Les matériaux ferroélectriques possédant des propriétés diélectriques géantes font l’objet d’une grande attention scientifique, en grande partie à cause de leur utilisation et potentialités futures dans les dispositifs technologiques, en particulier en microélectronique. L’intérêt de ce travail porte sur l’étude des propriétés vibrationnelles de deux systèmes modèles : le relaxeur PbMg1/3Nb2/3O3 (PMN) monocristallin, et des nano-céramiques de SrTiO3 de tailles de grains contrôlées. Les travaux ont été menés essentiellement par une technique non-linéaire optique originale, la diffusion hyper-Raman (HR) de la lumière, et une attention particulière a été portée aux vibrations de basse fréquence. Des modèles structuraux pu être développés pour relier les signatures spectrales aux propriétés diélectriques de ses systèmes.Dans un premier temps, la spectroscopie hyper-Raman a été poussée à ses limites et a permis d’aboutir à une description convaincante de la dynamique de la polarisation électrique de PMN dans le domaine THz et ce, sur une gamme de température couvrant l’ensemble des différents états relaxeurs. L’hypothèse très répandue mettant en avant l’existence de deux modes mous polaires a ainsi pu être exclue. Les anomalies spectrales à haute température s’expliquent par un couplage entre un mode mou ferroélectrique unique et une vibration non-polaire dont le comportement est indépendant de la température. En refroidissant, l’éclatement du mode mou ferroélectrique révèle la formation d’une anisotropie polaire en dessous de 400-500 K. L’ensemble des modifications spectrales a été capturé dans un modèle simple qui rend également compte de la dépendance en température de la permittivité diélectrique.Dans une seconde partie, les expériences hyper-Raman ont été menées dans deux céramiques de titanate de strontium (SrTiO3) possédant des tailles de grains différentes, 80 nm et 150 nm. L’avantage de la diffusion hyper-Raman pour cette étude est sa propension à sonder les réponses spectrales individuelles du coeur et de la coquille des grains, alors que les techniques usuelles d’absorption infrarouge sondent un milieu effectif moyen. L’analyse spectrale révèle des propriétés de grains identiques dans les deux céramiques : même valeur et même dépendance en température de l’épaisseur de la coquille entourant le cœur, et permittivités diélectriques de cœur ainsi que de coquille identiques dans les deux cas. Un modèle structural reliant les propriétés vibrationnelles et la permittivité diélectrique effective confirme l’existence de deux coquilles entourant le cœur des grains.Ces travaux montrent par ailleurs qu’entre 150 nm et 80 nm, la diminution des propriétés diélectriques est principalement liée au rapport des volumes entre le cœur (haute permittivité) et les coquilles (basses permittivités). / Materials that exhibit giant dielectric properties have received a huge amount of attention from the scientific and industrial communities due to their potentialities and applications in technological devices, in particular for microelectronic applications. In this work, we are interested in studying the vibrational properties of a prototypical relaxor single crystal, PbMg1/3Nb2/3O3 (PMN), as well as nanoceramics of SrTiO3 (STO) with controlled grain sizes. The experiments have been mostly performed by hyper-Raman scattering (HRS), an original non-linear inelastic light scattering spectroscopy. Special attention was devoted to low frequency vibrations, and structural models were developed to relate the vibrational signatures to the giant dielectric responses of these systems.HRS spectroscopy was pushed towards its limit and provided a comprehensive picture of the polarization dynamics of PMN in the THz-range within the whole temperature sequence of its characteristic relaxor states. The widespread hypothesis of two paraelectric soft modes is convincingly excluded. The observation of the split ferroelectric mode reveals the local anisotropy below about 400 K. In contrast, the spectral anomalies observed at higher temperatures are explained as due to avoided crossing of the primary polar soft mode with a temperature-independent, non-polar spectral feature. The temperature changes of the vibrational modes involved in the measured fluctuation spectra of PMN were captured in a simple model that accounts for the temperature dependence of the dielectric permittivity as well.On the other hand, HRS experiments were carried-out on STO single crystal and nano-ceramics of different grain sizes, 80nm and 150nm. Contrary to IR-absorption which gives an effective spectral response of the core and the shell(s) constituting the grains, we show that hyper-Raman provides information of the individual core and shell responses. The spectral analysis reveals that except from their volume, the grains in the two ceramics exhibit very similar structures and properties: same value and temperature dependence of the shell thickness surrounding the core, and same dielectric response of the core as well as of the shell in the two samples. Structural models relating the vibrational properties to the effective dielectric permittivity were developed, and confirm that in STO the grains are composed by one core and two surrounding shells. We demonstrate that between 150 nm and 80 nm the decrease of the effective dielectric permittivity with grain size relates mostly to a change in core and shell volume fraction.

Foi desenvolvido um novo método para a determinação da polarização ferroelétrica de polímeros ferroelétricos, em particular do PVDF estirado biaxialmente. A técnica consiste em manter constante a corrente através da amostra e medir a evolução da tensão (V versus t) entre os eletrodos da mesma. As medidas foram realizadas em ambiente com atmosfera de ar superseco o que permite desprezar a condução elétrica das amostras, simplificando sobremaneira as equações do problema. Considera-se a existência de dois tipos de polarização: a ferroelétrica, com tempo de chavamento desprezível, e uma polarização elétrica foram realizados em duas situações distintas: com chaveamento e sem chaveamento ferroelétrico. O procedimento permite avaliar a dependência da polarização ferroelétrica com o campo, determinada a partir da equação geral da corrente. Para se descrever a resposta da polarização elétrica dependente do tempo, usa-se o princípio da superposição e assume-se que ela está em fase com o campo elétrico aplicado (processo lento). Será mostrado que é possível determinar a polarização ferroelétrica do polímero PVDF estirado biaxialmente sem a necessidade de determinar a formas funcionais da polarização dependente do tempo e de se conhecer o valor da capacitância da amostra. / In this work a novel method was developed for the determination of the ferroelectric polarization of ferroelectric polymers, in particular of biaxially stretched PVDF. The technique consists of maintaining constant the current through the sample and to measure the evolution of the bias voltage (V versus t) on the electrodes. Measurements were performed in super-dry air, which allows one to neglect the electric surface and volumetric conduction of the samples, simplifying the equations of the problem. It is considered the existence of two polarization types: the ferroelectric, with a very short switching time, and a another polarization dependent of the time. The processes for studying the electric polarization were accomplished in two different situations: with switching and without ferroelectric switching. The procedure allows evaluating the dependence of the ferroelectric polarization on the electric field, determined starting from the general equation of the current. To describe the time dependent polarization it is used the principle of superposition and the polarization value assumed to be in phase with the applied electric field (slow process). It will be shown that it is possible to determine the ferroelectric polarization of the material without the need of the functional form of the polarization and the value of the capacitance of the sample.

Dans cette thèse, les interactions magnétoélectriques et optomagnétiques transmises par les contraintes dans les structures ferroélectriques/ferromagnétiques sont étudiées. Nous montrons que la dynamique des déformations du Pb(ZrxTi1-x)O3 aboutit à la manipulation électrique sous-coercitive de multi-états ferroélastiques rémanents. La mesure par une jauge résistive de ces états, ainsi que l'écriture et l'effacement électriques et le stockage ferroélastique, sont démontrés. La configuration des contraintes de matériaux ferroélectriques créée électriquement, permet de modifier l'anisotropie magnétique d'une couche ferromagnétique. Ce phénomène est utilisé pour contrôler le champ magnétique coercitif des composants magnétostrictifs des vannes de spin au moyen des déformations. L’irradiation lumineuse est également utilisée pour entraîner une photostriction rémanente dans le BiFeO3. Cette déformation rémanente est transférée à une couche ferromagnétique et permet un contrôle optique de la coercivité magnétique. Nous montrons comment les états magnétiques peuvent être écrits au moyen de la lumière et effacés par un champ électrique. / In this thesis, the strain-mediated magnetoelectric and optomagnetic interactions in ferroelectric/ferromagnetic structures are studied. The strain dynamics in Pb(ZrxTi1-x)O3 is shown to result in the sub-coercive electrical manipulation of its remanent ferroelastic multi-states. The resistive readout of these states provided by the strain gauge layers, together with the electrically-triggered ferroelastic writing, storage, and erasing, are demonstrated. These strain configurations created by electric fields in ferroelectrics can effectively impact the magnetic anisotropy of a ferromagnetic adlayer. This phenomenon is shown to control the magnetic coercive field of the magnetostrictive components of spin valves via the strain. Light irradiation is shown to result in remanent photostriction effect (photo-driven deformation) in BiFeO3. Such optically-induced remanent deformations can be transferred to a ferromagnetic adlayer and result in the optical control of the magnetic coercive force. It is shown here how magnetic states can be written by light and erased by an electric field.

Les supraconducteurs à haute température critique (HTS) sont des systèmes fortement corrélés. Dans ces matériaux, une petite modification du nombre de porteurs peut avoir de grandes conséquences sur leurs propriétés physiques. C'est le cas lorsqu'un matériau ferroélectrique est à proximité immédiate d'un HTS : de manière locale et rémanente, la polarisation ferroélectrique modifie par effet de champ électrostatique le nombre d'électrons dans le HTS, au voisinage de l'interface avec le ferroélectrique. Cet effet peut être exploité dans le but de définir des structures à géométrie variable : dans un premier cas, des jonctions qui consistent en deux électrodes supraconductrices, séparées par une zone où la supraconductivité a été affaiblie par effet de champ. Dans un second cas, des jonctions tunnel ferroélectriques - à nouveau avec des électrodes supraconductrices - où l'épaisseur de la barrière est modulable par effet de champ ferroélectrique. Les travaux menés au cours de cette thèse visent à développer les matériaux et les technologies de lithographie pour fabriquer de telles structures, et à caractériser les propriétés de transport de ces dernières. / In this experimental thesis, we fabricated ferroelectric field-effect devices based on high-Tc superconductors. We grew high-quality epitaxial heterostructures consisting of an ultra-thin (2 to 6 unit cells) film of YBCO and a thin ferroelectric film (BFO-Mn). We fabricated transport measurement microbridges and used a CT-AFM tip to polarise the BFO-Mn outwards or towards the BFO-Mn/YBCO interface. Due to the ferroelectric field-effect, the superconducting properties of the underlying YBCO film were consequently modified. We then used this effect locally in order to design weak links within the microbridges: two regions where the superconducting properties are enhanced are separated by a narrow region where they are depressed. We explored the conditions of existence of a Josephson coupling across this weak link. In parallel, we fabricated ferroelectric junctions. The barrier is an ultra-thin BFO-Mn film sandwiched between a high-Tc superconducting YBCO bottom electrode and a low-Tc superconducting top electrode. Both at room temperature and at low temperature, we characterised the transport properties across the barrier and the resistive switching resulting from the polarisation of the ferroelectric barrier.

Les céramiques piézoélectriques, comme le Titatano-Zirconate de Plomb (PZT), peuvent produire une tension électrique quand elles sont soumises à une contrainte mécanique et, inversement, se déforment sous l'effet d'un champ électrique. Ce couplage électromécanique peut être décrit par des équations de comportement linéaires pour des chargements modérés. Cependant, au-delà de certaines valeurs de champ électrique ou de contrainte mécanique, ce couplage devient fortement non linéaire à cause des phénomènes de réorientation de la polarisation électrique. Dans ce travail de thèse, un modèle phénoménologique, qui tient compte des réorientations ferroélectrique (par un champ électrique) et ferroélastique (par une contrainte mécanique) de la polarisation électrique, est proposé. Deux variables internes sont considérées pour décrire l'histoire du chargement et deux surfaces de charges électrique et mécanique sont définies pour déterminer les débuts des écoulements ferroélectrique et ferroélastique. Une version bi-dimensionnelle de ce modèle est développée également pour faire l'étude des structures piézoélectriques minces. Les deux versions 2D et 3D du modèle phénoménologique sont intégrées implicitement en adoptant la méthode de retour radial (prédiction/correction). Deux éléments finis coque et hexaédrique de premier ordre, qui intègrent ce modèle phénoménologique non linéaire, sont ensuite développés et implémentés dans le code de calcul par éléments finis Abaqus via la routine utilisateur UEL (User ELement) / Piezoceramics like lead zirconate titanate or PZT can produce an electric potential when they are subjected to a mechanical stress and deform in the presence of an electric field. This electromechanical coupling can be described by linear constitutive equations for moderate loadings. Nevertheless, this coupling becomes highly non linear when piezoceramics are subjected to high electromechanical loadings due to the electric polarization switching. In this thesis work, a phenomenological material constitutive model that describe the electric polarization ferroelectric switching (by an electric field) and ferroelastic switching (by a mechanical stress) is proposed. To describe the loading history, two internal variables are considered and two electric and mechanical loading surfaces are defined to indicate the onset of domain switchings. A bi-dimensional version of this model is developed to study thin piezoelectric structures. The phenomenological model 2D and 3D versions are implicitly integrated by adopting the return-mapping algorithm. Two shell and hexahedral first-order finite elements are then formulated and implemented into the commercial finite element code Abaqus via the user subroutine UEL (User ELement)

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第22451号 / 工博第4712号 / 新制||工||1736(附属図書館) / 京都大学大学院工学研究科材料化学専攻 / (主査)教授 田中 勝久, 教授 田中 功, 教授 陰山 洋 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

Diese Arbeit beschäftigt sich mit den elektrischen Eigenschaften der resistiven Schaltmaterialien SrTiO3 und NbO2. Im ersten Teil werden NbO2 (001)-Dünnschichten untersucht. Bisher sind die für NbO2-Dünnschichten in der isolierenden Phase gemessenen spezifische Widerstände um einen Faktor von 200 niedriger als der in NbO2-Einkristallen gemessene 10 kΩ cm Widerstand. In dieser Arbeit wird der spezifische Widerstand von NbO2-Dünnschichten auf 945 Ω cm erhöht. Es wird gezeigt, dass leitfähige Perkolationspfade entlang der Korngrenzen für die Abnahme des spezifischen Widerstandes verantwortlich sind. Durch temperaturabhängige Leitfähigkeitsmessungen wurden Defektzustände identifiziert, die für die Verringerung des spezifischen Widerstandes gegenüber dem theoretischen Wert verantwortlich sind. Im zweiten Teil wird der Einfluss des Ti-Antisite Defekts auf das resistive Schalten in SrTiO3 Dünnschichten untersucht, welche mit metallorganischer Dampfphasenepitaxie gezüchtet wurden. Dabei werden sowohl stoichiometrische als auch Strontium defizitäre Schichten untersucht. Es wird über temperaturabhängige Permittivitätsmessungen gezeigt, dass durch Kristalldefekte die weiche Phononenmode gestört wird und bei stark strontiumverarmten Schichten polare Nanoregionen gebildet werden, was auf die Bildung des TiSr Defekts zurückgeführt wurde. Darüber hinaus wird gezeigt, dass stark strontiumdefiziente SrTiO3 -Schichten ein stabiles resistives Schalten mit einem Ein-Aus-Verhältnis von 2e7 bei 10 K aufweisen, während stöchiometrische Dünnschichten kein stabiles Schalten zeigen. Es wird ein diodenartiger Transportmechanismus, der im hochohmigen Zustand auf Schottkyemission beruht und ihm niederohmigen Zustand durch defektassistierten Tunnelstrom dominiert wird, identifiziert. Daraus wurde ein neues Modell für das resistive Schalten, basierend auf dem TiSr Defekt und der induzierten Ferroelektrizität, entwickelt. / In this work, the impact of crystal defects on the resistive switching materials SrTiO3 and NbO2 is investigated. The work is divided into two parts. In the first part, NbO2 (001) thin films are studied. So far, resistivities measured for NbO2 thin films in the insulating phase are by a factor of 200 lower than the 10 kΩ cm resistivity measured in NbO2 single crystals. To make this material applicable for resistive switching, the resistivity in the insulating phase has to be increased to effectively block the current in the high resistive state. Throughout the investigations presented in this work, the resistivity of NbO2 thin films is increased to 945 Ω cm. It is shown that conductive percolation paths along the grain boundaries are responsible for the decrease in resistivity. Temperature-dependent conductivity measurements identified defect states responsible for the reduction in resistivity from the theoretical value. In the second part of this work, the influence of the Ti anti-site defect on resistive switching in SrTiO3 thin films grown by metal-organic vapor phase epitaxy is studied. Both stoichiometric and strontium deficient thin films are studied. It is shown via temperature-dependent permittivity measurements that crystal defects harden the soft phonon mode and polar nano regions are formed in highly strontium deficient films, which was attributed to the formation of Ti antisite defects. In addition, highly strontium deficient SrTiO3 films are shown to exhibit stable resistive switching with an on-off ratio of 2e7 at 10 K, whereas stoichiometric thin-films do not show stable switching. A diode-like transport mechanism based on Schottky emission in the high-resistance state and dominated by defect-assisted tunneling current in the low-resistance state is identified. From this, a new model for resistive switching based on the Ti antisite defect and the induced ferroelectricity is developed.

Electron emission from ferroelectrics (FEE) is a promising source for electrons. Although extensive studies have shown that the emission is inititated by a variation of the spontaneous polarization, the exact underlying emission process remained unclear to date. The focus of this work is to analyze and improve the electron emission process from ferroelectric materials. To achieve low operation voltages thin films and low coercive voltage ferroelectric relaxor single crystals were used. The emission was measured under ultrahigh vacuum (UHV) conditions with a single electron detector. The ferroelectric thin films were prepared with a structured top electrode, with nanometer-sized regularly arranged apertures. The emission from lead zirconate titanate (PZT) thin films was achieved at excitation voltages as low as 10 V. The voltage dependent polarization state within the emission apertures of PZT was imaged using piezoresponse force microscopy (PFM). The PFM measurements revealed that an increased fraction of the free surface area is switched by an increased applied voltage. Additional, as a thin film electron emitter, bismuth ferrite (BFO) films were investigated. Ferroelectric relaxor lead magnesium niobate - lead titanate (PMN-PT) was used as single crystal electron emitter due to its low coercive field. The time-dependent electron emission process from PMN-PT was clarified with the help of exciting voltage pulses of variable duration. It is demonstrated that FEE from PMN-PT can be described in the framework of a random field model for relaxors, with the measured electron flux correlating with the amount of reversed polarization. The time-resolved analysis gives insight into the polarization switching and screening processes within PMN-PT. The local electron emission from PMN-PT single crystals has been investigated, in the nanometer regime, with the help of an AFM tip serving as an electron detector. Additionally, the influence of the aperture size in the top electrode on the emission has been investigated. It is found that the electron emission is strongly influenced by the electric field distribution in the aperture. An optimum aperture width for electron emission from PMN-PT, which is much smaller than the apertures used so far, was found. Comparative investigations of the electron emission process from relaxors with barium titanate showed that the emission from PMN-PT is much more complex than the emission from a conventional ferroelectric. General conclusions on the future applications of FEE can be drawn.

In the thesis the structure-property relations of newly synthesised liquid crystalline compounds have been investigated with respect to the length of the non-chiral chain, number of lactate units or the lateral substitutions in the molecular core. Such promising for applications phenomena as orthoconic properties and de Vries behaviour have been found and studied for certain compounds. Besides, the antiferroelectric, TGB, hexatic phases have been observed for some compounds of the studied series. Such unique phenomenon as re-entrancy has been observed for a few compounds of the series under investigation and binary mixtures of one compound with neighbouring homologues have been prepared and studied.

Solid-state cooling based on the electrocaloric effect might be a promising alternative to vapor-compressed refrigeration, not only for its increased efficiency but also for its role in preventing the emission of hazardous gases. The electrocaloric effect (ECE) refers to the reversible adiabatic temperature change that occurs in polar materials when an external electric field is applied or varied. In ferroelectric materials, the ECE is particularly pronounced at the transition temperature between the ferroelectric and paraelectric phases. It was shown recently that ferroelectric thin films in general exhibit excellent electrocaloric properties due to their capacity to withstand high electric fields, which typically results in an increase in the adiabatic temperature change. Therefore, the major aim of this thesis was to study environmentally friendly lead-free compounds for their feasibility as electrocaloric active layers in epitaxial film architectures prepared by pulsed laser deposition. Reports in literature on bulk materials suggest that Na0.5Bi0.5TiO3 (NBTO) compounds may be suitable for electrocaloric cooling. Therefore, the growth of epitaxial NBTO-based thin films was studied, which helps to study the correlation between composition, microstructure, and functional properties of this material. Epitaxial films were deposited on different single crystalline substrates applying a thin epitaxial La0.5Sr0.5CoO3 layer as the bottom electrode for subsequent electric measurements. Structural investigation by X-ray diffraction revealed an undisturbed epitaxial growth on LaAlO3, whereas a significantly smaller temperature window for epitaxy was found on YAlO3. The differences might be explained by the lattice misfit resulting in a higher defect density of the intermediate buffer layer on YAlO3. For all samples, a columnar structure with additional pores was found leading to substantial surface roughness. Dielectric measurements revealed significantly decreased permittivity values and increased losses at elevated temperatures if compared to bulk samples. While polarization loops at -100 °C indicated a distinct ferroelectric behavior, ambient temperature data revealed significant resistive contributions due to high leakage currents. As a result, it was not possible to determine the electrocaloric properties for all NBTO-based thin films deposited with the indirect method. In the second part of the thesis, the correlation between structural properties and the electrocaloric effect was investigated in lead-free epitaxial Ba1-xSrxTiO3 (BSTO) thin films. Here, BSTO thin films with Sr contents ranging from x = 0 to x = 0.3 were deposited on SrRuO3 buffered SrTiO3 single crystalline substrates. X-ray diffraction analysis verified a pure epitaxial growth for all Sr concentrations and film thicknesses indicating a larger tetragonal distortion if compared to the bulk material. Dense layers with a low surface roughness were found in microstructural studies. Temperature and frequency-dependent dielectric measurements indicate a diffuse phase transition for all samples, where thicker films showed larger permittivity values. The temperature of maximum permittivity decreases as Sr concentration increases. Polarization curves demonstrate a relaxor-like behavior, particularly above room-temperature. The adiabatic temperature change due to the ECE was determined with the indirect method showing |ΔT| values of up to 2.9 K for an electric field change of 750 kV cm-1.