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L'étude du transport électronique dans des nano-objets métalliques et magnétiques issus de la chimie est un challenge en spintronique. En particulier, le manque de résultats expérimentaux révèle la difficulté à positionner ces nano-objets entre des électrodes de mesures tout en préservant leurs propriétés (magnétisme, intégrité des barrières tunnel organiques...). Ce travail de thèse vise à contourner ces difficultés et à étudier le magnétotransport dans ces systèmes. Pour cela, nous avons conçu et développé à l'intérieur d'une boîte à gants couplée à un bâti de pulvérisation cathodique des systèmes expérimentaux d'assemblages de nano-objets. Nous avons étudié les mécanismes mis en jeu lors de l'assemblage par la technique de dip coating, et réussi à déposer des monocouches de nanoparticules (NPs) de natures différentes (FeCo, Fe, Co) sur des surfaces d'Au, de SiO2 et de résine fine (40 nm). Ces résultats, couplés à une technique de nanoindentation, ont permis de mesurer quelques - voire une- NP(s). Une autre technique, la diélectrophorèse, s'est révélée simple et efficace pour piéger et orienter des nano-objets de taille, de nature, et de forme différentes entre des électrodes. Grâce à cette technique et au dépôt d'une couche protectrice d'alumine, nous avons étudié les propriétés de magnétotransport de plusieurs types de nano-objets sensibles à l'oxydation ou à la vapeur d'eau: NPs de Fe, de Co, FeCo et [Fe(H-trz)2(trz)](BF4)] (composés à transition de spin). Trois jeux de barrières tunnel organiques greffés sur des NPs de fer ont présenté de la magnétorésistance tunnel jusqu'à température ambiante. De plus, des nano-objets de [Fe(H-trz)2(trz)](BF4)] de facteurs de forme variable, ont montré une variation de la conductance liée à la transition de spin. Enfin, nous avons étudié l'influence de la longueur des ligands sur les propriétés de conductions de NPs de Cobalt, qui a validé nos méthodes d'échange de ligands et ont pu être analysées quantitativement. Nos travaux rendent désormais envisageable l'utilisation de NPs issues de la chimie dans différents domaines de la spintronique / The study of charge transport in metallic and magnetic nano-objects chemically synthesized is a challenge in spintronic. Particularly, the lack of experimental results reveals the difficulty in locating such nano-objects in between electrodes while preserving their good properties. This thesis aims to overcome these difficulties in order to study the magnetotransport in such systems. Therefore, we have designed and developed technical processes which induce the self-assembly of the nano-objects inside a glove box-sputtering system. After studying the mechanisms involved in the self-assembly obtained by dip coating, we succeeded to deposit monolayers of nanoparticles (NPs) of different materials (FeCo, Fe, Co) on gold surfaces, SiO2 and thin resin film (40 nm). These results, coupled with a nanoindentation technique allows us to measure a few or a unique NP(s). Another technique, called dielectrophoresis, has been proved to be a simple and versatile way to trap (and align) nano-objects with different (aspect ratio), size, nature, and shape in between the electrodes. Thanks to this technique and the deposit of a protective capping layer of alumina, we studied the magnetotransport properties of a large number of nano-objects sensitive to oxidation or humidity: Fe, Co, FeCo and [Fe(H-trz)2(trz)](BF4)] (spin crossover compounds). Three sets of organic tunnel barriers surrounding different Fe NPs presented tunnel magnetoresistance up to room temperature. Moreover, [Fe(H-trz)2(trz)](BF4)] nano-objects with different aspect ratio, highlighted a change in conductance connected to the spin transition. Finally, we validated our ligands exchange methods by studying the influence of the ligands length on the conduction properties of Co NPs, which have been analyzed quantitatively. Our works demonstrate the possibility to use the chemical NPs in different fields of spintronics

Les jonctions tunnel magnétiques épitaxiées Fe/MgO/Fe(001) présentent des comportements remarquables dans la limite des faibles ou des fortes épaisseurs de MgO. Ainsi, dans le premier cas, une interaction antiferromagnétique entre les deux couches de fer est observée ; dans le second, des effets de filtrage en symétrie conduisent à l?obtention de fortes valeurs de magnétorésistance. Les expériences réalisées au cours de cette thèse visent à étudier et mettre en relation ces deux régimes de propriétés extrêmes. Des analyses en tension et en température nous permettent d?étudier les conséquences d?une modulation de la structure cristalline des électrodes et/ou de l?interface sur le transport polarisé en spin. Dans ce cadre, nous nous intéressons à trois systèmes : des jonctions hybrides Fe/MgO(001)/CoFeB, où l?électrode de CoFeB est déposée par pulvérisation cathodique puis cristallisée in situ, des jonctions Fe/MgO/Fe à texture (001), ainsi que des jonctions Fe/MgO/Fe monocristallines présentant une rugosité artificielle à l?interface barrière/électrode. Le couplage antiferromagnétique dans des systèmes Fe/MgO/Fe(001) à barrière fine est étudié grâce à des mesures de magnétométrie sur la gamme de température [5 K ; 500 K]. Nous considérons aussi l?effet de modifications structurales et/ou chimiques de l?interface par le biais de l?introduction d?une rugosité contrôlée ou d?un contaminant. Les résultats les plus originaux de cette thèse sont obtenus grâce à l?introduction d?une rugosité artificielle à l?interface Fe/MgO ; contre toute attente, ce désordre contrôlé peut en effet augmenter l?effet de magnétorésistance ou l?intensité du couplage antiferromagnétique / Epitaxial magnetic tunnel junctions Fe/MgO/Fe(001) exhibit noteworthy behaviors for both small and large MgO thicknesses. In the first case, a strong antiferromagnetic interaction between Fe layers is observed, whereas symmetry filtering effects occur for large barriers, leading to high TMR values. The aim of the experiments performed during this thesis is to study and link these two behaviors. We consider the effect of a modulation of electrodes and/or interfaces crystalline structure on spin-polarized transport, by means of temperature and voltage analyses. In this framework, we focus on three systems: Fe/MgO(001)/CoFeB hybrid junctions, where the CoFeB electrode is grown by sputtering and in situ recrystallized, textured Fe/MgO/Fe(001) junctions, and finally single crystalline (001)Fe/MgO/Fe junctions with an artificial roughness at the electrode/barrier interface. The antiferromagnetic coupling in epitaxial Fe/MgO/Fe(001) systems with a thin MgO barrier is studied by magnetometry measurements in the [5 K; 500 K] range. We also consider the effect of structural and/or chemical changes resulting from a controlled roughness or a contamination on the coupling. The most interesting results of this thesis are obtained with the introduction of an artificial roughness at the Fe/MgO interface. Unexpectedly, this controlled disorder can improve the magnetoresistance effect or the coupling intensity. In both cases, this result is attributed to a Fe-O hybridization, which emphasizes the role of oxygen in MgO based magnetic tunnel junctions for both behaviors associated with extremely thin or thick barriers

Os objetivos do presente trabalho consistem na síntese de dois polímeros - poli(2,1,3-benzotiadiazol-4,7-ilenovinileno-alt-9,9-n-dioctil-2,7-fluorenilenovinileno) (PBTDV-alt-PDO27FV) e poli[4,7-(2,1,3-benzotiadiazolileno)-alt-(2,5-dioctilóxi-1,4- fenilenovinileno)] (PBTDV-alt-PDOPPV) - cinco oligômeros - 4,7-bis(2-(9,9-n-dioctil-9H-fluoren-2-il)vinil)2,1,3-benzotiadiazol (FBF); 2,7-bis(2-(2,1,3-benzotiadiazol-4-il)vinil)9,9-n-dioctil-9H-fluoreno (BFB); 4-(2-(9,9-n-dioctil-9H-fluoren-2-il)vinil)2,1,3-benzotiadiazol (FB); 1,2-bis(9,9-n-dioctil-9H-fluoren)2-vinileno (FF) e 2,2\'-(2,2\'-(9,9-n-dioctil-9H-fluoreno)2,7-bis(vinileno)bis(9,9-n-dioctil-9H-fluoreno) (FFF) - e três small molecules - 7,7\'-(4,4-bis(2-etil-hexil)-4H-silolo[3,2-b-:4,5-b\']ditiofen-2,6-diil)bis(6-fluoro-4-(5\'-hexil-[2\'-tiofeno-2\'-tiazol]-5-il)benzo[c][1,2,5]tiadiazol (G37FBT); 7,7\'-(4,4-bis(2-etil-hexil)-4H-silolo[3,2-b-:4,5-b\']ditiofen-2,6-diil)bis(6-fluoro-4-(2\'-isobutil-[5\',2\'-ditiazol])-5-il)benzo[c][1,2,5]tiadiazol (J1) e 7,7\'-(4,4-bis(2-etil-hexil)-4H-silolo[3,2-b-:4,5-b\']ditiofen-2,6-diil)bis(5\'-hexil-[2,2\'-bitiofeno]-5- bis(il)benzo[c][1,2,5]tiadiazol) (J2) - seguida da aplicação desses materiais em dispositivos fotovoltaicos e magnetorresistivos e em sensores de gás. Os polímeros e oligômeros preparados são derivados de poli(p-fenilenovinileno) e contêm unidades de fluoreno e/ou 2,1,3-benzotiadiazol, tendo sido empregada, na etapa final de cada síntese, reação de Wittig visando ao acoplamento e à formação das duplas vinilênicas. As small molecules, preparadas por meio de sucessivas formações de organo-estananas e reações de Stille, também foram sintetizadas com sucesso e em rendimentos apreciáveis. A espécie J2 não foi preparada com sucesso, obtendo-se, em contrapartida, um análogo dessa molécula, que foi devidamente isolado e caracterizado. No que tange à aplicação dos materiais em dispositivos fotovoltaicos, os resultados até então obtidos mostraram-se inconclusivos, sendo digno de nota, no entanto, que o estudo continua sendo conduzido por colaboradores. O trabalho de aplicação dos materiais em dispositivos magnetorresistivos apresentou resultados interessantes para os oligômeros FBF, BFB, FFF e FF, visto que dispositivos fabricados a partir das referidas espécies mostraram variação em seus valores de corrente elétrica quando submetidos a campo magnético de 200 mT. Por fim, foram desenvolvidos narizes eletrônicos com duas finalidades distintas: (a) identificação de diferentes espécies de madeira de interesse ambiental e (b) estudo da influência da atividade física no indivíduo por meio da análise do suor. Os sensores de gás foram preparados por meio da deposição de finos filmes de materiais orgânicos conjugados dopados sobre a superfície de eletrodos interdigitados. No estudo acerca da identificação de espécies de madeira, a análise de leave-one-out revelou 100 % de taxa de acerto na diferenciação entre as espécies angelim, cedro-rosa, imbuia e perobinha. O estudo da influência da atividade física no indivíduo também se mostrou promissor na medida em que identificou os diferentes estágios do exercício físico com taxa de acerto de 93 %, também definida pela técnica estatística de leave-one-out. / The present work consists on the synthesis of two polymers - poly(2,1,3- benzothiadiazole-4,7-ylenevinylene-alt-9,9-n-dioctyl-2,7-fluorenylenevinylene) (PBTDV-alt-PDO27FV) and poly[4,7-(2,1,3-benzothiadiazole-ylene)-alt-(2,5-dioctyloxy-1,4-phenylenevinylene)] (PBTDV-alt-PDOPPV) - five oligomers - 4,7-bis(2-(9,9-n-dioctyl- 9H-fluorene-2-yl)vinyl)2,1,3-benzothiadiazole (FBF); 2,7-bis(2-(2,1,3- benzothiadiazole-4-yl)vinyl)9,9-n-dioctyl-9H-fluorene (BFB); 4-(2-(9,9-n-dioctyl-9H-fluorene-2-yl)vinyl)2,1,3-benzothiadiazole (FB); 1,2-bis(9,9-n-dioctyl-9H-fluorene)2- vinylene (FF) and 2,2\'-(2,2\'-(9,9-n-dioctyl-9H-fluorene)2,7-bis(vinylene)bis(9,9-n-dioctyl-9H-fluorene) (FFF) - and three small molecules - 7,7\'-(4,4-bis(2-ethyl-hexyl)-4H-silolo[3,2-b-:4,5-b\']dithiophene-2,6-diyl)bis(6-fluoro-4-(5\'-hexyl-[2\'-thiophene-2\'- thiazole]-5-yl)benzo[c][1,2,5]thiadiazole (G37FBT); 7,7\'-(4,4-bis(2-ethyl-hexyl)-4H-silolo[3,2-b-:4,5-b\']dithiophene-2,6-diyl)bis(6-fluoro-4-(2\'-isobutyl-[5\',2\'-dithiazole]-5-yl)benzo[c][1,2,5]thiadiazole (J1) and 7,7\'-(4,4-bis(2-ethyl-hexyl)-4H-silolo[3,2-b-:4,5-b\']dithiophene-2,6-diyl)bis(5\'-hexyl-[2,2-bithiophene]-5-bis(yl)benzo[c][1,2,5]thiadiazole) (J2) - followed by the application of such materials in organic photovoltaics (OPV), magnetoresistive devices and gas sensors. The polymers and oligomers are all poli(p-phenylenevinylene)s derivatives containing 2,1,3-benzothiadiazole and/or substituted fluorene units in the main chain. Those materials\' preparation comprises a coupling Wittig reaction as the key step, through which the vinylenic bonds are formed. The small molecules mentioned above were synthesized by successive organostannane preparations followed by cross-coupling Stille reactions, leading to the target-compounds in considerably high yields. Although the route is a very useful methodology for synthesizing organic conjugated small molecules, J2 was not successfully prepared. The reaction led to a J2 analogue instead, that was properly isolated and characterized. The application of the cited materials in organic photovoltaics (OPVs) is still in progress, since the study has shown inconclusive results so far. The study of the organic magnetoresistive properties of the synthesized materials has led to interesting results for the oligomers FBF, BFB, FFF and FF. The devices containing those compounds in their active layers presented considerable variations in their electric current values when submitted to a magnetic field of 150 mT magnitude. Finally, electronic noses for two different applications were developed: (a) the identification of wood species that can be easily mistaken and (b) the study of exercise physiology through analyses of sweat samples. The gas sensors were prepared by the deposition of doped organic conjugated materials thin films onto the surface of intedigitated electrodes. The study on the differentiation of several species of wood showed 100 % rate of hits determined by leave-one-out statistics analysis. The study on the exercise physiology was also promising since it allowed the identification of the three different stages of the physical exercise with a 93 % rate of hits, also determined by leave-one-out methods. Thus, both studies suggest that the electronic nose can be a powerful tool to study many different targets in which the release of volatile compounds is involved

Os objetivos do presente trabalho consistem na síntese de dois polímeros - poli(2,1,3-benzotiadiazol-4,7-ilenovinileno-alt-9,9-n-dioctil-2,7-fluorenilenovinileno) (PBTDV-alt-PDO27FV) e poli[4,7-(2,1,3-benzotiadiazolileno)-alt-(2,5-dioctilóxi-1,4- fenilenovinileno)] (PBTDV-alt-PDOPPV) - cinco oligômeros - 4,7-bis(2-(9,9-n-dioctil-9H-fluoren-2-il)vinil)2,1,3-benzotiadiazol (FBF); 2,7-bis(2-(2,1,3-benzotiadiazol-4-il)vinil)9,9-n-dioctil-9H-fluoreno (BFB); 4-(2-(9,9-n-dioctil-9H-fluoren-2-il)vinil)2,1,3-benzotiadiazol (FB); 1,2-bis(9,9-n-dioctil-9H-fluoren)2-vinileno (FF) e 2,2\'-(2,2\'-(9,9-n-dioctil-9H-fluoreno)2,7-bis(vinileno)bis(9,9-n-dioctil-9H-fluoreno) (FFF) - e três small molecules - 7,7\'-(4,4-bis(2-etil-hexil)-4H-silolo[3,2-b-:4,5-b\']ditiofen-2,6-diil)bis(6-fluoro-4-(5\'-hexil-[2\'-tiofeno-2\'-tiazol]-5-il)benzo[c][1,2,5]tiadiazol (G37FBT); 7,7\'-(4,4-bis(2-etil-hexil)-4H-silolo[3,2-b-:4,5-b\']ditiofen-2,6-diil)bis(6-fluoro-4-(2\'-isobutil-[5\',2\'-ditiazol])-5-il)benzo[c][1,2,5]tiadiazol (J1) e 7,7\'-(4,4-bis(2-etil-hexil)-4H-silolo[3,2-b-:4,5-b\']ditiofen-2,6-diil)bis(5\'-hexil-[2,2\'-bitiofeno]-5- bis(il)benzo[c][1,2,5]tiadiazol) (J2) - seguida da aplicação desses materiais em dispositivos fotovoltaicos e magnetorresistivos e em sensores de gás. Os polímeros e oligômeros preparados são derivados de poli(p-fenilenovinileno) e contêm unidades de fluoreno e/ou 2,1,3-benzotiadiazol, tendo sido empregada, na etapa final de cada síntese, reação de Wittig visando ao acoplamento e à formação das duplas vinilênicas. As small molecules, preparadas por meio de sucessivas formações de organo-estananas e reações de Stille, também foram sintetizadas com sucesso e em rendimentos apreciáveis. A espécie J2 não foi preparada com sucesso, obtendo-se, em contrapartida, um análogo dessa molécula, que foi devidamente isolado e caracterizado. No que tange à aplicação dos materiais em dispositivos fotovoltaicos, os resultados até então obtidos mostraram-se inconclusivos, sendo digno de nota, no entanto, que o estudo continua sendo conduzido por colaboradores. O trabalho de aplicação dos materiais em dispositivos magnetorresistivos apresentou resultados interessantes para os oligômeros FBF, BFB, FFF e FF, visto que dispositivos fabricados a partir das referidas espécies mostraram variação em seus valores de corrente elétrica quando submetidos a campo magnético de 200 mT. Por fim, foram desenvolvidos narizes eletrônicos com duas finalidades distintas: (a) identificação de diferentes espécies de madeira de interesse ambiental e (b) estudo da influência da atividade física no indivíduo por meio da análise do suor. Os sensores de gás foram preparados por meio da deposição de finos filmes de materiais orgânicos conjugados dopados sobre a superfície de eletrodos interdigitados. No estudo acerca da identificação de espécies de madeira, a análise de leave-one-out revelou 100 % de taxa de acerto na diferenciação entre as espécies angelim, cedro-rosa, imbuia e perobinha. O estudo da influência da atividade física no indivíduo também se mostrou promissor na medida em que identificou os diferentes estágios do exercício físico com taxa de acerto de 93 %, também definida pela técnica estatística de leave-one-out. / The present work consists on the synthesis of two polymers - poly(2,1,3- benzothiadiazole-4,7-ylenevinylene-alt-9,9-n-dioctyl-2,7-fluorenylenevinylene) (PBTDV-alt-PDO27FV) and poly[4,7-(2,1,3-benzothiadiazole-ylene)-alt-(2,5-dioctyloxy-1,4-phenylenevinylene)] (PBTDV-alt-PDOPPV) - five oligomers - 4,7-bis(2-(9,9-n-dioctyl- 9H-fluorene-2-yl)vinyl)2,1,3-benzothiadiazole (FBF); 2,7-bis(2-(2,1,3- benzothiadiazole-4-yl)vinyl)9,9-n-dioctyl-9H-fluorene (BFB); 4-(2-(9,9-n-dioctyl-9H-fluorene-2-yl)vinyl)2,1,3-benzothiadiazole (FB); 1,2-bis(9,9-n-dioctyl-9H-fluorene)2- vinylene (FF) and 2,2\'-(2,2\'-(9,9-n-dioctyl-9H-fluorene)2,7-bis(vinylene)bis(9,9-n-dioctyl-9H-fluorene) (FFF) - and three small molecules - 7,7\'-(4,4-bis(2-ethyl-hexyl)-4H-silolo[3,2-b-:4,5-b\']dithiophene-2,6-diyl)bis(6-fluoro-4-(5\'-hexyl-[2\'-thiophene-2\'- thiazole]-5-yl)benzo[c][1,2,5]thiadiazole (G37FBT); 7,7\'-(4,4-bis(2-ethyl-hexyl)-4H-silolo[3,2-b-:4,5-b\']dithiophene-2,6-diyl)bis(6-fluoro-4-(2\'-isobutyl-[5\',2\'-dithiazole]-5-yl)benzo[c][1,2,5]thiadiazole (J1) and 7,7\'-(4,4-bis(2-ethyl-hexyl)-4H-silolo[3,2-b-:4,5-b\']dithiophene-2,6-diyl)bis(5\'-hexyl-[2,2-bithiophene]-5-bis(yl)benzo[c][1,2,5]thiadiazole) (J2) - followed by the application of such materials in organic photovoltaics (OPV), magnetoresistive devices and gas sensors. The polymers and oligomers are all poli(p-phenylenevinylene)s derivatives containing 2,1,3-benzothiadiazole and/or substituted fluorene units in the main chain. Those materials\' preparation comprises a coupling Wittig reaction as the key step, through which the vinylenic bonds are formed. The small molecules mentioned above were synthesized by successive organostannane preparations followed by cross-coupling Stille reactions, leading to the target-compounds in considerably high yields. Although the route is a very useful methodology for synthesizing organic conjugated small molecules, J2 was not successfully prepared. The reaction led to a J2 analogue instead, that was properly isolated and characterized. The application of the cited materials in organic photovoltaics (OPVs) is still in progress, since the study has shown inconclusive results so far. The study of the organic magnetoresistive properties of the synthesized materials has led to interesting results for the oligomers FBF, BFB, FFF and FF. The devices containing those compounds in their active layers presented considerable variations in their electric current values when submitted to a magnetic field of 150 mT magnitude. Finally, electronic noses for two different applications were developed: (a) the identification of wood species that can be easily mistaken and (b) the study of exercise physiology through analyses of sweat samples. The gas sensors were prepared by the deposition of doped organic conjugated materials thin films onto the surface of intedigitated electrodes. The study on the differentiation of several species of wood showed 100 % rate of hits determined by leave-one-out statistics analysis. The study on the exercise physiology was also promising since it allowed the identification of the three different stages of the physical exercise with a 93 % rate of hits, also determined by leave-one-out methods. Thus, both studies suggest that the electronic nose can be a powerful tool to study many different targets in which the release of volatile compounds is involved

Studies on perovskite rare earth manganites of general formula R1-xAxMnO3 (where R is a trivalent rare earth ion such as La3+, Pr3+ etc. and A is a divalent alkaline earth ion such as Ca2+, Sr2+, Ba2+, have been a very active research area in the last few years in condensed matter physics. Manganites have a distorted perovskite crystal structure with R and A ions situated at the cube corners, oxygen ions at the edge centers of the cube and Mn ions at the centres of the oxygen octahedra. In these manganites the Mn ions are found to be in mixed valence state i.e., in Mn3+ and Mn4+ states. In the octahedral crystal field of oxygen ions the single ion energy levels are split into t2g and eg levels. Mn3+ being a Jahn-Teller ion, the eg level is further split due to the Jahn-Teller effect. A strong Hund’s coupling between the spins in the t2g and eg levels renders the Mn3+ ions to be in the high spin state. The interplay of competing super exchange between Mn ions which determines the antiferromagnetism, orbital ordering and insulating behavior and double exchange between Mn ions which leads to ferromagnetism and metallicity gives rise to very complex phase diagrams of manganites as a function of composition, temperature and magnetic field. The strength of these interactions is determined by various factors such as the A-site cation radius and the Jahn-Teller distortion due to the presence of Mn3+ ions. The strongly coupled charge, spin, lattice and orbital degrees of freedom in manganites gives rise to complex phenomena such as colossal magnetoresistance (CMR), charge order (CO) and orbital order (OO) and phase separation (PS) etc. The properties of these materials are sensitive functions of external stimuli such as the doping, temperature and pressure [1-5] and have been extensively studied both experimentally and theoretically in single crystal, bulk polycrystalline and thin film forms of the samples [6-9]. Charge ordering is one of the fascinating properties exhibited by manganites. Charge ordering has historically been viewed as a precursor to the complex ordering of the Mn 3d orbitals, which in turn determine the magnetic interactions and these magnetic interactions are the driving force for charge localization and orbital order. This ordering of Mn3+ / Mn4+ charges can be destabilized by many methods. An external magnetic field can destabilize the charge ordered phase and drive the phase transition to the ferromagnetic metallic state [10-11]. Other than magnetic field, charge ordering can also be ‘melted’ by a variety of perturbations like electric field [12, 13], hydrostatic and chemical pressure [14-16], irradiation by X-rays [17], substitution at the Mn -site [18 -21] and A-site [22]. Of these, A-site substitution with bigger cations like barium is particularly of great interest since it does not interrupt the conduction path in the “MnO3” frame work Recently attention has been drawn towards the properties of nanoscale manganites. The nanoscale materials are expected to behave quite differently from extended solids due to quantum confinement effects and high surface/volume ratio. Nanoscale CMR manganites have been fabricated using diverse methods in the form of particles, wires, tubes and various other forms by different groups. It has been shown that the properties of CMR manganites can be tuned by reducing the particle size down to nanometer range and by changing the morphology [23-27]. As mentioned above, charge order is an interesting phase of manganites and these CO mangnites in the form of nanowires and nanoparticles show drastic changes in their properties compared to bulk. In contrast to the studies on the CMR compounds, there are very few reports on charge ordering nano manganites except on nanowires of Pr0.5Ca0..5MnO3 [28] and nanoparticles of Nd0.5Ca0.5MnO3 [29] and Pr0.5Sr0..5MnO3 [30]. This thesis is an effort in understanding certain aspects of charge order destabilization by two different methods, namely, doping bigger size cation (barium) in A-site (external perturbation) and by reducing the particle size to nano scale ( intrinsic). For this purpose we have selected the charge ordering system Pr1-xCaxMnO3 (PCMO) with composition x = 0.43. The reason behind choosing this composition is the observation [31] that CO is particularly weak for this value of x. We have prepared bulk, nanoparticles and nanowires of Pr0.57Ca0.41Ba0.02MnO3 manganite and have carried out microstructure, magnetic, magneto transport and EMR measurements to understand the nature of CO destabilization and also to understand other aspects such as magneto transport and magnetic anisotropy . Apart from destabilization of the charge order in PCMO we have also studied the bismuth based manganite Bi0.5Ca0.5MnO3. The reason behind choosing this system is the robust charge order of Bi0.5Ca0.5MnO3 compared to rare earth based manganites. So far no attempt has been made in comparing the electron paramagnetic resonance properties of bismuth based manganites with those of the rare earth based manganites. We have studied the magnetic, transport and electron paramagnetic resonance properties of Bi0.5Ca0.5MnO3 prepared by solid state reaction method and compared the results with those of Pr0.5Ca0.5MnO3 . In the following we present a chapter wise summary of the thesis. Chapter 1 of the thesis contains a brief introduction to the general features of manganites describing various interesting phenomena exhibited by them and the underlying interactions . Chapter 2 contains a detailed review of EPR studies on manganites describing the current level of understanding in the area. In this chapter we have also described the different experimental methodology adopted in this thesis. Chapter 3 reports the effect of a small amount (2%) of barium doped in the charge ordered antiferromagnetic insulating manganite Pr0.57Ca0.43MnO3. The samples were prepared by solid state synthesis and charecterized by various techniques like XRD, EDXA. The results of magnetization, magnetotransport and EPR/EMR experiments on both Pr0.57Ca0.43MnO3 and Pr0.57Ca0.41Ba0.02MnO3 are compared. The magnetization studies show that barium doping induces ferromagnetic phase in place of the CO-antiferromagnetic phase of the pristine sample at low temperatures as reported earlier by Zhu et al.,[31]. The transport studies show insulator to metal transition. The EPR parameters viz line width, intensity and ‘g’ value of Pr0.57Ca0.43MnO3 and Pr0.57Ca0.41Ba0.02MnO3 are compared. The magnetization and EPR studies reveal that the CO transition temperature TCO has shifted to a slightly lower value accompanied by a small decrease in the strength of the charge order. Thus a small amount of barium affects the CO phase of Pr0.57Ca0.43MnO3 and it also induces a ferromagnetic metallic phase at low temperature. Another most important and unexpected result of EMR experiment is the observation of high field signals, i.e. two EMR signals are observed at low temperatures in the ferromagnetic phase of Pr0.57Ca0.41Ba0.02MnO3. The appearance of the high field signals are understood in terms of the effects of magneto crystalline anisotropy. Chapter 4, reports the microstructure, magnetization and EMR studies of Pr0.57Ca0.41Ba0.02MnO3 nanoparticles prepared by sol-gel method. We have mainly focused on the effect of size on the charge ordered phase. The samples were characterized by different techniques like XRD, EDXA and TEM. The obtained particle size of the samples are 30, 60 and 100 nm respectively. We have compared the magnetic, magneto transport and EMR results of these nano samples with the bulk properties. The 30 nm particles do not show the CO phase whereas the 60 and 100 nm particles show CO signatures in DC- magnetization measurements. The EPR intensity also shows a similar trend. These results confirm that charge ordering can also be destabilized by reducing the particle size to nano scale. But the EPR linewidth which reflects the spin dynamics shows a change in the slope near the CO temperature and there by indicates the presence of premonitory charge ordering fluctuations in smaller particles. We also observed that the EMR linewidth increases with the decrease of particle size. Another striking result is the disappearance of high field signals in all the nanosamples. This is understood in terms of a decrease in the magnetic anisotropy in nanoparticles. Part of the result of this chapter is published [32]. Chapter 5, reports the morphological, magnetic and electron paramagnetic resonance studies of Pr0.57Ca0.41Ba0.02MnO3 nanowires. Recently our group has studied the nanowires of Pr0.5Ca0..5MnO3 [28]. In the nanowire sample of Pr0.5Ca0..5MnO3 only a partial suppression of CO is observed. This raises the question about the incomplete suppression of the CO in the nanowires: is this a consequence of the material being microscopic in one dimension and is it necessary to have a 3-dimensional nano material to have full suppression of the charge order ? In the present work we attempt to provide an answer to this question. PCBM nanowires of diameter 80-90 nm and length of ∼ 3.5 μm were synthesized by a low reaction temperature hydrothermal method. We have confirmed the single phase nature of the sample by XRD experiments. Scanning electron microscopy (SEM) and trasmission electron microscopy (TEM) were used to characterize the morphology and microstructures of the nanowires. The surface of nanowires was composed of particles of different grain size and interestingly some particles were hexagonal in shape. The bulk PCBM manganite exhibits charge order at 230 K along with a ferromagnetic transition at 110 K. However, SQUID measurements on PCBM nano-wires show a complete melting of the charge ordering and a ferromagnetic transition at 115 K. The magnetization observed in the nanowires was less compared to that in the bulk. EPR intensity measurements also support this result. Characteristic differences were observed in linewidth and ‘g’ factor behaviors of nanowires when compared with those of the bulk. EPR linewidth which reflects the spin dynamics shows a slope change near the CO temperature (like in nanoparticles) possibly due to charge order fluctuations in nanowires. The high field signals were absent in nanowires as well. Part of the result of this chapter is published [33]. Chapter 6 deals with the magnetic and electron paramagnetic resonance studies on Pr0.5Ca0.5MnO3 and Bi0.5Ca0.5MnO3. These manganites are prepared by solid state reaction method and characterized by different techniques like XRD and EDXA. Further, we have compared the results of magnetization and electron paramagnetic resonance properties of Pr0.5Ca0.5MnO3 with those of Bi0.5Ca0.5MnO3 manganite in the temperature range of 10- 300 K. The two charge ordered manganites show significant differences in their behavior. The temperature dependence of the EPR parameters i.e. line width, central field and intensity of Bi0.5Ca0.5MnO3 are quite different from the rare earth based manganite i.e. Pr0.5Ca0.5MnO3. Linewidth of BCMO is large compared to PCMO manganite and interestingly the temperature dependence of the central fields (CF) of PCMO and BCMO show opposite behavior. The CF of PCMO decreases with decrease in temperature as found in a large number of other CO systems, whereas CF of BCMO increases with decrease in temperature. This unusual behavior of resonance field is attributed to the different magnetic structure of BCMO system at low temperatures. Chapter 7 sums up the results reported in the thesis. The insight gained from the present work in understanding the destabilization of charge order by chemical doping and size reduction is discussed as well as the differences in the properties of bismuth and rare earth manganites. Further, we have indicated possible future directions of research in this area.

Electrical conductance in the ferromagnetic insulating (FMI) phase of manganites has been experimentally investigated. The investigations were performed on single crystals of compositions La0.82Ca0.18MnO3 and Nd0.7Pb0.3MnO3. The nature of electrical conductance is determined to be Shklovskii–Efros variable range hopping (SE–VRH). Further, at high bias levels, non–linear conductance (NLC) is observed. A “hot electron” model, that quantitatively explains the bias and temperature dependence of the NLC, consistent with the SE–VRH nature of electrical conductance, is presented. The limits of validity of the model are discussed.

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.

The main focus of this thesis is in understanding the role of phase separation and microstructure in determining the physical properties of manganites. We also aim to be able to tune certain material properties using appropriate control mechanisms. For this, an understanding of the local electronic properties of manganites is essential. We thus set out to study the local electronic states in manganites using a highly sensitive probe: the scanning tunneling microscope (STM). The chapter 1 of the thesis gives an introduction to manganites, and of how manganites are susceptible to various perturbations due to closely lying ground states and an intricate interplay of their charge, spin and lattice degrees of freedom. Chapter 2 of this thesis gives a detailed account of various experimental methods used in the current investigation. In particular, we describe the design and fabrication of a variable temperature ultra-high vacuum scanning tunneling microscope (UHV-STM) which was used to carry out spatially resolved measurements on various manganite systems. This chapter also describes sample fabrication techniques by which strain and microstructure of thin films can be controlled. Other characterization techiniques, such as tranport and magnetotransport measurements, are also described in detail. Chapter 3 presents our investigation of the role of microstructure and phase separation on the DOS and local electronic properties of manganite thin films. We describe various spatially resolved STM/STS measurements carried out on La0.67Sr0.33MnO3 and La0.67Ca0.33MnO3 films having different micrsotructure and varying degrees of phase separation. We also present a theoretical model used in interpreting STS data to account for finite temperature effects and explain the existing data in this context. We use this model to gain insight into the behaviour of the DOS at EF near the MIT where thermal smearing can often give rise to misleading inferences. Chapter 4 presents our investigation on the density of states in a typical charge ordered manganite system, Pr1-xCaxMnO3. We describe STS measurements carried out on this system to study the occurrence and evolution of the charge ordering (CO) gap as a function if temperature as well as tunneling current. We report the observation of destabilization of the CO gap using tunnel current injection by an STM tip. Chapter 5 presents our investigation into the controlled and localized “nanoscale” phase separation in Pr1-xCaxMnO3 (PCMO) using an STM tip. The investigations were carried out on PCMO single crystal and PCMO epitaxial films. Our results raise the possibility of nano-fabrication of metallic nanoislands in a CO matrix using an STM tip. We demonstrate some examples of this and also raise the relevance of intrinsic phase separation in this context. We show that the “melting” of CO using tunnel current injection by an STM tip is analogous to the magnetic field-induced melting of CO on a microscopic scale. Chapter 6 summarizes the important results of this thesis work and suggests the scope for future experiments.