Dissertations / Theses on the topic 'BiFeO3'

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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)
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.

Ce travail porte sur l’étude de l’effet des contraintes d’épitaxie sur la structure magnétique de couches minces du multiferroïque BiFeO3 (BFO). Des films minces de BFO ont été déposés par ablation laser pulsée sur plusieurs substrats d’oxyde possédant des paramètres de mailles différents permettant de balayer une large gamme de désaccord paramétrique allant des contraintes de compression (ε= -1,7%) aux contraintes de tension (ε= +1%). L’étude par spectrométrie Mössbauer et diffusion nucléaire résonante du 57Fe à température ambiante a montré que l’ordre antiferromagnétique dans les couches minces évolue en fonction des contraintes d’épitaxie. La modulation cycloïdale des moments magnétiques de Fe3+ du BFO massif subsiste dans les couches minces soumises à une faible contrainte de compression, avec une direction de propagation du type [1-10]. De fortes contraintes d’épitaxie déstabilisent la cycloïde vers un ordre antiferromagnétique colinéaire avec une orientation préférentielle des moments magnétiques dépendant du type de contrainte. Dans le cas d’une faible contrainte de tension, une nouvelle direction de propagation de la cycloïde selon [110], inédite dans le massif, est mise en évidence. Les différentes structures magnétiques sont stables à basse température et la température de Néel de ces films ne dépend que faiblement des contraintes d’épitaxie. L’application d’un champ magnétique selon la normale au plan des couches permet de déstabiliser l’ordre magnétique cycloïdal vers un ordre colinéaire pour une valeur critique de champ magnétique appliqué bien inférieure à celle du composé massif
This work is devoted to the study of epitaxial strain effects on the magnetic structure of BiFeO3 (BFO) multiferroic thin films. The BFO thin layers have been deposited by Pulsed Laser Deposition on various oxide substrates having different lattice parameters spanning a strain range from compressive (ε= -1. 7%) to tensile strain (ε= +1%). 57Fe Mössbauer spectrometry and nuclear resonant scattering measurements at room temperature showed that antiferromagnetic order is very sensitive to epitaxial strain. The bulk-like Fe3+ spins cycloidal modulation of BFO survives at low compressive strain, with a propagation direction along [1-10]. High values of epitaxial strain destabilize the cycloid towards a collinear antiferromagnetic order with a preferential orientation depending on the sign of the strain. At low tensile strain, a new cycloid state is evidenced, with a propagation wave vector along [110]. The various magnetic structures are stable at low temperature, and the Néel temperature of the films hardly varies with strain. Application of an external magnetic field along the normal of the film destabilizes the cycloidal magnetic order towards a collinear state for a critical value of the applied magnetic field much lower than that of the bulk compound

Magnetic spin structures in epitaxial BiFeO3 single layer and an epitaxial BaTiO3/BiFeO3 multilayer thin film have been studied by means of nuclear resonant scattering of synchrotron radiation. We demonstrate a spin reorientation in the 15 x[BaTiO3/BiFeO3] multilayer compared to the single BiFeO3 thin film. Where as in the BiFeO3 film, the net magnetic moment m→ lies in the (1–10) plane, identical to the bulk, m→ in the multilayer points to different polar and azimuthal directions. This spin reorientation indicates that strain and interfaces play a significant role in tuning the magnetic spin order. Furthermore, large difference in the magnetic field dependence of the magnetoelectric coefficient observed between the BiFeO3 single layer and multilayer can be associated with this magnetic spin reorientation.

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

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.

La technique d’optique ultra-rapide pompe-sonde, qui repose sur l’emploi de lasers à impulsion ultracourte(femtoseconde), permet de déclencher et étudier des processus ultrarapides dans la matière. L’acoustique picoseconde concerne pour sa part l’étude des processus de génération et détection de phonons acoustiques haute fréquence ainsi quel’analyse des nanomatériaux avec ces phonons (nanoéchographie). Les travaux de recherche de cette thèse avaient pourbut l’étude des couplages électronphonon acoustique dans le matériau ferroélectrique BiFeO3 par acoustique ultrarapide. Nous avons pu mettre en évidence que selon l’orientation du cristal photoexcité, l’émission des phonons acoustiques cohérents longitudinaux (LA) et transverses (TA) pouvait être modulée. De manière spectaculaire, nous avons purévéler un couplage électron-phonon acoustique transverse très efficace comme cela n’avait jamais été observé jusqu’alors dans les métaux, semiconducteurs ou nanostructures artificielles. Une étude détaillée indique que le mécanismepiézoélectrique inverse semble être le moteur de ce couplage électron-phonon (Lejman et al, Nature Communications, 2014). Dans une seconde partie, nous avons montré que BFO, ainsi qu’un autre ferroélectrique biréfringent LiNbO3 (LNO), peuvent être utilisés pour la conversion de mode ultra-rapide par processus acousto-optique (manipulation de la polarisation de la lumière à l’échelle de la picoseconde avec des phonons acoustiques). Cet effet, jamais mis enévidence jusqu’alors dans le domaine GHz, pourrait potentiellement être exploité dans de nouveaux dispositifs photoniques/phononiques pour des modulations acousto-optiques à haute cadence
Ultrafast optical pump-probe technique, by exploiting ultrashort laser pulses (femtosecond), allows to initiate and monitor ultrafast processes in matter. Picosecond acoustics is a research field that focuses on the generation and detection mechanisms of high frequency coherent acoustic phonons in different media, as well as on their application in testing of nanomaterials and nanostructures. This PhDs research project was devoted to study of electron-acoustic phonon coupling in ferroelectric BiFeO3 (bismuth ferrite, BFO) by ultrafast acoustics. We have evidenced that depending on the BFO crystal orientation it was possible to tune the coherent phonons spectrum with in particular variable amplitude of longitudinal (LA) and transverse (TA) acoustic modes. In some grains with particular crystallographic orientations much stronger TA than LA signal was observed. Spectacularly, we have revealed an efficient coupling between electron and transverse acousticphonon. Such high ratio never reported before in any metal, semiconductor or nanostructure before, can be principally attributed to the photoinduced inverse piezoelectric effect (Lejman et al Nature Communications 2014). In a second part, we have shown that BFO as well as another birefringent ferroelectric LiNbO3 (LNO) can be used for ultrafast acousto-optic modeconversion (manipulation of light polarization at the picosecond time scale with coherent acoustic phonons). This effect, never reported at GHz up to now, can be potentially applied in photonics for ultrafast manipulation of light polarization bycoherent acoustic phonons in next generation photonic/phononic devices

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.

Bismuth ferrite (BiFeO₃) is a magneto-electric material which exhibits simultaneously ferroelectric and antiferromagnetic properties. We have used high-field electron spin resonance (ESR) as a local probe of the magnetic order in the magnetic range of 0-25 Tesla. With increasing magnetic field, an induced transition has been found between incommensurately modulated cycloidal antiferromagnetic and homogeneous magnetized spin state. The data reveal a number of interesting changes with increasing field, including: (i) significant changes in the ESR spectra; (ii) hysteresis in the spectra near the critical field. We have analyzed the changes in the ESR spectra by taking into account the magnetic anisotropy of the crystal and the homogeneous anti-symmetric Dzyaloshinsky-Moria exchange. We have also investigated phase induced transition by epitaxial constraint, and substituent and cystalline solution effects. Variously oriented BiFeO₃ epitaxial thin films have been deposited by pulsed laser deposition. Dramatically enhanced polarization has been found for (001)c, (110)c, and (111)c films, relative to that of BiFeO₃ crystals. The easy axis of spontaneous polarization lies close to (111)c for the variously oriented films. BiFeO₃ films grown on (111)c have a rhombohedral structure, identical to that of single crystals. Whereas, films grown on (110)c or (001)c are explained in terms of an epitaxially-induced transition between cycloidal and homogeneous spin states, via magneto-electric interactions. Finally, lanthanum modified BiFeO₃-xPbTiO₃ crystalline solutions have been found to have a large linear magneto-electric coefficient, ∝p. The value of ∝p (2.5x10⁻⁹ s/m or C/m²-Oe) is ∼10x greater than that of any other material (cg., Cr₂O₃ ∼2.5x10⁻¹⁰ s/m), and many order(s) of magnitude higher than unmodified BiFeO₃ crystals. The data also reveal: (i) that ∝p is due to a linear coupling between polarization and magnetization; and (ii) that ∝p is independent of dc magnetic bias and ac magnetic field. We show that the ME effect is significantly enhanced due to the breaking of the transitional invariance of a long-period spiral spin structure, via randomly distributed charged imperfections.
Master of Science

Mestrado em Engenharia Física
Esta dissertação teve como objetivo a produção e caracterização física de fibras e nanotubos de BiFeO3 e FeNbO4. Para o desenvolvimento destes materiais utilizou-se a técnica de fusão com laser (LFZ), o método sol-gel (Pechini) e o método de poros absorventes. As amostras obtidas foram sujeitas a uma caracterização estrutural por difração de raios-X e espetroscopia de Raman, morfológica por microscopia electrónica de varrimento e elétrica por medidas de constante dielétrica. Os resultados obtidos com a técnica de difração de raios-X mostraram que o gel com tratamento a 750 ºC é polifásico. Para conseguir produzir nanotubos escolheu-se o LaCoO3 como material alternativo. Usando a técnica de fusão de zona com laser (LFZ) obtiveram-se fibras de BiFeO3, FeNbO4 e compósitos de BiFeO3+FeNbO4. Com esta técnica foram crescidas fibras a várias velocidades (5, 10, 25, 50, 100 e 200 mm/h), tendo os resultados obtidos com a difração de raios-X evidenciado que todas as amostras obtidas são polifásicas, sendo a amostra de 10 mm/h para o BiFeO3 e a de 5 mm/h para o FeNbO4 as que apresentam melhores propriedades. As amostras de 5 mm/h de todos os compósitos são aquelas que possuem menor quantidade de segundas fases e portanto foram alvo de estudo mais aprofundado. A caracterização dielétrica permitiu verificar que todas as amostras apresentam fenómenos de relaxação dielétrica. Verifica-se também que para o BiFeO3 a constante dielétrica é superior na amostra crescida à velocidade de 10 mm/h, para o FeNbO4 é superior na amostra crescida a 5 mm/h e nos compósitos a amostra com 75% de BiFeO3 e 25% de FeNbO4 apresenta um comportamento diferente das restantes, eventualmente devido à sua microestrutura singular.
In this work, BiFeO3 and FeNbO4 fibers and nanotubes were prepared and characterized. The samples were obtained using three different preparation techniques: laser floating zone technique (LFZ), sol-gel method (Pechini) and the wetting pore method. Structural characterization of the samples was made using the X-ray diffraction (XRD) and Raman spectroscopy techniques, morphologic characterization by scanning electron microscopy (SEM) and electrical characterization by impedance spectroscopy. The XRD patterns showed that the BiFeO3 gel heat-treatment at 750 °C is polycrystalline. To produce nanotubes, by the wetting pore method, LaCoO3 was used as an alternative material. With the LFZ technique, BiFeO3 and FeNbO4 fibers and BiFeO3 + FeNbO4 composites were prepared. The fibers were grown at various pulling speeds (5, 10, 25, 50, 100 and 200 mm/h), and the XRD patterns demonstrated that all samples are polycrystalline. The BiFeO3 samples growth at 10 mm/h and the FeNbO4 samples growth at 5 mm/h were chosen to be analysed electrically. The composite samples growth at 5 mm/h are those having the least amount of secondary phases, and therefore were subjected to further studies. The dielectric characterization shown that all the samples have a dielectric relaxation phenomenon, thermally activated. It was also verifyed that for the BiFeO3 sample the dielectric constant is higher for the growth speed of 10 mm/h and for the FeNbO4 is higher for the grown speed of 5 mm/h. The composite sample 75% BiFeO3-25% FeNbO4 (% wt) behaves differently from the others, possibly due to its unique microstructure.

The downscaling of transistors is assumed to come to an end within the next years, and the semiconductor nonvolatile memories are facing the same physical downscaling challenge. Therefore, it is necessary to consider new computing paradigms and new memory concepts. Resistive switching devices (also referred to as memristive switches) are two-terminal passive device, which offer a nonvolatile switching behavior by applying short bias pulses. They have been considered as one of the most promising candidates for next generation memory and nonvolatile logic applications. They provide the possibility to carry out the information processing and storage simultaneously using the same resistive switching device. This dissertation focuses on the fabrication and characterization of BiFeO3 (BFO)-based metal-insulator-metal (MIM) devices in order to exploit the potential applications in nonvolatile memory and nonvolatile reconfigurable logics. Electroforming-free bipolar resistive switching was observed in MIM structures with BFO single layer thin film. The resistive switching mechanism is understood by a model of a tunable bottom Schottky barrier. The oxygen vacancies act as the mobile donors which can be redistributed under the writing bias to change the bottom Schottky barrier height and consequently change the resistance of the MIM structures. The Ti atoms diffusing from the bottom electrode act as the fixed donors which can effectively trap and release oxygen vacancies and consequently stabilize the resistive switching characteristics. The resistive switching behavior can be engineered by Ti implantation of the bottom electrodes. MIM structures with BiFeO3/Ti:BiFeO3 (BFO/BFTO) bilayer thin films show nonvolatile resistive switching behavior in both positive and negative bias range without electroforming process. The resistance state of BFO/BFTO bilayer structures depends not only on the writing bias, but also on the polarity of reading bias. For reconfigurable logic applications, the polarity of the reading bias can be used as an additional logic variable, which makes it feasible to program and store all 16 Boolean logic functions simultaneously into the same single cell of BFO/BFTO bilayer MIM structure in three logic cycles
Die Herunterskalierung von Transistoren für die Informationsverarbeitung in der Halbleiterindustrie wird in den nächsten Jahren zu einem Ende kommen. Auch die Herunterskalierung von nichtflüchtigen Speichern für die Informationsspeicherung sieht ähnlichen Herausforderungen entgegen. Es ist daher notwendig, neue IT-Paradigmen und neue Speicherkonzepte zu entwickeln. Das Widerstandsschaltbauelement ist ein elektrisches passives Bauelement, in dem ein der Widerstand mittels elektrischer Spannungspulse geändert wird. Solche Widerstandsschaltbauelemente zählen zu den aussichtsreichsten Kandidaten für die nächste Generation von nichtflüchtigen Speichern sowie für eine rekonfigurierbare Logik. Sie bieten die Möglichkeit zur gleichzeitigen Informationsverarbeitung und -speicherung. Der Fokus der vorliegenden Arbeit liegt bei der Herstellung und der Charakterisierung von BiFeO 3 (BFO)-basierenden Metal-insulator-Metall (MIM) Strukturen, um zukünftig deren Anwendung in nichtflüchtigen Speichern und in rekonfigurierbaren Logikschaltungen zu ermöglichen. Das Widerstandsschalten wurde in MIM-Strukturen mit einer BFO-Einzelschicht untersucht. Ein besonderes Merkmal von BFO-basierten MIM-Strukturen ist es, dass keine elektrische Formierung notwendig ist. Der Widerstandsschaltmechnismus wird durch das Modell einer variierten Schottky-Barriere erklärt. Dabei dienen Sauerstoff-Vakanzen im BFO als beweglichen Donatoren, die unter der Wirkung eines elektrischen Schreibspannungspulses nichtflüchtig umverteilt werden und die Schottky-Barriere des Bottom-Metallkontaktes ändern. Dabei spielen die während der Herstellung von BFO substitutionell eingebaute Ti-Donatoren in der Nähe des Bottom-Metallkontaktes eine wesentliche Rolle. Die Ti-Donatoren fangen Sauerstoff-Vakanzen beim Anlegen eines positiven elektrischen Schreibspannungspulses ein oder lassen diese beim Anlegen eines negativen elektrischen Schreibspannungspules wieder frei. Es wurde gezeigt, dass die Ti-Donatoren auch durch Ti-Implantation der Bottom-Elektrode in das System eingebracht werden können. MIM-Strukturen mit BiFeO 3 /Ti:BiFeO 3 (BFO/BFTO) Zweischichten weisen substitutionell eingebaute Ti-Donatoren sowohl nahe der Bottom-Elektrode als auch nahe der Top-Elektrode auf. Sie zeigen nichtflüchtiges, komplementäres Widerstandsschalten mit einer komplementär variierbaren Schottky-Barriere an der Bottom-Elektrode und an der Top-Elektrode ohne elektrische Formierung. Der Widerstand der BFO/BFTO-MIM-Strukturen hängt nicht nur von der Schreibspannung, sondern auch von der Polarität der Lesespannung ab. Für die rekonfigurierbaren logischen Anwendungen kann die Polarität der Lesespannung als zusätzliche Logikvariable verwendet werden. Damit gelingt die Programmierung und Speicherung aller 16 Booleschen Logik-Funktionen mit drei logischen Zyklen in dieselbe BFTO/BFO MIM-Struktur

Orientador: Eudes Borges de Araújo
Resumo: O objetivo do trabalho foi estudar as propriedades estruturais dos filmes finos de ferrita de bismuto (BFO) ao se adicionar excesso de nitrato de ferro ao invés de nitrato de bismuto conforme muitas referências na literatura vêm praticando com a intenção de obter um BFO puro. Filmes finos de BFO foram preparados sobre substratos Pt/TiO2/SiO2/Si(100) usando o método de Pechini pertencente a rota química sol-gel polimérica. Foram produzidos filmes de estequiometria nominal e de variação de 2, 4, 6, 8, 10 e 12 mol% de excesso de nitrato de Ferro. O processo de síntese dos filmes passou por quatro deposições, quatro pirólises a 300 ºC por 20 minutos e cristalização a 600 ºC por 40 minutos. As propriedades físicas dos filmes foram investigadas usando técnicas de MEV, DRX, Raman e EDS. Rietveld foi usado para calcular os parâmetros de rede e o modelo de Williamson-Hall foi usado para calcular o tamanho do cristalito e o microstrain. Resultados do DRX revelaram o aparecimento da fase secundária Bi2O3, ela aparece quando há o excesso de bismuto. Resultados do EDS confirmam o excesso de Bi. A técnica de EDS apontou uma maior At% do bismuto em relação ao ferro em todas as amostras, sendo que, a de 12 mol% foi a que apresentou características mais próxima de uma estequiometria desejável para a produção de um BFO puro.
Abstract: The objective of this work was to study the structural properties of bismuth ferrite (BFO) thin films by adding excess iron nitrate instead of bismuth nitrate as many references in the literature have been practicing with the intention of obtaining a pure BFO. BFO thin films were prepared on Pt / TiO2 / SiO2 / Si (100) substrates using the Pechini method belonging to the polymeric sol-gel chemical route. Films of nominal stoichiometry and variation of 2, 4, 6, 8, 10 and 12 mol% of iron nitrate excess were produced. The synthesis process of the films went through four depositions, four pyrolysis at 300 ºC for 20 minutes and crystallization at 600 ºC for 40 minutes. The physical properties of the films were investigated using SEM, XRD, Raman and EDS techniques. Rietveld was used to calculate lattice parameters and the Williamson-Hall model was used to calculate crystallite size and microstrain. XRD results revealed the appearance of the secondary phase Bi2O3, it appears when there is excess bismuth. EDS results confirm excess Bi. The EDS technique showed a higher At% of bismuth in relation to iron in all samples, and the 12 mol% was the one that presented characteristics closer to a desirable stoichiometry for the production of a pure BFO.
Mestre

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

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.

博士
國立交通大學
材料科學與工程學系所
105
BiFeO3 is the only single-phase material that exhibits two ferroic orderings – antiferromagnetism and ferroelectricity above room temperature[1]. Moreover, it also shows a robust magnetoelectric coupling – a weak ferromagnetism induced by the antisymmetric exchange in the antiferromagnetic spin structure described by the Dzyaloshinskii-Moriya (DM) interaction. Due to this superior property, BiFeO3 has become the most popular and studied material in multiferroic society[2][3][4][5]. Thus, inevitably, controlling and understanding the ferroelectricity in BiFeO3 are the crucial issues in this field. In this dissertation, I will focus on the engineering of ferroelectricity in BiFeO3 thin films and understanding the fundamental physics behind the following three major phenomena – the proximity effect between ferroelectricity and superconductivity, anomalous microwave absorption induced by ferroelectric domain wall, and the role of ferroelectricity plays in water splitting process. The first part of this dissertation reviews the background history and knowledge that will provide a comprehensive picture for readers to have clear ideas of the contents in the following chapters. I will begin with the introduction of ferroic order parameters, ferroic domain and domain wall, and domain wall engineering. The second part of this dissertation devotes to the study of anisotropic superconductivity in YBa2Cu3O7−x induced by periodic multiferroic domain patterns, including 109◦ and 71◦[6] patterns. The anisotropic superconductivity can be observed in YBa2Cu3O7−x on both 109◦ and 71◦ domain patterns. The third part of this dissertation will discuss the anomalous microwave absorption at 71 domain pattern of BiFeO3 probed by microwave impedance microscopy. In this part, I will explore the domain wall motion of BiFeO3 in microwave frequency regime with spatial resolution, which elucidates the contribution from the domain wall. Finally, I will demonstrate the application of ferroelectricity engineering in water splitting process[7]. This part will focus on understanding the role of ferroelectricity plays in the water splitting process via controlling the spontaneous polarization direction and the facets of BiFeO3 .

碩士
國立成功大學
物理學系碩博士班
100
In this study, the dielectric dispersion of mixed-phase BiFeO3 is investigated. Mixed-phase BiFeO3, which has tetragonal (T) and rhombohedral (R) phases, is similar to the relaxor system, such as (1-x)PbMg1/3Nb2/3O3 - xPbTiO3 (PMN - PT) in several aspects: (1) They both possess monoclinic phases of the morphotropic phase boundary (MPB). (2) Their dielectric constant-temperature dependence (ε´-T) show the frequency dispersion behaviors. (3) They both consist of nanodomain structures. Therefore, mixed–phase BiFeO3 exhibits the relaxor-like behavior. In this study, we observed ε´-T spectrum include the dielectric dispersion and phase transition parts. The former is contributed to dielectric relaxation, and the latter is associated with monoclinic phase transformation from M_C to M_A. We also discovered three main mechanisms in dielectric spectrum. There are low frequency conductive relaxation, middle frequency dielectric relaxation, and high frequency relaxation. They come from leakage current, dipolar reorientation polarization, and interfaces of micro-regions, respectively. Further, we use equivalent lumped circuit to analyze the relaxation time and the distribution of potential energy. Calculation results indicated 0.11 eV and 0.23 eV for activation energy of dielectric relaxation and conductive relaxation, respectively, and we inferred the dielectric dispersion is due to the small activation energy. Based on the nano-scale conductive mapping, we can conclude the conductive phenomenon comes from the phase boundaries of mixed-phase. The conductive state can be controllable by switching polarization with applying DC bias.

An attempt has been made in preparing BiFeO3 articles based on gel casting system. For this purpose, BiFeO3 was synthesized by glycine-nitrate auto combustion route. The powders were calcined between 450-550oC with different soaking time. Phase pure BiFeO3 was obtained at 550oC with a soaking time of 2 hours. Phase purity was authenticated by X- ray diffraction technique. Employing BET 5 point method, surface area was measured. The surface area calculated is about 10.36m2/g which is suitable for powder dispersion in gel casting system. Then the phase pure powder was used in gelcasting based on Acrylamide (AM) - N,N/-methylene bisacrylamide (MBAM) premix solution. First, the ratio of AM-MBAM was optimized. Then, stable dispersion of BiFeO3 in that premix solution was also achieved with Triammonium citrate (TAC) as dispersing agent with pH ~9. Then the slurry was gelcasted in humid atmosphere between 50-60oC. The gelcasted bodies have good green strength and free of surface exfoliation defect. Differential scanning calorimetry was employed to study the thermal behavior of green bodies. Gelcasted bodies were first machined in green state and sintered at different temperature between 750-825oC to obtain the final product. The density of gelcasted body was measured and phases are again identified using X-ray diffraction. It has been observed that with increase in sintering temperature from 750 to 800oC theoretical density increased from 92.5% to 94%. A small amount of impurity was also present in sintered product.

碩士
國立交通大學
電子物理系所
102
In this thesis, we study the ultrafast dynamics of YBCO/BFO heterostructures by using dual-color pump-probe spectroscopy. The relaxation behavior of photoexcited carriers in superconducting state and normal state of YBCO can be revealed by measuring the transient reflectivity changes (ΔR/R) at various temperatures; meanwhile, the anisotropy of quasiparticle relaxation in YBCO/BFO heterostructures can be observed through varying the thickness of the BFO layer. At first, we measured the ultrafast spectra of a single-layer YBCO thin film and a single-layer BFO thin film to verify the complicate ΔR/R signals in YBCO/BFO heterostructures, which are composed of two components, i.e., positive (ΔR/R>0 before 15 ps) and negative (ΔR/R<0 after 15 ps) signals. By fitting with the exponential decay functions, we found that the positive component in ΔR/R is similar to that of a single-layer YBCO thin film. On the other hand, the negative component in ΔR/R is due to the BFO layer under the YBCO layer. Moreover, the BFO layer with stripped ferroelectric domains and the net magnetic moments at the domain walls would further lead to the anisotropy of quasiparticle relaxation and superconducting transition temperatures (Tc) in YBCO/BFO heterostructures.

碩士
國立高雄大學
應用物理學系碩士班
101
In this study, we investigated the structural and electrical properties of La doped BiFeO3. We prepared a series of multiferroic Bi1-xLaxFeO3 (0 ￡ x ￡ 0.20) samples by solid-state reaction method to systematically study the effect of La-doping on their structural and electrical properties. The microstructures of samples were examined with a typical x-ray diffraction (XRD) system and a scanning electron microscopy (SEM). The leakage current and polarization measurements were performed using a ferroelectric test system. From the results of XRD patterns of the Bi1-xLaxFeO3 (0 ￡ x ￡ 0.20) samples, it is found that the structures transform from rhombohedral to cubic when the doping concentration x between 0.15 and 0.2. Moreover, we observed that the substitution of Bi3+-ion by La3+-ion can suppress the leakage current in Bi1-xLaxFeO3 samples powerfully. In addition, the dominant leakage mechanisms of Bi1-xLaxFeO3(0 ￡ x ￡ 0.20) samples were the Ohmic conduction and space charge limit conduction. The large leakage current mainly comes from the space charge limit conduction. We also found that the space charge limit conduction could be suppressed at low temperature for Bi1-xLaxFeO3(0 ￡ x ￡ 0.20) samples. Meanwhile, we can respectively extract reliable values of the remanent polarization for all samples by the experimental data and theoretical analysis.

碩士
國立屏東教育大學
應用物理系
100
There are several high-pressure modifications of BiFeO3 (Bismuth ferrite, BFO) whose symmetry and structures are still a matter of debate in the literature. In order to evaluate the effect of impurity substitution on phase transition of BFO, a hydrothermal synthesis route as well as flux-grown technique was utilized to fabricate pure BFO and doped BFO samples. In order to reveal the affecting factor of impurity on the BFO structure, the comparative high-pressure X-ray diffraction on these powders have been be performed through DAC device. The experimental results indicate that the pure BFO as well as doped BFO powder has been successively synthesized through those two synthetic routes. In addition, we obtain a small decrease in BFO particle size with an increase of the Ba-, Pb- and Ca-ion doping concentration in the flux-grown method. X-ray diffraction experiments on pure and doped BiFeO3 powders in diamond-anvil cells show that phase transitions take place in a similar way for pure and doped BFO. As a result, modifications in phase transition behavior due to minor ion incorporation highlight the low sensitivity of BFO to variations of chemical composition. The similar procedures (R3c → C2/m+OIII → Pnma) of reconstructive phas transition are observed in pure, Ca-doped and Pb-doped BFO samples. However, it is shown that the intermediate phase OIII is not detected in Ba-doped sample. Therefore, the present study suggests that a significant structural distortion induced by big Ba ion incorporation inhibits the crystallization of OIII phase. The bulk moduli of R3c were obtained by fitting cell-volume data with a second-order Birch-Murnaghan equation-of-state with K′ (= ∂K/∂P) fixed at 4, and were found to be K=155.8(0.3) GPa, 159.4(0.4) GPa, 205.5(1.0) GPa, 105.8(0.3) GPa for pure, Ca-, Pb-, and Ba-doped BFO, respectively. It is suggested that the remarkable difference in bulk modulus of R3c could be attributed to the size-induced effect.

碩士
國立成功大學
物理學系碩博士班
100
The special feature of the interface between the oxide composite materials and conduction at the interface had attracted great interests in recent years. In this study, we investigated the conductivity mechanism in magneto-electric nano-composites composed of multiferroic BiFeO3 (BFO) and ferrite CoFe2O4 (CFO). By using conductive atomic force microscopy (c-AFM), the current distribution and the barrier for carrier transportation were analyzed. Meanwhile, the defect levels were observed by using photoluminescence spectrum. The results show that the BFO-CFO interface is more conductive than the matrices, and the conduction states are affected by external magnetic fields, irradiated laser light, and applied voltages. The conduction mechanism at the interface is thus discussed by measuring the variation of photoluminescence, which shows changes in defect levels, under different external parameters.

Multiferroic BiFeO3 and the cobalt doped BiFeO3 sample is prepared by sol – gel combustion technique using 2–methoxy ethanol and ethylene glycol as fuel. X-ray diffraction results confirm the formation of BiFeO3 as major phase with small amount of impurity phases, which are subsequently removed by leaching the sample with dilute nitric acid. The Rietveld refinement of all compositions shows there is a gradual decrease in lattice parameter with increase in cobalt concentration. DSC and TG plots of the samples shows there is a structural phase change from R3c to P3mm at 8290C and at higher temperatures appearance of impurity phases due to bismuth loss and liquid phases are seen. The SEM images of the sintered pellet show that the sample so prepared are having distribution of grain sizes which varies from 100nm - 1m. The grains shrink to smaller size, when the sample is doped with cobalt. EDXS of sample shows that the elemental composition is free from any foreign elements contamination. Dielectric measurement is done for all the samples from 100Hz to 1MHz. The low frequency dielectric constant of undoped sample is 36 and increases to 202 for 10% cobalt doped sample. i

碩士
輔仁大學
物理學系碩士班
102
This work study BiFeO3-100x% (Bi0.5Na0.5)TiO3 (x=0.005,0.05) multiferroic ceramics and their photovoltaic responses, structure, dielectric permittivity, conductivity, magnetic properties, ferroelectric, and X-ray absorption spectroscopy. Open-circuit voltage, short-circuit current, and the I-V curve without illumination had been measured, using P-N junction model to do the data fitting. Ferroelectric (Bi0.5Na0.5)TiO3 doping maybe can do the high E-field poling in order to increase the conversion efficiency. Use XRD & SEM to analyze the structure, α angle, and lattice parameter at plane (110), and to compare the grain sizes due to different NBT compounds. The temperature point closes to the Nèel temperature (TN), suggested the magnetoelectric coupling. Conductivity can be calculate by the imagine part of dielectric permittivity. The enhanced ferromagnetism in BFO-5%BNT may due to variation of oxidation valences of Fe ion (Fe3+=>Fe4+) and/or the angle changing of Fe-O-Fe, we use the X-ray absorption spectroscopy to determine.

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
國立成功大學
物理學系碩博士班
98
Significant changes of physical properties are usually observed in perovskite materials with mixed phases. This phenomenon may come from the couplings between the multiple phases or the intermediate structures. In this study, we investigated the BiFeO3 (BFO) films with mixed phases driven by compressive stress from the substrate. In these films, different phases, including monoclinic rhombohedral-like phase (R), and tetragonal-like phase (T), formed periodical strained patterns, and the R/T ratio relaxed with the film thickness. We explored the BFO crystal structures by Raman scattering. The Raman spectrum of samples with different R/T phase ratios showed the change of lattice distortion with thickness. The evolution of phonon behaviors during phase transformation from room temperature to 600 ℃ was investigated. Moreover, the phase transformation with the bias, which was confirmed at nanoscale by scanning probe microscopy, was also revealed by the Raman spectrum. The enhancement of the rhombohedral phonon was observed in the thickest film. In the processes of increasing thicknesses, the Bi-O bonds became longer and the octahedra rotated around [110] axis. The R-phonon about 225, 238 and 362 cm-1 disappeared at the R-T phase transition at low temperature. In contrast, the T phase in the Raman spectrum last to our highest measured temperature 600 ℃ and the Bi-O bonds distance decreased. Under the electric field, the Bi-O bonds shrank and the intensity of Fe-O octahedra-related phonons decreased, which corresponded with the phase transformation of R to T.

碩士
東海大學
物理學系
98
Multiferroic materials exhibit the multifunctional properties. Among them, BiFeO3 compound shows both a ferroelectric phase transition at the high Curie temperature of TC = 1103 K and a magnetic phase transition at the Néel temperature of TN=643 K. BiFeO3 film has a strong ferroelectric at room temperature. Therefore, BiFeO3 film has drawn much attention due to potential applications in the spintronic and memory devices that can be addressed both electrically and magnetically. In this study, BiFeO3 films were deposited by RF sputtering on glass and Pt/Ti/SiO2/Si substrate, respectively, and effect of experimental parameters, including the sputtering power (10-180 W), substrate temperature (300-700 ℃), the ratio of pressure of Ar and O2 (3:1-9:1), and film thickness (50-380nm), on the structural evolution of BiFeO3 (BFO) thin films have been investigated. The experimental results show that pure BFO phase is present for the thickness larger than 300 nm on the amorphous glass substrate. On the other hand, on the Pt/Ti/SiO2/Si(100) substrate, pure single BiFeO3 phase with perovskite (R3c) structure could be easier to obtain for wide experimental parameters, including thickness of 50-380 nm, sputtering power of 20-180 W, the Ar/O2 pressure ratio 3:1-9:1; heat treatment temperature of 450-575 oC. Besides, summarized with XRD, SEM, and AFM results, it is also found that different growth state occurs for various growth temperatures.

碩士
國立成功大學
材料科學及工程學系碩博士班
94
In this study, BiFeO3 thin films were deposited by reactive magnetron sputtering system as the ferroelectric films in FeRAM. Because of the significant leakage of BiFeO3, three (BiFeO3)x-(BaTiO3)1-x solid solution targets of various compositions were prepared for film deposition and the material properties of the (BiFeO3)x-(BaTiO3)1-x films were characterized. 　The crystalline structure of thin films was characterized by glancing incident angle X-ray diffraction(GIAXRD). The composition and the mass density were determined by Rutherford backscattering spectrometry (RBS). The chemical bonding structure of films was determined by X-ray photoelectron spectroscopy (XPS). Magnetization of the sample was measured by using the vibrating sample magnetometer (VSM). In addition, the sample was made to the Pt /(BiFeO3)x -(BaTiO3)1-x /Pt/Ti/SiO2/Si structure for electrical measurements. Ferro- electric analyzer was used to measure the ferroelectric property of thin films, and picoampere meter was used to determine the leakage current. 　After calcining powders of Bi2O3 and Fe2O3, nitric acid was used to wash away the secondary phases. By this way, we can obtain the pure BiFeO3 target. According to GIAXRD result, the as deposited films are amorphous, but diffraction peaks are observed in the samples after annealing at 700oC for 10 min. from the result of RBS, analysis reveals that the mass density of (BiFeO3)0.7-(BaTiO3)0.3 thin films increases when the BaTiO3 content increases. The VSM result indicates that (BiFeO3)0.7-(BaTiO3)0.3 thin film possesses remenant magnetization after annealing at 700 oC. As heat treatment time increases, the degree of remenant magnetization increases, too. In PE curves of all annealed thin films do not present any hystersis loop. Finally, IV measurement shows that the leakage current of (BiFeO3)x-(BaTiO3)1-x films increases as the annealing temperature increases.

Dissertação de Mestrado em Física e Química para o Ensino, apresentada à Universidade de Trás-os-Montes e Alto Douro
Os materiais multiferróicos, como o caso do BiFeO3, são bastante promissores em termos tecnológicos, possuindo uma potencial aplicação em sensores, memórias não voláteis e actuadores. A perovesquite BiFeO3 apresenta vantagens relativamente a outros compostos multiferróicos: elevada temperatura de Curie (TC=1100 K); elevada temperatura de Néel (TN=640 K); não contém chumbo na sua composição. No entanto a fase pura de BiFeO3 é difícil de sintetizar, formando simultaneamente diversas fases secundárias como Bi2O3, Bi2Fe4O9 e Bi25FeO39. Neste trabalho sintetizaram-se cerâmicos maciços de BiFeO3 através do método sol-gel com combustão de ureia, partindo de uma mistura estequiométrica de Bi2O3 e Fe2O3. O pó obtido foi calcinado a diferentes temperaturas (300-840ºC) e diferentes tempos (1-64 horas) para investigar quais as melhores condições de síntese do material. O material obtido foi analisado por termogravimetria, espectroscopia de infravermelho com transformada de Fourier, difracção de raios X, microscopia electrónica de varrimento e microscopia electrónica de transmissão. Investigaram-se também os efeitos da dopagem de lantânio na estrutura cristalina, utilizando composições do tipo Bi1-xLaxFeO3 com x≤0,30. A quantificação de fases foi obtida através do refinamento de Rietveld dos espectros de difracção de raios X das amostras, utilizando o programa PowderCell. Este processo revelou ser uma ferramenta útil na determinação dos parâmetros das estruturas cristalinas e na quantificação de fases, permitindo monitorizar a evolução das reacções de formação e decomposição das diversas fases. Verificou-se que os tratamentos térmicos mais rápidos, com o máximo de 1 hora, minimizavam a formação de fases secundárias, tendo sido obtido um máximo da fase BiFeO3 de 99% molar à temperatura de 600ºC. Tratamentos térmicos mais prolongados a 600ºC, quer em ar ou em árgon, levaram à decomposição da fase BiFeO3 nas fases secundárias Bi2Fe4O9 e Bi25FeO39. A interpretação desta decomposição de acordo com o modelo de Avrami-Erofeev sugere uma cinética a uma dimensão (n=1), compatível com os estudos em SEM onde não foi possível detectar as fases secundárias mesmo quando estas estavam em maioria (tratamento térmico de 64 horas). Para as amostras dopadas com lantânio, verificou-se que a estrutura cristalina do Bi1-xLaxFeO3 sofre uma alteração gradual de romboédrica (R3c em x=0) para ortorrômbica (Pnma em x=0,30). A variação dos valores das correntes de fuga com o campo aplicado seguem os modelos de Poole-Frenkel e de Space Charge Limited. Os menores valores para a corrente de fuga verificaram-se nas amostras de BiFeO3 a 700ºC (5x10-11 A/cm2 para um campo de 1 kV/cm) e de Bi0,9La0,10FeO3 a 800ºC (3x10-9 A/cm2 para um campo de 1 kV/cm).
Multiferroic materials, such as BiFeO3, have a promising technologic application in sensors, non volatile memory and actuators. The perovskite BiFeO3 doesn’t have lead in its composition and exhibits high Curie temperature (TC=1100 K) and high Néel temperature (TN=640 K) which present advantages when compared with other multiferroic materials. Phase pure BiFeO3 compound is very difficult to achieve. Secondary phases like Bi2O3, Bi2Fe4O9 and Bi25FeO39 are reported to systematically appear. We prepared several bulk samples of BiFeO3 by the urea sol-gel combustion method, yielding brownish powders. These powders were calcinated at different temperatures (300-840ºC) and times (1-64 hours) to investigate the best synthesis conditions of the material. The resulting materials were analysed by infrared spectroscopic, thermogravimetric analysis, X-ray diffraction, scanning electron microscopy and transmition electron microscopy. We also investigated the effects of the lanthanum substitution on the structure, Bi1-xLaxFeO3, for the composition range of x≤0,30. In order to quantify the phases present we use the Rietveld refinement method and the software PowderCell, which was a powerful tool to determine the parameters of the crystalline structures and in phase quantification. This study reveals that fast thermal treatments, with a maximum of one hour, minimize the appearance of secondary phases. We achieved 99% molar of BiFeO3 phase with a thermal treatment of 600ºC in air for one hour. Further treatments at 600ºC, in air or in argon, yielded decomposition of BiFeO3 into Bi2Fe4O9 and Bi25FeO39 phases. Avrami plots of the decomposition process indicated a slope near one suggesting that the reaction follows a one dimensional process, which is in accordance with the EDS analysis made with scanning electron microscopy. The substitution of lanthanium on Bi1-xLaxFeO3 changes the cristalline structure gradually from rhombohedral (R3c at x=0) to orthrhombic (Pnma at x=0,30) . The leakage current follows predominantly the Poole-Frenkel and Space Charge Limited conduction mechanism. The lowest density leakage current achieved was v 5x10-11 A/cm2 (to an applied field of 1 kV/cm) for BiFeO3 at 700ºC. For Bi0,9La0,10FeO3 at 800ºC, it was obtained 3x10-9 A/cm2 (for an applied field of 1 kV/cm).

碩士
國立成功大學
材料科學及工程學系
104
In this study, we attempted to grow epitaxial Nd1.85Ce0.15CuO4 (NCCO) film on the (001) SrTiO3 substrate as the bottom electrode for the growth of the multiferroic BiFeO3 (BFO) film. The NCCO and BFO targets used for the deposition were prepared by the solid state reaction method. Both the RF magnetron sputtering and pulsed laser deposition (PLD) methods were studied for the NCCO film growth, while the growth of BFO films was only attempted by the sputtering method. Various efforts were made in order to obtain the epitaxial NCCO films, including direct formation of the desired phase at the heated substrate, deposition at room temperature (RT) followed by the post-annealing at high temperature, and varying the target composition in favor of some elements. The optimal NCCO films were obtained with the PLD method, which were deposited at RT from a stoichiometric target and then annealed in air at 950 C for 2 hours. The films grown by such a PLD process were epitaxial and showed a full width at half maximum of 0.134。 in the X-ray diffraction (004) rocking curve. They had a pure phase and were absent of the secondary phase, Nd0.5Ce0.5O1.75, which was often observed in the NCCO films grown by the sputtering method. The electric measurement showed that the resistivity of the PLD grown NCCO films decreased as the temperature decreased, i.e. a metallic behavior, with the RT value being 8.163×10-2 Ω.cm, which is good enough as the electrode. Furthermore, the surface roughness of the PLD grown NCCO films was 1.53 nm, suitable for the subsequent growth of BFO. Attempts have been made to grow epitaxial BFO film on NCCO. The preliminary results were encouraging, which showed that (001) oriented BFO was able to grown on the (001) NCCO film surface.

碩士
大葉大學
電機工程學系
99
BiFeO3(BFO) thin films were grown by radio frequency(RF) magnetron sputter deposition on a (111) SrTiO3(STO) substrate., These films were grown with Fe:Bi ratio =1:1.02 of target,, and at different argon environmental pressures and different growth time to grow, the growth of pressure used in the experiments were 20 × 10-2 torr. and 60 × 10-2 torr., the substrate temperature of 600 ℃ under the growth of this film. By X-ray diffraction for analysis, observed perpendicular to the films surface of the X-ray diffraction, showing the BFO thin film of STO (111) epitaxial properties , BFO lattice parameter matching (111) peak, the diffraction peak of the angle is 40 °, the growth of the BFO film with time will affect the structure, the growth in the relatively long time under the BFO thin film structure will be more obvious, the film growth will significantly affect the pressure structural phase BFO with the mixed phase (Bi25FeO40 , Bi2Fe4O9), the growth in the low pressure of the growth of the BFO film with the structure will be better, and also in the mixed phase is relatively good improvement on the film surface analysis using atomic force microscope (AFM) and scanning electron microscopy (SEM) to do measurements, we found that crystallization and surface roughness at a relatively low under the pressure of growth are more favorable, and in the electrical aspects of the I-V curves show only films with Ohmic nature of the relationship, the other part of the activation energy of the film in a relatively high growth under the pressure of a relatively high activation energy. Keywords: magnetron sputter, epitaxial, atomic force (AFM ), scanning electron microscope ( SEM)

碩士
國立中正大學
物理學系暨研究所
99
Multiferroics BiFeO3 (BFO) with perovskite structure, exhibiting simultaneously ferroelectricity (TC~1100 K) and anti-ferromagnetism (TN ~640 K) at room temperature, has attracted extensive attention. However, most of researches adopted the pulse laser deposition and chemical solution deposition to fabricate BFO films on the SrRuO3-buffered SrTiO3 substrate, but very few studies were reported on the sputtering due to the complexity of fabrication for BFO. In this study, the rf-magnetron sputtering is adopted to fabricate BFO films on the glass and Pt/Ti/SiO2/Si(100) substrate. The experimental results showed that the pure isotropic BFO phase can be obtained for wide experimental parameters, including thickness of 50-400 nm, sputtering power of 20-120 W, the ratio of pressure of Ar and O2 of 1/1-9/1, deposition temperature of 350-400 oC, and working pressure of 2.5-60 mTorr. For the optimized 200-nm-thick BFO films grown on different underlayers, Pt(111) underlayer suppresses BiFeO3 phase, and Pt/FePt underlayer results in isotropic growth of it. Single phase perovskite BFO with strong (001) texture and fine grain size was formed on L10 FePt(001) buffer. In addition, highly (001)-textured BFO(001)/FePt(001) thin films exhibited excellent ferroelectric properties, 2Pr = 94.2μC/cm2 for 200 nm BFO/30 nm FePt and 2Pr = 50 μC/cm2 for 150 nm BFO(001)/20 nm FePt(001), respectively. Furthermore, the exchange bias phenomenon is observed in Co/ polycrystalline BFO systems through proper field cooling, and large exchange bias field HE = 400 Oe and Hc=2650 Oe can be obtained for 5 nm Co/ polycrystalline BFO films.

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.

Superconductor BSCCO and BFO composite sample is prepared by sol-gel combustion technique using glycine as the fuel. X-ray diffraction results confirm the formation of BSCCO as major phase with some moderate amount of impurity phases. The SEM images of the sintered BSCCO and BFO composite pellet show that the samples so prepared are having distribution of grain sizes. EDXS of sample shows that the elemental composition is free from any foreign elements contamination. R-T measurement is done for all the samples. The TC for BSCCO is coming out to be around 60 K. The BFO added BSCCO samples show semiconducting behaviour as high concentration of magnetic impurity decouples the cooper pairs.

The phenomena of superconductivity are low temperature business. The low temperature works started with liquefaction of natural gases in the late 1800. Successful liquefaction of O2 was done by French scientist Caillettet in 1877. In 1911, superconductivity was discovered in mercury at liquid Helium temperature by Dutch physicists H. K. Onnes [1]. He was awarded with Nobel prize for his work. Since its discovery in 1911, even today, superconductivity is an important area of research in solid state physics. The disappearance of resistivity in a material is called superconductivity, and the temperature at which it occurs is called as critical temperature (TC). The discovery of superconductivity opened a new area for research.

This thesis presents the study of structural, surface morphology, electric, magnetic, magnetoelectric and magnetodielectric properties of Cobalt substituted multiferroic BiFeO3. Since their discovery, multiferroics have brought tremendous interest among the researchers due to the coexistence of various ferroic order parameters. The synchronization of the magnetic and electric order parameter, hence generating magnetoelectric coupling, has been of importance in particular. Various functional devices aiming at the coupling between the ferroelectric and ferromagnetic order parameter are underway. Among all, the perovskite oxides (ABO3) based multiferroics are of prime interest due to their ease of synthesis and easy to understand physical interactions due to their simple structure. Bismuth Ferrite (BiFeO3), is a prototype ABO3 type multiferroic material, possessing the ferroelectric Curie temperature (Tc) ~ 1103K and antiferromagnetic Neel temperature (TN) ~ 643K. It exhibits a weak net magnetization as the G- type magnetic ordering is superimposed with an incommensurate cycloidal spin structure having the periodicity of 62nm. The chemical substitution is considered as one of the alternatives for enhancing the net magnetization via disruption of the cycloidal chain. Among the various B–site substitution, the Co substituted BiFeO3 is found to be very rare in the literature.

The multiferroic perovskite solid solution xBiFeO3-(1-x)PbTiO3 (BF-PT) exhibits very unique features such as giant tetragonality (c/a ~ 1.19), coexistence of ferroelectric and magnetic order (in certain composition range) and a high Curie point. The system has an added advantage of being semiconducting in nature with an optical band gap of ~ 2 eV which makes it interesting from the viewpoint of photocatalyst and photovoltaic applications. The end members BiFeO3 and PbTiO3 show rhombohedral (R3c) and tetragonal (P4mm) structures, respectively. The system is reported to exhibit morphotropic phase boundary (MPB) separating the rhombohedral and tetragonal phases near x ~ 0.73. Despite considerable research, the morphotropic phase boundary (MPB) region characterized by the coexistence of coexistence of the tetragonal and rhombohedral phases, is still an unresolved issue. In this work we have examined (i) the factors that leads to the uncertainty in the MPB region of this system, (ii) the pressure induced structural transformation behavior of the tetragonal compositions, (iii) photocatalytic performance of the different phases, and finally (iv) an extensive study on the ferroelectric, piezoelectric and high-field electrostrain behavior of La-modified BF-PT. The thesis has seven chapters. Chapter 1 presents the fundamental concepts and definitions which are relevant to understand the results presented in the thesis. It also contains a summary of the literature pertaining to BiFeO3 and its various derivatives. Chapter 2 explains the details of the experimental methodology and analysis used in this work. The main results of the thesis are presented in the next four chapters (3-6). Chapter 3 deals with the erratic phase formation behavior in a wide range of compositions in (1-x)BiFeO3-(x)PbTiO3 (BF-PT) system in a certain composition range, often referred to as the MPB region. Under similar sintering conditions, sometimes the pellets would spontaneously disintegrate to powder (completely/partially) after cooling and sometimes not. Structural analysis revealed that the disintegrated powder was invariably tetragonal (P4mm) phase and the pellet which survives exhibit coexistence of P4mm and R3c phases. Detailed microstructural investigation revealed that this different in the phase formation behavior is intimately related to the size of the grains. For the composition x=0.29, a composition within the reported MPB region of this system, the spontaneously disintegrated powder grains exhibit size of ~ 10 microns. When the size of the grains was reduced to ~ 0.5 μm by mechanical grinding, the same powder specimen shows a rhombohedral (R3c) phase after annealing at ~ 700 oC (annealing was done to get rid of the residual stress induced effect during the grinding process, if any). Neutron diffraction experiment revealed that the size induced rhombohedral phase is also antiferromagnetic in nature. This suggests a strong coupling of the antiferromagnetic phase with the rhombohedral (R3c) structure. Based on the structural results, we argue that the driving force for this size driven coupled inter-ferroelectric-magnetic transformation is the large depolarizing and the stress fields due to large polarization and domain walls energy (which are intimately related to the giant tetragonality). Chapter 4 is an extension of the findings reported in Chapter 3. Keeping in view the fact that BiFeO3 and its derivatives are semiconducting ferroelectric with a band gap of ~ 2 eV, we investigated the behavior of the different phases (tetragonal and rhombohedral) regarding their photocatalytic properties. As discussed in Chapter 3, reducing the size of the grains to below 0.5 micron switches the ground state from tetragonal to rhombohedral. But then this was possible only after the ground specimen was annealed above the Curie point, i.e. after taking the system to the paraelectric state. When the annealing was not done, the as-ground 0.5 micron grains still retained the tetragonal phase. The pinned tetragonal domain walls which are already present in the large grains act as barriers for the bulk tetragonal regions to transform to rhombohedral when the size is reduced physically at room temperature. By taking the system to the paraelectric phase, the tetragonal domain walls are “dissolved”. In the absence of the tetragonal domain wall as barriers, the system lands in the rhombohedral (R3c) ground state during cooling. The trapped tetragonal phase at room temperature in the 0.5 microns grains are therefore metastable in nature. We therefore had the opportunity to examine the catalytic performance of the same powder in its stable rhombohedral phase (obtained after annealing) and metastable tetragonal phase (before annealing). For the sake of reference, we also made compositions with stable tetragonal phase (with grains having the same specific area as the metastable tetragonal phase). The photocatalysis experiments were carried out using these powders as catalysts to degrade typical organic contaminants. We found that when the metastable phase was used as the catalyst, the rate of die degradation increased by nearly five times as compared to its stable phase counterparts. Our results suggest that the metastable ferroelectric phase either increase the availability of the photogenerated charge carriers to participate in the redox reaction associated with dye degradation or increase the adsorption rate of the dyes on the surface of the particles. Chapter 5 gives the details of the investigation pertaining to pressure induced structural transformations in the tetragonal phase of (x)BiFeO3-(1-x)PbTiO3. High pressure experiments were carried out using complementary Raman and x-ray diffraction techniques to capture the structural changes on both the local and global length scales. We found two different types of pressure induced phase transformations in two different composition ranges. We established a correlation of the transition pressure with the tetragonality of the parent phase at the ambient pressure and temperature conditions. While the compositions (x < 0.4) with relatively low tetragonality show a transition from tetragonal P4mm to a non-polar rhombohedral R3̅c phase, the compositions in the range 0.4 < x < 0.71 with a relatively high tetragonality undergo a transformation first to polar rhombohedral (R3c) phase before transforming to the non-polar R3̅c phase. The transition pressure at which the composition loses its tetragonal structure decreases with the increasing BF content. Our study confirmed that the MPB at room temperature can be stabilized even by pressure in tetragonal systems with large tetragonality. Chapter 6 reports the discovery of an extraordinary large electrostrain (~ 1.3%) in polycrystalline specimens of La modified BF-PT, more specifically in the system with chemical formula 0.55(Bi0.7La0.3)FeO3-0.45PbTiO3. This is the largest electrostrain value reported so far in a polycrystalline ceramic specimen. We carried out a detailed investigation to understand the mechanism associated with this ultrahigh electrostrain response using Raman, XRD, neutron powder diffraction, and electron microscopy with specimen subjected to poling field. We found that the composition y = 0.30 exhibiting the ultrahigh electrostrain exhibit a cubic-like structure in the unpoled state. On poling it transforms to a majority tetragonal phase. Neutron diffraction revealed very weak superlattice reflections, characteristic of antiphase tilted octahedra in both the unpoled and poled specimen. Detailed analysis using group theoretical ideas and Rietveld analysis of the neutron diffraction data, revealed that the true structure of this system is monoclinic (space group Cc) although the pseudocubic lattice parameters are tetragonal-like. Raman and electron microscopic studies revealed that what appears as a cubic-like to tetragonal-like transformation on application of the poling field is primarily a manifestation of increase in the coherence length of the tetragonal-like domains. This composition behaves as a relaxor ferroelectric and the HAADF-STEM analysis revealed that the cubic—like phase is associated with the presence of considerable positional disorder, the degree of which is noticeably reduced after poling. An extensive XRD study in-situ with electric field was also carried out to understand the domain switching behavior with field. This study proved that the ultrahigh electrostrain of the critical composition (y = 0.30) is primarily associated with (i) the large reverse switching of the tetragonal-like non-180o ferroelectric-ferroelastic domain walls and (ii) the large tetragonality (c/a ~ 1.23) of the tetragonal-like phase. The important results of the thesis and the scope for further studies are summarized in Chapter 7.

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.

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
國立高雄師範大學
物理學系
97
Multiferroic BiFeO3 is considered to be one of the prospective materials due to the coexistence of electric order and magnetic order at room temperature. It has been intensively studied for ten years and the theory foundations are established well. The size effect on BiFeO3 has also been studied, especially for the magnetic property. In this thesis, the size effect on dielectric properties is also investigated. The BiFeO3 samples with various particle sizes were prepared by conventional solid-state reaction and the grinding machine. According to data of X-ray powder diffraction and scanning electron microscopy, all samples were polycrystalline due to the mismatch between grain size and particle size. The finest particle did not exhibit drastic change from antiferromagnetism to ferromagnetism, but magnetic susceptibility increased with the length of grinding duration. Besides magnetism was size-dependent, dielectric properties of BiFeO3 were related to particle size as well. But the only advantage for finely grinded BiFeO3 was the enhancement of dielectric permittivity. Resistivity and dissipation factor were negatively affected. The main problem might arise from the increased grain boundary and porosity in these samples.

碩士
東海大學
物理學系
102
In this present work, effect of BFO growth temperature (Tg) and BFO thickness (tBFO) and CoPt underlayer thickness (tCoPt) on the structure, surface morphology, ferroelectric, and photovoltaic properties of sputtered BFO films on CoPt(111) buffered on the glass substrates are investigated. The experimental results show that the pervoskite sturcture BFO phase with highly preferred(110) orientation is found for 200-nm thick BFO on 20-nm CoPt bottom layer at Tg = 350-500 oC. The decrease of coherent scattering domain size (dcsd) with Tg from 13.0 nm at Tg = 350 oC to 43.5 nm at Tg = 500 oC indicates the suppressed structural defect with Tg. Ferroelectric behavior is found for 200-nm BFO at Tg = 350-500 oC, where better ferroelectric properties of 2Pr = 155 μC/cm2 and EC = 454 kV/cm is obtained at Tg = 450 oC, which might be resulted from better crystallinity of BFO and low leakage current density. In addition to ferroelectric behavior, 200-nm BFO films at Tg = 350-500 oC also exhibit significant photovoltaic (PV) effect by the illumination of laser with wavelength of 405 nm, and short-circuit current density (JSC) increases with increasing illumination intensity. The short-circuit current density at the maximum intensity of 138 mW/cm2 (Jmax) almost linearly increases with dcsd, indicating that the structural defects suppress PV effect in this studied ITO/BFO(110) polycrystalline system. Post annealing above samples remarkably enhances the JSC, which might be related to the suppressed structural defect of ITO layer. In this series films, better PV properties of Jmax = 23 μA/cm2 is obtained at Tg = 450 oC. Besides, effect of tBFO at fixed Tg = 450 oC is also studied. A pure BFO phase with (110) preferred orientation is also found by reducing tBFO from 200 nm to 50 nm, but ferroelectric behavior is found at tBFO = 75-200 nm. Interestingly, Jmax increases from 23 to 42 μA/cm2 with decreasing tBFO from 200 to 75 nm. Finally, ferroelectric and photovoltaic properties of 75-nm thick BFO with various CoPt underlayer thicknesses are also studied. Increasing tCoPt from 10 nm to 30 nm not only keeps (110) preferred orientation of BFO phase but enhances crystalline of BFO, revealing suppressed structural defects of BFO with increasing tCoPt. Moreover, the films with tCoPt = 20-30 nm show ferroelectric behavior. Most importantly, Jmax is further enhanced to 79 μA/cm2 tCoPt = 30 nm, which is superior to the epitaxial BFO(110) films reported by Seidel et al [PRL107, 126805 (2011)]. The results of present study suggest that CoPt(111) textured underlayer with good crystalline and flat surface is helpful in enhancing crystalline of BFO polycrystalline thin films (tBFO = 75 nm) with (110) preferred orientation, and therefore showing both ferroelectric behavior and outstanding photovoltaic properties.