ce, Pr, Nd) apresentam uma superestrutura cúbica de corpo centrado com a¿~2a. Os compostos de Yb e Eu mostram um comportamento anômalo associado a um estado de valência diferente das outras terras raras. Estudos de difração de pó a baixas temperaturas para os compostos R3Co4Sn13 (R= La, Ce, Pr, Nd e Gd) mostram uma transição abaixo de T=150 K para uma superestrutura cúbica de corpo centrado com parâmetro de cela a¿~2a. Os compostos R3Ir4Sn13 não apresentam transições de fase até 20 K. As propriedades estruturais e magnéticas das cobaltitas Nd1-xCaxCoO3 (0 =x =0,4), foram estudadas usando difração de pó, magnetização dc e susceptibilidade ac em função da temperatura (2-800 K). Todos os compostos cristalizam com uma estrutura ortorrômbica (Pnma) no intervalo de temperatura estudado (20-800 K). Para x =0,1 o sistema mostra um comportamento paramagnético até 2 K, com os íons Co3+ e Co 4+ em estados de baixo spin (LS) para T<180 K. Para 0,2 =x =0,4, duas transições magnéticas são observadas para T<60 K. Estas transições estão associadas a estados de "spin glass" ou "cluster glass". Um diagrama de fases magnéticas e de estados de spin foi construído e comparado com outros sistemas similares. Foi realizada a síntese do composto GdBaCo2O5,50 e estudadas as propriedades estruturais e de transporte em função da temperatura (2-800 K). Sucessivas transições de fase magnéticas foram observadas a: TPM= 70 K, TN~ 260 K; TC~ 273 K. Observamos uma transição metal isolante a TMI~ 360 K, simultânea com uma transição de spin dos íons de Co3+ . Os resultados magnéticos são compatíveis com estados de spin: 50%Co3+ (IS)+50% Co3+ (LS) para T<TMI e 50% Co3+ (IS)+50% Co3+ (HS) para T > TMI. Estudos estruturais usando difração de pó com fonte convencional e luz de síncrotron, mostram que a TMI uma transição de spin LS ÎHS ocorre para os íons Co3+ nos octaedros. Uma transição ortorrômbica-tetragonal, de tipo ordem-desordem, foi observada a TOD~ 760 K / Abstract: A series of new compounds have been synthesized and their physical properties studied using X-ray diffraction and magnetic measurements as a function of temperature. In this type of compounds, structural properties are strongly correlated with their magnetic and transport properties. In this work we present details of the synthesis as well as the structural and magnetic characterization of nanostructured metallic alloys (Cu-Fe), intermetallic compounds of the type R3M4Sn13 (R=rare earth, M= Co, Ir), and the cobaltates Nd1-xCaxCo03 and GdBaCo2O5,5. The lattice parameters, crystallite size and lattice strain of nanostructured Cu1-xFex(0< x <0,5) alloys prepared by the ball milling method were determined from X-ray powder diffraction analysis. All samples present the Cu FCC structure. The lattice parameter of the Cu1-xFex phases increases with x. The X-ray diffraction peaks were analyzed using the Warren-Averbach and Integral Breadth methods. The crystallite size decreases and the lattice strain increases with Fe concentration. Crystal structures of the R3M4Sn13 (R= rare earth, M=Co, Ir) intermetallic compounds were determined from single crystal X-ray diffraction at room temperature. All R3Co4Sn13 and Eu3Ir4Sn13 compounds crystallize in the cubic Pm-3n space group with cell parameters a~9,5-9,7 ÿ. The R3IrSn13(R= La, Ce, Pr, Nd) compounds present a body centered cubic superstructure with aÂ' ~2a. In this series, the Yb and Eu compounds exhibit an anomalous behavior associated to valence states different from those of the other rare earths. Low temperature X-ray powder diffraction studies for R3Co4S n13(R= La, Ce, Pr, Nd and Gd) show a phase transition below 150 K from a simple cubic to a body centered cubic superstructure with cell parameter aÂ'~ 2 a. The temperature of the phase transition decreases with the rare earth ionic radii. No phase transitions were observed for the R3Ir4Sn13 series for temperatures down to 20 K. The structural and magnetic properties of the cobaltates Nd1-xCaxCoO3(0 =x =0,4), were studied using powder X-ray diffraction, dc magnetization and ac susceptibility measurements as a function of temperature (2-800 K). All compounds crystallize with an orthorhombic (Pnma) structure in the temperature range (20- 800 K). For x =0,1 the Nd1-xCaxCoO3system shows a paramagnetic behavior down to 2 K, with the Co3+e Co4+ions in low spin state (LS) for T<180 K. For 0,2 =x =0,4, two magnetic transitions were observed for T<60 K. These transitions are associated to a spin or cluster glass states. A magnetic phase and spin state diagram was constructed for comparison with similar systems. The compound GdBaCo2 O5.50was synthesized and the structural and transport properties were studied as a function of temperature [2-800 K]. Successive magnetic phase transitions were observed: TPM= 70 K, TN~ 260 K; TC~ 273 K. A metal-insulator transition is observed at TMI~ 360 K, with a simultaneous spin transition of the Co 3+ions. The magnetic measurements results are compatible with the spin states 50%Co3+(IS)+50% Co3+(LS) for T<TMI and 50%Co3+(IS)+50%Co3+(HS) for T>TMI. Structural studies using powder diffraction with a conventional X-ray source and synchrotron radiation show that the LS Ã?HS spin state transition occurs only for the Co3+ions in the octahedrons. An orthorhombic-tetragonal transition of the order-disorder type was observed at TOD~ 760 K / Doutorado / Física da Matéria Condensada / Doutor em Ciências

In this thesis, results of the investigation of a new low-dimensional cobaltates Ba2-xSrxCoO4 are presented. The synthesis of both polycrystalline and single crystalline compounds using the methods of conventional solid state chemical reaction and floating-zone optical furnace is first introduced. Besides making polycrystalline powders, we successfully, for the first time, synthesized large single crystals of Ba2CoO4. Single crystals were also obtained for Sr doped Ba2-xSrxCoO4. Powder and single crystal x-ray diffraction results indicate that pure Ba2CoO4 has a monoclinic structure at room temperature. With Sr doping, the lattice structure changes to orthorhombic when x ≥ 0.5 and to tetragonal when x = 2.0. In addition, Ba2CoO4 and Sr2CoO4, have completely different basic building blocks in the structure. One is CoO4 tetrahedron and the later is CoO6 octahedron, respectively. Electronic and magnetic properties were characterized and discussed. The magnetic susceptibility, specific heat and thermal conductivity show that Ba2CoO4 has an antiferromagnetic (AF) ground state with an AF ordering temperature TN = 25 K. However, the magnitude of the Néel temperature TN is significantly lower than the Curie-Weiss temperature (|θ| ~ 110 K), suggesting either reduced-dimensional magnetic interactions and/or the existence of magnetic frustration. The AF interaction persists in all the samples with different doping concentrations. The Néel temperature doesn’t vary much in the monoclinic structure regime but decreases when the system enters orthorhombic. Magnetically, Ba2CoO4 has an AF insulating ground state while Sr2CoO4 has a ferromagnetic (FM) metallic ground state. Neutron powder refinement results indicate a magnetic structure with the spin mostly aligned along the a-axis. The result from a μ-spin rotation/relaxation (μ+SR) experiment agrees with our refinement. It confirms the AF order in the ab-plane. We also studied the spin dynamics and its anisotropy in the AF phase. The results from inelastic neutron scattering show that spin waves have a clear dispersion along a-axis but not along c-axis, indicating spin anisotropy. This work finds the strong spin-lattice coupling in this novel complex material. The interplay between the two degrees of freedom results an interesting phase diagram. Further research is needed when large single crystal samples are available.

Ce mémoire de thèse porte sur l'étude par résonance magnétique nucléaire (noyaux 59Co et 23Na) des oxydes de cobalt NaxCoO2, dont les propriétés remarquables de supraconductivité et de fort pouvoir thermoélectrique suscitent un grand intérêt.<br />Les deux compositions extrêmes x=0 et x=1 ont été étudiées. Na1CoO2 est un isolant de bande non magnétique : tous les sites de cobalt ont un état d'oxydation +3, et un état bas spin S=0. Par contre, le composé CoO2, où la valence formelle du Co est +4, se révèle être très proche de la transition (isolant-métal) de Mott, ce qui montre que les corrélations électroniques sont fortes, à la base, dans ce système.<br />Dans la zone 0.65

The study of the co-existence of singlet superconductivity and ferromagnetism in bulk materials has been a long standing and intriguing problem in condensed matter physics since the superconductivity and ferromagnetism are quantum mechanically antagonistic to each other (i.e. parallel alignment of spins in the ferromagnet and Cooper pairs with oppositely aligned spins in the superconductor).Though it is incompatible to have the coexistence of singlet superconductivity and ferromagnetism in bulk compound, it is highly possible to artificially fabricate superconductor (S)/ferromagnet (F) heterostructures using various thin film deposition techniques and to study the interplay between the two antagonistic quantum phases over their characteristic length scales. The mutual interaction between the two competing order parameters at the interface in hybrid S/F heterostructures give rise to a variety of novel exotic physical phenomena. Moreover, the spin polarized transport and tunneling experiments in S/F heterostructures seem to be very much useful for providing important information on the spin dependent electronic properties of high Tc superconductors below and above the transition temperature. This can help a lot to understand the long debated unusual electronic properties and pairing mechanism of high Tc superconductors. In addition to the rich fundamental aspects buried in the study of S/F heterostructures, one can also use the spin dependent properties of high Tc superconductors in S/F heterostructures to design new spintronics devices from the application point of view. In this thesis an attempt is made to understand the spin polarized electron transport across S/F heterostructures where the superconductor used is YBa2Cu3O7-δand the ferromagnets are La0.5Sr0.5CoO3, La0.7Sr0.3MnO3, and La0.7Ca0.3MnO3. In addition, the magnetic properties of the La1-x SrxCoO3 system is also investigated in detail. The thesis is organized in six chapters and a brief summary of each chapter is given below. Chapter1 gives a brief introduction to the superconductivity, ferromagnetism and the interplay between superconductivity and ferromagnetism at the interface of S/F heterostructures. It also describes various exotic phenomena and the proximity effect that emerges at the S/F interface due to competing interactions. In addition, it also includes a discussion on various types of indirect magnetic interactions and basic idea about the spin glass ordering in magnetic materials. Chapter 2 outlines the basic principles of various experimental techniques employed for the work presented in this thesis. Chapter 3 describes an extensive magnetic and magnetotransport study of the La1-xSrxCoO3 system to understand the manifestation of various magnetic phases associated with it. The first section of this chapter aims at understanding the phase separation scenario in La0.85Sr0.15CoO3. Since the magnetic behavior of La0.85Sr0.15CoO3 is in the border area of spin glass (SG) and ferromagnetic (F) region in the x-T phase diagram; it has been subjected to a controversial debate for the last several years; while some groups show evidence for magnetic phase separation (PS), others show SG behavior. However, the experimental results presented in this thesis clearly demonstrate that the instability towards PS with inhomogeneous states or competing phases in La0.85Sr0.15CoO3 is not inherent or intrinsic to this compound; rather it is a consequence of the heat treatment condition during the preparation method. It is realized that low temperature annealed sample shows PS whereas the high temperature annealed sample shows the characteristics of canonical SG behavior. The second section of this chapter deals with a detailed study about the possible existence of various magnetic phases of La1-xSrxCoO3 in the range 0 ≤x ≤0.5. The dc magnetization study for x ≥0.18 exhibits the characteristic of ferromagnetic like behavior and for x<0.18, the SG behavior. More strikingly, the dc magnetization studies for x<0.18 rules out the existence of any ferromagnetic correlation that gives rise to irreversible line in the spin glass regime. The ac susceptibility study for x<0.18, exhibits a considerable frequency dependent peak shift, time-dependent memory effect, and the characteristic spin relaxation time scale τo ~10-13s, all pointing towards the characteristics of SG behavior. On the other hand, the ac susceptibility study in the higher doping ferromagnetic side exhibits the coexistence of glassy and ferromagnetic behavior. The glassiness is interpreted in terms of inter-cluster interaction. The reciprocal susceptibility vs. T plot in the paramagnetic side adheres strictly to Curie-Weis behavior and does not provide any signature for the pre-formation of ferromagnetic clusters well above the Curie temperature. The magnetotransport study reveals a cross over from metallic behavior to semiconducting like behavior for x ≤0.18 and the system exhibits a peak in MR in the vicinity of Tc on the metallic side and a large value of MR at low temperature on the semiconducting side. Such high value of MR in the semiconducting spin glass regime is strongly believed due to spin dependent part of random potential distribution. Based on the present experimental findings, a revised phase diagram has been constructed and each phase has been characterized with its associated properties. Chapter 4 deals with a comprehensive study of thickness dependent structural, magnetic and magnetotransport properties of oriented La0.5Sr0.5CoO3 thin films grown on LaAlO3 by pulsed laser deposition. The films are found to undergo a reduction in Curie temperature with decrease in film thicknesses and it is primarily caused by the finite size effect since the finite scaling law holds good over the studied thickness range. The contribution from strain induced suppression of the Curie temperature with decreasing film thickness is ruled out since all the films exhibit a constant out of plane tensile strain (0.5%) irrespective of their thickness. The coercivity of the films is observed to be an order of magnitude higher than that of the bulk. This is attributed to the local variation of the internal strain that introduces strong pinning sites (via. magnetoelastic interaction) for the magnetization reversal. In addition, an increase in the electrical resistivity and coercivity is observed with decrease in film thickness and it is strongly believed to be due to the interface effect. Chapter5 reports on the investigation of the effect of ferromagnetic layer on (i) pair breaking effect and (ii) vortex dynamics in different superconducting(S)/ ferromagnetic (F) bi-layers grown by pulsed laser deposition. The current (I) dependent electrical transport studies in the S/F bi-layers exhibit a significant reduction in the superconducting transition temperature with the increase in applied current as compared to single YBa2Cu3O7-δlayer and it follows I2/3 dependence in accordance with the pair breaking effect. Moreover, the superconducting transition temperature in YBa2Cu3O7-δ/ La0.7Sr0.3MnO3 bilayer is surprisingly found to be much larger than the YBa2Cu3O7-δ/La0.5Sr0.5CoO3. It appears that the current driven from a material with low spin polarization (-11%) like La0.5Sr0.5CoO3 can also suppress the superconductivity to a larger extent. This indicates that the degree of spin polarization of the ferromagnetic electrode is not the only criteria to determine the suppression of superconductivity by pair breaking effect in superconductor/ferromagnet hybrid structures; rather the transparency of the interface for the spin polarization, the formation of vortex state due to the stray field of ferromagnetic layer and the ferromagnetic domain patterns might play significant roles to determine such effect. More interestingly, the spin diffusion length in YBa2Cu3O7-δis found have a much longer length scale than that reported earlier in the study of F/ S heterostructures. The activation energy (U) for the vortex motion in S/F bilayers is reduced remarkably by the presence of the F layers. In addition, the U exhibits a logarithmic dependence on the applied magnetic field in the S/F bilayers suggesting the existence of decoupled 2D pancake vortices. This result is discussed in terms of the reduction in the effective S layer thickness and the weakening of the S coherence length due to the presence of F layers. Chapter 6 deals with the magnetotransport study on two different kind of F/S/F trilayers viz. La0.7Sr0.3MnO3/YBa2Cu3O7-δ/La0.7Sr0.3MnO3 and La0.5Sr0.5CoO3/YBa2Cu3O7-δ/La0.7Ca0.3MnO3 with changes in superconducting and ferromagnetic layer thickness. The activation energy for the vortex motion in F/S/F trilayer is found to decrease considerably as compared to S/F bilayer and it also exhibits a logarithmic dependence on magnetic field which gives the signature of existence of decoupled 2D pancake vortices. The magnetotransport study reveals that a much lower magnetic field is required to suppress the superconductivity in trilayer as compared to single YBCO layer. Moreover, the transport study also reveals that a threshold thickness of YBCO is required for the onset of superconductivity in trilayer structure and the onset of superconducting Tc increases with increase in YBCO thickness. More strikingly, a remarkable unconventional anisotropic superconducting Tc (Tc H║c-axis<Tc H⊥c-axis) is observed in La0.5Sr0.5CoO3/YBa2Cu3O7-δ/La0.7Ca0.3MnO3 trilayer for the magnetic field applied parallel and perpendicular to c-axis. The trilayer system also exhibits a huge positive magnetoresistance (MR) below superconducting Tc and it could arise due to vortex dissipation in liquid state of superconductor in the tri-layer structure. Finally, the thesis concludes with a general conclusion and an outlook in this area of research.

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