京都大学 / 新制・課程博士 / 博士(理学) / 甲第23688号 / 理博第4778号 / 新制||理||1684(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 柳瀬 陽一, 教授 川上 則雄, 教授 松田 祐司 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

In recent years, two-dimensional (2D) van der Waals (vdW) materials have aroused much interest for their unique structural, thermal, optical, and electronic properties and have become a hot topic in condensed matter physics and material science. Many research methods, including scanning tunneling microscopy (STM), transmission electron microscopy (TEM), optical and transport measurements, have been used to investigate these unique properties. Among them, STM stands out as a powerful characterization tool with atomic resolution and is capable of simultaneously revealing both atomic structures and local electronic properties. This dissertation focuses on scanning tunneling microscopy and spectroscopy (STM/S) investigation of 2D metal chalcogenides and heterostructures. The first part of the dissertation focuses on the continuous interface in WS2/MoS2 heterostructures grown by the chemical vapor deposition (CVD) method. We observed a closed interface between the MoS2 monolayer and the heterobilayer with atomic resolution. Furthermore, our scanning tunneling spectroscopy (STS) results and density functional theory (DFT) calculations revealed band gaps of the heterobilayer and the MoS2 monolayer agree with previously reported values for MoS2 monolayer and MoS2/WS2 heterobilayer on SiO2 fabricated through the mechanical exfoliation method. The results could deepen our understanding of the growth mechanism, interlayer interactions and electronic structures of 2D transition metal dichalcogenides (TMD) heterostructures synthesized via CVD. The second part of the dissertation focuses on phase transformation in 2D In2Se3. We observed that 2D In2Se3 layers with thickness ranging from single to ~20 layers stabilized at the beta phase with a superstructure at room temperature. After cooling down to around 180 K, the beta phase converted to a more stable beta' phase that was distinct from previously reported phases in 2D In2Se3. The kinetics of the reversible thermally driven beta-to-beta' phase transformation was investigated by temperature dependent transmission electron microscopy and Raman spectroscopy, combined with the expected minimum-energy pathways obtained from our first-principles calculations. Furthermore, DFT calculations reveal in-plane ferroelectricity in the beta' phase. STS measurements show that the indirect bandgap of monolayer beta' In2Se3 is 2.50 eV, which is larger than that of the multilayer form with a measured value of 2.05 eV. Our results on the reversible thermally driven phase transformation in 2D In2Se3 will provide insights to tune the functionalities of 2D In2Se3 and other emerging 2D ferroelectric materials and shed light on their numerous potential applications like non-volatile memory devices. The third part of the dissertation focuses on domain boundaries in 2D ferroelectric In2Se3. The atomic structure of domain boundaries in two-dimensional (2D) ferroelectric beta' In2Se3 is visualized with scanning tunneling microscopy and spectroscopy (STM/S) combined with DFT calculations. A double-barrier energy potential across the 60° tail to tail domain boundaries in monolayer beta' In2Se3 is also revealed. The results will deepen our understanding of domain boundaries in 2D ferroelectric materials and stimulate innovative applications of these materials. / Doctor of Philosophy / Two-dimensional (2D) materials are materials consisting of a single layer or a few layers of atoms. They exhibit unique and interesting properties distinct from their bulk counterparts. Over the past decade, much effort has been devoted to a large family of 2D materials — 2D metal chalcogenides that exhibit fascinating structural and electronic properties. These 2D metal chalcogenides can also be stacked together to form various heterostructures. The scanning tunneling microscope (STM) is a powerful tool to study these materials with atomic resolution and is capable of simultaneously revealing both atomic structures and local electronic properties. It can also be used to manipulate nanometer-scale structures on the material surface. In this dissertation, we use scanning tunneling microscopy and spectroscopy (STM/S) to investigate 2D metal chalcogenides and heterostructures. The first part of the dissertation focuses on WS2/MoS2 heterostructures grown by the chemical vapor deposition (CVD) method. We observed a closed interface between the MoS2 monolayer and the heterobilayer with atomic resolution. Furthermore, our scanning tunneling spectroscopy (STS) results and density functional theory (DFT) calculations revealed band gaps of the heterobilayer and the MoS2 monolayer. The results could deepen our understanding of the growth mechanism, interlayer interactions and electronic structures of 2D transition metal dichalcogenides (TMD) heterostructures synthesized via CVD. The second part of the dissertation focuses on phase transformation in 2D In2Se3. We observed that 2D In2Se3 layers transform from beta phase to a more stable beta' phase when the sample is cooled down from room temperature to 77 K. This thermally driven beta-to-beta' phase transformation was found to be reversible by temperature dependent transmission electron microscopy and Raman spectroscopy, corroborated with the expected minimum-energy pathways obtained from our first-principles calculations. Furthermore, DFT calculations reveal in-plane ferroelectricity in the beta' phase. Our results on the reversible thermally driven phase transformation in 2D In2Se3 will provide insights to tune the functionalities of 2D In2Se3 and other emerging 2D ferroelectric materials. The third part of the dissertation focuses on domain boundaries in 2D ferroelectric In2Se3. The atomic structure of domain boundaries in 2D ferroelectric beta' In2Se3 is visualized by using STM/S combined with DFT calculations. A double-barrier energy potential across the 60° tail to tail domain boundaries in monolayer beta' In2Se3 is also revealed. The results will deepen our understanding of domain boundaries in 2D ferroelectric materials and stimulate innovative applications of these materials.

The integration of sensors, actuators, and control systems is an ongoing process in a wide range of applications covering automotive, medical, military, and consumer electronic markets. Four major families of ceramic and metallic actuators are under development: piezoelectrics, electrostrictors, magnetostrictors, and shape-memory alloys. All of these materials undergo at least two phase transformations with coupled thermodynamic order parameters. These transformations lead to complex domain wall behaviors, which are driven by electric fields (ferroelectrics), magnetic fields (ferromagnetics), or mechanical stress (ferroelastics) as they transform from nonferroic to ferroic states, contributing to the sensing and actuating capabilities. This research focuses on two multiferroic crystals, Pb(Mg1/3Nb2/3)O3-PbTiO3 and Fe-Ga, which are characterized by the co-existence and coupling of ferroelectric polarization and ferroelastic strain, or ferro-magnetization and ferroelastic strain. These materials break the conventional boundary between piezoelectric and electrostrictors, or magnetostrictors and shape-memory alloys. Upon applying field or in a poled condition, they yield not only a large strain but also a large strain over field ratio, which is desired and much benefits for advanced actuator and sensor applications. In this thesis, particular attention has been given to understand the structure-property relationships of these two types of materials from atomic to the nano/macro scale. X-ray and neutron diffraction were used to obtain the lattice structure and phase transformation characteristics. Piezoresponse and magnetic force microscopy were performed to establish the dependence of domain configurations on composition, thermal history and applied fields. It has been found that polar nano regions (PNRs) make significant contributions to the enhanced electromechanical properties of PMN-x%PT crystals via assisting intermediate phase transformation. With increasing PT concentration, an evolution of PNRï  PND (polar nano domains)-> micron-domains-> macro-domains was found. In addition, a domain hierarchy was observed for the compositions near a morphotropic phase boundary (MPB) on various length scales ranging from nanometer to millimeter. The existence of a domain hierarchy down to the nm scale fulfills the requirement of low domain wall energy, which is necessary for polarization rotation. Thus, upon applying an E-field along <001> direction(s) in a composition near the MPB, low symmetry phase transitions (monoclinic or orthorhombic) can easily be induced. For PMN-30%PT, a complete E-T (electric field vs temperature) diagram has been established. As for Fe-x at.% Ga alloys, short-range Ga-pairs serve as both magnetic and magnetoelastic defects, coupling magnetic domains with bulk elastic strain, and contributing to enhanced magnetostriction. Such short-range ordering was evidenced by a clear 2theta peak broadening on neutron scattering profiles near A2-DO3 phase boundary. In addition, a strong degree of preferred [100] orientation was found in the magnetic domains of Fe-12 at.%Ga and Fe-20 at.%Ga alloys with the A2 or A2+DO3 structures, which clearly indicates a deviation from cubic symmetry; however, no domain alignment was found in Fe-25 at.%Ga with the DO3 structure. Furthermore, an increasing degree of domain fluctuations was found during magnetization rotation, which may be related to short-range Ga-pairs cluster with a large local anisotropy constant, due to a lower-symmetry structure. / Ph. D.

Ce mémoire de thèse porte sur la caractérisation de l'effet électrocalorique des matériaux ferroélectriques solides et cristaux liquides. La découverte récente d'un effet électrocalorique qualifié de "géant" a relancé l'intérêt pour l'étude et la caractérisation des propriétés électrocaloriques des matériaux. Au-delà de la recherche des matériaux performants, ce domaine de recherche concerne également le développement des techniques de caractérisations appropriées et la réalisation des prototypes de réfrigération électrocalorique. Dans ce contexte, notre étude se focalise sur le développement de nouvelles techniques de caractérisation, la méthode photopyroélectrique (indirecte) et la calorimétrie (directe). La technique photopyroélectrique, développée au sein du laboratoire a été utilisée pour la détermination des propriétés thermiques du matériau pyroélectrique lui-même. L'exploitation de cette technique nous a permis également de déterminer les propriétés pyroélectriques du matériau, en particulier le rapport entre le coefficient pyroélectrique et la capacité calorifique, en fonction de la température et du champ électrique appliqué, nécessaire pour une évaluation indirecte de l'effet électrocalorique. Plusieurs matériaux ferroélectriques solides et liquides ont été étudiés à l'aide de cette méthode, en particulier, un monocristal de TriGlycine Sulfate et deux cristaux liquides. L'effet électrocalorique a été évalué autour de la température de transition de phase que présentent chacun de ces matériaux. Pour valider les résultats obtenus, nous avons procédé à des mesures indirectes de la polarisation par la méthode usuelle du courant de dépolarisation. Dans ce travail, nous avons également développé une technique de mesure indirecte de l'effet électrocalorique, par mesure calorimétrique, à l'aide d'un nouveau dispositif. Outre l'étude des transitions de phase et de la capacité calorifique, cet instrument permet une mesure directe de la température et de la quantité de chaleur absorbée ou cédée avec le milieu environnant. Une première étude de l'effet électrocalorique a été réalisée sur un matériau multicouche à base de titanate de baryum. Les résultats obtenus par cette approche ont été ensuite comparés à d'autres techniques directes et indirectes existantes dans la littérature. Ces deux nouvelles approches permettent d'élargir les possibilités d'étude de futurs matériaux électrocaloriques et de mesurer à la fois les propriétés thermiques et pyroélectriques nécessaires pour l'étude de l'effet électrocalorique. / This thesis work deals with the characterization of the electrocaloric effect in solid and liquid crystal ferroelectric materials. Following the 2006 discovery of Mischenko and al., the characterization techniques of the electrocaloric effect and the exploration of new caloric materials have attracted much attention. This discovery showed also that electrocaloric materials can be used for efficient innovative solution for refrigeration devices. This PhD dissertation focuses on the development of new techniques used to evaluate the electrocaloric effect by the photopyroelectric technique and by calorimetry. Few years ago, a new particular configuration based on the photopyroelectric technique, developed in our laboratory, was described for measuring thermal parameters of pyroelectric materials themselves. By means of this technique, we indirectly investigate the electrocaloric effect in ferroelectric materials by measuring the ratio of the pyroelectric coefficient to the volumetric heat capacity, as function of temperature and applied field, using Maxwell's relation. Measurements were carried out on ferroelectric solid materials (TriGlycine Sulfate) and liquid crystals. Electrocaloric effect has been evaluated around the phase transition temperature of each sample. To further validate the accuracy to the evaluated adiabatic temperature changes, we proceeded to indirect measurements by using the polarization reversal current technique. In the present work, we also developed a calorimeter in order to directly evaluate the electrocaloric effect. This technique is mainly used to measure with high resolution the heat capacity and enthalpy near phase transition temperature. In addition, this technique allows us to directly measure the temperature and the amount of heat absorbed or transferred from the material to the surrounding environment. A primary study of the electrocaloric effect was carried out on a multilayer material based on barium titanate. The results obtained by this approach have been then compared to conventional direct and indirect measurements. Those two new approaches give access to the measurement of both thermal and pyroelectric properties allowing the evaluation of the electrocaloric effect.

Diese Arbeit untersucht die vielfältigen Wechselwirkungen zwischen elektronischen und strukturellen Eigenschaften in Perovskit-Oxiden und in einem molekularen Kristall. Optische Anregung mit ultrakurzen Lichtimpulsen verändert die elektronische Struktur und die Dynamik der damit verbundenen reversiblen Gitterveränderung wird mit zeitaufgelöster Femtosekunden Röntgenbeugung direkt aufgezeichnet. Eine Nanostruktur aus metallischen und ferromagnetischen Strontium Ruthenat (SRO) und dielektrischen Strontium Titanat Schichten dient als Modellsystem, um optisch induzierten Druck auf einer subpikosekunden Zeitskala zu untersuchen. In der ferromagnetischen Phase zeigen phononischer und magnetostriktiver Druck eine vergleichbare ultraschnelle Dynamik und eine ähnliche Größe unterschiedlichen Vorzeichens. Die Amplitude des magnetischen Drucks folgt dem Quadrat der temperaturabhängigen Magnetisierung. In einem weiteren Doppelschichtsystem komprimiert der sich ultraschnell aufbauende phononische Druck in SRO benachbarte ferroelektrische Blei Zirkonat Titanat Schichten. Dies reduziert die tetragonale Verzerrung von bis zu 2 Prozent innerhalb 1.5 Pikosekunden und koppelt an die ferroelektrische "weiche Mode", beziehungsweise an die Ionenverschiebung innerhalb der Einheitszelle. Damit verbunden wird die makroskopische Polarisation bis zu 100 Prozent reduziert; aufgrund der Anharmonizität der Kopplung mit einer Verzögerung von 500 Femtosekunden. Femtosekunden Photoanregung von Chromophoren in einem molekularen Kristall induziert eine Änderung des Diopolmomentes durch intramolekularen Ladungstransfer. Die Änderung der gestreuten Röntgenintensität weist auf eine Molekül-Rotationsbewegung in der Umgebung angeregte Dipole hin, welche der 10 Pikosekunden Dynamik des Ladungstransfer folgt. Die transienten Röntgenstreusignale werden vollständig von der kollektiven Solvatation bestimmt und verdecken lokale, intramolekulare Strukturänderungen. / This thesis investigates the manifold couplings between electronic and structural properties in crystalline Perovskite oxides and a polar molecular crystal. Ultrashort optical excitation changes the electronic structure and the dynamics of the connected reversible lattice rearrangement is imaged in real time by femtosecond X-ray scattering experiments. An epitaxially grown superlattice consisting of alternating nanolayers of metallic and ferromagnetic strontium ruthenate (SRO) and dielectric strontium titanate serves as a model system to study optically generated stress. In the ferromagnetic phase, phonon-mediated and magnetostrictive stress in SRO display similar sub-picosecond dynamics, similar strengths but opposite sign and different excitation spectra. The amplitude of the magnetic component follows the temperature dependent magnetization square, whereas the strength of phononic stress is determined by the amount of deposited energy only. The ultrafast, phonon-mediated stress in SRO compresses ferroelectric nanolayers of lead zirconate titanate in a further superlattice system. This change of tetragonal distortion of the ferroelectric layer reaches up to 2 percent within 1.5 picoseconds and couples to the ferroelectric soft mode, or ion displacement within the unit cell. As a result, the macroscopic polarization is reduced by up to 100 percent with a 500 femtosecond delay that is due to final elongation time of the two anharmonically coupled modes. Femtosecond photoexcitation of organic chromophores in a molecular, polar crystal induces strong changes of the electronic dipole moment via intramolecular charge transfer. Ultrafast changes of transmitted X-ray intensity evidence an angular rotation of molecules around excited dipoles following the 10 picosecond kinetics of the charge transfer reaction. Transient X-ray scattering is governed by solvation, masking changes of the chromophore''s molecular structure.

Dans ce travail, nous avons étudié la croissance de films d’oxydes Bi2FeCrO6 (BFCO) en utilisant les techniques de sol-gel et dépôt par laser pulsé (PLD). Dans le cas de la voie chimique, des précurseurs en solution ont été préparés, puis déposés par centrifugation sur des substrats de silicium (100) ou de quartz. Les nombreuses analyses structurelles (DRX) et d'imagerie (SEM, TEM) effectuées sur ces films BFCO ont montré que les films sont assez homogènes mais présentent de nombreuses phases parasites qui peuvent être éliminés partiellement par recuit thermique rapide. Des dispositifs tests à base de films BFCO par sol-gel ont été préparés et qui ont montré des propriétés électriques limitées à cause des nombreux défauts. Des films BFCO ont également été produits par la technique PLD sur des substrats STO et NbSTO. Les propriétés structurelles, optiques et électriques sont présentées. La diffusion épitaxiale de haute qualité et les films en phase pure sont démontrés par diffraction des rayons X. Nous avons étudié l'évolution de paramètres tels que la bande interdite en fonction des conditions de croissance, montrant qu'elle peut être ajustée de 1, 9 à 2,6 eV. Ce comportement a été corroboré par des calculs théoriques sur l’arrangement atomique dans la structure BFCO. Les propriétés ferroélectriques sont étudiées par microscopie à force piézoélectrique. La lumière s'est avérée avoir un effet sur la polarisation. Il a également été démontré que la mémoire de la polarisation affecte la réponse photovoltaïque. Enfin, des dispositifs basés sur BFCO sont fabriqués et leurs propriétés photovoltaïques sont analysées. Des valeurs de tension de circuit ouvert de 600mV sont encourageantes pour la nouvelle génération de cellules solaires. / In this work, we have produced Bi2FeCrO6 oxides (BFCO) by sol-gel technique and pulsed laser deposition (PLD). By sol-gel, precursors in solution were prepared, which are then deposited by centrifugation on silicon or quartz substrates. The numerous structural (XRD) and optical images (SEM, TEM) analyses carried out on these BFCO films show that the films are fairly homogeneous but exhibit many parasitic phases, which they can be partly eliminated by rapid thermal annealing. Finally, we present the first results obtained on BFCO-SG perovskite devices. On the other hand, BFCO films were deposited on STO and Nb:STO substrates. Their structural, optical and electrical properties are presented. High-quality epitaxial growth and pure-phase films are demonstrated by X-ray diffraction. We show that the band gap of the PLD-BFCO films can be tuned from 1, 9 to 2.6 eV thanks to the variation of growth conditions. Theoretical calculations has confirmed the observed behavior and highlight the importance of the ordering phase. The ferroelectric properties of the PLD films are studied by the piezoresponse force microscopy. Illumination is shown to have a strong effect on polarization. We show that the polarization memory affects the photovoltaic response. Finally, devices based on BFCO are manufactured and their photovoltaic properties are analyzed.

Des facteurs fondamentaux pour la performance de matériaux à haute permittivité basés sur des pérovskites sont étudiés par des techniques ab-initio. L’anomalie diélectrique géante à 0K des pérovskites est investiguée aux niveaux Hartree-Fock, Théorie de la Fonctionnelle de la Densité et méthodes hybrides, en termes de la stabilité thermodynamique des phases, des contributions structurales et vibrationnelles et des charges effectives de Born. Le mode mou actif en infra-rouge est responsable par l’anomalie et des méthodes hybrides sont nécessaires pour reproduire les données expérimentales. L’évolution de l’instabilité ferroélectrique dans les pérovskites ABO3 avec la substitution de l’ion A est reliée de façon systématique aux rayons ioniques ainsi qu’aux degrés d’hybridation dans les liaisons pour des tantalates, niobates et titanates. / Ab-initio techniques are used to investigate key factors influencing the dielectric performance of high permittivity materials based on perovskites. The pressure-induced giant dielectric anomaly at 0K of ABO3 perovskites is investigated at the Hartree-Fock, density-functional theory and hybrid levels. Its mechanism is analyzed in terms of thermodynamic phase stability, structural and phonon contributions and Born effective charges. It is shown that the IR-active soft phonon is responsible for the anomaly and the use of hybrid functionals is required for agreement with experimental data. The evolution of the ferroelectric instability in ABO3 perovskites with substitution of the ion in the A site is systematically investigated for tantalates, niobates and titanates and correlated with the ionic radii as well as the degree of hybridization in the bonds.

The increasing miniaturization of electric and mechanical components makes the synthesis and assembly of nanoscale structures an important step in modern technology. Functional materials, such as the ferroelectric perovskites, are vital to the integration and utility value of nanotechnology in the future. In the present work, chemical methods to synthesize one-dimensional (1D) nanostructures of ferroelectric perovskites have been studied. To successfully and controllably make 1D nanostructures by chemical methods it is very important to understand the growth mechanism of these nanostructures, in order to design the structures for use in various applications. For the integration of 1D nanostructures into devices it is also very important to be able to make arrays and large-area designed structures from the building blocks that single nanostructures constitute. As functional materials, it is of course also vital to study the properties of the nanostructures. The characterization of properties of single nanostructures is challenging, but essential to the use of such structures. The aim of this work has been to synthesize high quality single-crystalline 1D nanostructures of ferroelectric perovskites with emphasis on PbTiO3 , to make arrays or hierarchical nanostructures of 1D nanostructures on substrates, to understand the growth mechanisms of the 1D nanostructures, and to investigate the ferroelectric and piezoelectric properties of the 1D nanostructures. In Paper I, a molten salt synthesis route, previously reported to yield BaTiO3 , PbTiO3 and Na2Ti6O13 nanorods, was re-examined in order to elucidate the role of volatile chlorides. A precursor mixture containing barium (or lead) and titaniumwas annealed in the presence of NaCl at 760 °C or 820 °C. The main products were respectively isometric nanocrystalline BaTiO3 and PbTiO3. Nanorods were also detected, but electron diffraction revealed that the composition of the nanorods was respectively BaTi2O5/BaTi5O11 and Na2Ti6O13 for the two different systems, in contradiction to the previous studies. It was shown that NaCl reacted with BaO(PbO) resulting in loss of volatile BaCl2 (PbCl2 ) and formation and preferential growth of titanium oxide-rich nanorods instead of the target phase BaTiO3 (or PbTiO3 ). The molten salt synthesis route may therefore not necessarily yield nanorods of the target ternary oxide as reported previously. In addition, the importance of NaCl(g) for the growth of nanorods below the melting point of NaCl was demonstrated in a special experimental setup, where NaCl and the precursors were physically separated. In Paper II and III, a hydrothermal synthesis method to grow arrays and hierarchical nanostructures of PbTiO3 nanorods and platelets on substrates is presented. Hydrothermal treatment of an amorphous PbTiO3 precursor in the presence of a surfactant and PbTiO3 or SrTiO3 substrates resulted in the growth of PbTiO3 nanorods and platelets aligned in the crystallographic &lt;100&gt; orientations of the SrTiO3 substrates. PbTiO3 nanorods oriented perpendicular to the substrate surface could also be grown directly on the substrate by a modified synthesis method. The hydrothermal method described in Paper II and III was developed on the basis of the method described in Appendices I and II. In Paper IV, a template-assisted method to make PbTiO3 nanotubes is presented. An equimolar Pb-Ti sol was dropped onto porous alumina membranes and penetrated into the channels of the template. Single-phase PbTiO3 perovskite nanotubes were obtained by annealing at 700 °C for 6 h. The nanotubes haddiameters of 200 - 400 nm with a wall thickness of approximately 20 nm. Excess PbO or annealing in a Pb-containing atmosphere was not necessary in order to achieve single phase PbTiO3 nanotubes. The influence of the heating procedure and the sol concentration is discussed. In Paper V, a piezoresponse force microscopy study of single PbTiO3 nanorods is presented. The piezoelectric properties were studied in both vertical and lateral mode. Piezoelectric activity and polarization switching was observed in the vertical mode, demonstrating the ferroelectric nature of the nanorods. The nanorods decomposed after repeated cycling of the dc bias at one spot on the nanorod, which resulted in parts of the nanorod disappearing and/or accumulation of particles on the surface of the nanorod. In Paper VI, a method to contact single nanorods by electron beam induced deposition of platinum is presented. An organometallic compound, (trimethyl)-methylcyclopentadienylplatinum(IV), was used as precursor. A home-made apparatus was constructed for the purpose and was mounted onto a scanning electron microscope. Calculations based on apparatus geometry and molecular flow were used to estimate the deposition time and the height of the deposits. The location and height of the deposits were controlled so that single nanorods could be successfully contacted at the ends of the nanorods. Fabrication of a sample device for piezoresponse force microscopy studies of single nanorods using an axial dc bias setup is described in Appendix IV. A proposed experimental setup for such studies is also presented.

Thesis (Ph. D.)--University of Washington, 1997. / Vita. Includes bibliographical references (p. [153]-167).

Orientadores: Gustavo Fernandes Lopes, Eduardo Granado Monteiro da Silva / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-14T22:47:18Z (GMT). No. of bitstreams: 1 Fabbris_GilbertoFernandesLopes_M.pdf: 3782519 bytes, checksum: d66cd36be4756310d108388eaef23b5e (MD5) Previous issue date: 2009 / Resumo: Materiais multiferróicos apresentam pelo menos duas propriedades ferróicas na mesma temperatura: (anti) ferroeletricidade, (anti) ferromagnetismo, ou ferroelasticidade. Esses materiais têm atraído grande atenção da comunidade científica nos últimos anos devido às suas potenciais aplicações e ao complexo acoplamento entre suas propriedades, o que ainda não é bem compreendido. Neste trabalho, nos concentramos nos materiais multiferróicos da família RMn2O5, que possuem estrutura ortorrômbica com grupo espacial Pbam. Nestes compostos, a transição antiferromagnética acontece a ~40K e a ferroelétrica em torno de 39K para todos os membros da família. A ferroeletricidade vista nesse material é incompatível com o centro de simetria existente no grupo espacial Pbam. Resultados de EXAFS para o composto TbMn2O5 existentes na literatura indicam uma distribuição bimodal de Tb-O na primeira camada de coordenação. Este desdobramento da primeira camada de coordenação independe da temperatura e indica uma possível variação do grupo espacial Pbam. Tais resultados nos motivaram a empreender o estudo da estrutura local da família RMn2O5 a fim de entendermos a correlação entre a estrutura local e a natureza do íon R. Medidas do espectro de XAFS como função da temperatura foram realizadas na linha XAFS2 do Laboratório Nacional de Luz Síncrotron (LNLS). Foram realizadas varreduras na borda K do Mn e em torno da borda L3 dos íons R (Bi, Gd, Pr, Tb). Os resultados para a borda do manganês em todos os compostos indicam que os poliedros de MnO são rígidos. Já na borda do íon R há comportamentos distintos. Enquanto no composto com bismuto observamos uma possível vibração rígida dos poliedros de MnO, para os outros compostos estudados identificamos uma distorção rígida desses poliedros. O comportamento distinto do composto com Bi, em relação aos demais membros da família, parece estar relacionado ao lone pair do bismuto / Abstract: Multiferroic materials present, at the same temperature, at least two of the so-called ferroic properties: (anti) ferroelectricity, (anti) ferromagnetism and ferroelasticity. They have attracted great attention in the last few years due to their potential applications as well as from the basic science point of view, given the intricate coupling between their physical properties, which remains poorly understood. In this work, we have focused on the study of the RMn2O5 family multiferroic materials. Their crystalline structure belongs to the Pbam orthorhombic space group, with an antiferromagnetic and ferroelectric phase transition temperatures below 40 K and 39K, respectively. Such ferroelectric phase is incompatible with the inversion center of symmetry in the Pbam space group. Recently published EXAFS results for TbM n2O5 revealed a first coordination shell with a bimodal Tb-O bond length distribution. Such bond length splitting is temperature-independent and may be related to a change of the Pbam space group. Such results were our main motivation to undertake a systematic study of the local structure of the RMn2O5 multiferroic family, aiming at a better understanding the correlation between the ion R and the local structure. Mn K-Edge and R-ion L3 Edge (R=Bi, Gd, Pr, Tb) temperature-dependent XAFS measurements were performed at the Brazilian Synchrotron Light Laboratory XAFS2 beam line. Results for the Mn K-edge in all studied compounds reveal the Mn-O coordination polyhedral are rigid. The R-ion results reveal distinct behaviours. For the R=Bi compound, we identified that low energy Mn-O rigid unit vibrational modes are likely to be operative. For the other compounds, we have identified a rigid distortion of the Mn-O polyhedra. The distinct behavior of the BiMn2O5 seems to be related to the Bi 6s2 lone pair / Mestrado / Física da Matéria Condensada / Mestre em Física