Les dernières années les composés multiferroïques ont attiré un grand intérêt en raison de leurs propriétés uniques qui rendent ces matériaux très intéressants pour des applications réelles, par exemple des condensateurs, des détecteurs et des actuateurs ou des dispositifs de mémoire d'ordinateur. Dans la recherche de multiferroïques, PbVO3 est un matériel très prometteur. PbVO3 est isostructural avec PbTiO3 qui est un matériel ferroélectrique très bien connu et étudié et il contient également des ions de vanadium qui portent un spin 1/2, donc, un certain type d'ordre magnétique peut être attendu. Cependant, aucun ordre magnétique n'a pu être observé ce qui pose les deux questions suivantes:1) pourquoi PbVO3 n'est pas magnétique ?2) est-ce que PbVO3 peut être réglé de sorte qu'il devienne magnétique ?L'objectif du projet était de fournir une réponse aux questions ci-dessus par l'étude de la synthèse et des propriétés physiques des oxydes potentiellement multiferroïques de type Pb(V1-xMx)O3 où M = Ti, Fe.Nous avons préparé deux types d'échantillons, échantillons polycristallins et monocristaux. Le batch polycristallin est plus grand et il contient deux séries: la série de titane et de la série de fer avec les compositions chimiques suivantes:PbV1-xTixO3 où x = 0, 0.1, 0.25, 0.5, 0.6, 0.75, 0.8, 1PbV1-xFexO3 où x = 0.1, 0.25, 0.3, 0.4, 0.5, 0.55, 0.6 0.65, 0.75.Des monocristaux ont été préparés uniquement sous la forme de PbVO3 dans lequel aucune substitution n'a été essayée.La structure et les propriétés physiques des échantillons ont été étudiés et on a tenté de corréler les résultats et de formuler un modèle qui pourrait expliquer les comportements fascinants et souvent apparemment contradictoires de nos composés.Le document est structuré en cinq chapitres. Le premier chapitre passe en revue les concepts fondamentaux de la physique de derrière les composés multiferroïques, mettant l'accent sur les propriétés / phénomènes qui sont connectés à nos composés. Le premier chapitre s'occupe également des données déjà publiées dans la littérature pour PbVO3 et les composés substitués.Le deuxième chapitre décrit les méthodes expérimentales et les techniques de mesure utilisées au cours de l'étude.Le troisième chapitre présente les méthodes de préparation, les équipements de haute pression - hautes températures employées et les conditions de réaction nécessaires pour la synthèse de PbVO3 (et ses homologues substitués). La discussion est continuée avec les premières investigations et les résultats qui traitent de la pureté de phase.Le quatrième chapitre traite avec plus de détails les propriétés structurales des échantillons et le cinquième chapitre traite les propriétés magnétiques et diélectriques des composés potentiellement multiferroïques, Pb(V1-xMx)O3.Comme les propriétés physiques de ces matériaux sont fortement dépendantes de la structure des échantillons, les chapitres s'entrecroisent parfois, dans chaque chapitre, des résultats des autres chapitres sont mentionnés et une certaine redondance est donc inévitable. Les mesures d'absorption des rayons X forment le noyau de l'étude parce que ces mesures ont confirmé les états d'oxydation des cations du site B (où la substitution prend place). A partir de ces observations, presque toutes les propriétés structurales et physiques peuvent être expliquées. La substitution avec du titane, est donc révélée isovalente ce qui conduit à la formation de la solution solide PbVO3-PbTiO3 et à la dilution du réseau magnétique de PbVO3. D'autre part, la substitution avec du fer n'est pas isovalente et donc la solution solide PbVO3-PbFeO3 (ce dernier composé n'a jamais été signalé) s'arrête à x = 0.5. Le désordre introduit par la substitution avec du fer conduit à la formation d'un état magnétique similaire à un verre de spin et d'un comportement diélectrique typique pour un relaxeur ferroélectrique. / Recently, multiferroic compounds attracted huge interest due to their unique properties which make such materials very interesting for real life applications, from capacitors, sensors and actuators to computer memory devices. PbVO3 is a very promising material. It is isostructural with PbTiO3 which is a very well known and studied ferroelectric material and it also contains vanadium ions which carry a 1/2 spin so magnetic ordering can be expected. However, no sign of magnetic ordering could be observed which raises the following questions:1) why is PbVO3 not magnetic? and2) could it be tuned so that it becomes magnetic?The objective of the project was to provide some answer to the above questions by studying the synthesis and investigating of the physical properties of the Pb(V,M)O3, M = Ti, Fe potentially multiferroic oxides.We prepared two kinds of samples, polycrystalline and single crystals. The polycrystalline batch is larger containing two series, the titanium series and the iron series with the chemical compositions as follows.PbV1-xTixO3 with x = 0, 0.1, 0.25, 0.5 0.6, 0.75, 0.8, 1PbV1-xFexO3 with x = 0.1, 0.25, 0.3, 0.4, 0.5, 0.55, 0.6 0.65, 0.75Single crystals were prepared only in the form of clean PbVO3 in which no substitution was attempted.The structure and the physical properties of the samples were studied and an attempt was made to correlate the results and try to formulate a model which could explain the intriguing and often apparently contradicting behaviours of our compounds. The present work discusses the results obtained during the study and attempts to shed some light on the subject, without asserting that it reaches a final and definitive conclusion.The work is structured on five chapters.The first chapter reviews the fundamental concepts of the physics behind the multiferroic compounds, emphasizing the exotic properties / phenomena that are connected to our compounds. The first chapter also deals with the data already published in literature for PbVO3 and some substitution compounds.The second chapter describes the experimental methods and investigation techniques used during the study.The third chapter presents the preparation methods, the high pressure - high temperature equipments employed and the reaction conditions required for the synthesis of PbVO3 (and the substitution counterparts). The discussion continues with the first investigations and results which are concerned with phase purity.The fourth chapter discusses with somewhat greater details the structural properties of the samples.The fifth chapter deals with the magnetic and dielectric properties of the potentially multiferroic Pb(V1-xMx)O3 compounds.Since the physical properties of these materials are strongly dependent on the structure of the samples, the chapters intertwine at times, in each chapter results from the other chapters being mentioned and some redundancy is thus unavoidable. The measurements of X-Ray absorption form the core of the work as these measurements confirmed the oxidation states of the B site cations (where the substitution takes places). From these observations almost the entire set of structural and physical properties can be explained. The substitution with titanium is thus shown to be isovalent which leads to the formation of the PbVO3-PbTiO3 solid solution and the dilution of the magnetic network of PbVO3. On the other hand the substitution with iron is not isovalent and therefore the solid solution PbVO3-PbFeO3 (the latter compound was never reported) stops at x = 0.5. The disorder caused by the iron substitution leads to the formation of spin glass like magnetic states and ferroelectric relaxor states on these compounds.

Orientador: Prof. Dr. José Antonio Souza / Tese (doutorado) - Universidade Federal do ABC, Programa de Pós-Graduação em Física, 2014. / Neste trabalho, foram estudados tres sistemas de metais de oxidos de transicao os quais apresentam propriedades fisicas fortemente correlacionadas. O primeiro sistema foi uma serie da familia das manganitas La1-xCaxMnO3, com x = 0.20, 0.25, 0.30, 0.34, 0.40 e 0.45. Um estudo sistematico das propriedades magneticas e de transporte eletrico foi realizado nessa serie. O mecanismo de transporte eletrico usando o modelo de hopping de pequenos polarons no regime nao adiabatico foi revisitado considerando termos de ordem superior na equacao que descreve a probabilidade de hopping. Foi obtida uma equacao mais generica para descrever satisfatoriamente a dependencia com a temperatura da resistividade eletrica dos compostos com maior dopagem (x = 0.40 e 0.45). A analise dos parametros fisicos obtidos do ajuste indica que o regime nao adiabatico considerando termos de ordem superior e necessario para descrever o mecanismo de transporte eletrico em compostos com resistividade eletrica elevada. Por outro lado, um desvio da lei de Curie-Weiss foi observado no estado paramagnetico nesta serie indicando a presenca de interacoes magneticas de curto alcance. Um estudo sistematico do momento magnetico efetivo em funcao da temperatura e dos portadores de carga indica que tal desvio nao pode ser causado pela formacao de clusters ferromagneticos, onde os ions de Mn3+ e Mn4+ interagem via o mecanismo de dupla troca, como comumente sugerido na literatura. Os resultados revelam que os eletrons eg estao localizados no ion de Mn3+ independentemente da introducao de buracos no sistema sugerindo a presenca de interacoes magneticas do tipo supertroca. O segundo sistema estudado e a ferrita de bismuto com propriedades multiferroicas Bi2Fe4O9. A influencia da introducao de desordem quimica no composto Bi2Fe4O9, atraves da substituicao parcial do Fe pelo Mn [Bi2Fe4-xMnxO9+¿Ã, com x = 0, 1.0, 2.0, 3.0 e 4.0], sobre as propriedades fisicas foi investigada. Foi encontrado que a desordem quimica causa alteracoes em varias propriedades: (1) surgimento de uma coexistencia de duas fases cristalograficas tipo-Bi2Mn4O10 e tipo-Bi2Fe4O9 para x = 1.0 e 2.0; (2) inducao de uma transicao antiferromagnetica em temperaturas muito baixas onde a temperatura de Neel (TN) varia com x; (3) diminuicao do valor da resistividade eletrica alcancando o seu minimo valor em x = 3.0. Foi observado que o mecanismo de transporte eletrico de todas as amostras obedece ao regime adiabatico do modelo de hopping de pequenos polarons. A energia de ativacao e a frequencia de hopping de algumas amostras exibem uma alteracao em altas temperaturas. Foi sugerido que tais alteracoes sao induzidas devido a variacoes nas posicoes de equilibrio dos ions de oxigenio, caracterizadas atraves de medidas locais usando a tecnica de correlacao angular perturbada. Alem disso, um campo magnetico hiperfino foi encontrado, o qual surge devido a transicao antiferromagnetica de longo alcance das amostras Bi2Fe4O9 (250 K) e Bi2Mn4O10 (39 K). Evidencias do acoplamento magnetoeletrico nas amostras Bi2Fe4O9 e Bi2Mn4O10 foram reveladas atraves de uma variacao anomala na dependencia com a temperatura da frequencia quadrupolar (¿ËQ) e do parametro de assimetria (¿Å). O terceiro composto estudado e a ferrita de bismuto BiFeO3 a qual tambem exibe propriedades multiferroicas. Um estudo sistematico das propriedades magneticas e eletricas de nanoparticulas de BiFeO3 com dois tamanhos levemente diferentes (S1 e S2) foi realizado. Medidas de susceptibilidade magnetica em funcao da temperatura sugerem a presenca de um estado do tipo spin-glass em baixas temperaturas nas duas amostras. Foi mostrado que a presenca de fase magnetica espuria em porcentagens muito pequenas (nao detectaveis por difracao de raios x) pode afetar fortemente as propriedades magneticas e eletrica de nanoparticulas de BiFeO3. Mostrou-se que as propriedades eletricas (resistividade eletrica, impedancia, polarizacao e constate dieletrica) desse composto podem ser influenciadas pela adsorcao quimica e fisica de moleculas de agua na superficie da amostra. Medidas de resistividade eletrica obtidas em atmosfera ambiente revelaram que a principal contribuicao ao mecanismo de conducao e a delocalizacao de vacancias de oxigenio. Ja as medidas realizadas com fluxo de argonio mostraram uma queda abrupta em forma de cascata da resistividade eletrica em funcao da temperatura. Esse processo e reversivel com a diminuicao da temperatura sugerindo uma transicao de fase. Acredita-se que nas medidas em atmosfera ambiente, esse efeito e suprimido devido ao aumento da resistividade eletrica causado pela dessorcao de moleculas de agua em altas temperaturas. Alem disso, a polarizacao eletrica e constante dieletrica assumem valores colossais em altas temperaturas e baixas frequencias. / In this work, we have studied three systems of transition metal oxides which exhibit strongly correlated physical properties. The first system was the magnetoresistive series La1-xCaxMnO3, with x = 0.20, 0.25, 0.30, 0.34, 0.40 and 0.45. A systematic study of the magnetic and electrical transport properties has been done in this series. The non-adiabatic regime of the small polaron model was revisited considering higher order terms in the hopping probability equation. We have obtained a more general equation in order to describe the temperature dependence of the electrical resistivity for compounds with higher doping (x = 0.40 to 0:45). The analysis of the physical parameters obtained from the fitting indicates that the non-adiabatic regime with higher order terms is needed to describe the electric transport mechanism in compounds with high electrical resistivity. On the other hand, it was observed a deviation from the Curie-Weiss law in the paramagnetic state indicating the presence of short-range magnetic interactions in this series. A systematic study of the effective magnetic moment as a function of temperature and charge carriers indicates that such deviation may not be caused by the formation of ferromagnetic clusters, where the ions Mn3+ and Mn4+ interact via double exchange mechanism, as commonly suggested in the literature. The results reveal that electrons eg are localized in the Mn3+ ion regardless of the introduction of holes in the system suggesting the presence of super-exchange like magnetic interactions. The second studied system was the bismuth ferrite Bi2Fe4O9 with multiferroic properties. The influence of the chemical disorder, produced by partial substitution of Fe in the Mn-site [Bi2Fe4-xMnxO9+å, with x = 0, 1.0, 2.0, 3.0 and 4.0], on the physical properties of Bi2Fe4O9 was studied. We have found that several physical properties are altered by the chemical disorder: (1) appearance of coexistence of two crystallographic phases Bi2Fe4O9-type and Bi2Mn4O10-type for x = 1.0 and 2.0; (2) presence of an antiferromagnetic transition at very low temperatures where the Neel temperature (TN) depends on x; (3) reduction of electrical resistivity value which reaches its minimum value at x = 3.0. It has been observed that the electric transport mechanism for all samples follows the adiabatic regime of the small polaron model. Interesting, the values of the activation energy and hopping frequency of some samples are not constant. We suggested that this result is caused by variation in the equilibrium positions of oxygen ions. Such variations were characterized by using the perturbed angular correlation local technique. Furthermore, we have observed that a hyperfine magnetic field arises simultaneously with the long-range antiferromagnetic transition in the samples Bi2Fe4O9 (TN = 250 K) and Bi2Mn4O10 (TN = 39 K). An anomalous behavior in the temperature dependence of the quadrupolar frequency (íQ) and asymmetry parameter (ç) provided evidence about the existence of magnetoelectric coupling in the compounds Bi2Fe4O9 and Bi2Mn4O10. The third studied system was the bismuth ferrite BiFeO3 which also exhibits multiferroic properties. We have performed an electrical and magnetic properties comprehensive study on two sets of BiFeO3 nanoparticles with slightly different sizes (S1 and S2). It was observed that the presence of spurious magnetic phase in very small amounts (not detectable by x-ray diffraction) can strongly affect the magnetic and electrical properties of BiFeO3 nanoparticles. We have also observed that the electrical properties (resistivity, impedance, polarization, and dielectric constant) of this compound can be influenced by chemical and physical adsorption of water molecules on the sample surface. Furthermore, the analysis of the electrical resistivity measurements obtained at ambient atmosphere suggests that the delocalization of oxygen vacancies is the main contribution to the transport mechanism. On the other hand, the measurements obtained with argon flow revealed that the electrical resistivity undergoes an abrupt decrease such as cascade-like behavior. This process is reversible on warming and cooling curves suggesting a phase transition. We believe that in the measurements obtained at ambient atmosphere, this effect is suppressed due to increase of the electrical resistivity caused by desorption of water molecules at high temperatures. The electrical polarization and dielectric constant exhibit colossal values at high temperatures and low frequencies.

Fundação de Apoio a Pesquisa e à Inovação Tecnológica do Estado de Sergipe - FAPITEC/SE / The RMnO3 (R=Y,Lu) compounds, members of the family of hexagonal manganites, are materials which exhibit strong magneto-electric coupling characteristic for multiferroic compounds. Despite the numerous studies with objective to reveal the origin of this phenomenon, the mechanism that is causing it is still not fully understood. The question that attracts special attention of scientific community is about the cause of ferroelectric distortion that occurs at certain temperatures in these materials. Although this issue is discussed in many papers, there is still no consensus what mechanism is responsible for it: (1) hybridization between the Mn dz2 orbital and O pz orbital (Mn d0 –ness model), (2) hybridization between the R dz2 orbital and O pz orbital (R d0 –ness model), (3) geometric effects or (4) charge transfer from Mn-O bonds to R-O bonds. Understanding of the mechanism that leads to ferroelectric polarization is necessary to speed application of these materials in ferroelectric memories or spintronics devices. Objective of the present work was to evaluate the first two possible mechanisms that might cause the ferroelectric distortion in RMnO3. Adopted strategy was to investigate what happens with the Mn-O and R-O chemical bonds after the structural phase transition from paraelectric to ferroelectric phase that occurs at high temperature, at which both phases are characterized by paramagnetic order of the Mn magnetic moments. For that purpose, the first-principles calculations based on density functional theory were carried on, with usage of the most modern exchange-correlation potentials. The chemical bonds were evaluated (1) qualitatively, by analysis of density of electronic states (DOS) and maps of electronic density along the bonds, and (2) quantitatively, in terms of topological analysis of Bader. The results revealed that no significant change occurred with Mn-O bonds, while the R-O bonds were affected by phase transition in the sense that R dz2 and O pz orbital exhibited enhanced hybridization in the ferroelectric phase. Therefore, the present study substantiates the R d0 –ness mechanism as the probable cause of ferroelectric distortions in RMnO3 compounds. / The RMnO3 (R=Y,Lu) compounds, members of the family of hexagonal manganites, are materials which exhibit strong magneto-electric coupling characteristic for multiferroic compounds. Despite the numerous studies with objective to reveal the origin of this phenomenon, the mechanism that is causing it is still not fully understood. The question that attracts special attention of scientific community is about the cause of ferroelectric distortion that occurs at certain temperatures in these materials. Although this issue is discussed in many papers, there is still no consensus what mechanism is responsible for it: (1) hybridization between the Mn dz2 orbital and O pz orbital (Mn d0 –ness model), (2) hybridization between the R dz2 orbital and O pz orbital (R d0 –ness model), (3) geometric effects or (4) charge transfer from Mn-O bonds to R-O bonds. Understanding of the mechanism that leads to ferroelectric polarization is necessary to speed application of these materials in ferroelectric memories or spintronics devices. Objective of the present work was to evaluate the first two possible mechanisms that might cause the ferroelectric distortion in RMnO3. Adopted strategy was to investigate what happens with the Mn-O and R-O chemical bonds after the structural phase transition from paraelectric to ferroelectric phase that occurs at high temperature, at which both phases are characterized by paramagnetic order of the Mn magnetic moments. For that purpose, the first-principles calculations based on density functional theory were carried on, with usage of the most modern exchange-correlation potentials. The chemical bonds were evaluated (1) qualitatively, by analysis of density of electronic states (DOS) and maps of electronic density along the bonds, and (2) quantitatively, in terms of topological analysis of Bader. The results revealed that no significant change occurred with Mn-O bonds, while the R-O bonds were affected by phase transition in the sense that R dz2 and O pz orbital exhibited enhanced hybridization in the ferroelectric phase. Therefore, the present study substantiates the R d0 –ness mechanism as the probable cause of ferroelectric distortions in RMnO3 compounds.