Photoferroelectric effects in polycrystalline bismuth ferrite

Photoferroelectrics effects include a variety of properties observed in materials exhibiting both ferroelectric and photoresponsive properties. Some examples of these effects include the bulk photovoltaic effect, photostriction, and photorefraction. These effects display a potential for light-driven applications, such as light-induced ferroelectric switching, light-driven actuators, or holographic data storage. Bismuth ferrite is a prototypical photoferrolectric material due to its high remanent ferroelectric polarization and relatively narrow bandgap. In addition, the high-temperature stability of the ferroelectric phase and high birefringence make bismuth ferrite an interesting candidate for electro-optic modulation. The bulk photovoltaic and electro-optic properties, combined with the large dark conductivity of bismuth ferrite, could make it suitable for transient photorefractive applications, such as reconfigurable waveguides. Consequently, this thesis reports on the synthesis and photoferroelectric properties of polycrystalline bismuth ferrite films fabricated using solution-deposition methods. High-quality polycrystalline bismuth ferrite films were grown by precise doping and control of the microstructure using seeding strategies. The photovoltaic properties were measured, showing that the bulk photovoltaic effect is the main light-induced charge transport mechanism at zero fields. The influence of stress and doping on the bulk photovoltaic properties was studied. Additionally, the electro-optic properties were measured using a modified Teng-Man set-up. Larger Pockels coefficients were measured in films under compressive stress. A drastic enhancement of the electro-optic response is achieved by combining the Pockels effect and transient ferroelectric switching contributions. The results discussed in this thesis highlight the potential of low-cost polycrystalline bismuth ferrite films for light-driven applications.PACE (Photovoltaics: Advanced Concepts for high Efficiency, PRIDE17/12246511/PACE)7. Affordable and clean energy9. Industry, innovation and infrastructure13. Climate actio

Nanoscale Ferroic Materials—Ferroelectric, Piezoelectric, Magnetic, and Multiferroic Materials

Ferroic materials, including ferroelectric, piezoelectric, magnetic, and multiferroic materials, are receiving great scientific attention due to their rich physical properties. They have shown their great advantages in diverse fields of application, such as information storage, sensor/actuator/transducers, energy harvesters/storage, and even environmental pollution control. At present, ferroic nanostructures have been widely acknowledged to advance and improve currently existing electronic devices as well as to develop future ones. This Special Issue covers the characterization of crystal and microstructure, the design and tailoring of ferro/piezo/dielectric, magnetic, and multiferroic properties, and the presentation of related applications. These papers present various kinds of nanomaterials, such as ferroelectric/piezoelectric thin films, dielectric storage thin film, dielectric gate layer, and magnonic metamaterials. These nanomaterials are expected to have applications in ferroelectric non-volatile memory, ferroelectric tunneling junction memory, energy-storage pulsed-power capacitors, metal oxide semiconductor field-effect-transistor devices, humidity sensors, environmental pollutant remediation, and spin-wave devices. The purpose of this Special Issue is to communicate the recent developments in research on nanoscale ferroic materials

Metal Oxide Thin Films: Synthesis, Characterization and Applications

This Special Issue will compile recent developments in the field of metal oxide thin film deposition. The articles presented in this Special Issue will cover various topics, ranging from, but not limited to, the optimization of deposition methods, thin film preparations, the functionalization of surfaces with targeted applications, nanosensors, catalysis, electronic devices, biocidal coating, and the synthesis of nanostructures via the accurate control of thin film deposition methods, among others. Topics are open to metal oxide thin film deposition and characterization for the development of applications

Desenho de materiais funcionais 2D para futuras aplicações em microeletrónica

Doutoramento em Ciência e Engenharia de MateriaisDevido à redução de dimensões e ao aumento da velocidade de processamento de dados nos dispositivos microeletrónicos baseados em semicondutores convencionais, estão a ser exploradas abordagens inovadoras envolvendo novos materiais tais como óxidos funcionais. Com o rápido desenvolvimento da indústria eletrónica existe uma maior necessidade de elevado desempenho, de elevada fiabilidade, e de componentes eletrónicos miniaturizados integrados em vários dispositivos. A fim de tornar os dispositivos amplamente acessíveis e de fácil utilização, requisitos adicionais devem ser considerados: o tamanho e peso desejados, o custo reduzido, o baixo consumo de energia e a portabilidade. Materiais funcionais de baixa dimensionalidade são muito promissores para cumprir essas exigências. Em particular, os ferroeléctricos de filmes finos bidimensionais (2D) têm recebido grande atenção devido à sua crescente utilização em memórias não voláteis, detectores piroelétricos, transdutores piezoeléctricos miniaturizados e dispositivos sintonizáveis de micro-ondas. A temperatura de cristalização é um parâmetro chave na preparação de ferroelétricos 2D. Muitos filmes finos ferroelétricos são cristalizados a temperaturas >600 °C. Esses valores estão acima da temperatura que certos elementos do dispositivo funcional podem suportar. Recentemente, este facto tornou-se ainda mais importante, devido às promissoras aplicações que podem ser consideradas caso os ferroeléctricos 2D sejam compatíveis com substratos poliméricos flexíveis de baixo custo e de baixo ponto de fusão. A compatibilidade de filmes finos ferróicos com estes últimos tipos de substratos é muito difícil, mas se conseguida pode ampliar acentuadamente a gama de aplicações para os mais recentes requisitos de eletrónica flexível e microeletrónica, onde dispositivos leves e baratos são exigidos. Neste trabalho, é implementada uma combinação da modificação da química de precursores e assistência por luz UV, com promoção simultânea da cristalização pela introdução de sementes nanocristalinas na solução precursora, para a fabricação de filmes finos ferróicos sem chumbo - Método de Precursores Fotossensíveis Semeados. Neste contexto, o principal objetivo deste trabalho foi fabricar filmes finos sem chumbo BiFeO3 (BFO) e Na0.5Bi0.5TiO3 (NBT) a baixas temperaturas (~300 °C) com uma resposta ferroelétrica competitiva. Além disso, a investigação do efeito do elétrodo-base sobre as propriedades dielétricas e ferroelétricas de filmes finos de BFO foi levada a cabo, e a comparação entre o comportamento de condensadores de BFO com base em IrO2, LaNiO3 (LNO) e Pt foi estabelecida. Adicionalmente, os efeitos dos vários eléctrodos sobre a microestrutura de filmes finos ferroeléctricos de BFO foram estudados por microscopia eletrónica de transmissão (TEM) de alta resolução. Primeiramente, filmes finos finos de perovesquite BFO e NBT foram preparados sobre substratos de silício revestidos com Pt, por deposição de solução química. Os filmes finos de BFO foram preparados a temperaturas na gama de 400-500 °C, a partir de soluções de precursores estequiométricas e com excesso de Bi. Os filmes de BFO cristalinos foram obtidos a 400 °C, o limite inferior de temperatura. Os filmes preparadas com excesso de Bi possuem curvas de histerese ferroelétrica mais definidas do que aqueles sem qualquer excesso, para filmes com espessuras ~150 nm. Uma vez que as densidades de corrente de fuga nos filmes finos diminuem com a diminuição da temperatura de processamento, a polarização de filmes finos de BFO preparados com excesso Bi e recozidos a 400 e 450 °C pode ser efetivamente comutada à temperatura ambiente. Obtiveram-se valores de polarização remanescente de Pr ~10 e ~60 μC/cm2 com campos coercivos de EC ~ 205 e 235 kV/cm para os filmes finos preparados a 400 e 450 °C, respectivamente. Os filmes finos de NBT foram preparados a temperaturas entre 400 e 650 °C. As propriedades estruturais e ferroelétricas dos filmes foram examinadas. A constante dieléctrica observada e as perdas dieléctricas a 100 kHz são 616 e 0,032, respectivamente, enquanto que a polarização remanescente observada e o campo coercivo são Pr ~ 24 μC/cm2 e EC ~ 215 kV/cm, respectivamente para o filme de NBT recozido a 650 °C. O recozimento térmico, em atmosfera de oxigénio após cada camada de revestimento, é eficaz na promoção da cristalização do filme na fase de perovesquite romboédrica a uma baixa temperatura de 400 °C. No entanto, obteve-se um ciclo P-E quase linear para os filmes NBT cristalizados a 400 °C devido à sua incipiente cristalinidade. Os filmes finos de BFO foram depositados numa gama de elétrodos para determinar o seu papel no controlo da formação de fases e da microestrutura. A cristalização em elétrodos de óxido seguiu a sequência: amorfa → Bi2O2(CO3) → perovesquite, enquanto que nos elétrodos de Pt cristalizaram diretamente a partir da fase amorfa. Os elétrodos de IrO2 promoveram a formação da fase de perovesquite à temperatura mais baixa e o LNO induziu adicionalmente o crescimento epitaxial local. O LNO tem a estrutura de perovesquite com o parâmetro de rede a = 0.384 nm, compatível com o de BFO, a = 0.396 nm, e assim a epitaxia é mais provável. Todas as composições exibiram precipitados inteiramente coerentes ricos em Fe dentro do interior de grão da matriz de perovesquite, enquanto que a incoerente segunda fase de Bi2Fe4O9 foi também observada nos limites de grão de BFO crescido em eléctrodos de Pt. Esta última pode ser observada por difração de raios X, bem como TEM, mas os precipitados coerentes foram observados apenas por TEM, principalmente evidenciados pelo seu contraste Z em imagens de campo escuro anular. Estes dados têm consequências acentuadas permitindo alargar a utilização de filmes de BFO sob campo aplicado, a aplicações como atuadores, sensores e aplicações de memória. Em seguida, os filmes finos de BFO foram depositados em substratos de Si com elétrodos distintos, como Pt, LNO e IrO2, para investigar o efeito do elétrodo-base sobre o crescimento e as propriedades elétricas do BFO. Todas os filmes de BFO são compostos por grãos colunares cujo tamanho é dependente do elétrodo-base. Não se observou textura para filmes de 320 nm de espessura fabricados em Pt orientado (111). Os filmes sobre eléctrodos de óxido, em particular sobre LNO são altamente orientados no plano (012). A grande polarização remanescente em BFO/Pt e BFO/IrO2 é atribuída à alta contribuição de corrente de fuga. Os filmes BFO de 400 nm de espessura em LNO possuem uma baixa densidade de corrente de fuga ~4 × 10-6 A/cm2, uma grande polarização remanescente de 50 μC/cm2 e um pequeno campo coercitivo de 180 kV/cm à temperatura ambiente. Demonstramos que as camadas de LNO aumentam a cristalinidade e a orientação de filmes finos BFO, o que se reflete nas suas propriedades funcionais. Este estudo mostra que, além da simples necessidade de filmes monofásicos, os elétrodos de óxido de metal têm um impacto relevante no desenvolvimento de filmes finos BFO de alta qualidade fabricados por métodos químicos de deposição de solução. Estes resultados têm uma implicação grande para a fabricação de dispositivos BFO baseados em filmes finos. Finalmente, provamos que é possível fabricar diretamente filmes finos de BFO sem chumbo em substratos flexíveis de poliamida com funcionalidades ferroelétricas e magnéticas (multiferroicidade) à temperatura ambiente. O nosso método inovador, baseado em soluções de Precursores Fotossensíveis e nanosementes cristalinas, foi usado com sucesso para diminuir a temperatura de cristalização de filmes finos de BFO até uma temperatura tão baixa quanto 300 °C, a mais baixa temperatura reportada até agora para a preparação de filmes finos multiferróicos de BFO. Apesar deste excepcionalmente baixo nível térmico, obtém-se uma polarização remanescente Pr de 2.8 μC/cm2 para os filmes semeados + UV, com um campo coercitivo EC de 300 kV/cm. A estratégia de síntese baseada na utilização de precursores fotossensíveis sementados pode ser transferida para qualquer outra família de óxidos metálicos funcionais.With the dimensions reduction and data processing speeds increasing of conventional semiconductor based microelectronic devices, innovative approaches involving new materials such as functional oxides are being explored. With the rapid development of the electronics industry there is a need for high performance, high reliability and miniaturized electronic components integrated into various devices. In order to make the devices user friendly and widely accessible, additional requirements should be considered: the desired size and weight, low cost, low power consumption, and portability in addition to high levels of functionality. Low dimensional functional materials hold great promises to fulfil those requirements. In particular, two-dimensional (2D) thin film ferroelectrics have received wide attention because of their growing use as non-volatile memories, pyroelectric detectors, miniaturized piezoelectric transducers and tunable microwave devices. Crystallization temperature is a key parameter in preparation of 2D-ferroelectrics. Many ferroelectric thin films are crystallized at temperatures >600 °C. This is above the temperature that certain elements of the functional device can withstand. Recently it became even more important due to promising applications that can be envisaged if 2D-ferroelectrics will be compatible with low cost, low melting temperature flexible polymeric substrates. The compatibility of ferroic thin films with those last types of substrates can markedly widen the range of applications towards the most recent requirements of flexible electronics and microelectronics, where lightweight and cheap devices are demanded. In this work, a combination of the modification of precursor chemistry and the assistance of UV-light, with simultaneous promotion of crystallization by introducing nanocrystalline seeds in the precursor solution, is implemented to fabricate lead-free ferroic thin films - Seeded Photosensitive Precursor Method. Within this context, the main objective of this work was to fabricate lead-free BiFeO3 (BFO) and Na0.5Bi0.5TiO3 (NBT) thin films with a competitive ferroelectric response at low temperatures. Moreover, investigations of the effect of the bottom electrode on the dielectric and ferroelectric properties of BFO thin films was conducted and the comparison between the behavior of IrO2, LaNiO3 (LNO) and Pt based BFO capacitors established. Additionally, the effects of these various bottom electrodes on the microstructure of BiFeO3 ferroelectric films was studied by high-resolution TEM. Firstly, BFO and NBT perovskite thin films were prepared on Pt-coated silicon substrates by chemical solution deposition. BFO was prepared at temperatures in the range 400-500 °C, and from stoichiometric and Bi excess precursor solutions. Crystalline BFO films were obtained at the lowest temperature limit of 400 °C. The films prepared with Bi excess possess more defined ferroelectric hysteresis loops than those without any excess; for films with thicknesses ~150 nm. As the leakage current densities in the films decrease with decreasing the processing temperature, polarization of BFO films prepared with Bi excess and annealed at 400 and 450 °C can be effectively switched at room temperature. Remanent polarization values of Pr ~ 10 and ~60 μC/cm2 with coercive fields of EC ~ 205 and 235 kV/cm were obtained for the films prepared at 400 and 450 °C, respectively. NBT thin films were prepared at temperatures from 400 to 650 °C. Structural and ferroelectric properties of the films were examined. The observed dielectric constant and dielectric losses at 100 kHz are 616 and 0.032, respectively, while the observed remanent polarization and coercive field are Pr ~ 24 μC/cm2 and EC ~ 215 kV/cm, respectively for the NBT film annealed at 650 °C. Thermal annealing in an oxygen atmosphere after each layer of coating is effective in promoting crystallization of the film into rhombohedral perovskite phase at a low temperature of 400 °C. However, almost linear, P-E loop was obtained for those NBT films crystallized at 400 °C due to incipient crystallinity. BFO thin films were grown on a range of electrodes to determine their role in controlling phase formation and microstructure. The crystallization on oxide electrodes followed the sequence: amorphous → Bi2O2(CO3) → perovskite, while those on Pt crystallized directly from the amorphous phase. IrO2 electrodes promoted perovskite phase formation at the lowest temperature and LaNiO3 additionally induced local epitaxial growth. LNO has the perovskite structure with lattice parameter a = 0.384 nm, compatible with that of BFO, a = 0.396 nm and thus epitaxy is more likely. It was observed for the first time that all compositions exhibited fully coherent Fe-rich precipitates within the grain interior of the perovskite matrix, whereas incoherent Bi2Fe4O9 second phase was also observed at the grain boundaries of BFO grown on Pt electrodes. The latter could be observed by X-ray diffraction as well as transmission electron microscopy (TEM) but coherent precipitates were only observed by TEM, principally evidenced by their Z contrast in annular dark field images. These data have pronounced consequences for the extended use of BFO films under applied field for actuator, sensor and memory applications. Then, BFO thin films were deposited on Si-based substrates with distinct electrodes, such as Pt, LNO, and IrO2, in order to investigate the effect of bottom electrode on the growth and electrical properties of BFO. All BFO films are composed of columnar grains which size is dependent on the bottom electrode. No texture was observed for 320 nm thick films fabricated on (111) oriented Pt. Films on oxide electrodes, in particular on LNO are highly (012) oriented. The large remanent polarization in BFO/Pt and BFO/IrO2 is attributed to the high leakage current contribution. 400 nm thick BFO films on LNO possess a low leakage current density ~4 × 10-6 A/cm2, a large remanent polarization of 50 μC/cm2 and a small coercive field of 180 kV/cm at room temperature. We demonstrate that LNO layers enhance the crystallinity and orientation of BFO thin films, which is reflected in their functional properties. This study shows that besides the simple need of monophasic films metal oxide electrodes have a relevant impact on the development of high quality BFO thin films fabricated by chemical solution deposition methods. These results have a broad implication for the fabrication of BFO thin film based devices. Finally, we prove that it is possible to directly fabricate lead-free BFO thin films on flexible polyamide substrates with ferroelectric and magnetic functionalites (multiferroicity) at room temperature. Our own proprietary novel solution-based Seeded Photosensitive Precursor Method was successfully used to decrease the crystallization temperature of BFO thin films down to a temperature as low as 300 °C, the lowest reported up to now for the preparation of multiferroic BFO thin films. Despite this exceptionally low thermal budget a remanent polarization Pr of 2.8 μC/cm2 is obtained for the seeded + UV films, with a coercive field EC of 300 kV/cm. The synthesis strategy based on the use of seeded photosensitive precursors can be transferred to any family of functional metal oxide

Magnetoelectric Coupling in BaTiO3-BiFeO3 Multilayers: Growth Optimization and Characterization

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

Functional Oxides with Nitride Buffer Layers for Heteroepitaxial Devices

As conventional memory technologies approach their limit of scalability, there is a quest to find new technologies to replace existing memories. Of the emerging switching phenomena, ferroelectric switching and resistive switching have been considered for this work. Ferroelectricity is a property by which a material develops a spontaneous polarization that can be reversed by and external electric field. Resistive switching, the basis for the novel “memristor” devices, is a property that enables a device switch to a low or high resistance state depending on the magnitude and polarity of the applied voltage. In this work, various nanostructures have been explored to achieve property enhancement in functional oxides. For example, vertically aligned nanocomposite structures consist of two different materials that are simultaneously deposited onto a single substrate, and grow as two distinct phases. Vertically aligned nanocomposite structures offer the advantage of strain tuning through the vertical interfaces between phases. First, to improve the ferroelectric properties of BaTiO3, a conventional ferroelectric material, epitaxial vertically aligned nanocomposite BaTiO3-CeO2 films have been deposited on SrTiO3 substrates. These films exhibit a columnar structure with high epitaxial quality. The films show a similar ferroelectric response as that of pure thin film BaTiO3, but with an improved Curie temperature, despite the incorporation of CeO2. These nanocomposite structures have been replicated on Si substrates using a double buffer layer of SrTiO3/TiN to achieve the eventual integration of these films on Si. No reduction in ferroelectric properties has been observed, but the films again showed an improvement in the Curie temperature. Second, a simple resistive switching device has been demonstrated by the in situ partial oxidation of a TiN film under three different oxidation time periods. The oxidized region consists of near stoichiometric TiO2, and serves as the oxide layer, while the unoxidized TiN serves as the bottom electrode. All films exhibit bipolar resistive switching and all films are forming-free. The forming-free property is attributed to an oxygen deficient TiO2-x layer at the interface between the oxide and nitride regions. Third, ZnO, a piezoelectric, has been selected as another complementary second phase material for BaTiO3. Epitaxial and highly textured vertically aligned BaTiO3-ZnO composite films have been deposited on SrTiO3 substrates and SrTiO3/TiN buffered Si substrates, respectively. Electrical characterization shows that the films grown on both substrates are ferroelectric at room temperature and exhibit similar properties. Composition analysis shows that both the laser fluence and the oxygen partial pressure can modulate the Ba/Ti cation stoichiometry which, in turn, impacts the ferroelectric properties. This is the first demonstration of the vertically aligned nanocomposite of BaTiO3 and ZnO and its silicon based integration. Finally, based on the excellent buffer layer and diffusion barrier properties of TiN for integrating functional oxides on Si, TiN has been applied as a protective layer on metal surfaces. A 500 nm thick TiN layer has been demonstrated to serve as an excellent diffusion barrier in extreme environments

New modalities of strain-based engineering of ferroelectrics: domain structure and properties of PbZr1-xTixO3 thin films

Epitaxial strain has been widely used to modify the crystal and domain structure, and ultimately the dielectric, ferroelectric, and pyroelectric responses of ferroelectric thin-films for a wide variety of applications including memories, transducers, energy harvesters, sensors, and many more. Traditionally, the ability to engineer materials using epitaxial strain has been confined to a limited range of materials systems which are closely lattice matched to commercially available substrates. In turn, considering the PbZr1-xTixO3 system, a model ferroelectric, study of strain effects has been primarily limited to the Ti-rich variants where a wealth of closely lattice matched substrates (~±1%) exists, enabling nearly-coherently-strained films to be obtained. While these studies have generated a wealth of knowledge on the basic effects of epitaxial strain and have demonstrated the ability to enhance ferroelectric susceptibilities, these improvements have only been incremental. In the present work, we seek to expand the bounds of epitaxial strain engineering through the use of chemistry, controlled epitaxial strain (and relaxation), and compositional and strain gradients with the directive of generating phase competition and high-energy ferroelastic domain structures to enhance ferroelectric susceptibilities. We, for the first time, grow epitaxial thin films of PbZr1-xTixO3 across the compositional phase diagram, and show that the rate of strain relaxation is significantly enhanced at the morphotropic phase boundary (PbZr0.52Ti0.48O3), as the result of the high adaptability of the crystal and domain structure. Despite the appearance of nearly “relaxed” crystal structures, PbZr0.52Ti0.48O3 thin films were shown to exhibit significantly different dielectric responses when grown on various substrates. Highlighting the more nuanced effects of partial strain relaxation in tuning ferroelectric susceptibilities. We then proceed to extend the bounds of epitaxial strain by growing compositionally-graded heterostructures which facilitates the retention of strains in excess of 3.5%, large strain gradients (~4.35x10-5 m-1), and has the ability to stabilize the tetragonal crystal and domain structure, in PbZr1-xTixO3 solid solutions that are rhombohedral in the bulk. Furthermore, we show that by varying the compositional-gradient form (in terms of the nature of the compositional gradient and heterostructure thickness) that it is possible to generate large built-in potentials, which significantly suppresses the dielectric response, but has minimal detrimental impact on the ferroelectric and pyroelectric susceptibilities, giving rise to dramatically enhanced pyroelectric figures of merit. Additionally, we show that the magnitude of the built-in potential does not follow predictions based on the magnitude of the strain gradient alone, and instead, is enhanced by local strain gradients which occur when traversing chemistries associated with structural phase boundaries (where there are abrupt changes in the lattice parameter) and at/near ferroelastic domain boundaries. Finally, we explore the nanoscale response of these ferroelastic domains in compositionally-graded heterostructures using a combination of transmission electron microscopy-based nanobeam diffraction strain mapping and multi-dimensional band-excitation switching spectroscopy. We observe that the presence of compositional and strain gradients can preferentially stabilize highly-energetic, needle-like domains, which under electrical excitation are highly labile in the out-of-plane direction (while remaining spatially fixed in the plane). These labile domain walls act like domain wall springs (enhancing the magnitude of the built-in potential), transferring elastic energy between the a and c domains (depending on the direction of the applied bias), and giving rise to locally enhanced piezoresponse at the a domains, as the result of a switching mechanism where the a domains expand towards the free surface or are nearly excluded from the film (once again, depending on the direction of applied bias). These studies demonstrate the efficacy of compositional and strain gradients as a new modality of strain engineering capable of significantly altering the crystal and domain structure, and ferroelectric susceptibilities of ferroelectric thin films inconceivable in the absence of compositional/strain gradients

New ways to tune the multi-functionality of oxide thin films for memory applications

This thesis presents studies on engineering the electrical and magnetic properties in oxide thin films using vertically-aligned nanocomposites (VAN). The works aim to enhance their multi-functionality for various memory applications. First, electroforming-free resistive switching (RS) with high ON/OFF ratio is realized in a novel model VAN system based on self-assembled Sm-doped CeO2 and SrTiO3 films that allow separate tailoring of nano-scale ionic and electronic channels at a high density (1012 inch-2). These devices allow precise engineering of the resistance states, thus enabling large and tunable ON/OFF ratios and high stability for acting as resistive random access memory (RRAM) devices. Second, the ferromagnetic insulating (FMI) and metallic (FMM) properties of La0.9Ba0.1MnO3 (LBMO) are tuned using VAN. La0.9Ba0.1MnO3 is FMI in bulk but usually shows metallicity in plain films. By using VAN consisting of CeO2 nanocolumns embedded in a La0.9Ba0.1MnO3 matrix, the FMI property is maintained in thin film form. The CeO2 phase acts as strain-controlling nanocolumns and at the same time, produces light Ce doping of the LBMO and thus filling of intrinsic cation vacancies. Together these reduce the unwanted double exchange (DE) coupling. This is hard to realize in plain LBMO films which contain cation vacancies, and have several strain-relaxing LBMO phases which result in an enhanced Mn4+/Mn3+ ratio, and hence to DE coupling and metallicity. Besides, by varying the growth temperature of the LBMO–CeO2 VAN, the system is engineered from a FMI to a FMM and the magnetoresistance is highly tunable, which are correlated to the tuning of the lateral size of both phases. These effects are attributed to a dimension change-induced change in the electronic band structure. The tunable properties of LBMO–CeO2 VAN make it a good candidate as low-power-consumption, high-Tc FMI and FMM components in magnetic random access memory (MRAM) and spintronic devices. Last, in-situ electric field tuning of magnetic properties is studied in La0.9Ba0.1MnO3-ZnO VAN, a novel candidate for magnetoelectric random access memory (MERAM) devices. The M-H curves and the remanence are tuned by applying electric fields at a low temperature (10 K). The possible origins for the magnetic modifications are discussed rationally, which include Joule heating, piezoelectric strain, current-induced induction field and charge trapping/detrapping related to a resistive switching effect (which is found to be the most likely mechanism for a hysteretic tuning of the remanence). All these effects are correlated to the existence of the ZnO phase. This work helps to understand the charge doping effect in manganite-ZnO VAN systems.Cambridge Commonwealth, European & International Trus

Magnetism and Magnetotransport in Complex Oxide Thin Film Heterostructures

The nature of magnetism at thin film surfaces and interfaces is not yet fully understood, yet it is quite important for both fundamental studies and technological applications. In this dissertation, I present a study of the magnetism and magnetotransport in single thin film layers as well as at interfaces of Fe3O4/spinel chromite/LSMO and Fe3O4/spinel chromite/Fe3O4 heterostructures. To begin with, investigations of single layer thin films on metallic oxides such as perovskite structure SrRuO3 and spinel structure LiTi2O4 elucidate the dependence of transport properties on parameters such as thickness, film strain state, and crystal orientation. In addition, the magnetism of CoFe2O4 thin films is examined while dynamically altering the strain state via the temperature-dependent lattice parameter of piezoelectric BaTiO3 substrates. Detailed spectroscopy experiments indicate that magnetism at the (110) LSMO and (111) LSMO surfaces are not suppressed compared to (001) LSMO interfaces. In addition, no magnetic coupling was observed between LSMO and spinel chromite layers above 100K. In contrast, the (110) Fe3O4 surface exhibited a significant change in anisotropy accompanied by an enhanced magnetization in the spinel chromite layer to beyond room temperature. At the isostructural interface, there is strong ferromagnetic coupling between Fe and Cr ions in bilayers. Our results on Fe3O4 and LSMO surfaces, combined with measurements on the angular, field and temperature dependence of junctions with LSMO and Fe3O4 electrodes, indicate that spin polarization is not intrinsically suppressed at a surface or interface but that magnetization and spin polarization depends on the crystal surface orientation, strain state and surface or interface reconstruction.This research was supported by the Office of Naval Research (N00014-97-1-0564) and the Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231