Dissertations / Theses on the topic 'The multiferroic BiFeO3'

For over half a century, the technological promise of spins manipulable by a small voltage has captivated the interest of experimental and theoretical researchers alike. However, if thin-film multiferroics are to be incorporated into future data storage devices, a much greater understanding of their behaviour and how they differ from their bulk counterparts is required. In this thesis, we probe the fundamental multiferroic properties of BiFeO₃ films through a combination of state-of-the-art diffraction and microscopy techniques. We investigate the coupling between magnetic, ferroelectric, and structural order, with a focus on domains, and how the domain structure may be manipulated in order to tailor the multiferroic properties of the material. Using non-resonant magnetic x-ray scattering (NXMS) and neutron diffraction, we study the magnetic and structural properties of (111)_pc-oriented BiFeO₃ films. Contrary to the general belief that to they grow as a rhombohedral monodomain, we find that they comprise a sub-micron texture of monoclinic domains. The magnetic structure is found to be intimately coupled to the structure, resulting in the propagation vector being locked to the monoclinic b-axis. This magnetoelastic coupling opens up a route to strain-engineer the magnetic domains via epitaxial strain. By growing BiFeO₃ films on a lower-symmetry, TbScO₃ substrate, we are able to engineer a magnetic, structural and ferroelectric monodomain, coherent over the entire film, constituting an increase in the domain size by over five orders of magnitude. We directly demonstrate the coupling between ferroelectric and magnetic order parameters of the cycloidal magnetic structure. Using NXMS polarimetry to measure directly the magnetic polarity, we show that upon switching the ferroelectric polarisation, the magnetic polarity switches accordingly---a major rearrangement of the magnetic structure, with each spin rotating by 90 degrees on average. This goes counter to idea that magnetic and ferroelectric order parameters are only weakly coupled in type-I multiferroics. Finally, using photoemission electron microscopy we are able to directly image the sub-micron magnetostructural domain structure. We further show that there is a strong interfacial coupling between the magnetostructural domains of BiFeO₃ with a ferromagnetic overlayer. The BiFeO₃ domains are found to impose a uniaxial anisotropy in the overlayer, opening up a route to control ferromagnetic domains.

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Foram preparados filmes finos, de Ferrita de Bismuto (BiFeO3), considerado um dos principais multiferróico que são classes de materiais que apresentam ferroeletricidade e ferromagnetismo simultaneamente. Os filmes foram preparados por um rota química chamada de Sol-gel modificado, variando-se a quantidade de % de mol do Bismuto, depositados em substratos de platina Pt/TiO2/SiO2/Si(100), variando-se a temperatura de cristalização entre 400°C a 600°C, com o objetivo de eliminar algumas fases indesejadas encontradas na literatura. Alguns filmes finos passaram pelo tratamento térmico em atmosférica de O2, com o intuito de diminuir a condutividade, causada pelas vacâncias de oxigênio no material. Pelos resultados obtidos foi possível conseguir filmes finos sem as fases indesejadas e com condutividade não tão alta, sendo possível realizar análises elétricas. Assim, tornou-se possível analisar o comportamento da permissividade, impedância e condutividade em função do campo aplicado e da temperatura. Com tais resultados mostra-se a indicação de polarização iônica nestes filmes. Eles apresentam uma energia de ativação parecida com filme finos encontrados na literatura. Além disso, também mostra que o comportamento das propriedades físicas são os mesmos quando varia a temperatura e o campo.
Bismuth Ferrite (BiFeO3) thin films were prepared, considered one of the main multiferroic that are classes of materials that present ferroelectricity and ferromagnetism simultaneously. The films were prepared by a chemical path called modified sol-gel, varying the amount of Bismuth mol percentage, deposited on Pt/TiO2/SiO2/Si(100) platinum substrates, varying the crystallization temperature between 400 °C to 600 °C, with the aim of eliminating some unwanted phases found in literature. Some thin films underwent the thermal treatment in atmospheric O2, in order to reduce the conductivity, caused by the oxygen vacancies in the material. By the results obtained, it was possible to obtain thin films without the undesired phases and with not so high conductivity, being possible to perform electrical analysis. This way it was possible to analyze the behavior of the permissiveness, impedance and conductivity in function of the applied field and temperature. With these results, it is shown an indication of ionic polarization in these films. They have an activation energy similar to thin films found in literature. It is also shown that the behavior of the physical properties are the same when temperature and the field change.

Esta tesis trata sobre los magnetoel ectricos multiferroicos, una clase relativamente nueva de materiales descubiertos a mediados del siglo pasado, que presentan simultaneamente ferroelectricidad y magnetismo. El BiFeO3 (BFO) es un oxido con estructura perovskita, el cual es uno de los pocos materiales multiferroicos a temperatura ambiente. Sin embargo, como sus temperaturas de ordenamiento ferroel ectrico y anti-ferromagn etico son relativamente altas (alrededor de 1100 K y 640 K, respectivamente), las respuestas electromec anica y magnetoel ectrica del BFO son relativamente peque~nas en condiciones ambientales. En esta tesis se utilizamos m etodos ab-initio, basados en la teor a del funcional de la densidad (DFT), para estudiar las propiedades del BFO, y proponemos una posible estrategia para la mejora de su respuesta. Hemos utilizado m etodos de primeros principios para llevar a cabo una b usqueda sistem atica de las fases potencialmente estables de este compuesto. En la que consideramos las distorsiones m as comunes entre los oxidos de tipo perovskita y encontrando un gran n umero de m nimos locales de la energ a. En este trabajo se discute la gran variedad de estructuras de baja simetr a descubiertas, as como las implicaciones de estos hallazgos en cuanto a los trabajos experimentales mas recientes sobre este compuesto. Tambi en se llev o acabo un estudio de la soluci on s olida Bi1􀀀��xLax FeO3 (BLFO) formada por la BFO y la LaFeO3 (LFO)antiferromagnetica parael ectrica. Se discuten las transformaciones estructurales que sufre BLFO en funci on del contenido de La, y la conexi on de nuestros resultados con los estudios cristalogr a cos existentes. Hemos encontrado que, en una amplia gama de composiciones intermedias, la BLFO presenta fases que son esencialmente degeneradas en energ a. Adem as, los resultados sugieren que para este compuesto, dentro de esta regi on morfotr opica inusual, se puede utilizar un campo el ectrico para inducir transiciones parael ectrico a ferroel ectrico. Tambi en se discuten las propiedades de respuesta de la BLFO y se demuestra que se pueden mejorar signi cativamente en los materiales puros BFO y LFO, mediante la sustituci on parcial de los atomos Bi y La . Por otra parte, se presenta tambi en un estudio de primeros principios de la BFO a altas presiones. En el cual explicamos la naturaleza de las transiciones de fase del BFO, que simult aneamente involucran un colapso del volumen, un cambio en el estado de spin de High spin a Low spin y una metalizaci on producto del desorden magn etico en la nueva fase. Por ultimo presentamos los resultados preliminares de un proyecto en marcha, en el cual estamos modelando la energ etica de las rotaciones de los octaedros de oxigeno en los oxidos de estructura perovskita. Para ello se ha expandido la energ a en funci on de los par ametros de orden que caracterizan dichas rotaciones hasta cuarto orden. Hemos teado el modelo a los resultados de nuestros c alculos de primeros principios y realizado una comprobaci on cuidadosa de su valides, determinando que es necesario recurrir a ordenes mas altos en nuestra teor a.
This work is about magnetoeltric multiferroics, a relatively new class of ma- terials discovered by the mid of the past century, which involve simultaneously ferroelectricity and magnetism. Perovskite oxide BiFeO3 (BFO) is one of the few multiferroic materials at room temperature. However, as its ferroelectric and anti- ferromagnetic transition temperatures are relatively high (about 1100 K and 640 K, respectively), BFO's electromechanical and magnetoelectric responses are small at ambient conditions. In this thesis we used ab-initio methods, based on density functional theory, to study the basic properties of BFO and proposed possible strategies for enhancing its response. We used rst-principles methods to perform a systematic search for potentially stable phases of BFO. We considered the distortions that are most common among perovskite oxides and found a large number of local minima of the energy. We discussed the variety of low-symmetry structures discovered, as well as the implications of these ndings as regards current experimental work on this compound. We also carried out a study of the Bi1􀀀�xLaxFeO3 (BLFO) solid solution formed by multiferroic BFO and the paraelectric antiferromagnet LaFeO3 (LFO). We dis- cussed the structural transformations that BLFO undergoes as a function of La content and the connection of our results with the existing crystallographic stud- ies. We found that, in a wide range of intermediate compositions, BLFO presents competitive phases that are essentially degenerate in energy. Further, our results suggested that, within this unusual morphotropic region, an electric eld might be used to induce various types of paraelectric-to-ferroelectric transitions in the compound. We also discussed BLFO's response properties and showed that they can be signi cantly enhanced by partial substitution of Bi/La atoms in the pure BFO and LFO materials. We analyzed the atomistic mechanisms responsible for such improved properties and showed that the e ects can be captured by simple phenomenological models that treat explicitly the composition x in a Landau-like potential. Furthermore, we performed a rst-principles study of BFO at high pressures. Our work revealed the main structural change in Bi's coordination and suppression of the ferroelectric distortion, electronic spin crossover and metallization, and mag- netic loss of order e ects favored by compression and how they are connected. Our results are consistent with and explain the striking manifold transitions observed experimentally We conclude our thesis presenting the preliminary results of an ongoing project in which we are modeling the energetics of the oxygen octahedra rotations in per- ovskite oxides. The model is tted to the rst-principles results and a careful check of its validity is carried out.

De tous les matériaux multiferroïques, BiFeO3 est celui qui est le plus étudié. C’est un ferroélectrique, antiferromagnétique dont les températures de transition sont bien au-dessus de la température ambiante. De plus, le couplage magnétoélectrique entre ces deux paramètres d’ordre a été observé aussi bien dans les cristaux que dans les couches minces. BiFeO3 possède également la plus grande polarisation ferroélectrique jamais mesurée, 100µC/cm². De gros efforts sont fournis pour comprendre et exploiter les propriétés physiques de ce matériau. Dans ce but, il est important de pouvoir contrôler sa structure en domaines afin d’étudier les phénomènes émergeant aux parois de ces domaines. C’est l’objectif de cette thèse : étudier quelques une des propriétés de BiFeO3, comme la photoélectricité et le magnétisme, tout en prêtant en parallèle une attention particulière à la caractérisation de ces propriétés, dans un domaine et dans une paroi, avec des techniques originales telles que la microscopie de photocourants à balayage (MPB) et le rayonnement synchrotron ou les champs magnétiques intenses. Les images obtenues par MPB, révèlent qu’un champ dépolarisant proche d’une paroi de domaine à 180° peut améliorer de manière significative le rendement des effets photoélectriques : les parois de domaines peuvent être générées et positionnées dans le but de contrôler localement le rendement de l’effet photoélectrique. De plus, l’imagerie de la figure de diffraction de surface d’un réseau de parois de domaines dans des couches minces, par diffusion magnétique résonante de rayons X, permet de montrer que les parois de domaines entraînent la formation de structures magnétiques particulières qui pourraient donner lieu à une aimantation
Among all multiferroics, BiFeO3 is a material of choice because its two ordering temperatures are well above 300K. It is a ferroelectric antiferromagnet, and magnetoelectric coupling has been demonstrated in bulk and in thin films. Remarkably, BiFeO3 has the largest polarization of all known ferroelectrics (100µC/cm²). A huge research effort is carried out worldwide to understand and exploit the physical properties of this material which requires to design and tailor BiFeO3 on many scales. In this sense, developing methods and tools to control the domain structure is essential to explore new emergent phenomena arising at domain walls. This is the aim of the present PhD work. Some of the original properties of BiFeO3 have been investigated including its photoelectric and magnetic properties. A particular attention is given to characterize in a parallel fashion bulk properties and domain walls properties, using original techniques of characterization such as Scanning Photocurrent Microscopy (SPCM), scattering synchrotron facilities or high field pulses. SPCM mapping reveals that depolarizing fields in the vicinity of a 180° domain wall can significantly improve the photovoltaic efficiency. Thus domain walls can be generated and precisely positioned in order to tailor the local photovoltaic efficiency. Moreover, X-ray resonant magnetic scattering on thin films with periodic domain structure shows that domain walls generate specific magnetic structures with possible uncompensated magnetization

Bulk BiFe03 samples prepared by the conventional mixed oxide route were investigated. High purity Bi203 and Fe203 powders were weighed according to stoichiometry and milled for 20 hours. The powders were pressed into cylinders (10mm diameter by 6mm thickness) at 100 MPa. The cylinders were heated at a rate of 3 degrees C/min at temperatures of between 700 degrees C and 900°C for times between 7.5 minutes and 48 hours. XRD spectra collected from both the as-sintered and 'bulk' (internal) surfaces showed the formation of additional Bi2Fe40, and Bi25Fe04o secondary phases coexisting alongside the main BiFeOs phase.

The thesis presented here is focused on the synthesis of iron-bismuth alkoxides and siloxides as precursors for multiferroic BiFeO₃ systems. Spectrum of novel cyclopentadienyl substituted iron-bismuth complexes of the general type [{Cp^y(CO)₂Fe}BiX₂], as potential precursors for cyclopentadienyl iron-bismuth alkoxides or siloxides [{Cp^y(CO)₂Fe}Bi(OR)₂] (R-O^tBu, OSiMe₂^tBu), were obtained and characterised. The use of wide range of cyclopentadienyl rings in the iron carbonyl compounds allowed for a comprehensive analysis of its influence on structure, reactivity as well as solubility of the studied complexes, which are crucial features of potential precursors. The results fill the gap in the chemistry of cyclopentadienyl iron-bismuth complexes. In this work a new method of preparation of novel alkoxides or siloxides iron-bismuth complexes has been developed. In the reaction of Fe₂(CO)₉ with Bi(O^tBu)₃ or Bi(OSiMe₂^tBu)₃ molecular precursors for preparation of heterobimetallic oxides were obtained. Moreover, characterised compounds allowed to extend the knowledge about existence of iron-bismuth clusters and open new ways for the further investigations on the carbonyl iron-bismuth siloxides and alkoxides. The resulting compounds are good single source precursors for the BiFeO₃ materials. The presented synthetic route can be generalized and other heterobimetallic compounds can be obtained. This work should also be helpful in the designing new precursors for synthesis of metal oxides.

Multiferroic (BaTiO3-BiFeO3) × 15 multilayer heterostructures show high magnetoelectric (ME) coefficients aME up to αME up to 24 V/cm·Oe at 300 K. This value is much higher than that of a single-phase BiFeO3 reference film (αME = 4.2 V/cm·Oe). We found clear correlation of ME coefficients with increasing oxygen partial pressure during growth. ME coupling is highest for lower density of oxygen vacancy-related defects. Detailed scanning transmission electron microscopy and selected area electron diffraction microstructural investigations at 300K revealed antiphase rotations of the oxygen octahedra in the BaTiO3 single layers, which are an additional correlated defect structure of the multilayers.

Le multiferroїque BiFeO3 est l'un des matériaux ferroïques les plus étudiés à ce jour du fait de la coexistence à température ambiante d'un état ferroélectrique et antiferromagnétique. Il présente de plus une réponse photovoltaïque dont l'origine précise n'est actuellement pas comprise. Le but principal de cette thèse est donc d'étudier les propriétés photovoltaïques de films épitaxiés BiFeO3. Préalablement à l'investigation des propriétés photovoltaïques une étude des mécanismes de conduction a été entreprise. Un transport polaronique de saut via les défauts les plus proches voisins a été mis en évidence et une transition est observée à 253K. En dessous de cette transition les états associés aux défauts proches du niveau de Fermi contribuent principalement et une longueur de saut variable émerge. Cette observation semble être corrélée à la réponse photovoltaïque avec un changement de régime de la tension photo-induite autour de 253K. Cette réponse photovoltaïque est provoquée par l'état ferroélectrique et peut être basculée à l'aide d'un champ électrique. Afin de reproduire artificiellement l'état en domaines associé à l'effet photovoltaïque de BiFeO3 des super-réseaux BiFeO3/SrRuO3 ont été fabriqués et une étude préliminaire de la structure a été entreprise. Nous observons un changement structural d'une phase rhomboédrique vers une phase pseudo quadratique dans ces super-réseaux de période variable et attribuons cette transition à l'influence prépondérante des contraintes élastiques induites dans le plan
The multiferroic BiFeO3 is one of the most studied material because of the room temperature coexisting ferroelectric and antiferromagnetic state. It also shows a photovoltaic response not yet understood. The main objective of this thesis is therefore to investigate the the photovoltaic properties of epitaxial BiFeO3 thin films. Preliminary to photovoltaic studies an investigation of the conduction mechanism has been performed. A polaronic transport with next nearest hopping mechanism is evidenced with a change of regime below 253K. Below 253K variable range hopping transport is observed and involves defects states near the Fermi level. This transport behavior seems connected to the photovoltaic response and change observed at 253K in the photo-induced voltage. Interestingly the photovoltaic response is induced by the ferroelectric state and we demonstrate a switchable photovoltaic effect by an applied electric field. In order to artificially reproduce the domain structure involved in the photovoltaic effect in BiFeO3 BiFeO3/SrRuO3 superlattices have been fabricated and a preliminary structural investigation is presented. A structural change is evidenced from a rhombohedral structure to pseudo-tetragonal state in the superlattices with variable periodicities and we attribute this transition to the influence of the induced in-plane elastic strain

Le matériau BiFeO3 (BFO) est le sujet de très nombreuses études fondamentales dans le domaine des matériaux multiferroïques. Cet intérêt est du au fait que cet oxyde présente deux ordres à longue distance à la température ambiante : ferroélectricité et antiferromagnétisme de type G (ce dernier est aussi non colinéaire avec la présence de faible ferromagnétisme ainsi qu'une modulation de spin de type cycloïdale possédant une longueur d'onde de 620 angstrœm). Il est alors possible d'étudier les comportements de couplage entre les propriétés électrique et magnétique. Ce travail concerne principalement la synthèse, les structures haute température, et les propriétés physiques (électronique et magnétique principalement) du matériau BiFeO3 ayant subi des recuits de différentes pressions partielles d'oxygène. La première étape de ce travail concerne l'étude de la synthèse afin de déterminer le protocole optimal de réalisation des céramiques. Les recuits sous atmosphère ont eu pour but de modifier la stœchiométrie en oxygène du matériau, afin d'affecter ses propriétés physiques. Des modifications de faible amplitude de certaines propriétés ont été détectées, mais à l'inverse, la température de Néel et la température de Curie ne sont pas affectées.Concernant la nature des structures haute température, les phases beta et gamma, sujettes à de nombreuses controverses dans la littérature, ont été étudiées par diffraction des rayons X et analyse DSC sur BFO pur ou avec excès de bismuth. Cet excès a permis de stabiliser la phase gamma entre 940 et 950°C, en évitant sa décomposition. Pour compléter ce travail sur BFO en phase pure, nous avons dopé des céramiques avec 10 % de Zr4+ pour étudier le comportement structurale à haute température, ainsi que les propriétés magnétiques et électriques de cette nouvelle composition. Enfin, des simulations numériques sur le composé stœchiométrique, lacunaire en bismuth ou en oxygène ont été réalisées pour comprendre les évolutions structurale, électronique et magnétique du matériau suite aux recuits. La dernière partie est une étude sur le comportement basse température de BFO pur sous différentes formes : nanotubes, céramiques et monocristaux. Nous avons analysé le comportement électrique (impédance, pyroélectricité, RPE et électrostriction), magnétique (aimantation en fonction de la température et du champ magnétique) et structurale (rayon X en thêta-2thêta et rasant, DSC, microRaman et résonance d'ultrasons). Suite à ces études, trois températures sont observées comme présentant un comportement particulier : 140 et 200 K, qui semblent liées par de nombreuses techniques d'analyses et ressortent comme étant une transition à la surface de BFO, mais aussi 180 K où nous avons un écart à la linéarité de la dilatation thermique et un effet d'électrostriction.

There has been significant interest in BiFeO₃ over the past decade. This interest has focused on the magnetic and electrical properties, which in the long term may prove useful in device applications. This thesis focuses on the synthesis, electrical characterisation, and structural origin of the electrical properties of rare earth doped bismuth ferrite. Two systems have been studied: BiFeO₃ doped with lanthanum and neodymium (Bi₁₋ₓREₓFeO₃ RE= La, Nd). Specific examples have been highlighted focusing on a detailed structural analysis of a lanthanum doped bismuth ferrite, Bi₀.₅La₀.₅FeO₃, and a neodymium analogue, Bi₀.₇Nd₀.₃FeO₃. Both adopt an orthorhombic GdFeO₃-type structure (space group: Pnma) with G-type antiferromagnetism. Structural variations were investigated by Rietveld refinement of temperature dependent powder neutron diffraction using a combination of both conventional “bond angle/bond length” and symmetry-mode analysis. The latter was particularly useful as it allowed the effects of A-site displacements and octahedral tilts/distortions to be considered separately. This in-depth structural analysis was complemented with ac-immittance spectroscopy using the multi-formulism approach of combined impedance and modulus data to correlate structural changes with the bulk electrical properties. This approach was essential due to the complex nature of the electrical response with contributions from different electroactive regions. The structural variations occur due to a changing balance between magnetic properties and other bonding contributions in the respective systems. This results in changes in the magnitude of the octahedral tilts, and A-site displacements giving rise to phenomena such as negative thermal expansion and invariant lattice parameters i.e., the invar effect. More specifically, analysis of Bi₀.₅La₀.₅FeO₃ highlights a structural link between changes in the relative dielectric permittivity and changes in the FeO₆ octahedral tilt magnitudes, accompanied by a structural distortion of the octahedra with corresponding A-site displacement along the c-axis; this behaviour is unusual due to an increasing in-phase tilt mode with increasing temperature. The anomalous orthorhombic distortion is driven by magnetostriction at the onset of antiferromagnetic ordering resulting in an Invar effect along the magnetic c-axis and anisotropic displacement of the A-site Bi³⁺ and La³⁺ along the a-axis. This contrasts with the neodymium analogue Bi₀.₇Nd₀.₃FeO₃ in which a combination of increasing A-site displacements in the ac-plane and decrease in both in-phase and anti-phase tilts combine with superexchange giving rise to negative thermal expansion at low temperature. The A-site displacements correlate with the orthorhombic strain. By carefully changing the synthesis conditions, a significant change in bulk conductivity was observed for a number for Bi₁₋ₓLaₓFeO₃ compositions. A series of Bi₀.₆La0.₄FeO₃ samples are discussed, where changes in the second step of the synthesis result in significantly different bulk conductivities. This behaviour is also observed in other compositions e.g. Bi₀.₇₅La₀.₂₅FeO₃. Changes in the electrical behaviour as a function of temperature are discussed in terms of phase composition and concentration gradients of defects. Activation energies associated with the conduction process(es) in Bi₁₋ₓLaₓFeO₃ samples, regardless of composition, fall within one of two broad regimes, circa. 0.5 eV or 1.0 eV, associated with polaron hopping or migration of charge via oxygen vacancies, respectively. The use of symmetry-mode analysis, in combination with conventional crystallographic analysis and electrical analysis using multi-formulism approach, presents a new paradigm for investigation of structure-property relationships in rare earth doped BiFeO₃.

Ce travail de thèse porte sur la réalisation de deux instruments adaptés à l’étude de couches minces par spectrométrie Mössbauer du 57Fe par électrons de conversion (CEMS) et à leur utilisation pour la caractérisation de films épitaxiés de ferrite de bismuth BiFeO3 (BFO). Le premier dispositif est constitué d’un compteur proportionnel couplé à un module thermoélectrique. Il permet l’acquisition de manière simple et économique de spectres Mössbauer sur une gamme de température variant de 245 à 375K et sous une induction magnétique externe allant jusqu’à 1,4 T. Un second dispositif a été développé sur la base d’un channeltron™et d’un cryostat à circulation d’hélium pour des acquisitions allant jusqu’à 4 K. Les analyses CEMS ont été réalisées sur des couches minces de différentes épaisseurs de BFO (110) et (001) épitaxiées sur LaAlO3 et SrTiO3. Au-delà d’une épaisseur critique, les couches de BFO (110) présentent un mélange de phases magnétiques colinéaire et cycloïdale. La phase colinéaire présente un axe d’anisotropie suivant [001] dans le plan de la couche et lamodulation cycloïdale se propage dans un plan perpendiculaire à celui-ci. Des effets combinés de contraintes et dimensionnalité ont été avancés pour expliquer la déstabilisation de la cycloïde pour les couches les plus fines. Dans les couches minces de BFO (001) présentant une phase tétragonale de BFO, les mesures CEMS ont montré que la température de mise en ordre magnétique se rapproche de l’ambiante lorsque l’épaisseur des couches diminue
This work is devoted to the development of two Mössbauer detectors dedicated to thin films studies by conversion electron Mössbauer spectrometry (CEMS), and to their use for the characterization of bismuth ferrite BiFeO3 (BFO) epitaxials thin films. The first designed instrument is composed of a proportional counter and a thermoelectric module. It allows CEMS acquisitions of Mössbauer spectra from 245 to 375K with an external magnetic field upto 1.4 T. The second device is based on a commercial channeltron™ and a continuous flow cryostat allowing measurements downto 4 K. The CEMS measurements have been performed on (110) and (001) oriented BFO layers with various thickness deposited on LaAlO3 et SrTiO3 substrates. Beyond a critical thickness, the (110) BFO exhibits a mixing of collinear and cycloidal magnetic phases. The collinear phase shows an anisotropy axis [001] direction which is located in the sample plane. The cycloid propagation plane have been found to be perpendicular to the sample plane. Both epitaxial strain and size effects have been proposed to explain the cycloid destabilization in the thinner films. In (001) BFO thin films, exhibiting a BFO tetragonal phase, the CEMS measurements have shown that the magnetic ordering temperature tends to decrease with the layer thickness

Orientadora: Profa. Dra. Marcia Tsuyama Escote
Tese (doutorado) - Universidade Federal do ABC, Programa de Pós-Graduação em Nanociências e Materiais Avançados, Santo André, 2017.
Neste trabalho foram estudados os efeitos da substituição do Bi por um elemento terra-rara (R = Pr, Dy) e da adição de polímeros nas propriedades físicas de compostos de BiFeO3 (BFO) sintetizados pelo método hidrotermal assistido por micro-ondas. Inicialmente, amostras de BFO foram preparadas em diferentes condições de síntese (tempo, temperatura e concentração de KOH) e com este estudo escolheu-se os parâmetros de síntese utilizados neste trabalho para síntese de todas as amostras. As sínteses hidrotermais foram realizadas a 200 °C por 120 min com concentração de KOH de 4 M. Os compostos preparados foram avaliados por meio de medidas de difração de raios X (DRX) e de imagens de microscopia eletrônica de varredura (MEV). Na segunda etapa, o efeito da substituição de bismuto (Bi) por praseodímio (Pr) ou disprósio (Dy) foi investigado por meio de medidas das propriedades físicas caracterizadas por medidas de DRX e análise pelo método de refinamento de Rietveld, imagens de MEV, espec-tros de absorção na região UV-Vis, medidas de constante dielétrica em função da frequência e medidas de magnetização em função do campo magnético aplicado (MxH) e da temperatura (MxT). Por meio das análises de DRX das amostras de Bi1-xRxFeO3, foi observado que com a substituição de Bi por R as amostras tendem a cristalizar-se de forma polimórfica, apresentando duas simetrias: uma romboédrica (R3c) e outra monoclínica (Cc), sendo que a proporção da simetria monoclínica tende a aumentar com o aumento de x. Este polimorfismo, em geral, está associado a presença de Fe2+ na estrutura do Bi1-xRxFeO3, que exerce forte influência nas pro-priedades magnéticas destes compostos. As medidas elétricas mostraram uma melhora dos va-lores da constante dielétrica destas amostras quando comparadas a amostras sem substituição e com resultados listados na literatura para compostos BiFeO3 dopados preparados por outras metodologias. Resultados de absorção na região UV-Vis dos compostos Bi1-xRxFeO3 eviden-ciam uma diminuição do gap de energia de 2,1 eV para a amostra com x = 0 a 1,7eV para com x = 0,3 (Pr). Por fim, no estudo do efeito da adição de polímeros ou surfactantes, foram adici-onados os seguintes materiais: polietilenoglicol (PEG), polivinilpirrolidona (PVP), carboxime-tilcelulose de sódio (NaCMC) ou brometo de cetiltrimetilamonio (CTAB) com o objetivo de verificar a influência de diferentes morfologias nas propriedades físicas do BFO. De fato, o surfactante na síntese do BiFeO3 modificou a morfologia destes compostos, sendo que o resul-tado diferencial foi a obtenção do BiFeO3 na forma de nanobastões utilizando o CTAB. Os demais surfactantes apresentaram formatos similares aqueles já descritos na literatura. As me-didas de UV-Vis revelaram que o valor do gap de energia variou de 1,7 a 2,1 eV com a variação da morfologia do BFO, sendo que este resultado já foi observado em compostos de BFO com diferentes morfologias na literatura. As medidas de constante dielétrica em função da frequên-cia apresentaram um comportamento similar àqueles observados para o BFO preparado sem surfactante. As caracterizações magnéticas revelaram modificações nas curvas de MxT e MxH na região de baixa temperatura (<50 K), o que foi atribuído a presença de fases adicionais nestas amostras.
In this work, the effects of chemical substitution and addition of polymers on the physical prop-erties of BiFeO3 (BFO) compounds synthesized by microwave-assisted hydrothermal method were studied. Firstly, samples of BFO were prepared using different synthesis conditions (time, temperature, KOH concentration), with this study we chose the synthesis parameters used in this work to produce all samples. In order to obtain the parameters that allow the production of compounds with the desired crystalline phase. Hydrothermal syntheses were performed at 200°C during 120 min with KOH concentration of 4M. The compounds were evaluated by X-ray diffraction (XRD) measurements and images of Scanning Electronic Microscopy (SEM). In the second step, the effect of bismuth (Bi) substitution by praseodymium (Pr) or dysprosium (Dy) was investigated by measurements of the physical properties characterized by XRD meas-urements, and analysis by the Rietveld method of refinement, SEM images, absorption spectra in the UV-Vis region, dielectric constant measurements as a function of frequency, and mag-netization measurements as a function of the applied magnetic field (MxH) and temperature (MxT). By means of the XRD analysis of Bi1-xRxFeO3 samples, it was observed that with Bi for R substitution these samples are likely to crystallize in a polymorphic way, which present a rhomboedric (R3c) and a monoclinic (Cc) symmetry. The proportion of monoclinic symmetry tends to increase with the increasing of x. In general, such polymorphism is related to the Fe2+ content in the Bi1-xRxFeO3 structure, which provides a strong influence in the magnetic proper-ties of these compounds. Electrical measurements of the samples show dielectric constants val-ues similar to values observed for undopped and dopped-BiFeO3 prepared by other methodologies. UV-vis absorption results of Bi1-xRxFeO3 compounds revealed a decrease of energy gap from 2.1 eV for sample with x =0 to 1.7 eV for x = 0.3 (Pr). Finally, study of the effect of polymers or surfactants addition, the following materials were added: polyethylene glycol (PEG), polyvinylpyrrolidone (PVP), sodium carboxymethylcellulose (NaCMC) or cetyltrimethylammonium bromide (CTAB) to verify the influence of different morphologies on the physical properties of BFO. In fact, the morphology of BFO was modified through the sur-factant addition, the most remarkable results is the nanostick shape observed for BFO samples prepared with CTAB. Samples prepared using the other surfactants revealed different mor-phology than those reported in literature. UV-vis measurements revealed energy gap varying from 1.7 to 2.1 eV for BFO samples with different morphologies. Dielectric constant measure-ments as function of frequency presents similar behavior than those observed for BFO without surfactant. Magnetic characterizations revealed changes in low temperature region (<50 K), which is attributed to the presence of additional phases in these samples.

Nous menons trois études expérimentales du comportement de renversement de l’aimantation (RM) dans trois types différents de bicouches, et sous différents types de contraintes. Nous étudions l’influence sur les propriétés magnétiques de l’état structural du BiFe03, de contraintes mécaniques magnétoélastiques dans le Fe81Ga19, couplées ensuite à des contraintes électriques et même thermiques.Une bicouche polycristalline composée d’un ferromagnétique Ni81Fe19, et d’un multiferroïque intrinsèque BiFe03, est déposée par pulvérisation cathodique. Sa structure et sa morphologie sont caractérisées par diffraction des rayons X, et microscopie électronique à transmission, révélant deux états structuraux fondamentalement différents du BiFe03 dûs à des défauts. Le RM est analysé par magnétométrie à échantillon vibrant, fournissant des mesures angulaires à température ambiante. L’état parasité avec la phase parasite Bi2O3 augmente les valeurs du champ d’échange en fonction de la concentration de celle-ci, qui est contrôlable. Un état mésoporeux est aussi mis en évidence, et empêche l’établissement de l’anisotropie unidirectionnelle du couplage d’échange.Des couches minces magnétostrictives de Fe81Ga19 sont déposées sur des substrats de verre. Leurs caractérisations mettent en évidence une dépendance en épaisseur des propriétés magnétiques, en correspondance avec l’état structural.Deux directions cristallographiques remarquables pour toutes les épaisseurs permettent un RM cohérent. La couche la plus mince présente un coefficient de magnétostriction de 20 ppm, qui diminue pour les couches plus épaisses. Cette tendance est associée à une texture de surface prédominante qui se réduit au profit du volume polycristallin sans orientation préférentielle.De telles couches de Fe81Ga19 sont déposées sur des substrats monocristallins ferroélectriques de PMN-PZT pour former un multiferroïque extrinsèque. Le RM et le caractère d’anisotropie sont contrôlés par un champ électrique. Le composite révèle un fort couplage magnétoélectrique inverse entre les deux phases piézoélectrique et magnétostrictive, de valeur parmi les meilleurs rapportées à ce jour. Des mesures à basses températures montrent un effet magnéto-mécanique dû à la contrainte thermique et imposé par la nature du substrat
We conducted three experimental studies of magnetization reversal (MR) behavior in three different types of bilayers, under different types of strain. We studied the influence on the magnetic properties of the structural state in the BiFe03, of magnetoelastic mechanical strain in the Fe81Ga19, which we then coupled to electrical and even thermal strainA bilayer consisted of using a ferromagnetic Ni81Fe19, and an intrinsic multiferroic BiFe03. These polycrystalline thin films are deposited by sputtering. Their structure and morphology are characterized by X-ray diffraction, and transmission electron microscopy, revealing two fundamentally different structural states of the BiFeO3 due to defects. The MR is analyzed by vibrating sample vector magnetometry, providing angular measurements it room temperature. The parasitic state with the parasitic phase Bi2O3 increases the values of the exchange field according to its concentration, which we can control. A mesoporous state is also highlighted, and prevents the establishment of the unidirectional anisotropy.Magnetostrictive thin films of Fe81Ga19 are deposited on glass substrates. Their characterizations reveal thicknessdependent magnetic properties, in correspondence with the structural state. Two remarkable crystallographic directions for the whole range of thicknesses allow a coherent MR. The thinner films have a magnetostriction coefficient value of 20 ppm, which decreases for the thicker films. This trend is associated with a predominant surface texture which is reduced in favor of the polycrystalline volume with non-preferential orientation.Such Fe81Ga19 films are deposited on single-cristalline ferroelectric substrates of PMN-PZT to form an extrinsic multiferroic.The MR and the anisotropy character are controlled by an electric field. The composite reveals a strong inverse magnetoelectric coupling αCME between the two piezoelectric and magnetostrictive phases, of value among the best reported so far. Measurements at low temperatures show a magnetomechanical effect due to thermal stress, and imposed by the nature of the substrate

Thesis (Ph. D.) -- University of Maryland, College Park, 2005.
Thesis research directed by: Material Science and Engineering. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

Ce travail de thèse expérimentale a été consacré à la synthèse par des méthodes de chimie par voie humide de nanoparticules à base du multiferroïque BiFeO3 et à leur caractérisation, avec comme objectif finale des applications photocatalytiques. Ce matériau présente une bande interdite, avec un gap de 2.6eV, qui permet la photo-génération de porteurs de charges dans le visible faisant ainsi de BiFeO3 un système intéressant pour des processus photo-induits. Ce travail s’est en particulier focalisé à caractériser les propriétés de nanoparticules à base de BiFeO3 en vue de comprendre l’effet de ses propriétés sur leur potentiel dans des applications liées à la photocatalyse. Tout d’abord, l’étude des effets de taille sur les propriétés structurales, de transitions de phase, et physico-chimiques des particules a été réalisée, en gardant comme principal objectif de découpler les propriétés liées à la surface de celles du massif/cœur de la particule. Pour cela, une maîtrise et une optimisation des procédés de synthèse de particules aux échelles nano- et micro-micrométriques de BiFeO3 a été nécessaire pour obtenir des composés de taille variable et de très bonne qualité cristalline. Malgré la diminution de la taille des particules, on constate que, grâce au contrôle de paramètres de synthèse, nos nanoparticules présentent des propriétés très proches à celles du massif de BiFeO3, gardant la structure rhomboédrique R3c avec des faibles effets de contrainte. Afin de contrôler indirectement par le dopage les propriétés optiques des composés à base de BiFeO3, on a réussi à réaliser un dopage très homogène en La3+, et un dopage partiel en Ca2+, sur le site de Bi3+. Les propriétés optiques des nanoparticules et leurs applications dans les premières expériences photocatalytiques sur la dégradation du colorant rhodamine B ont montré la complexité de la physico-chimie de leur surface et du processus d’interaction lumière-particule. Après analyse des données d’absorbance optique en fonction de la taille de nanoparticules, on observe que la bande interdite déduite pour ces différentes particules n’est pas le facteur prédominant sur les performances photocatalytiques. D’autres facteurs ont pu être identifiés comme étant à l’origine de la localisation de charges photo-générées, tels que des états de surface liés à une fine couche de peau ou skin layer sur les nanoparticules, présentant des défauts structuraux, une réduction de l’état d’oxydation du Fe3+ vers le Fe2+ et la stabilisation d’autres adsorbats, tels que FeOOH ; tous ces facteurs peuvent contribuer au changement dans les performances photocatalytiques. Les résultats photocatalytiques restent très encourageants pour poursuivre les études de nanoparticules à base de BiFeO3, montrant une dégradation de la rhodamine B à 50% au bout de 4h de réaction photocatalytique pour certaines des nanoparticules étudiées
This experimental PhD work has been dedicated to the synthesis, by wet chemistry methods, and characterization of nanoparticles based on multiferroic BiFeO3, with the aim of using them for photocatalytic applications. This material presents a bandgap of 2.6eV, which allows the charge carrier photoexcitation in the visible range, making BiFeO3 a very interesting system for photoinduced processes. This thesis has been particularly focused on characterizing the properties of BiFeO3 nanoparticles in view of understanding the relationship of their properties on their potential use for photocatalytic applications. First of all, the topic of the size effect on the structural properties, phase transitions, and physics and chemistry of the particles has been developed, keeping as first aim to separate the properties related to the surface from those arising from the bulk/core of the particle. To do so, the mastering and optimization of the synthesis processes of BiFeO3 particles at the nano and microscale were needed, to finally obtain different size compounds with high crystalline quality. Despite the size reduction of the particles, we notice that, thanks to the control of the synthesis process, our BiFeO3 nanoparticles present properties very close to those of the bulk BiFeO3 material, keeping the rhombohedral structure R3c with weak strain effects. In order to indirectly tune the optical properties exploiting the doping, we have succeeded in realizing a homogenous La3+ doping, and a partial Ca2+ doping, on the Bi3+ site. The optical properties of the nanoparticles and their use on the first photocatalytic experiments for degrading rhodamine B dye have shown the complexity of the physics and chemistry phenomena at their surface and of the light-particle processes. After analyzing optical absorbance data as a function of the particle size, we observe that the deduced bandgap for different particles is not the main parameter directing the photocatalytic performances. Other factors have been identified to be at the origin of the localization of the photoexcited charges, as the surface states linked to the skin layer of the nanoparticles, depicting structural defects, a reduction of the oxidation state of Fe3+ towards Fe2+ and the stabilization of other adsorbates, such as FeOOH; all these parameters may contribute to the change on the photocatalytic performances. The photocatalytic results are very encouraging, motivating to continue the study of BiFeO3 based nanoparticles, though depicting a 50% rhodamine B degradation after 4h of photocatalytic reaction using some of the present nanoparticles

Room temperature magnetoelectric BiFeO3-BaTiO3 superlattices with strong out-of-plane magnetic anisotropy have been prepared by pulsed laser deposition. We show that the out-ofplane magnetization component increases with the increasing number of double layers. Moreover, the magnetoelectric voltage coefficient can be tuned by varying the number of interfaces, reaching a maximum value of 29 V/cmOe for the20×BiFeO3-BaTiO3 superlattice. This enhancement is accompanied by a high degree of perpendicular magnetic anisotropy, making the latter an ideal candidate for the next generation of data storage devices.

Les matériaux antiferromagnétiques suscitent un intérêt croissant pour la spintronique de par leur insensibilité aux champs magnétiques parasites et leur dynamique magnétique ultrarapide. Cependant, la lecture et le contrôle de l’ordre antiferromagnétique restent des verrous pour le développement des dispositifs. Dans les matériaux multiferroïques, le couplage magnétoélectrique entre les ordres électrique et magnétique pourrait permettre de contrôler l’antiferromagnétisme avec un champ électrique. Dans cette thèse, nous imageons une grande variété de textures antiferromagnétiques que nous contrôlons par l’ingénierie des contraintes et le champ électrique pour l’archétype des matériaux multiferroïques, BiFeO₃. Nous élaborons des films minces sous différentes contraintes d’épitaxie, maîtrisant ainsi la texture de domaines ferroélectriques, telle qu’imagée par microscopie à force piézoélectrique. De plus, nous montrons qu’une transition de phase inverse peut être utilisée pour accroître l’ordre électrique global, d’une configuration labyrinthique de domaines vers un réseau périodique en bandes rectilignes. La magnétométrie à centre NV nous permet de corréler les textures antiferromagnétiques et ferroélectriques. Nous démontrons que les contraintes stabilisent différents types de cycloïdes ainsi qu’un ordre antiferromagnétique colinéaire. La diffraction X élastique résonante permet de confirmer macroscopiquement l’existence de deux types de cycloïdes. Enfin, nous contrôlons électriquement ces textures antiferromagnétiques, passant d’une cycloïde à une autre ou transformant un ordre colinéaire en cycloïde. Sur la base d’un substrat imposant une contrainte anisotrope, nous stabilisons des films ne présentant qu’un seul domaine ferroélectrique associé à un unique domaine antiferromagnétique. Ceci ouvre de larges perspectives pour explorer le couplage entre l’antiferromagnétisme non-colinéaire et le transport de spin
Antiferromagnetic materials are generating a growing interest for spintronics due to important assets such as their insensitivity to spurious magnetic fields and fast magnetization dynamics. A major bottleneck for functional devices is the readout and electric control of the antiferromagnetic order. In multiferroics, the magnetoelectric coupling between ferroelectric and antiferromagnetic orders may represent an efficient way to control antiferromagnetism with an electric field. In this thesis, we observe a wide variety of antiferromagnetic textures that we control by strain engineering and electric field in the archetypical multiferroic, BiFeO₃. We elaborate epitaxial BiFeO₃ thin films, harbouring various ferroelectric domain landscapes, as imaged by piezoresponse force microscopy. Furthermore, we resort on an inverse phase transition to improve the global electrical order from maze to perfect array of striped ferroelectric domains. Using scanning NV magnetometry, we correlate the antiferromagnetic landscapes to the ferroelectric ones. We demonstrate that strain stabilizes bulk or exotic spin cycloids, as well as collinear antiferromagnetic order. With resonant X-ray elastic scattering, we macroscopically confirm the existence of two types of cycloid. Furthermore, we electrically design antiferromagnetic landscapes on demand, changing one type of cycloid to another or turning collinear states into non-collinear ones. Finally, resorting on anisotropic strain, we stabilize a single domain ferroelectric state, in which a single spin cycloid propagates. This opens a fantastic avenue to investigate the coupling between non-collinear antiferromagnetism and spin transport

Le défis principal de l'industrie de la micro électronique est de créer d'augmenter la capacité de stockage mais aussi la vitesse des ordinateurs. Pour atteindre cette objectif, lkes composants électroniques doivent être miniaturisés à l'échelle du nanomètre. À cette échelle, les propriétés de la matière sont encore mal connues.Les matériaux les plus prometteurs dans cette recherche sont les multiferroïques où l'ordre magnétique et l'ordre ferroélectrique sont couplés. Ils pourraient amener des composants électroniques plus rapide et moins consommateur d'énergie dans des composants tels que les Random Access Memory. Ce travail traite de l'étude d'un multiferroïque typique BiFeO3 (BFO) en se concentrant sur les couplages entre les ordres magnétiques, ferroélectriques et le contrainte dans des systèmes de taille nanométrique

Ce manuscrit est consacré à l’analyse théorique et expérimentale d’oxydes Ln2Ti2O7 (Ln = La, Nd, Sm, Gd) et BiFeO3.Les propriétés physiques de La2Ti2O7 et Nd2Ti2O7 ont été investiguées au moyen de calculs ab initio, confirmant ainsi leur ferroélectricité. D’autres oxydes de la famille Ln2Ti2O7, Sm2Ti2O7 et Gd2Ti2O7, ont ensuite été étudiés selon les mêmes méthodes théoriques. Nos calculs ont révélé une meilleure amplitude de polarisation pour ces composés par rapport au La2Ti2O7 et au Nd2Ti2O7. La deuxième partie de ce travail est consacrée aux propriétés structurales, électroniques et ferroélectriques du BiFeO3. L’évolution de ses propriétés lorsqu’il est soumis à une contrainte épitaxiale ont été investiguées au moyen de calculs ab-initio et de mesures en microscopie à champ proche réalisées sur des couches minces déposées sur un substrat de SrTiO3(001). Nos résultats mettent en évidence une modification de la structure interne du matériau sous effet de contrainte, qui se traduit par une réorientation progressive de la polarisation spontanée suivant la direction [001]. Notre étude s’est ensuite tournée vers l’élaboration et l’analyse des propriétés structurales et ferroélectriques de superréseaux (SrTiO3)n(BiFeO3)m. Nos calculs ont mis en évidence que la contrainte épitaxiale imposée au superréseau offrait un contrôle accru des propriétés du BiFeO3 par rapport à son comportement lorsqu’il est déposé seul en couches minces. Les analyses en microscopie à champ proche ont montré une réduction de la tension coercitive de tels films par rapport à celle mesurée sur des bicouches SrTiO3/BiFeO3 ou sur une couche mince de BiFeO3
In this work, first-principles calculations and experimental measurements have been done in order to investiguate the structural, electroniq and ferroelectric properties of Ln2Ti2O7 (Ln = La, Nd, Sm, Gd) and BiFeO3 oxydes. Calculations on La2Ti2O7 and Nd2Ti2O7 confirmed their ferroelectricity. Other oxydes belonging to the Ln2Ti2O7 family have also been investigated. The results showed an enhancement of the spontaneous polarization within these compounds compared to that of La2Ti2O7 and Nd2Ti2O7. The second part of this work is related to the structural and ferroelectric properties of bismuth ferrite BiFeO3. The evolution of its properties when undergoing an epitaxial strain have been investigated by ab initio calculations and piezoresponse force microscopy measurements on thin films deposited on a (001)-SrTiO3 substrate. Our results showed a modification of the inner structure of BiFeO3 under stain, leading to a continuous reorientation of the spontaneous polarization vector towards [001]. The third part of our study consists in the computational design and synthesis of (SrTiO3)n(BiFeO3)m superlattices. Our calculations showed that epitaxial strain imposed to the superlattice brings a further control of physical properties of BiFeO3 as compared with its behaviour when deposited alone in a thin film. PFM analysis showed a decrease of the coercive field for STO/LNO/(STO)n(BFO)m superlattices as compared with those measured on STO/BFO bi-layers and on BiFeO3 thin films

碩士
輔仁大學
物理學系
99
This work is to investigate the structural, electrical, and magnetic properties of various A-site ion substations in BiFeO3. The samples were fabricated by the solid state reaction method (SSR). The experiment methods include the high-resolution synchrotron XRD, dielectric constant, SEM(grain size), and magnetic properties. The (Bi0.95La0.05)FeO3 and (Bi0.95Nd0.05)FeO3 ceramics exhibit a rhombohedral-orthorhombic-cubic phase transition. The dielectric permittivities of (Bi0.95La0.05)FeO3 and (Bi0.95Nd0.05)FeO3 ceramics are 71.045 and 74.937 at room temperature, respectively. The dielectric loss of (Bi0.95La0.05)FeO3 and (Bi0.95La0.05)FeO3 are about 0.0127 and 0.1415 at room temperature, respectively. The frequency dependent dielectric maximum in 600~800 K is likely activated by the antiferromagnetic transition which takes place at the Néel temperature (TN). This phenomenon associates with a local minimum in rhombohedral distortion angle αR near TN.

碩士
國立成功大學
物理學系碩博士班
96
In this study, I observe the domain structure and growth at nanoscale by the piezoresponse force microscopy (PFM) in the multiferroic BiFeO3 thin films. The topography, in-plane (IP) and out-of-plane (OP) components of domains for BFO thin films can be revealed simultaneously. The effects of free carriers exist at grain boundaries, where free carries are assumed to screen the depolarization fields in rough epitaxial and polycrystalline samples. The effect of free carriers also provide the explanations for that BFO ferroelectric domains are usually larger than theory expected. The stripe-like domains formed as normal states by considering ferroelectric ordering, magnetoelectric coupling, and the depolarization energy. When applying lower voltage pulses, the domain grows logarithmically with time, which suggests the observed domain wall follows the creep motion in (111) epitaxial sample. When applying higher voltage pulses (close to the macroscopically saturation voltage), the observed states are in equilibrium so that the domain size is determined by minimizing the domain free energies, which include the contributions from (1) the depolarization energy from the bound charges on the domain wall; (2) the surface energy of the domain wall and (3) interaction energy between the domain and the tip fields. The threshold electric field of nonequlibrium creep wall for negative bias (~0.81-0.91 MV/cm) is smaller than that (~1.71-1.93 MV/cm) for positive bias. It is reasonable since the original polarizations of the films tend to direct toward the bottom electrodes.

碩士
輔仁大學
物理學系碩士班
101
In this thesis, the synthesizing process of BiFeO3 (BFO) polycrystalline multiferroic ceramic by solid-state reaction method has been explained systematically. The as prepared BFO ceramic sample shows high purity single phase without any traces of secondary phases. In order to study the photovoltaic effect, ITO/BFO/Au heterostructure (with electrodes of Indium tin oxide and Au films) has been prepared. Photovoltaic responses under near-ultraviolet illumination at λ= 405 nm exhibit nonlinear dependence on light intensity, whereas light illumination at λ = 532 nm does not show any significant response due to its energy band gap of about 2.7 eV. Under the illumination at λ= 405 nm, the measured photovoltage, and photovoltaic current density are 0.83V and 0.25 A/m2 respectively for a chosen sample thickness of 0.2 mm. The maximal power conversion efficiency is about 0.0289% at illumination intensity of 9.2 W/m2. It is also verified that the photovoltaic responses increases as the sample thickness decreases and it can be enhanced after dc E-field poling. A model based on the PN junction theory has been proposed in order to explain the photovoltaic effects. The model gives good agreement between the theoretical and experimental values.

碩士
輔仁大學
物理學系
100
This study used the solid state reaction to produce BiFeO3 multiferroic ceramics. The processes include mixing powders, ball milling, calcining, high -energy ball milling, granulation, pressing, and sintering. XRD of BiFeO3 ceramics show high purity without obvious second phases. Room -temperature dielectric permittivity is about 48 (for f=1 MHz). The maximum dielectric-permittivities occur between 650-800 K and show obvious frequency-dependent dispersion. Dielectric loss increases rapidly when temperature is above 630 K because of the thermal-active conductivity. In one-dimension barrier model, a turning point of conductivity appears around 610 K, which is close to the Nèel temperature. The maximum of dielectric permittivity from the barrier model is consistent with the experiment data. Probably, the main reason is due to transition from antiferromagnetism to paramagnetism. Comparing with two different diode lasers, the photovoltaic responses of 373 nm laser is better than the green diode laser (=532 nm). The smaller photovoltaic phenomena are mainly due to inefficiently photonic energy for electronic excitation. The thickness of ceramic sample will also affect photovoltaic effect, in which the thinner sample exhibit better photovoltaic effect. With poling by external electric field, the photovoltaic effects under illumination of =405 nm increase with rising of poling intensity.

博士
國立清華大學
材料科學工程學系
94
Multiferroics BiFeO3 (BFO), exhibiting simultaneously ferroelectricity (Tc~1100K) and anti-ferromagnetism (TN~640K), have attracted extensively attention for their coupled electric, magnetic, and structure order parameters in the same phase. The crystal structure, chemical configuration, nanoscale characterization, electric and magnetic properties were investigated is this study. The pure perovskite phase of BFO films were deposited by rf-magnetron sputtering at low processing temperature. The crystal structure of the BFO films was significantly influenced by the substrate and the bottom electrodes. The BFO film was grown with random orientation on Pt/TiOx/SiO2/Si (Pt), whereas highly (100)- and (111)-oriented ones were obtained on LaNiO3/Pt/TiOx/SiO2/Si (LNO) and BaPbO3/Pt/TiOx/SiO2/Si (BPO), respectively. The BFO-based films were hetero-epitaxially grown on the LaNiO3/LaFeO/MgO single crystal substrates. The chemical configuration of the films, which significantly depended on working pressure and temperature, was enhanced by well-controlled processing parameters. The orientation dependence in the crystal growth, electric properties and magnetic behavior of BFO films were examined. The film/electrode interface and chemical homogeneity of the films were characterized by the scanning transmission electron microscope high-angle annular dark-field imaging (STEM-HAADF). Nanoscale characterization of the BFO films was studied by scanning probe microscopy (SPM). With the partial substitution of lanthanum (La) ions for bismuth ions, the significant enhancement in the dielectric, ferroelectric and magnetic performance of BFO films was attributed to the improved crystallinity, smooth surface, and increased lattice volume.

碩士
國立交通大學
電子物理系所
102
BFO-CFO vertically align nanocomposite (VAN) was successfully made by utilizing the different wetting conditions from BFO and CFO film when growing on STO substrate at the same time in the PLD system. From the XRD result, there was strain effect from STO substrate to BFO and CFO film. Especially for BFO-CFO/STO VAN, CFO pillar relaxed the strain of BFO film, and shifted magnetic phase transitions on BFO/STO thin film to around 30 K and 160 K as compared to that of BFO powders where the transitions occured around 55 K and 200 K, respectively. From M-T and C-T measurements on BFO/CFO/STO bilayer and BFO-CFO/STO VAN, antiferromagnetic-ferromagnetic coupling (BFO-CFO) is stronger than antiferromagnetic-ferroelectric coupling (BFO). making the magnetization and capacitance anomalies unobservable in those systems. Furthermore, the C-T beharviors of BFO/STO and BFO/CFO/STO are in general similar, but different in subtle details, which presumably originates from the quenching of spin reorientation in BFO due to ferromagnetic coupling from CFO. The situation is even more complicated in the CFO pillar embedded in BFO matrix sample. Finally, from R-T measurement, the high-density BFO-CFO/STO VAN film showed an apparent insulator-metal transition around 30 K, which is similar to that observed in BFO film under strong external magnetic fields which done by another reseach before. The result suggests that when the pillar density is large enough it may generate strong enough local magnetic field to modify the ferroelectric domain structures in BFO matrix. Further investigations are certainly in order to delineate the interisting emergent phenomena observed in the present study.

碩士
國立成功大學
材料科學及工程學系碩博士班
97
One-dimensional nano-structures have generated considerable interest, due to their unique and superior qualities opposite bulk structures. The investigation in BiFeO3(BFO) material is to transform its morphology into nanowires. The BiFeO3 compound is one of the few materials with coexistent ferroelectricity (TC =810℃) and antiferromagnetism (Neel temperature TN =380℃) at room temperature. Therefore, BFO material is considered to have the great potential for applications in magnetic as well as in ferroelectric devices. Though BiFeO3 was discovered in the 1960’s, but it has the week use in the field of electronics for several reasons: one is that Bi and Fe atoms make it hard form BiFeO3 single phase and ionic iron valence changes from tervalency to bivalence easily result in BiFeO3 has quite high leakage current. In the other hand BiFeO3 its low dielectric constant and resistance make it hard to measure hysteresis loop; furthermore BiFeO3 single phase cannot be synthesized by solid state method and it often includes Bi2Fe4O9、Bi25FeO40 etc impurity phases. Recently, it has generated considerable interest, being a material with great potential applications, by a combination of its magnetic and electric properties in the field of electronics: radio, television, audio-video and digital recording, and as permanent magnets. Thus, the synthesis of bismuth ferrite materials subject of renewed research and attempts have been made to obtain a pure phase. There are two major preparation technique, solid-state reactions based on Bi2O3 andFe2O3 have been used, with thermal treatments around 800°-830℃, but unreacted Bi2O3/ Bi2Fe4O9 were present and removed by washing with HNO3.. Another chemical method includes simultaneous precipitation / coprecipitation involving starting solutions such as Bi and Fe nitrates with ammonium hydroxide and sol-gel method to obtain pure-phase BiFeO3 .Nano-stucture synthesis is the new technique for the last decades. So, the subject of the research is to change BFO material structure by recently nano-techniques and hope to get better physical qualities. In the literature[40], BFO nanowires synthesis has successful done by NCA (porous nanochannel alumina) template, but in the study represents another BFO nanowires synthesis in Si substrate without using NCA template.

碩士
國立交通大學
材料科學與工程學系
98
Magnetoelectric multiferroic materials which have coupled electric, and magnetic, that result ferroelectricity and ferromagnetism . These compounds are explored in transition-metal-oxide that present great opportunities for applications in information storage, sensors and green materials. BiFeO3 (BFO)is a room-temperature , single- phase magnetoelectric multiferroic which present ferroelectric polarizations along &lt;111> directions and G-type anti-ferromagnetism . Our study suggests an isostructural change can be induced by epitaxial strain, which is usually driven by temperature or high pressure in solids. Such a transition can induce large displacement of ions that causes high polarizations and show great potential for green piezoelectrics. By using substrates with different lattice parameters for the growth of BFO thin films ,we can get different strain states. In order to fully understand how these strains affect the structure, polarization rotation, ferroelectric domains and environment of ions, we have several techniques to build the framework. X-ray analysis ,such as reciprocal space mapping has been used to understand the structure correlation with strain states. Piezoresponce microscopy has been used to probe the ferroelectric domains .We can also find how the local environment of iron and oxygen ions changed by using X-ray absorption near edge structure (XANES).

碩士
輔仁大學
物理學系
98
In-situ high-resolution synchrotron X-ray diffraction reveals a local minimum in rhombohedral distortion angle αR (associated with an inflection in the lattice constant aR) in the region of 350 ~ 400 oC in BiFeO3 (BFO) and 5 mol% BaTiO3-doped BiFeO3 (BiFeO3(0.95)-BaTiO3(0.05)). It confirms a coupling of ferroelectric and magnetic parameters near the Néel temperature, which is responsible for frequency-dependent maxima in dielectric permittivity. A rhombohedral–cubic structure transition occurs near 850 and 830 oC in BiFeO3 and BiFeO3(0.95)-BaTiO3(0.05), respectively. The BaTiO3-doping can enhance dielectric and ferromagnetic responses, and reduce electric leakage. The dielectric loss of BiFeO3(0.95)-BaTiO3(0.05) remains less than 0.04 below 150 oC.

博士
輔仁大學
應用科學與工程研究所博士班
100
A one-dimensional conductivity barrier model with intrinsic barrier B every lattice constant a and extrinsic barrier B+Δ is introduced to describe the dielectric response and conductivity as functions of temperature and frequency. Temperature- and frequency-dependent dielectric permittivity (ε′) and conductivity (σ′) have been studied on multiferroic BiFeO3 (BFO), (1-x)BiFeO3-(x)BaTiO3 (BFO-BT), and (Bi1-xBax)(Fe1-xTix)O3 [BFO-(Ba,Ti)] ceramics, which were synthesized by the solid state reaction method. A frequency-dependent dispersion of dielectric maximum appears in the lower temperature region. This phenomenon is likely activated by the antiferromagnetic (AFM)-paramagnetic (PM) transition. Good qualitative fits of dielectric permittivity and conductivity are obtained with the interior grain sizes d of 20-40 nm. BFO-BT and BFO-(Ba,Ti) ceramics show higher intrinsic barriers B about 11000 K than B=8200 K in BiFeO3. The phase transition of BFO is rhombohedral (R)-orthorhombic (O)-cubic (C) and the Curie temperature (Tc) is near 850 ℃. The structure transition sequence of BFO-BT and BFO-(Ba,Ti) ceramics is R to C upon heating. The Curie temperatures in BFO-BT and BFO-(Ba,Ti) ceramics shift toward lower temperatures due to the contents of (Ba,Ti). The lattice constant in BFO is about 3.9699 Å and increases with increasing the (Ba,Ti) contents. The local minima of R distortion angle (αR) in BFO, BFO-BT, and BFO-(Ba,Ti) occur in the region of 300-500 ℃, implying ionic displacements. The temperature regions also associate with Nèel temperature (TN). This anomaly is likely resulted from the antiferromagnetic (AFM)–paramagnetic (PM) transition and is responsible for the broad frequency-dependent dielectric maximum. The dielectric permittivities are about 33 in BFO and 321 in BFO-30%BT for 1 MHz at room temperature. The dielectric loss decreases as BT increases. The real part of conductivity in BFO shows a deviation from the linear relation near Tm and αR. The deviation temperature shifts to lower temperature with increasing BT contents. BFO and BFO-BT ceramics exhibit a similar antiferromagnetic (AFM) behavior. BFO-5%(Ba,Ti) shows the weak ferromagnetic behavior. The magnetic hysteresis loop of possible iron oxide in BFO-10%(Ba,Ti) was observed due to higher sintering temperature.

Rhombohedral perovskite BiFeO3 is a single phase multiferroic compound exhibiting both magnetic (Neel temperature ~370˚C) and ferroelectric (Curie point ~840˚C) ordering well above the room temperature. Ferroelectricity in BiFeO3 is due to stereochemically active 6slone pair in Biion which causes large relative displacements of Bi and O ions along the [111] direction. Long range spiral modulation of the canted antiferromagnetic spin arrangement in Feeffectively cancels the macroscopic magnetization due to Dzyaloshinskii–Moriya interaction and thereby prevents linear magneto-electric effect. Synthesizing dense pure BiFeO3 by conventional solid state method is difficult due to the formation of thermodynamically stable secondary phases such as Bi2Fe4O9, Bi25FeO39 and Bi46Fe2O72. To stabilize the perovskite phase and to suppress the cycloid several groups have adopted different strategies such as thin film growth, different synthesis methods and chemical substitution. Of the various substitutions reported in the literature, PbTiO3 substitution has shown very interesting features, such as (i) unusually large tetragonality (c/a~1.19), (ii) formation of morphotropic phase boundary (MPB) and (iii) high curie point Tc~650C. MPB ferroelectric systems such as lead zirconate titanate (PZT) are known to exhibit high piezoelectric response due to the coupling between strain and polarization. Hence the existence of magnetic ordering in BiFeO3-PbTiO3 offers an interesting scenario where polarization, strain and magnetization may couple together. The high Curie point also makes the system an interesting candidate for high temperature piezoelectric application. However its potential as a high temperature piezoelectric material has not been realized yet. A detailed review of literature suggests a lack of clear agreement with regards to the composition range of the reported MPB itself. Different research groups have reported different composition range of MPB for this system even for almost similar synthesis conditions. The present thesis deals with broadly two parts, firstly with the preparation of pure BiFeO3 by co-precipitation and hydrothermal methods and its thermal stability and secondly resolving the cause of discrepancy in range of MPB reported in BiFeO3-PbTiO3 solid solution. Detailed examination of this system (BiFeO3-PbTiO3) around the reported MPB composition by temperature dependent X-ray, electron and neutron diffraction techniques, in conjunction with a systematic correlation of sintering temperature and time with microstructural and phase formation behavior revealed the fact that the formation of MPB or the single ferroelectric phase is critically dependent on the grain size. This phenomenon is also intimately related to the abnormal grain growth in this system. Chapter 1 gives the brief overview of the literature on the topics relevant to the present study. The literature survey starts with a brief introduction about the perovskite oxides; their ferroelectric, magnetic and multiferroic properties were discussed in further sections. A brief outline on the grain growth mechanism is described. An overview of BiFeO3 and various synthesis methods, different chemical substitutions and their effect on properties are provided. A brief review of published literature on BiFeO3-PbTiO3 solid solution and its properties is also presented. Chapter 2 deals with the synthesis of pure BiFeO3, heat treatment and characterisation. BiFeO3 was synthesised by (a) co-precipitation and (b) hydrothermal methods. In co-precipitation method, calcination of precipitate at different temperature resulted in the formation of BiFeO3 along with secondary phases (Bi2Fe4O9 and Bi24FeO39). The optimum calcination temperature to prepare pure BiFeO3 was found to be 560C. The synthesized pure BiFeO3 exhibits weak ferromagnetic hysteresis at room temperature, the degree of which increases slightly at 10K (-263C). The hydrothermal treatment was carried out in (a) carbonate and (b) hydroxide precipitates with KOH as mineralizer. BiFeO3 prepared using hydroxide precipitate was stable till 800C whereas with carbonate precipitate it was stable only till 600C. Chapter 3 deals with the stability of phases in (1-x)BiFeO3 -(x)PbTiO3 solid solution. Samples prepared by conventional solid state route sometimes remain as dense pellet and on certain occasions it disintegrate completely into powder observed after sintering. Irrespective of the composition, sintering time and temperature, powder X-ray Diffraction (XRD) pattern of the survived pellet (crushed into powder) shows coexistence of rhombohedral (R3c) and tetragonal (P4mm) phases and the disintegrated powder (without crushing) show 100% tetragonal (P4mm) phase. Very high spontaneous tetragonal strain (c/a-1) ~0.19 at MPB is believed to be the origin for disintegration. But in all the survived pellets at least a minor fraction of rhombohedral phase (5-7%) is present. Systematic sintering studies with the time and temperature shows, decreasing the sintering temperature and time will increase the lifetime of the pellet and by increasing the sintering temperature and time the pellet will disintegrate. In this work we have conclusively proved that the wide composition range of MPB reported in the literature is due to kinetic arrest of the metastable rhombohedral phase and that if sufficient temperature and time is given, the metastable phase disappears. The suppression/formation of minor rhombohedral phase is expected due to the play of local kinetic factors during the transformation process. This makes the system behave in an unpredictable way with regard to the fraction of rhombohedral phase that is observed at room temperature. A systematic X-ray and neutron powder diffraction study of the giant tetragonality multiferroic (1-x)BiFeO3 -(x)PbTiO3 have shown that the compositions close to the morphotropic phase boundary of this system present two different structural phase transition scenarios on cooling from the cubic phase: (i) Pm3m P4mm(T2)+P4mm(T1) P4mm (T1) and (ii) Pm3m P4mm(T2) + P4mm(T1) + R3c P4mm (T1) + R3c. The comparatively larger tetragonality of the T1 phase as compared to the coexisting isostructural T2 phase is shown to be a result of significantly greater degree of overlap of the Pb/Bi-6s and Ti/Fe-3d with the O-2p orbitals as compared to that in the T2 phase. High temperature electron diffraction studies show that the metastable rhombohedral phase is present in the cubic matrix well above the Curie point as nuclei. Life time of the metastable R3c nuclei is very sensitive to composition and temperature, and nearly diverges at x → 0.27. MPB like state appears only if the system is cooled before the metastable R3c nuclei could vanish. Issue of the metastable rhombohedral state is developed further in Chapter 4. A one-to-one correlation was found between the grain size and phase formation behavior. Fine grained (~1µm) microstructure (usually pellets) shows phase coexistence (R3c+P4mm) and the disintegrated coarse grains (~10µm) show tetragonal (P4mm) phase. Microstructural analysis revealed the disintegration was caused by abnormal grain growth along with the disappearance of metastable rhombohedral phase. Abnormal grain growth starts at the periphery/crack i.e., at the free surface and move towards the canter of the pellet. Size reduction of disintegrated coarse grains (~10µm) to fine grains (~1µm) by crushing the sample showed that the system switching form pure tetragonal (P4mm) state to the MPB state comprising of tetragonal and rhombohedral phases (R3c+P4mm). In another approach the smaller sized particles of x=0.20 were synthesized by sol gel method. It was reported that in conventional solid state route x=0.20 exhibits pure rhombohedral phase. The sol-gel sample calcined at 500C (particle size ~15nm) stabilizes tetragonal metastable phase along with the stable rhombohedral phase, the morphotropic phase boundary state. Samples calcined at higher temperature, 800C (particle size ~50nm) also showed stable rhombohedral phase. Ferromagnetic behavior was observed in the sample having phase coexistence and the sample with pure rhombohedral phase showed antiferromagnetic behavior. Hence this material is a promising candidate which can be tuned to exhibit different behavior just by adopting different grain size. Chapter 5 deals with the magnetic structure of (1-x)BiFeO3 -xPbTiO3 solid solution with change in composition and temperature. Magnetic structure was studied using powder neutron diffraction in the composition range x=0.05 -0.35. Rietveld analysis was carried out for the nuclear and magnetic phases, by considering R3c phase for the nuclear structure. To account for the magnetic Bragg peak at d=4.59Å, three antiferromagnetic models were considered for the magnetic structure: (i) helical spin arrangement as in BiFeO3, (ii) commensurate G-type antiferromagnetic ordering with moments in the a-b plane (of the hexagonal cell), and (iii) commensurate G-type ordering with moments parallel to the c-axis (of the hexagonal cell). The third model was found to be suitable to explain the magnetic peak accurately and the better fitting of magnetic peak was observed in this model compared to others. At room temperature the MPB compositions have rhombohedral and tetragonal nuclear phases along with the rhombohedral magnetic phase. Addition of PbTiO3 in BiFeO3 not only changes the magnetic structure but also reduces the magnetic moment due to the substitution of Ti in Fesite. High temperature neutron diffraction studies reveal the magnetic transition at ~300C for x=0.20, ~95C for x=0.27 and ~150C for x=0.35. The Neel temperature observed in neutron diffraction studies were also confirmed by DSC and by temperature dependent dielectric studies. For x=0.20, anomalous variation in the lattice parameters and the octahedral tilt angle was observed across the magnetic transition temperature. In the magnetic phase, the c-parameter was contracted and the octahedral tilt angle slightly increased. This result suggests a coupling between spin, lattice and structural degrees of freedom around the transition temperature. Temperature dependent powder neutron diffraction study at low temperature from 300K (27C) to 4K (-269C) in x=0.35 shows the evolution of tetragonal magnetic phase at 200K (-73C) whose intensity is increasing with decrease in temperature. Below 200K, x=0.35 has rhombohedral and tetragonal magnetic and nuclear phases. While in x=0.27 at low temperature, rhombohedral magnetic and nuclear phases are present along with the tetragonal nuclear phase alone (the tetragonal magnetic phase is absent). We propose this discrepancy in the Neel temperature and the magnetic phase formation can be due to the probabilistic nature of the existence of metastable rhombohedral phase which was discussed earlier.

Rhombohedral perovskite BiFeO3 is a single phase multiferroic compound exhibiting both magnetic (Neel temperature ~370˚C) and ferroelectric (Curie point ~840˚C) ordering well above the room temperature. Ferroelectricity in BiFeO3 is due to stereochemically active 6slone pair in Biion which causes large relative displacements of Bi and O ions along the [111] direction. Long range spiral modulation of the canted antiferromagnetic spin arrangement in Feeffectively cancels the macroscopic magnetization due to Dzyaloshinskii–Moriya interaction and thereby prevents linear magneto-electric effect. Synthesizing dense pure BiFeO3 by conventional solid state method is difficult due to the formation of thermodynamically stable secondary phases such as Bi2Fe4O9, Bi25FeO39 and Bi46Fe2O72. To stabilize the perovskite phase and to suppress the cycloid several groups have adopted different strategies such as thin film growth, different synthesis methods and chemical substitution. Of the various substitutions reported in the literature, PbTiO3 substitution has shown very interesting features, such as (i) unusually large tetragonality (c/a~1.19), (ii) formation of morphotropic phase boundary (MPB) and (iii) high curie point Tc~650C. MPB ferroelectric systems such as lead zirconate titanate (PZT) are known to exhibit high piezoelectric response due to the coupling between strain and polarization. Hence the existence of magnetic ordering in BiFeO3-PbTiO3 offers an interesting scenario where polarization, strain and magnetization may couple together. The high Curie point also makes the system an interesting candidate for high temperature piezoelectric application. However its potential as a high temperature piezoelectric material has not been realized yet. A detailed review of literature suggests a lack of clear agreement with regards to the composition range of the reported MPB itself. Different research groups have reported different composition range of MPB for this system even for almost similar synthesis conditions. The present thesis deals with broadly two parts, firstly with the preparation of pure BiFeO3 by co-precipitation and hydrothermal methods and its thermal stability and secondly resolving the cause of discrepancy in range of MPB reported in BiFeO3-PbTiO3 solid solution. Detailed examination of this system (BiFeO3-PbTiO3) around the reported MPB composition by temperature dependent X-ray, electron and neutron diffraction techniques, in conjunction with a systematic correlation of sintering temperature and time with microstructural and phase formation behavior revealed the fact that the formation of MPB or the single ferroelectric phase is critically dependent on the grain size. This phenomenon is also intimately related to the abnormal grain growth in this system. Chapter 1 gives the brief overview of the literature on the topics relevant to the present study. The literature survey starts with a brief introduction about the perovskite oxides; their ferroelectric, magnetic and multiferroic properties were discussed in further sections. A brief outline on the grain growth mechanism is described. An overview of BiFeO3 and various synthesis methods, different chemical substitutions and their effect on properties are provided. A brief review of published literature on BiFeO3-PbTiO3 solid solution and its properties is also presented. Chapter 2 deals with the synthesis of pure BiFeO3, heat treatment and characterisation. BiFeO3 was synthesised by (a) co-precipitation and (b) hydrothermal methods. In co-precipitation method, calcination of precipitate at different temperature resulted in the formation of BiFeO3 along with secondary phases (Bi2Fe4O9 and Bi24FeO39). The optimum calcination temperature to prepare pure BiFeO3 was found to be 560C. The synthesized pure BiFeO3 exhibits weak ferromagnetic hysteresis at room temperature, the degree of which increases slightly at 10K (-263C). The hydrothermal treatment was carried out in (a) carbonate and (b) hydroxide precipitates with KOH as mineralizer. BiFeO3 prepared using hydroxide precipitate was stable till 800C whereas with carbonate precipitate it was stable only till 600C. Chapter 3 deals with the stability of phases in (1-x)BiFeO3 -(x)PbTiO3 solid solution. Samples prepared by conventional solid state route sometimes remain as dense pellet and on certain occasions it disintegrate completely into powder observed after sintering. Irrespective of the composition, sintering time and temperature, powder X-ray Diffraction (XRD) pattern of the survived pellet (crushed into powder) shows coexistence of rhombohedral (R3c) and tetragonal (P4mm) phases and the disintegrated powder (without crushing) show 100% tetragonal (P4mm) phase. Very high spontaneous tetragonal strain (c/a-1) ~0.19 at MPB is believed to be the origin for disintegration. But in all the survived pellets at least a minor fraction of rhombohedral phase (5-7%) is present. Systematic sintering studies with the time and temperature shows, decreasing the sintering temperature and time will increase the lifetime of the pellet and by increasing the sintering temperature and time the pellet will disintegrate. In this work we have conclusively proved that the wide composition range of MPB reported in the literature is due to kinetic arrest of the metastable rhombohedral phase and that if sufficient temperature and time is given, the metastable phase disappears. The suppression/formation of minor rhombohedral phase is expected due to the play of local kinetic factors during the transformation process. This makes the system behave in an unpredictable way with regard to the fraction of rhombohedral phase that is observed at room temperature. A systematic X-ray and neutron powder diffraction study of the giant tetragonality multiferroic (1-x)BiFeO3 -(x)PbTiO3 have shown that the compositions close to the morphotropic phase boundary of this system present two different structural phase transition scenarios on cooling from the cubic phase: (i) Pm3m P4mm(T2)+P4mm(T1) P4mm (T1) and (ii) Pm3m P4mm(T2) + P4mm(T1) + R3c P4mm (T1) + R3c. The comparatively larger tetragonality of the T1 phase as compared to the coexisting isostructural T2 phase is shown to be a result of significantly greater degree of overlap of the Pb/Bi-6s and Ti/Fe-3d with the O-2p orbitals as compared to that in the T2 phase. High temperature electron diffraction studies show that the metastable rhombohedral phase is present in the cubic matrix well above the Curie point as nuclei. Life time of the metastable R3c nuclei is very sensitive to composition and temperature, and nearly diverges at x → 0.27. MPB like state appears only if the system is cooled before the metastable R3c nuclei could vanish. Issue of the metastable rhombohedral state is developed further in Chapter 4. A one-to-one correlation was found between the grain size and phase formation behavior. Fine grained (~1µm) microstructure (usually pellets) shows phase coexistence (R3c+P4mm) and the disintegrated coarse grains (~10µm) show tetragonal (P4mm) phase. Microstructural analysis revealed the disintegration was caused by abnormal grain growth along with the disappearance of metastable rhombohedral phase. Abnormal grain growth starts at the periphery/crack i.e., at the free surface and move towards the canter of the pellet. Size reduction of disintegrated coarse grains (~10µm) to fine grains (~1µm) by crushing the sample showed that the system switching form pure tetragonal (P4mm) state to the MPB state comprising of tetragonal and rhombohedral phases (R3c+P4mm). In another approach the smaller sized particles of x=0.20 were synthesized by sol gel method. It was reported that in conventional solid state route x=0.20 exhibits pure rhombohedral phase. The sol-gel sample calcined at 500C (particle size ~15nm) stabilizes tetragonal metastable phase along with the stable rhombohedral phase, the morphotropic phase boundary state. Samples calcined at higher temperature, 800C (particle size ~50nm) also showed stable rhombohedral phase. Ferromagnetic behavior was observed in the sample having phase coexistence and the sample with pure rhombohedral phase showed antiferromagnetic behavior. Hence this material is a promising candidate which can be tuned to exhibit different behavior just by adopting different grain size. Chapter 5 deals with the magnetic structure of (1-x)BiFeO3 -xPbTiO3 solid solution with change in composition and temperature. Magnetic structure was studied using powder neutron diffraction in the composition range x=0.05 -0.35. Rietveld analysis was carried out for the nuclear and magnetic phases, by considering R3c phase for the nuclear structure. To account for the magnetic Bragg peak at d=4.59Å, three antiferromagnetic models were considered for the magnetic structure: (i) helical spin arrangement as in BiFeO3, (ii) commensurate G-type antiferromagnetic ordering with moments in the a-b plane (of the hexagonal cell), and (iii) commensurate G-type ordering with moments parallel to the c-axis (of the hexagonal cell). The third model was found to be suitable to explain the magnetic peak accurately and the better fitting of magnetic peak was observed in this model compared to others. At room temperature the MPB compositions have rhombohedral and tetragonal nuclear phases along with the rhombohedral magnetic phase. Addition of PbTiO3 in BiFeO3 not only changes the magnetic structure but also reduces the magnetic moment due to the substitution of Ti in Fesite. High temperature neutron diffraction studies reveal the magnetic transition at ~300C for x=0.20, ~95C for x=0.27 and ~150C for x=0.35. The Neel temperature observed in neutron diffraction studies were also confirmed by DSC and by temperature dependent dielectric studies. For x=0.20, anomalous variation in the lattice parameters and the octahedral tilt angle was observed across the magnetic transition temperature. In the magnetic phase, the c-parameter was contracted and the octahedral tilt angle slightly increased. This result suggests a coupling between spin, lattice and structural degrees of freedom around the transition temperature. Temperature dependent powder neutron diffraction study at low temperature from 300K (27C) to 4K (-269C) in x=0.35 shows the evolution of tetragonal magnetic phase at 200K (-73C) whose intensity is increasing with decrease in temperature. Below 200K, x=0.35 has rhombohedral and tetragonal magnetic and nuclear phases. While in x=0.27 at low temperature, rhombohedral magnetic and nuclear phases are present along with the tetragonal nuclear phase alone (the tetragonal magnetic phase is absent). We propose this discrepancy in the Neel temperature and the magnetic phase formation can be due to the probabilistic nature of the existence of metastable rhombohedral phase which was discussed earlier.

碩士
輔仁大學
物理學系碩士班
102
In this research, 5% and 10% Co-doped BFO multiferroic ceramics have been synthesized by solid state reaction (SSR) method with various sintering temperature and present single phase in XRD measurements. The average grain size of Co-doped BFO observed by SEM grows significantly as increasing sintering temperature. The ferromagnetic behavior at room temperature is enhanced as cobalt doped in BFO and grains demonstrate more rectangle shape with increasing Co content. The temperature- and frequency- dependent dielectric permittivity of Co-doped BFO show strong frequency dispersion and high electric conductivity around 250 oC. For photovoltaic (PV) effects measurement, the ITO and Au films were deposited on both sides of Co-doped BFO and the thickness of BFO is 0.2 mm. The diode laser of λ = 405 nm was used as an excitation source to measure open circuit voltage and short circuit current density. The photovoltaic phenomena can be explained by the developed model which is based on p-n junction concepts. On the other hand, the photovoltaic responses also depend on the average grain size and an application of magnetic field~3000 Oe. The relation between photovoltaic response and light intensity can be described by exponential equations Voc = Vb[1-exp(I/α)] and Jsc = Jb [1-exp(I/β)], where Voc, Jsc, Vb, and Jb are open-circuit voltage, short- circuit current density , saturated open-circuit voltage, and saturated short-circuit current density, respectively.

博士
國立交通大學
材料科學與工程學系所
103
Multiferroics - materials that exhibit coexistence of different ferroic order parameters - have offered a new route to create intriguing functionalities for next generation nanoelectronics. Bismuth ferrite, BiFeO3, is currently the most studied single-phase multiferric, because BiFeO3 is the only room temperature multiferroic in the world to date. In this dissertation, domain engineering of BiFeO3 is explored to create well-controlled domain patterns and desired domain walls, which are used to gain further understanding on the intriguing functionalities of this room-temperature multiferroic. In addition to domain engineering, epitaxial strain engineering is another main focus in this dissertation. The fabrication of newly-developed and non-equilibrium phases of BiFeO3 is achieved by choosing misfit substrates and suitable growth conditions, allowing original properties to be tailored by epitaxial strains. In this dissertation, a new orthorhombic phase of the multiferroic BiFeO3 is stabilized by exerting proper tensile strain, leading to the formation of functional 90o domain walls. On the other hand, with precisely controlled compressive strain, a highly strained BiFeO3 phase is found to exhibit electric controllable magnetism. This dissertation is written to offer a roadmap to reach advanced control on multiferroic BiFeO3, with the hope to explore new opportunities for novel multifunctional devices and nanoelectronics.

The multiferroics are an important class of multifunctional material which simultaneously possess spontaneous ferroelectric polarization and magnetic ordering. If there exists a coupling between the ferroelectricity and the magnetic ordering, the materials are known as magneto electric (ME) multiferroic materials. The coupling between the magnetic and electric order parameters allows to tune the magnetic properties by an electric field and vice versa. Multiferroic materials are promising candidate for designing new spintronic devices, advanced sensors, high density ferroelectric memory devices and the emerging category of four-state memory devices. In multiferroic memory devices, data can be written electrically using its ferroelectric property and can be read magnetically without causing any Joule heating. Depending on the origin of ferroelectricity and magnetic orderings, multiferroics can be divided into two categories: type I and type II multiferroics. The type I multiferroics have different sources of ferroelectricity and magnetism. On the other hand, ferroelectricity is induced by the magnetic ordering in type II multiferroic materials and they have a strong ME coupling. However, even after extensive investigations into different families of compounds, a multiferroic material with high-enough polarization and magnetization suitable for practical applications has not been realized yet. In order to overcome this problem, composite multiferroics are designed by combining a ferroelectric and a ferromagnetic material. Recently composite multiferroics have drawn significant attention due to its enormous design flexibility which can be used for a wide range of applications. In this thesis, a thorough study of the structural, electrical, and magnetic properties of multiferroic BiFeO3 and CuO epitaxial thin films is carried out. BiFeO3 is a type I multiferroic material with a perovskite distorted rhombohedral (R3c) crystal structure. It is ferroelectric (TC = 1123 K) and G-type antiferromagnetic (TN = 643 K) at room temperature. Antiferromagnetism in BiFeO3 arises from the Fe sublattice having d5 configuration whereas ferroelectricity appears due to the directional orientation of 6s lone pair electrons of the Bi3+ ion. We observed that the crystal structure of BiFeO3 thin film gets altered depending on lattice misfit stress caused by the substrate which in turn modifies its magnetic properties through strong magneto-structural coupling. Furthermore, a signature of magneto-(di)electric coupling and exchange bias effect were observed between the BiFeO3 and SrRuO3 layers of a heterostructure. On the other hand, CuO is a type II multiferroic material where ferroelectricity is generated between 213 K and 230 K due to incommensurate spiral magnetic spin ordering along its crystallographic ‘b’ axis. We found that CuO thin films can be grown in the direction of its static polarization axis by proper choice of substrate and the temperature dependent magnetic properties of CuO thin films vary depending on its crystallographic orientations due to strong magneto-structural coupling. Chapter 1 provides a general introduction to various physical phenomena, such as ferroelectricity, ferromagnetism, antiferromagnetism, multiferroicity, magneto-electric coupling, and different magnetic interactions, like Dzyaloshinskii-Moriya interaction, and exchange bias effect. Basic concepts of impedance spectroscopy, dielectrics and perovskite structures are also discussed. General introductions of different materials, which are studied in the thesis, and the motivation of choosing them are incorporated at the end of the chapter. Chapter 2 contains the description of thin film growth technique and different steps of device fabrication process. Different characterization techniques, the instruments used for the characterizations and the working-principle of those instruments have been summarized in the chapter. Chapter 3 focuses on the variation of magnetic properties and crystal structure with the thickness of BiFeO3 thin films. BiFeO3 thin films of different thicknesses, ranging from 16 nm to 60 nm, were grown on (001) SrTiO3 substrate by PLD technique. Detailed x-ray diffraction studies show that the 16 nm, 20 nm and 30 nm films have “R-like” crystallographic phase with an out-of-plane lattice parameter of 4.06 Å whereas the 45 nm and 60 nm films have “R-like” and ‘T-like” crystallographic phases simultaneously. The “T-like” phase has an out-of-plane lattice parameter of 4.65 Å and a c/a ratio of 1.25, resembling a tetragonal crystal structure. Off-specular reciprocal space mapping and azimuthal φ scan show that the “T-like” phase deviates from an ideal tetragonal crystal structure by a monoclinic tilt. The occurrence of the “T-like” phase is associated with the formation of a very thin layer of parasitic Bi2O3 phase which appears in between two film-thicknesses of 30 nm and 45 nm and BiFeO3 grows in “T-like” phase thereafter. High lattice mismatch between Bi2O3 phase and BiFeO3 phase causes more distorted unit cell in “T-like” phase with a high c/a ration. Parasitic Bi2O3 phase appears because of slightly higher partial oxygen pressure used during the growth which prevents the formation of the parasitic ferrimagnetic γFe2O3 phase in the films. Moreover, our XPS studies confirmed that the films contain Fe3+ only without any trace of Fe2+ within a resolution of few atomic percentages and the magnetic signals measured in our experiments are entirely from the BiFeO3 phase. The saturation magnetizations of the films were found to increase with decreasing thickness. At room temperature, the saturation magnetization of a 16 nm-thick BiFeO3 thin film is 87 emu/cc but it goes down to 9 emu/cc when the thickness increases to 60 nm. Moreover, it was observed that the 16 nm thick film is magnetically more anisotropic in comparison to the 60 nm thick film and there is an apparent out-of-plane magnetic hard axis in the 16 nm film. Summarizing the results obtained from the films with different thicknesses, it can be concluded that the vanishing magnetic anisotropy is related to the structural transformation of the film. Chapter 4 provides a detailed study of the variation of magnetic properties of a BiFeO3 thin film with its crystal structure. BiFeO3 thin films of different thicknesses were grown on orthorhombic (001) NdGaO3 substrate. In-depth x-ray diffraction studies and off-specular reciprocal space mapping show that a 15 nm thick BiFeO3 film grows with monoclinic crystal symmetry (Cm) with an out-of-plane lattice parameter of 4.187 Å on the NdGaO3 substrate. The crystal structure was further verified by the TEM studies which showed a good agreement with the results obtained from x-ray diffraction studies. To probe the ferroelectric nature of the monoclinic BiFeO3 film, piezo response force microscopy was performed. It was found that the oppositely oriented ferroelectric domains have 180° phase contrast and a phase vs. voltage hysteresis loop gets generated when the domains are switched between two antiparallel directions. DC magnetic measurements at room temperature showed that the saturation magnetization of the 15 nm film with Cm crystal symmetry is as high as ~250 emu/cc. Experimental evidence confirmed that the films are free from all magnetic parasitic phases and the high saturation magnetization comes solely from the BiFeO3 phase. For comparative study, BiFeO3 films of similar thickness were deposited on (001) SrTiO3 under identical conditions which grew in “R-like” crystal structures. We saw that “R-like” BiFeO3 films have saturation magnetization 2.5 times lower (~100 emu/cc) than that of the film with Cm structure grown on NdGaO3. Our observation was further supported by density functional theory calculations which show that BiFeO3 has a ferromagnetic ground state in the Cm crystal phase. The theoretically obtained magnetic moment is 266 emu/cc which is very close to magnetization values found experimentally. Chapter 5 deals with the magnetic interaction and the magneto-electric coupling between the BiFeO3 and SrRuO3 layers of a heterostructure. BiFeO3/SrRuO3 heterostructures were grown on (001) SrTiO3 substrate by PLD technique. The ferroelectric nature of the top BiFeO3 layer was probed by out-of-plane piezo response force microscopy technique. Temperature dependent magnetization measurements of the heterostructure show a sharp ferromagnetic to paramagnetic transition at 160 K which arises from the bottom SrRuO3 layer. Therefore, the heterostructure is ferroelectric and ferromagnetic below 160 K. Magnetic interactions between the two layers were investigated by isothermal magnetic hysteresis loop (M-H) measurement in a SQUID magnetometer. The M-H measurements at 10 K showed a two-step magnetic hysteresis loop which implies that magnetic moments of the SrRuO3 layer get pinned by the magnetic interaction between the two layers. During magnetization reversal process, the pinned magnetic moments switch at a higher magnetic field and generate the second step of the hysteresis loop whereas the first step appears at a lower magnetic field during the switching of the free SrRuO3 moments. The amount of the pinned SrRuO3 moments depends on the thickness of the BiFeO3 layer as the magnetic properties of a BiFeO3 thin film are related to its thickness. Moreover, evidence of the exchange bias effect was also found in the heterostructure. Field-cooled M-H measurement shows that the second step of the hysteresis loop shifts in two opposite directions along the magnetic field axis depending on the polarity of the cooling field whereas the first step doesn’t respond to the cooling field. This confirms that the exchange bias effect is directly related to the pinned magnetic moments of the SrRuO3 layer. The total amount of pinned moment and hence the exchange bias effect reduces with increasing temperature and disappears completely above 100 K. A strong coupling between the electrical properties of the BiFeO3 layer and the magnetic properties of the SrRuO3 layer was also observed in the heterostructure. To carry out electrical measurements, interdigitated gold electrodes were fabricated on the BiFeO3 layer of the heterostructure by standard photolithography, magnetron sputtering, and lift-off procedure. Temperature dependent resistance and reactance measurements of the heterostructure at different frequencies show anomalies at ferromagnetic TC of the bottom SrRuO3 layer. Moreover, temperature dependent capacitance measurement at 0 T and at 5 T magnetic fields also showed anomalies near 160 K which indicate that the electrical properties of the heterostructure are affected by the magnetic transition of the SrRuO3 layer. Furthermore, impedance spectroscopy measurements were carried out at different constant temperatures and the corresponding Nyquist plots were fitted with an equivalent circuit model. Remarkably, the capacitance and resistance of the equivalent circuit corresponding to the BiFeO3 layer of the heterostructure, show anomalies at 160 K. Absence of any dielectric anomaly at 160 K in pure BiFeO3 confirms that the observed ones appear because of the magnetic phase transition of the bottom SrRuO3 layer. Therefore, the BiFeO3/SrRuO3 heterostructure has ferroelectric and ferromagnetic properties along with a strong magneto-electric coupling between the layers which can be a promising candidate for the composite multiferroic. Chapter 6 describes a correlation between the crystal structure and magnetic properties of CuO thin film. CuO thin films were grown on (001) SrTiO3, (110) SrTiO3, and (111) Si substrate with MgO buffer layers by PLD technique. On (110) SrTiO3 substrate, CuO thin films grow along [010] direction, which is the direction of ferroelectric polarization of CuO, but growth direction becomes [111] when (001) SrTiO3 substrate is used. The CuO film becomes polycrystalline when it is grown on (111) Si substrate. To find the in-plane epitaxial relations between the substrate and the two layers, cross-sectional TEM of the heterostructure grown on (110) SrTiO3 was carried out. HRTEM images showed very sharp interfaces between the layers indicating high-quality growth of the heterostructure. The epitaxial relations were deduced from the SAED pattern and the FFT pattern of the HRTEM images. Distinctly different temperature dependent magnetic properties were found for three differently oriented CuO films. Two anomalies at 213 K and 230 K are clearly visible in temperature dependent magnetization (M vs. T) plot of the heterostructure with (010) CuO film which are associated with the two magnetic transitions of CuO. On the other hand, no such anomaly was observed in M vs. T plot of the heterostructure with (111) CuO film. The heterostructure with polycrystalline CuO film shows a very weak magnetic anomaly at 230 K in its M vs. T plot. It can be concluded from our studies that the contrasting magnetic behaviours of these three heterostructures are due to the difference in epitaxial orientations of the CuO layers. Moreover, CuO thin films can be successfully grown in the direction of static ferroelectric polarization which is the ‘b’ axis of its monoclinic crystal structure. Chapter 7 concludes with general findings pertaining to various observations made in the different chapters. Prospects for future work are briefly outlined in this chapter.

The thesis presented here is focused on the synthesis of iron-bismuth alkoxides and siloxides as precursors for multiferroic BiFeO₃ systems. Spectrum of novel cyclopentadienyl substituted iron-bismuth complexes of the general type [{Cp^y(CO)₂Fe}BiX₂], as potential precursors for cyclopentadienyl iron-bismuth alkoxides or siloxides [{Cp^y(CO)₂Fe}Bi(OR)₂] (R-O^tBu, OSiMe₂^tBu), were obtained and characterised. The use of wide range of cyclopentadienyl rings in the iron carbonyl compounds allowed for a comprehensive analysis of its influence on structure, reactivity as well as solubility of the studied complexes, which are crucial features of potential precursors. The results fill the gap in the chemistry of cyclopentadienyl iron-bismuth complexes. In this work a new method of preparation of novel alkoxides or siloxides iron-bismuth complexes has been developed. In the reaction of Fe₂(CO)₉ with Bi(O^tBu)₃ or Bi(OSiMe₂^tBu)₃ molecular precursors for preparation of heterobimetallic oxides were obtained. Moreover, characterised compounds allowed to extend the knowledge about existence of iron-bismuth clusters and open new ways for the further investigations on the carbonyl iron-bismuth siloxides and alkoxides. The resulting compounds are good single source precursors for the BiFeO₃ materials. The presented synthetic route can be generalized and other heterobimetallic compounds can be obtained. This work should also be helpful in the designing new precursors for synthesis of metal oxides.

Magnetoelectric multiferroics have triggered the attention of scientific community because of their intriguing fundamental physics and novel multifunctional device applications. BiFeO3 has been established as a prototype multiferroic materials having perovskite structure. However, the potentials of this material are yet to be realized due to several difficulties in synthesis, characterization and attaining the desired value of magnetoelectric coupling. The physical properties and the phase transition of this material vary as a function of intensive parameters (i.e., temperature, frequency, pressure, electric field and magnetic field). In the present dissertation, we have fabricated nanoceramic solid solution of (1-x)BiFeO3-xRMnO3 (R: Y3+, Gd3+, Dy3+) and Bi1-xBaxFe1-xZrxO3 with an aim to demonstrate enhanced magnetoelectric multiferroic properties. These modifications have induced a compositional driven structural phase transition and morphotropic phase boundary in the solid solution. The field emission scanning electron micrographs shows polycrystalline nature of microstructure. Temperature dependent dielectric study of BiFeO3 shows anomaly near antiferromagnetic transition temperature, 364 °C suggesting the signature of mgnetoelectric coupling in the material. The antiferromagnetic ordering temperature decreases towards room temperature with increase in composition. Improved ferroelectric hysteresis loops at room temperature have been observed. The grain and grain boundary contributions from the overall electrical properties have been separated using complex impedance spectroscopic analysis. The temperature variation of the bulk and grain boundary electrical conductivity obeyed the Arrhenius behavior suggesting the thermally activated conduction mechanism. The magnetization versus magnetic field curve for YMnO3 modified BiFeO3 samples has exhibited a switching behavior at low fields. Enhanced magnetization properties have been observed in GdMnO3 and DyMnO3 modified BiFeO3. A cross-over from antiferromagnetism to weak ferromagnetism is observed at x = 0.1 for Ba-Zr co-substituted BiFeO3with enhanced magnetization. The behavior of the magnetic hysteresis loops observed at room temperature suggests the suppression of space modulated spin structure. At room temperature, the dielectric permittivity of all the samples decreases with increasing magnetic field. The enhanced magnetoelectric coupling coefficient (ε(H)-ε(0))/ε(0) are found to be -5.5% (x = 0.2), -8% (x = 0.15) and -18% (x = 0.2) for YMnO3, GdMnO3 and DyMnO3 modification respectively at 2 Tesla. The temperature dependent dielectric properties, frequency dependent magneto-capacitance and magneto-impedance measurements demonstrate the signature of magnetoelectric coupling in the materials.

This thesis presents experimental and related theoretical studies of pyrochlore titanate oxides, boron nitride nanotubes, and multiferroic bismuth ferrite. We have investigated these systems at high pressures and at low temperatures using Raman spectroscopy. Below, we furnish a synoptic presentation of our work on these three systems. In Chapter 1, we introduce the systems studied in this thesis, viz. pyrochlores, boron nitride nanotubes, and multiferroic BiFeO3, with a review of the literature pertaining to their structural, electronic, vibrational, and mechanical properties. We also bring out our interests in these systems. Chapter 2 includes a brief description of the theory of Raman scattering and infrared absorption. This is followed by a short account of the experimental setups used for Raman and infrared measurements. We also present the technical details of high pressure technique including the alignment of diamond anvil cells, gasket preparation, calibration of the pressure, etc. Chapter 3 furnishes the results of our pressure-and temperature-dependent studies of pyrochlore oxides which has been divided into eight different parts. In recent years, magnetic and thermodynamic properties of pyrochlores have received a lot of attention. However, not much work has been reported to address the quasiparticle excitations, e.g., phonons and crystal-field excitations in these materials. A material that shows exotic magnetic behavior and high degree of degenerate ground states can be expected to have low-lying excitations with possible couplings with phonons, thereby, finger-printing various novel properties of the system. Raman and infrared absorption spectroscopies can, therefore, be used to comprehend the novel role of phonons and their role in various phenomena of frustrated magnetic pyrochlores. Recently, there have been reports on various novel properties of these systems; for example, Raman and absorption studies [Phys. Rev. B 77, 214310 (2008)] have revealed a loss of inversion symmetry in Tb2Ti2O7 at low temperatures which has been suggested as the key reason for this frustrated magnet to remain in spin-liquid state down to 70 mK. Powder neutron-diffraction experiments [Nature 420, 54 (2002)] have shown that an application of isostatic pressure of about 8.6 GPa in spin-liquid Tb2Ti2O7 induces a long-range magnetic order of the Tb3+ spins coexisting with the spin-liquid phase ascribing this transition to the breakdown of the delicate balance among the various fundamental interactions. Moreover, Raman and x-ray studies have shown that Tb2Ti2O7,Sm2Ti2O7,and Gd2Ti2O7 undergo a structural transition followed by an irreversible amorphization at very high pressures (~ 40 GPa or above) [Appl. Phys. Lett. 88, 031903 (2006)]. In this chapter, therefore, we present our temperature-and pressure-dependent Raman studies of A2Ti2O7 pyrochlores, where ‘A’ is a trivalent rare-earth element (A = Sm, Gd,Tb, Dy,Ho, Er,Yb, and Lu; and also Y). Since all the group theoretically predicted Raman modes of this cubic lattice are due to oxygen vibrations only, in Part (A), we revisit the phonon assignments of pyrochlore titanates by performing Raman measurements on the O16 /O18 − isotope based Dy2Ti2O7 and Lu2Ti2O7 and find that the vibrations with frequencies below 250 cm−1 do not involve oxygen atoms. Our results lead to a reassignment of the pyrochlore Raman phonons thus proposing that the mode with frequency ~ 200 cm−1, which has earlier been known as an F2g phonon due to oxygen vibration, is a vibration of Ti4+ ions. Moreover, we have performed lattice dynamical calculations using Shell model that help us to assign the Raman phonons. In Part (B), we have explored the temperature dependence of the Raman phonons of spin-ice Dy2Ti2O7 and compared with the results of two non-magnetic pyrochlores, Lu2Ti2O7 and Y2Ti2O7. Our results reveal anomalous red-shift of some of the phonons in both magnetic and non-magnetic pyrochlores as the temperature is lowered. The phonon anomalies can not be understood in terms of spin-phonon and crystal field transition-phonon couplings, thus attributing them to phonon-phonon anharmonic interactions. We also find that the anomaly of the disorder activated Ti4+ Raman vibration (~ 200 cm−1) is unusually high compared to other phonons due to the large vibrational amplitudes of Ti4+-ions rendered by the vacant Wyckoff sites in their neighborhood. Later, we have quantified the anharmonicity in Dy2Ti2O7. We have extended our studies on spin-ice compound Dy2Ti2O7 by performing simultaneous pressure-and temperature-dependent Raman measurements, presented in Part (C). We show that a new Raman mode appears at low temperatures below TC ~ 110 K, suggesting a structural transition, also supported by our x-ray measurements. There are reports [Phys. Rev. B 77, 214310 (2008), Phys.Rev.B 79, 214437 (2009)] in the literature where the new mode in Dy2Ti2O7 at low temperatures has been assigned to a crystal field transition. Here, we put forward evidences that suggest that the “new” mode is a phonon and not a crystal field transition. Moreover, the TC is found to depend on pressure with a positive coefficient. In Part (D), we have presented our results of temperature-and pressure-dependent Raman and x-ray measurements of spin-frustrated pyrochlores Gd2Ti2O7, Tb2Ti2O7,and Yb2Ti2O7. Here, we have estimated the quasiharmonic and anharmonic contributions to the anomalous change in phonon frequencies with temperature. Moreover, we find that Gd2Ti2O7 and Tb2Ti2O7 undergo a subtle structural transition at a pressure of ~ 9 GPa which is absent in Yb2Ti2O7. The implication of this structural transition in the context of a long-range magnetically ordered state coexisting with the spin-liquid phase in Tb2Ti2O7 at high pressure (8.6 GPa) and low temperature (1.5 K), observed by Mirebeau et al. [Nature 420, 54 (2002)], has been discussed. As we have established in the previous parts that the anomalous behavior of pyrochlore phonons is due to phonon-phonon anharmonic interactions, we have tuned the anharmonicity in the first pyrochlore of the A2Ti2O7 series, i.e., Sm2Ti2O7,by replacing Ti4+-ions with bigger Zr4+-ions, presented in Part (E). Our results suggest that the phonon anomalies have a very strong dependence on the ionic size and mass of the transition element (i.e., the B4+-ion in A2B2O7 pyrochlores). We have also observed signatures of coupling between a phonon and crystal-field transitions in Sm2Ti2O7. In Part (F), we have studied spin-ice compound Ho2Ti2O7 and compared the phonon anomalies with the stuffed spin-ice compounds, Ho2+xTi2−xO7−x/2 by stuffing Ho3+ ions into the sites of Ti4+ with appropriate oxygen stoichiometry. We find that as more and more Ho3+-ions are stuffed, there is an increase in the structural disorder of the pyrochlore lattice and the phonon anomalies gradually disappear with increasing Ho3+-ions. Moreover, a coupling between phonon and crystal field transition has also been observed. In Part (G), we have examined the temperature dependence of phonons of “dynamical spin-ice” compound Pr2Sn2O7 and compared with its non-pyrochlore (monoclinic) counterpart Pr2Ti2O7. Our results conclude that the anomalous behavior of phonons is an intrinsic property of pyrochlore structure having inherent vacant sites. We also find a coupling between phonon and crystal-field transitions in Pr2Sn2O7. In the last part of this chapter, Part (H), we present our Raman studies of Er2Ti2O7. Here, we show that in addition to the anomalous phonons, there are modes that originate from photoluminescence transitions and some of these luminescence lines show anomalous temperature dependence which have been understood using the theory of optical dephasing in crystals, developed by Hsu and Skinner [J. Chem. Phys. 81, 1604 (1984)]. Temperature dependence of a few Raman modes and photoluminescence bands suggest a phase transition at 130 K. In Chapter 4, we furnish our pressure-dependent Raman studies of boron nitride multi-walled nanotubes (BNNT) and hexagonal boron nitride (h-BN) and compare the results with those of their carbon counterparts. Using Raman spectroscopy, we show that BNNT undergo an irreversible transition at ~ 12 GPa while the carbon counterpart, multi-walled carbon nanotubes, show a similar transition at a much higher pressure of ~ 51 GPa. In sharp contrast, the layered form of both the systems (i.e. h-BN and graphite) undergo a hexagonal to wurtzite phase at nearly similar pressure (~ 13 GPa of h-BN and ~ 15 GPa for graphite). A molecular dynamical simulation on boron nitride single-walled nanotubes has also been undertaken that suggests that the polar nature of the B−N bonds may be responsible for the irreversibility of the pressure-induced transformations. It is interesting to see that in hexagonal phase both the systems have almost similar mechanical property, but once they are rolled up to make nanotubes, the property becomes quite different. Chapter 5 presents the temperature dependence of the Raman modes of multiferroic thin films of BiFeO3 and Bi0.7Tb0.2La0.1O3. Though there have been several Raman investigations of BiFeO3 in literature, here we emphasize the observation of unusually intense second order Raman phonons. Our results have motivated Waghmare et al. to suggest a theoretical model to explain the anomalously large second order Raman tensor of BiFeO3 in terms of an incipient metal-insulator transition. In Chapter 6, we summarize our findings on the three different systems, namely, pyrochlores, boron nitride nanotubes, and BiFeO3 and highlight a few possible experiments that may be undertaken in future to have a better understanding of these systems.

This thesis presents experimental and related theoretical studies of pyrochlore titanate oxides, boron nitride nanotubes, and multiferroic bismuth ferrite. We have investigated these systems at high pressures and at low temperatures using Raman spectroscopy. Below, we furnish a synoptic presentation of our work on these three systems. In Chapter 1, we introduce the systems studied in this thesis, viz. pyrochlores, boron nitride nanotubes, and multiferroic BiFeO3, with a review of the literature pertaining to their structural, electronic, vibrational, and mechanical properties. We also bring out our interests in these systems. Chapter 2 includes a brief description of the theory of Raman scattering and infrared absorption. This is followed by a short account of the experimental setups used for Raman and infrared measurements. We also present the technical details of high pressure technique including the alignment of diamond anvil cells, gasket preparation, calibration of the pressure, etc. Chapter 3 furnishes the results of our pressure-and temperature-dependent studies of pyrochlore oxides which has been divided into eight different parts. In recent years, magnetic and thermodynamic properties of pyrochlores have received a lot of attention. However, not much work has been reported to address the quasiparticle excitations, e.g., phonons and crystal-field excitations in these materials. A material that shows exotic magnetic behavior and high degree of degenerate ground states can be expected to have low-lying excitations with possible couplings with phonons, thereby, finger-printing various novel properties of the system. Raman and infrared absorption spectroscopies can, therefore, be used to comprehend the novel role of phonons and their role in various phenomena of frustrated magnetic pyrochlores. Recently, there have been reports on various novel properties of these systems; for example, Raman and absorption studies [Phys. Rev. B 77, 214310 (2008)] have revealed a loss of inversion symmetry in Tb2Ti2O7 at low temperatures which has been suggested as the key reason for this frustrated magnet to remain in spin-liquid state down to 70 mK. Powder neutron-diffraction experiments [Nature 420, 54 (2002)] have shown that an application of isostatic pressure of about 8.6 GPa in spin-liquid Tb2Ti2O7 induces a long-range magnetic order of the Tb3+ spins coexisting with the spin-liquid phase ascribing this transition to the breakdown of the delicate balance among the various fundamental interactions. Moreover, Raman and x-ray studies have shown that Tb2Ti2O7,Sm2Ti2O7,and Gd2Ti2O7 undergo a structural transition followed by an irreversible amorphization at very high pressures (~ 40 GPa or above) [Appl. Phys. Lett. 88, 031903 (2006)]. In this chapter, therefore, we present our temperature-and pressure-dependent Raman studies of A2Ti2O7 pyrochlores, where ‘A’ is a trivalent rare-earth element (A = Sm, Gd,Tb, Dy,Ho, Er,Yb, and Lu; and also Y). Since all the group theoretically predicted Raman modes of this cubic lattice are due to oxygen vibrations only, in Part (A), we revisit the phonon assignments of pyrochlore titanates by performing Raman measurements on the O16 /O18 − isotope based Dy2Ti2O7 and Lu2Ti2O7 and find that the vibrations with frequencies below 250 cm−1 do not involve oxygen atoms. Our results lead to a reassignment of the pyrochlore Raman phonons thus proposing that the mode with frequency ~ 200 cm−1, which has earlier been known as an F2g phonon due to oxygen vibration, is a vibration of Ti4+ ions. Moreover, we have performed lattice dynamical calculations using Shell model that help us to assign the Raman phonons. In Part (B), we have explored the temperature dependence of the Raman phonons of spin-ice Dy2Ti2O7 and compared with the results of two non-magnetic pyrochlores, Lu2Ti2O7 and Y2Ti2O7. Our results reveal anomalous red-shift of some of the phonons in both magnetic and non-magnetic pyrochlores as the temperature is lowered. The phonon anomalies can not be understood in terms of spin-phonon and crystal field transition-phonon couplings, thus attributing them to phonon-phonon anharmonic interactions. We also find that the anomaly of the disorder activated Ti4+ Raman vibration (~ 200 cm−1) is unusually high compared to other phonons due to the large vibrational amplitudes of Ti4+-ions rendered by the vacant Wyckoff sites in their neighborhood. Later, we have quantified the anharmonicity in Dy2Ti2O7. We have extended our studies on spin-ice compound Dy2Ti2O7 by performing simultaneous pressure-and temperature-dependent Raman measurements, presented in Part (C). We show that a new Raman mode appears at low temperatures below TC ~ 110 K, suggesting a structural transition, also supported by our x-ray measurements. There are reports [Phys. Rev. B 77, 214310 (2008), Phys.Rev.B 79, 214437 (2009)] in the literature where the new mode in Dy2Ti2O7 at low temperatures has been assigned to a crystal field transition. Here, we put forward evidences that suggest that the “new” mode is a phonon and not a crystal field transition. Moreover, the TC is found to depend on pressure with a positive coefficient. In Part (D), we have presented our results of temperature-and pressure-dependent Raman and x-ray measurements of spin-frustrated pyrochlores Gd2Ti2O7, Tb2Ti2O7,and Yb2Ti2O7. Here, we have estimated the quasiharmonic and anharmonic contributions to the anomalous change in phonon frequencies with temperature. Moreover, we find that Gd2Ti2O7 and Tb2Ti2O7 undergo a subtle structural transition at a pressure of ~ 9 GPa which is absent in Yb2Ti2O7. The implication of this structural transition in the context of a long-range magnetically ordered state coexisting with the spin-liquid phase in Tb2Ti2O7 at high pressure (8.6 GPa) and low temperature (1.5 K), observed by Mirebeau et al. [Nature 420, 54 (2002)], has been discussed. As we have established in the previous parts that the anomalous behavior of pyrochlore phonons is due to phonon-phonon anharmonic interactions, we have tuned the anharmonicity in the first pyrochlore of the A2Ti2O7 series, i.e., Sm2Ti2O7,by replacing Ti4+-ions with bigger Zr4+-ions, presented in Part (E). Our results suggest that the phonon anomalies have a very strong dependence on the ionic size and mass of the transition element (i.e., the B4+-ion in A2B2O7 pyrochlores). We have also observed signatures of coupling between a phonon and crystal-field transitions in Sm2Ti2O7. In Part (F), we have studied spin-ice compound Ho2Ti2O7 and compared the phonon anomalies with the stuffed spin-ice compounds, Ho2+xTi2−xO7−x/2 by stuffing Ho3+ ions into the sites of Ti4+ with appropriate oxygen stoichiometry. We find that as more and more Ho3+-ions are stuffed, there is an increase in the structural disorder of the pyrochlore lattice and the phonon anomalies gradually disappear with increasing Ho3+-ions. Moreover, a coupling between phonon and crystal field transition has also been observed. In Part (G), we have examined the temperature dependence of phonons of “dynamical spin-ice” compound Pr2Sn2O7 and compared with its non-pyrochlore (monoclinic) counterpart Pr2Ti2O7. Our results conclude that the anomalous behavior of phonons is an intrinsic property of pyrochlore structure having inherent vacant sites. We also find a coupling between phonon and crystal-field transitions in Pr2Sn2O7. In the last part of this chapter, Part (H), we present our Raman studies of Er2Ti2O7. Here, we show that in addition to the anomalous phonons, there are modes that originate from photoluminescence transitions and some of these luminescence lines show anomalous temperature dependence which have been understood using the theory of optical dephasing in crystals, developed by Hsu and Skinner [J. Chem. Phys. 81, 1604 (1984)]. Temperature dependence of a few Raman modes and photoluminescence bands suggest a phase transition at 130 K. In Chapter 4, we furnish our pressure-dependent Raman studies of boron nitride multi-walled nanotubes (BNNT) and hexagonal boron nitride (h-BN) and compare the results with those of their carbon counterparts. Using Raman spectroscopy, we show that BNNT undergo an irreversible transition at ~ 12 GPa while the carbon counterpart, multi-walled carbon nanotubes, show a similar transition at a much higher pressure of ~ 51 GPa. In sharp contrast, the layered form of both the systems (i.e. h-BN and graphite) undergo a hexagonal to wurtzite phase at nearly similar pressure (~ 13 GPa of h-BN and ~ 15 GPa for graphite). A molecular dynamical simulation on boron nitride single-walled nanotubes has also been undertaken that suggests that the polar nature of the B−N bonds may be responsible for the irreversibility of the pressure-induced transformations. It is interesting to see that in hexagonal phase both the systems have almost similar mechanical property, but once they are rolled up to make nanotubes, the property becomes quite different. Chapter 5 presents the temperature dependence of the Raman modes of multiferroic thin films of BiFeO3 and Bi0.7Tb0.2La0.1O3. Though there have been several Raman investigations of BiFeO3 in literature, here we emphasize the observation of unusually intense second order Raman phonons. Our results have motivated Waghmare et al. to suggest a theoretical model to explain the anomalously large second order Raman tensor of BiFeO3 in terms of an incipient metal-insulator transition. In Chapter 6, we summarize our findings on the three different systems, namely, pyrochlores, boron nitride nanotubes, and BiFeO3 and highlight a few possible experiments that may be undertaken in future to have a better understanding of these systems.

Bismuth ferrite (BFO) is a multiferroic material with cross-correlation between magnetic and electric orders. With no applied external fields the spin structure of BFO is anitferromagnetic and cycloidal. This ordering prevents the detection of the weak ferromagnetism known to exist in the material. The application of magnetic and electric fields of suitable strength and direction is capable of compelling the Fe3+ spins to align in a homogeneous, antiferromagnetic fashion. This report details how numerical methods were used to simulate the spin alignment of a BFO system under different fields. The results were compiled into electric field-magnetic field phase diagrams of BFO to show the divide between cycloidal and homogeneous systems.
Graduate
0607
0611
marca@uvic.ca

Over the last two decades, great progress has been made in the understanding of multiferroic materials, ones where multiple long-range orders simultaneously exist. However, much of the research has focused on bulk systems. If these materials are to be incorporated into devices, they would not be in bulk form, but would be miniaturized, such as in nanoparticle form. Accordingly, a better understanding of multiferroic nanoparticles is necessary. This manuscript examines the multiferroic phase diagram of multiferroic nanoparticles related to system size and surface-induced magnetic anisotropy. There is a particular focus on bismuth ferrite, the room-temperature antiferromagnetic-ferroelectric multiferroic. Theoretical results will be presented which show that at certain sizes, a bistability develops in the cycloidal wavevector. This implies bistability in the ferroelectric and magnetic moments of the nanoparticles. This novel magnetoelectric bistability may be of use in the creation of an electrically-written, magnetically-read memory element.
Graduate

One of the most widely used and useful method of preparation of BFO is the combustion synthesis route using a fuel.The fuel used may be glycine ,citric acid or urea.the precursor material used for the synthesis of BFO by autocombustion route are $BiNO_3$ and $FeNO_3$ solutions with a certain concentration level. Combustion synthesis is becoming one of the most popular methods for the preparation of a wide variety of materials, .The main advantage of using this tchnique is due to the simplicity, the broad applicability range, the self-purifying feature due to the high temperatures involved, the possibility of obtaining products in the desired size and shape. This method is rapidly emerging as one of the most-convenient methods for the preparation of oxide materials. An aqueous solution of a redox system constituted by the nitrate ions of the metal precursor, acting as oxidizer, and a fuel like urea, glycine, citric acid or many others is heated up to moderate temperatures and, upon dehydration, the strongly exothermic redox reaction develops, which is generally self-sustaining and provides the energy for the formation of the oxide. The XRD structure of the synthesized sample was taken after calcination.

碩士
國立成功大學
材料科學及工程學系碩博士班
94
Multiferroics BiFeO3 (BFO) thin films which simultaneously coexists ferroelectricity and antiferromagnetism, have attracted extensively attention to applying on memory, tunable sensor and spin transistor. Because of the significant leakage of BiFeO3, the ferroelectric hysteresis can’t measure at room temperature. Si-doped and La-doped BFO thin films were deposited LaNiO3/Si substrates by chemical solution method in this study. Doping effects on crystal structure, leakage current, dielectric, ferroelectric and magnetic properties were investigated. Because the radius of Si4+ ion was much smaller than that of Fe3+, a high temperature annealing process was necessary to induce Si4+ ion to substitute Fe3+ ion. The annealing temperature for Si-doped BFO thin films was 650℃, but the impurity phase (Bi2Fe4O9) was always occured in BFO thin films. The solubility of Si ion was about 5 mole%. Si-doped in BFO thin films would make films dense, reduce crystal size, vacancies, and surface roughness. From dielectric analyses, 5BFSO thin films got a smallest tanδ at low frequency. That’s why leakage current densities of 5BFSO thin films were reduced from 1.58×10-4 (A/cm2) to 1.09×10-7 (A/cm2). The remnant polarization and coercive field of 5BFSO thin films which measured at room temperature were 2.94 (μm/cm2) and 100.1(kV/cm), respectively. The remnant magnetization and coercive magnetic field were increased with increasing the amounts of Si-doped in BFO thin films. The dopant of La would inhibit the formation of Bi2Fe4O9 phase, resulting in increasing dielectric constant. But as the amounts of La-doped in BFO thin films were increased to 15 mole%, grains appeared to be discontinuous. That’s the reason why 15BLFO had a large leakage current density. 10BLFO had the lowest leakage current density of 2.02×10-5(A/cm2) for La-doped in BFO thin films. But this value was still too high to have complete P-E analyses. The remnant magnetization increased as the amounts of La-doped in BFO thin films increased, but the coercive magnetic field was decreased as the amounts of La-doped in BFO thin films increased.

Die vorliegende Arbeit befasst sich mit Züchtung und Untersuchung von dünnen Kompositschichten aus BaTiO3 und BiFeO3 sowie Sr2FeMoO6-Dünnschichten. Diese Materialien haben gemeinsam, dass sie Bausteine in neuartigen elektronischen Bauelemente z. B. in Computerspeicher-Modulen werden könnten. BiFeO3 ist ein magnetoelektrisches Multiferroikum bei Raumtemperatur, was bedeutet, dass es sowohl eine spontane magnetische als auch eine spontane ferroelektrische Ordnung besitzt. Zusätzlich sind diese aneinander gekoppelt (magnetoelektrisch). Das macht BiFeO3 zu einem sehr begehrten Forschungsobjekt. Da die magnetoelektrische Kopplung im BiFeO3 zu schwach ist, um anwendungsrelevant zu sein, werden im Rahmen dieser Arbeit Kompositmaterialien unter Zuhilfenahme des Ferroelektrikums BaTiO3 hergestellt und untersucht, mit welchen eine Steigerung der magnetoelektrischen opplungskonstante von 4,2 V/cmOe auf über 35 V/cmOe bei Raumtemperatur erreicht wird. Zudem wird das entdeckte Sauerstoffvakanz-Übergitter untersucht und die daraus erwachsende elektrische Polarisation beschrieben. Im zweiten Teil der Arbeit wird zunächst die Herrstellung von Sr2FeMoO6 per gepulster Laserplasmaabscheidung als Dünnfilm beschrieben und die Untersuchung des magnetfeldabhängigen elektrischen Widerstandes gezeigt. Dies geschieht in Abhängigkeit von verschiedenen Parametern wie Temperatur und Substratmaterial. Als Untersuchungsmethodenkommen unter anderem Raster- und Tunnelelektronenmikroskopie sowie Rasterkraftmikroskopie, Rutherford- ückstreuung und Röntgediffraktometrie für strukturelle Charakterisierungen und Strom-, Magnetisierungsund temperatur- und magnetfeldabhängige Spannungsmessungen für die elektrische und magnetische Charakterisierung der Dünnschichten zum Einsatz. Nach einer Einleitung mit der Motivation der Arbeit werden im zweiten Kapitel die teilweise übergreifenden experimentellen Methoden inklusive der Probenherstellung beschrieben. Das dritte und vierte Kapitel beinhaltet jeweils einen Überblick über den Stand der Forschung zu den einzelnen Materialien und die experimentellen Ergebnisse samt Diskussion. Das darauf folgende Kapitel liefert die Zusammenfassung und einen Ausblick.

The presented thesis explores the magnetoelectric (ME) coupling in multiferroic thin film multilayers of BaTiO3 (BTO) and BiFeO3 (BFO). Multiferroics possess more than one ferroic order parameter, in this case ferroelectricity and anti-ferromagnetism. Cross-coupling between these otherwise separate order parameters promises great advantages in the fields of multistate memory, spintronics and even medical applications. The first major challenge in this field of study is the rarity of multiferroics. Second, most known multiferroics, both intrinsic and extrinsic in nature, possess very low ME coupling coefficients. In previous studies conducted by our group, BTO-BFO multilayers deposited by pulsed laser deposition (PLD) showed a ME coupling coefficient αME enhanced by one order of magnitude, when compared to single-layers of the intrinsic multiferroic BFO. However, the mechanism of ME coupling in such heterostructures is poorly understood until now. In this thesis, we used a selection of structural, chemical, electrical and magnetic measurements to maximize the αME-coefficient and shed light on the origin of this enhanced ME effect. The comparison of BTO-BFO multilayers over single-layers revealed not only enhanced ME-coupling, but also reduced mosaicity, roughness and leakage current density in multilayers. Following a parametric sample optimization, we achieved an atomically smooth interface roughness and vast improvements in the ferroelectric properties by introducing a shadow mask in the PLD process. We measured the highest αME-value so far of 480 Vcm-1Oe-1 for a multilayer with a double-layer thickness of only 4.6 nm, two orders of magnitude larger than the coefficient of 4 Vcm-1Oe-1 measured for BFO single-layers. The αME-coefficient in these multilayers stands in an inverse correlation with the double-layer thickness ddl. The influence of oxygen pressure during growth and BTO-BFO ratio on αME was shown to be neglible in comparison to that of ddl. From the characteristic dependencies of αME on magnetic bias field, temperature and ddl, we concluded the existence of an interface-driven coupling mechanism in BTO-BFO multilayers.:1 Introduction 2 Theory of Multiferroic Magnetoelectrics 2.1 Primary Ferroic Properties 2.2 Magnetoelectric Coupling 3 Materials 3.1 The General Structure of Perovskites ABX3 3.2 Strontium Titanate SrTiO3 3.3 Barium Titanate BaTiO3 3.4 Bismuth Ferrite BiFeO3 3.5 Heterostructures Based on BiFeO3 4 Experimental Section 4.1 Thin Film Fabrication 4.2 X–Ray Diffraction 4.3 Microscopic Techniques 4.4 Chemical Analysis Techniques 4.5 Ferroelectric Characterization 4.6 Magnetic Property Measurements 4.7 Measurement of the Magnetoelectric Coupling Coefficient 5 BaTiO3–BiFeO3 Heterostructures 5.1 General Properties of Single-Layers and Multilayers of BTO and BFO 5.2 PLD–Growth of BaTiO3–BiFeO3 Multilayers 5.3 Manipulation of Multilayer Properties through Design 5.4 Effectiveness of Eclipse–PLD 5.5 Enhanced ME Effect in BaTiO3–BiFeO3 Multilayers 6 Summary and Outlook A Magnetoelectric Measurement Setup B Magnetic Background Measurements C Polarized Neutron Reflectometry Literature Own and Contributed Work Acknowledgement Erratum

Blending nanomaterials with liquid crystal (LC) is considered to be a prospective method to enhance the electro-optical properties of the host. Threshold voltage, driving voltage, residual dc, response time and rotational viscosity etc. are found to decrease in nano doped LC system and in turn the anisotropic order of the liquid crystalline host imparts order in the nano-sized guest particles. Proper selection of size, shape and crystallographic phase of nanomaterial is important to achieve desired improvements of the host. This dissertation presents results of doping different type of nanomaterials in nematic, ferro- and antiferroelectric liquid crystal medium. Conducting polymer nanotubes in nematic LC exhibits remarkable reduction in the threshold and driving voltage which is good from application point of view. The residual dc is also reduced significantly in the doped cell and the reduction is even more than that observed in the carbon nanotube doped same LC system. The influence of multiferroic bismuth ferrite nanoparticles on the electro-optical and dielectric properties of an antiferroelectric LC mixture is investigated in planar cells. It is shown that dispersion of nanoparticles lead to modifications of response time, spontaneous polarization, rotational viscosity and voltage required for switching the molecules between two ferroelectric states in the antiferroelectric phase. The large electric field exerted by the nanoparticles on LC molecules and probable charge transfer among them, causes weakening of the interlayer and intermolecular interactions of LC molecules in the antiferroelectric and ferroelectric phases, respectively. In a ferroelectric LC/ conducting polymer nanotubes composite system electrooptic study reveals a lower electrical response time, rotational viscosity and spontaneous polarization. By fitting the capacitance with voltage in a Preisach model, four dipolar species in both pure and nanocomposite system have been obtained. The orientation of the four dipolar species in the composites system is such that the effective dipole moment in the transverse direction of the FLC molecule is less than that in FLC compound. Another aspect of this dissertation is to study a newly synthesized orthoconic antiferroelectric liquid crystal which provides an excellent dark state because of its high tilt. A detailed investigation of the dielectric, electro-optical properties and X-ray studies of the partially fluorinated high-tilt antiferroelectric LC material revealed that the SmA* phase of this material is of de Vries type. Double-peak polarization current response and tristable-optical-switching studies have revealed an antiferroelectric molecular ordering in the de Vries SmA* phase of this material. Two distinct dielectric relaxation peaks in the SmA* phase also complement the antiferroelectric-like ordering of the molecule in the SmA* phase.
The research was conducted under the supervision of Prof. Subir Kumar Roy of the Spectroscopy division under SPS [School of Physical Sciences]
The research was carried out under IACS fellowship and DST research grant