Dissertations / Theses on the topic 'Multiferroic'

Multiferroic materials have recently begun to attract significant scientific interest due to their potential applications in the design of modern electronic devices. Currently, the magnetic properties of materials form the basis of our electronic data storage and have the potential to enhance the logic operations performed in electronic devices (such as computers and sensors). Non-volatile magnetic memory is used in data storage devices, such as the hard drives found in personal computers, where data is encoded via the magnetisation state of magnetic domains in the device with one of two states: either up or down (M ↑ or M ↓); the state is determined or changed by interacting with the magnetic flux about the domain. Furthermore, in current computing and sensor technology, logic operations are performed with arrays of transistors; however, in spintronics ("spin transport electronics") the electric current itself is spin polarised and there is data encoded in the current itself. Circuit elements in such a system are magnetic devices that interact with the electron spin.Magnetoelectric multiferroics are materials that have both a spontaneous ferroelectric polarisation (P) and magnetic magnetisation (M). Polarisation may be manipulated by an electric field and magnetisation by a magnetic field, hence the potential of multiferroics lies in the coupling between the two degrees of freedom and the manipulation of magnetisation by an applied electric field and vice versa. The properties of a magnetic device could be altered "on-the-fly" by applying an electric pulse, and in the context of the examples provided this would greatly diversify the logic elements in spintronic circuits. Furthermore, with both polarisation and magnetisation a multiferroic domain can take on one of four states (M ↑ P ↑, M ↑ P ↓, M ↓ P ↑, or M ↓ P ↓) dramatically increasing data storage density over the current binary system.Lutetium ferrite (LuFe2O4) is a multiferroic material in which both the magnetisation and polarisation arise from the iron sites and with strong iron-iron correlations the material is a promising candidate as a high temperature multiferroic. The material has a layered structure with bilayers of FeO separated by single layers of LuO on a hexagonal lattice. Frustrated 2D charge order exists below 550 K which transitions to 3D charge order below 330 K and simultaneously frustrated ferrimagnetic order exists in the multiferroic phase below 250 K. X-ray and neutron scattering experiments have been performed in order to characterise the ferroelectric and ferrimagnetic order and magnetoelectric coupling in this material.Resonant x-ray scattering (RXS) was performed on the Material Science beamline of the Swiss Light Source where the energy dependence of the superlattice reflections corresponding to the charge order was collected. Non-linear regression using a custom Levenberg-Marquadt algorithm was applied in order to extract the anomalous scattering factors which demonstrated the superlattice reflections were described by a charge order model. Furthermore, the chemical shift was shown to correspond to full Fe2+/Fe3+ charge disproportionation. The absence of any polarisation or azimuthal dependence, shown by resonant x-ray scattering data collected on the ID20 beamline of the European Synchrotron Radiation Facility, confirmed the prediction of Nagano et al. that the orbital moments of the Fe2+-sites exist in a disordered glassy state.X-ray absorption near edge structure (XANES) calculations were performed using the FDMNES program in order to assess the validity of the anomalous scattering factors obtained in the RXS experiment and to further test the charge order model. It was shown that the characteristic features of the experimentally determined functions can be qualitatively reproduced by calculations using the known charge order model. Furthermore, these functions were shown to reproduce the phase of the RXS data further demonstrating that the reflections result from a pure charge ordered phase.Inelastic neutron scattering performed on the PUMA triple axis spectrometer of the FRMII demonstrated that magnetic critical scattering is observed at 250 K. A broad peak in the temperature dependence is observed rather than the characteristic divergence of a magnetic transition: this is attributed to broadening of the transition by the distribution of oxygen stoichiometry in the sample and ferroelectric fluctuations integrated into the data due to poor c-axis resolution. Pyroelectric current and magnetometry measurements demonstrate a peak in the magnetic susceptibility and a step in the polarisation at approximately 215 K, well below the magnetic transition. Elastic neutron scattering experiments performed on the E2 flat cone diffractometer of the Helmholtz-Zentrum Berlin demonstrate these features correspond to a 2D-to-3D magnetic transition that has previously only been predicted by anomalies in other measurements.An applied field study performed by neutron scattering on the E2 flat cone diffractometer of the Helmholtz-Zentrum Berlin and x-ray scattering on the PX1 protein crystallography beamline of the Australian synchrotron demonstrate the control of the magnetic domain population with an electric field, contrary to other recent reports on this topic. Furthermore, the observed magnetoelectric coupling is inconsistent with current models of the magnetic structure of this system. The x-ray measurements demonstrate a disorder-to-order effect by the applied electric field as 3D order is preferred with an increase in the intensity of all satellites.Temperature dependent x-ray powder diffraction data collected on the Powder Diffraction (PD) beamline of the Australian Synchrotron has demonstrated anisotropic thermal expansion with negative thermal expansion of the c-axis in this material. Electron density mapping by Fourier analysis shows the disorder of the oxygen between the electrically static Lu ions and the neighbouring Fe ions, as electron hopping between Fe2+ and Fe3+ leading to a corresponding variation on the Fe-O bond length. Reversible structural distortions are observed indicating a piezoelectric effect in this material caused by the crushing during sample preparation. Furthermore, weak reflections in the x-ray patterns, corresponding to a monoclinic sublattice, suggest a monoclinic distortion of the oxygen sites which is supported by neutron powder diffraction collected on the ECHIDNA instrument of the OPAL reactor.

Orientador: Pascoal José Giglio Pagliuso
Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin
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Resumo: Há muitas décadas os óxidos de metais de transição são tema de grande interesse científico devido à grande variedade de propriedades físicas interessantes que apresentam, com suas possíveis aplicações tecnológicas. Mais recentemente, por exemplo, os óxidos de metais de transição com propriedades multiferróicas ganharam destaque na comunidade científica como potenciais dispositivos magneto-eletrônicos. Muitos óxidos de metais de transição se formam na estrutura cristalina chamada perovskita simples, com simetria cúbica ou distorcida. Muitos outros óxidos podem se cristalizar numa variante da perovskita simples, a chamada perovskita dupla ordenada (PDO), que possui fórmula geral A2B¿B¿¿O6, onde o íon A ocupa os vértices do cubo enquanto os cátions B¿ e B¿¿ se alternam nos centros dos octaedros de oxigênio. Dois compostos com estrutura PDO bastante estudados são o Sr2FeReO6 e Sr2FeMoO6 devido ao fato de apresentarem, entre outras propriedades interessantes, comportamento meio-metálico (halfmetal), magnetrorresistência por tunelamento à temperatura ambiente, ferrimagnetismo com TC acima de 400K em ambos os compostos. As propriedades estruturais, eletrônicas e magnéticas dessas PDOs estão altamente conectadas e são fortemente dependentes do grau de hibridização dos orbitais d dos cátions B¿¿. Assim, se fazem importantes os estudos de novos compostos PDO a fim de se investigar as idéias correntes propostas em literatura e, nesse contexto, reportamos aqui os resultados da síntese e caracterização das séries inéditas Ca2-xLaxFeIrO6 e Sr2-xLaxFeIrO6, onde o Ir, assim como o Re e Mo, é metal de transição, no caso com caráter 5d, e pode assumir diferentes estados de valência. As medidas de magnetização indicaram que estes sistemas tendem a evoluir de antiferromagnéticos nas extremidades das séries, x = 0 e x = 2, para ferrimagnéticos em regiões intermediárias da série. Medidas realizadas no composto de maior magnetização da série de Sr, o Sr1.2La0.8FeIrO6, indicaram que este composto se ordena ferrimagneticamente em torno de 700 K, sendo esta a mais elevada TC já reportada para perovskitas duplas. Medidas de resistividade em função da temperatura indicaram que os compostos apresentam comportamento isolante e praticamente nenhum efeito magneto-resistivo. No composto antiferromagnético Sr2FeIrO6 foi estudada a resistividade sob efeito de pressão e, embora não tenha ocorrido nenhuma transição metal-isolante, ocorre uma diminuição sistemática da resistência do material e da inclinação da curva à medida que a pressão aumenta, indicando um comportamento do tipo isolante de Mott nesse composto. Neste trabalho são apresentados também resultados dos estudos realizados na perovskita TbMnO3. Realizamos neste óxido medidas de susceptibilidade magnética, calor específico, Ressonância Paramagnética Eletrônica (EPR) e absorção de microondas para várias temperaturas. A susceptibilidade magnética e o calor específico confirmaram para a amostra estudada as temperaturas de transição de fase magnética (TN = 41 K) e ferroelétrica (Tlock) já reportadas em literatura. Os espectros de EPR mostraram para todo o intervalo de temperatura uma única linha consistente com uma forma de linha Lorentziana e um valor de g independente da temperatura g = 1.96(3) consistente com Mn3+ em um meio isolante. A largura de linha sofreu um alargamento com a temperatura seguindo uma lei do tipo C/T. Esse alargamento impediu a observação dos espectros de ressonância em torno das regiões de temperaturas das transições de fase magnética e ferroelétrica. Devido à forte dependência da constante dielétrica com a freqüência, as medidas realizadas com a cavidade de campo elétrico não permitiram a observação de qualquer anomalia em torno das temperaturas de transições
Abstract: For many decades the transition metal oxides are subject of great scientific interest because of the wide variety of interesting physical properties and their potential technological applications. More recently, for example, oxides of transition metals with multiferroic properties have been considered as potential magneto-electronic devices. Many transition metal oxides form in the perovskite crystalline structure, with cubic or distorted symmetry. Many other oxides can crystallize in a variant of the simple perovskite, called the ordered double perovskite (ODP), which has the general formula A2B'B''O6, where the A ion occupies the vertices of the cube while the cations B 'and B'' alternate in the centers of the oxygen octahedra. Sr2FeReO6 and Sr2FeMoO6 are two compounds with the ODP structure which were extensively studied due to their interesting properties such as half-metal behavior, tunneling magnetoresistance at room temperature and ferrimagnetic order (TC above 400 K). The structural, electronic and magnetic properties of these ODPs are highly correlated and are strongly dependent on the strong d orbitals hybridization of the of the B'' cations. Therefore, studies of new ODP compounds are important in order to investigate the current ideas proposed in the literature and improve the understanding of their physical properties. We report here our results of synthesis and characterization of the unpublished series Ca2-xLaxFeIrO6 and Sr2-xLaxFeIrO6, where the Ir such as Re and Mo are transition metal, with d character that can assume different valence states. The magnetic measurements indicated that those systems tend to evolve from antiferromagnetics at the ends of the series, x = 0 and x = 2, to ferrimagnetic for intermediate regions of the series. Measurements performed in the compound of higher magnetization in the Sr serie, Sr1.2La0.8FeIrO6 indicated that this compound orders ferrimagnetic around 700 K, which is the highest TC ever reported for double perovskites. Resistivity measurements as a function of temperature indicated that these compounds also exhibit insulating behavior and virtually no magneto-resistive effect. In the antiferromagnetic compound Sr2FeIrO6, the effect of pressure on the resistivity was investigated, and although no metal-insulator transition was seen, there is a systematic decrease of the resistance and the slope of the curve as the pressure increases, indicating a Mott insulator-like behavior in this compound. This work also presents results on the TbMnO3 perovskite. We have performed magnetic susceptibility, specific heat, Electron Paramagnetic Resonance (EPR) and microwave absorption measurements at various temperatures. Magnetic susceptibility and specific heat data confirmed the ocurrence of a magnetic (TN = 41 K) and ferroelectric (Tlock) phase transition. The EPR spectra showed, for the entire temperature range measured, a single Lorentzian line shape and T independent g-value = 1.96 (3), consistent with the resonance of Mn3+ in an insulating environment. The width line broadens with the decreasing temperature following a C/T law. This broadening prevented the observation of the resonance spectra near the magnetic and ferroelectric phase transitions. Because of the strong frequency dependence of the dielectric constant, the measurements performed with the electric field cavity also did not allow observation of any anomaly around the ferroelectric transition
Doutorado
Física da Matéria Condensada
Doutor em Ciências

The fundamental properties of hexagonal multiferric HoMnO3 films have been thoroughly investigated. The films are grown by pulsed laser deposition on Y:ZrO2(111) substrates. High quality epitaxial HoMnO3 films of 25 { 1000 nm thickness were successfully prepared. The film properties are compared to those of single-crystals. The magnetization measurements revealed that the films show a deviating magnetic behavior from the single-crystals in several ways. For instance, the films have a weakened antiferromagnetic Ho3+ order confirmed from magnetic susceptibility. The difierences are likely to be related to the modified (mostly larger) lattice parameters of films. An approximate phase diagram in comparison with the single-crystal's one is constructed. For multiferroicity investigations, Second Harmonic Generation (SHG; in collaboration with the group of M. Fiebig) has been employed. By SHG, the ferroelectric polar order of the films is obviously confirmed. The ferroelectric switching at room temperature could be clearly demonstrated, whereas leakage of films requires generally a more sophisticated approach
Die fundamentalen Eigenschaften von hexagonalen multiferroischen HoMnO3 Schichten werden eingehend untersucht. Die dünnen Schichten wurden mittels gepulster Laserdeposition auf Y:ZrO2(111)-Substraten gewachsen. Hochwertige epitaktische HoMnO3-Dünnschichten von 25 { 1000 nm Dicke wurden erfolgreich hergestellt. Die Dünnschichteigenschaften werden mit denen von Einkristallen verglichen. Die Magnitisierungsmessungen ergeben, dass die dünnen Schichten ein von den Einkristallen in verschiedener Weise abweichendes magnetischen Verhalten zeigen. Zum Beispiel haben die dünnen Schichten eine abgeschwächte antiferromagntetische Ho3+ Ordnung, die durch die magnetische Suszeptibilität bestätigt wird. Die Unterschiede sind wahrscheinlich auf die veränderten (meistens grösseren) Gitterparameter der dünnen Schichten zurückzuführen. Ein Phasendiagramm wird zum Vergleich mit Einkristallen konstruiert. Durch Second Harmonic Generation (SHG; in Zusammenarbeit mit der Gruppe von M. Fiebig) wird die ferroelektrische Ordnung der dünnen Schichten eindeutig bestätigt. Das ferroelektrische Umschalten bei Raumtemperatur kann eindeutig nachgewiesen werden, wobei durch den Leckstrom der dünnen Schichten allgemein eine detailliertere Vorgehensweise benötigt wird

Doutoramento em Física
The present PhD work aims the research and development of materials that exhibit multiferroic properties, in particular having a significant interaction between ferromagnetism and ferroelectricity; either directly within an intrinsic single phase or by combining extrinsic materials, achieving the coupling of properties through mechanic phenomena of the respective magnetostriction and piezoelectricity. These hybrid properties will allow the cross modification of magnetic and electric polarization states by the application of cross external magnetic and/or electric fields, giving way to a vast area for scientific investigation and potential technological applications in a new generation of electronic devices, such as computer memories, signal processing, transducers, sensors, etc. Initial experimental work consisted in chemical synthesis of nano powders oxides by urea pyrolysis method: A series of ceramic bulk composites with potential multiferroic properties comprised: of LuMnO3 with La0.7Sr0.3MnO3 and BaTiO3 with La0.7Ba0.3MnO3; and a series based on the intrinsic multiferroic LuMn1-zO3 phase modified with of Manganese vacancies. The acquisition of a new magnetron RF sputtering deposition system, in the Physics Department of Aveiro University, contributed to the proposal of an analogous experimental study in multiferroic thin films and multilayer samples. Besides the operational debut of this equipment several technical upgrades were completed like: the design and construction of the heater electrical contacts; specific shutters and supports for the magnetrons and for the substrate holder and; the addition of mass flow controllers, which allowed the introduction of N2 or O2 active atmosphere in the chamber; and the addition of a second RF generator, enabling co-deposition of different targets. Base study of the deposition conditions and resulting thin films characteristics in different substrates was made from an extensive list of targets. Particular attention was given to thin film deposition of magnetic phases La1-xSrxMnO3, La1-xBaxMnO3 and Ni2+x-yMn1-xGa1+y alloy, from the respective targets: La0.7Sr0.3MnO3, La0.7Ba0.3MnO3; and NiGa with NiMn. Main structural characterization of samples was performed by conventional and high resolution X-Ray Diffraction (XRD); chemical composition was determined by Electron Dispersion Spectroscopy (EDS); magnetization measurements recur to a Vibrating Sample Magnetometer (VSM) prototype; and surface probing (SPM) using Magnetic-Force (MFM) and Piezo-Response (PFM) Microscopy. Results clearly show that the composite bulk samples (LuM+LSM and BTO+LBM) feat the intended quality objectives in terms of phase composition and purity, having spurious contents below 0.5 %. SEM images confirm compact grain packaging and size distribution around the 50 nm scale. Electric conductivity, magnetization intensity and magneto impedance spreading response are coherent with the relative amount of magnetic phase in the sample. The existence of coupling between the functional phases is confirmed by the Magnetoelectric effect measurements of the sample “78%LuM+22%LSM” reaching 300% of electric response for 1 T at 100 kHz; while in the “78%BTO+22%LBM” sample the structural transitions of the magnetic phase at ~350 K result in a inversion of ME coefficient the behavior. A functional Magneto-Resistance measurement system was assembled from the concept stage until the, development and operational status; it enabled to test samples from 77 to 350 K, under an applied magnetic field up to 1 Tesla with 360º horizontal rotation; this system was also designed to measure Hall effect and has the potential to be further upgraded. Under collaboration protocols established with national and international institutions, complementary courses and sample characterization studies were performed using Magneto-Resistance (MR), Magneto-Impedance (MZ) and Magneto-Electric (ME) measurements; Raman and X-ray Photoelectron Spectroscopy (XPS); SQUID and VSM magnetization; Scanning Electron Microscopy (SEM) and Rutherford Back Scattering (RBS); Scan Probe Microscopy (SPM) with Band Excitation Probe Spectroscopy (BEPS); Neutron Powder Diffraction (NPD) and Perturbed Angular Correlations (PAC). Additional collaboration in research projects outside the scope of multiferroic materials provided further experience in sample preparation and characterization techniques, namely VSM and XPS measurements were performed in cubane molecular complex compounds and enable to identify the oxidation state of the integrating cluster of Ru ions; also, XRD and EDS/SEM analysis of the acquired targets and substrates implied the devolution of some items not in conformity with the specifications. Direct cooperation with parallel research projects regarding multiferroic materials, enable the assess to supplementary samples, namely a preliminary series of nanopowder Y1-x-yCaxØyMn1O3 and of Eu0.8Y0.2MnO3, a series of micropowder composites of LuMnO3 with La0.625Sr0.375MnO3 and of BaTiO3 with hexagonal ferrites; mono and polycrystalline samples of Pr1-xCaxMnO3, La1-xSrxMnO3 and La1-xCaxMnO3.
O trabalho de doutoramento presente tem por objectivo a pesquisa e desenvolvimento de materiais que manifestem propriedades multiferróicas, em particular com uma significativa interacção entre os fenómenos de ferromagnetismo e ferroelectricidade; seja de forma intrínseca em determinados materiais singulares, ou extrínseca ao combinar materiais que apresentam respectivamente fenómenos magnetoestritivo e de piezoelectricidade e em que geralmente o acoplamento se processa mecanicamente entre as fases. Esta hibridação de propriedades permite a modificação dos estados de polarização magnética ou eléctrica por aplicação dos campos externos complementares (eléctricos e/ou magnéticos), dando origem a uma vasta área de investigação científica e potenciais aplicações tecnológicas numa nova geração de dispositivos electrónicos como memórias, processadores, transdutores, sensores, etc. O trabalho experimental inicial consistiu na síntese química de óxidos sob a forma de pós nanométricos, pelo método de pirólise da ureia; As séries de compósitos maciços com potenciais propriedades multiferróicas compreendem: LuMnO3 com La0.7Sr0.3MnO3 e BaTiO3 com La0.7Ba0.3MnO3; e uma série baseada na modificação com lacunas de Manganésio da fase multiferróica intrínseca LuMn1-zO3. A aquisição de um novo sistema de deposição por RF sputtering, no Departamento de Física da Universidade de Aveiro, contribuiu para a proposta de estudo análogo de amostras multiferróicas sob a forma de filmes finos e multicamadas. Além da estreia operacional do equipamento foram efectuadas algumas melhorias técnicas e funcionais de que se destacam: o desenho e construção das ligações eléctricas do aquecedor; de portadas, protecções e respectivos suportes para os magnetrões e para o “porta substratos”; a adição de dois controladores de fluxo de gás permitindo a introdução controlada de Árgon e de atmosfera activa de O2 ou N2 durante a deposição; e a adição de uma segunda fonte e controlador RF permitindo a co-deposição simultânea de filmes a partir de dois alvos diferentes. O estudo base sobre as condições de deposição e das características dos filmes finos resultantes em diferentes substratos foi efectuada a partir de uma extensa lista de alvos. Atenção particular foi dada à deposição de filmes finos das fases magnéticas de La1-xSrxMnO3, La1-xBaxMnO3 e da liga Ni2+x-yMn1-xGa1+y a partir dos correspondentes alvos La0.7Sr0.3MnO3; La0.7Ba0.3MnO3 e NiGa com NiMn. A caracterização estrutural das amostras foi efectuada com Difractometria por Raios-X (XRD) convencional e de elevada resolução; determinação da composição química foi essencialmente realizada por Espectroscopia de Dispersão de Electrões (EDS); medidas de magnetização foram executadas com recurso a um protótipo de Magnetometro por Vibração da Amostra (VSM) e as medidas de análise de superfície utilizaram Microscopia de Ponta (SPM) nas vertentes de piezo resposta (PFM) e de força magnética (MFM). Os resultados obtidos nos compósitos maciços (LuM+LSM e BTO+LBM) demonstram claramente que as amostras satisfazem os objectivos propostos em termos de composição pureza das fases, com eventual conteúdo em óxidos espúrios inferior a 0.5%. Imagens obtidas por SEM confirmam a compactação dos grãos e distribuição de tamanhos em torno dos 50 nm. Condutividade eléctrica, intensidade da magnetização e a dispersão da resposta em Magneto-Impedância são coerentes com a proporção relativa da fase magnética em cada amostra. A existência de um acoplamento entre as fases funcionais é evidenciada por medidas de efeito Magneto-Eléctrico na amostra “78%LuM+22%LSM” que apresenta uma resposta eléctrica de ~300% para 1 Tesla a 100 kHz; enquanto que na amostra “78%BTO+22%LBM” se assinala a transição estrutural da fase magnética a ~350 K resulta na inversão do comportamento do coeficiente ME. Um sistema de Medidas de Magneto-Resistência foi totalmente desenvolvido e montado desde a fase conceptual até ao estado operacional; permite testar amostras de 77 a 350 K em função do campo magnético até 1 Tesla, e rotação horizontal de 360º; o sistema foi também desenhado para poder efectuar medidas de efeito de Hall e permitir upgrades. Ao abrigo de protocolos de colaboração estabelecidos com diversas instituições nacionais e internacionais, foram realizados cursos de formação complementar e caracterização de amostras em técnicas como Magneto Resistência (MR), Magneto Impedância (MZ) e efeito Magneto Eléctrico (ME); Espectroscopia Raman e Fotoelectrónica de Raios-X (XPS); Magnetização via sistemas SQUID e VSM; Microscopia de Ponta em Piezo resposta (PFM) e Espectroscopia de excitação em largura de banda (BEPS); Espectroscopia de Rutherford por Retro dispersão (RBS); Difracção de Neutrões em pós (NPD) e Correlações de Perturbação Angular (PAC) Colaboração em projectos de investigação fora do âmbito dos materiais multiferróicos permitiu ampliar e versatilizar experiencia em técnicas de preparação e caracterização de amostras, nomeadamente medidas de VSM e XPS permitiram identificar os estados de oxidação dos clusters de iões de Ruténio que integram complexos moleculares utilizados em catalisadores; A certificação por XRD e SEM/EDS do conjunto dos alvos e amostragem dos substratos adquiridos implicou a devolução de alguns itens com por falta de conformidade com as especificações. Cooperação directa em projectos de investigação paralelos sobre materiais multiferróicos permitiu o acesso a amostras suplementares, nomeadamente a uma série nano pós de Y1-x-yCaxØyMn1O3 e de Eu0.8Y0.2MnO3; a series de compósitos microestruturados de LuMnO3 com La0.625Sr0.375MnO3 e de BaTiO3 com ferrites hexagonais; e a diversas amostras poli- e mono-cristalinas de Pr1-xCaxMnO3, La1-xSrxMnO3 e La1-xCaxMnO3.
FCT - SFRH/BD/25011/2005

Thesis (Ph. D.) -- University of Maryland, College Park, 2008.
Thesis research directed by: Dept. of Physics. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

Materiais que apresentam um acoplamento entre as propriedades elétricas e magnéticas, conhecidos como magnetoelétricos, estão despertando a atenção da comunidade acadêmica nos últimos anos principalmente devido ao seu grande potencial tecnológico no desenvolvimento de materiais multifuncionais. Sendo um maior esforço concentrado na obtenção de novos materiais multiferróicos, que são aqueles que possuem duas ou mais ordens ferróicas, em especial os que apresentam ferroeletricidade e ferromagnetismo. Este trabalho tem como objetivo uma investigação das propriedades dielétricas e magnéticas de um grupo de materiais ainda pouco estudado a fim de se compreender os mecanismos responsáveis pelo efeito magnetoelétrico, de forma a contribuir com a pesquisa nessa área. As amostras estudadas são os óxidos de titânio e cromo Nd(1-x)A(x)CrTiO5 (A=Y, La) e GdCrTiO5, todos na forma de cerâmicas policristalinas monofásicas. Foram discutidas de que forma a diluição da subrede magnética do neodímio afeta o caráter magnetoelétrico observado nesse material e qual a importância de cada uma das subredes (Nd3+/Gd3+ e Cr3+) nas propriedades dielétricas e magnéticas. Verificamos que o efeito magnetoelétrico é sensível a variações na rede do Nd3+. Entretanto, nossos resultados no composto GdCrTiO5 indicam que as subredes magnéticas são pouco acopladas, o que indica que o ion Cr3+ talvez seja o responsável por dirigir o efeito magnetoelétrico nesta família de materiais.
The coupling of the electric and magnetic properties in the magnetoelectric compounds is attracting a great deal of interest in the scientific community due to the huge potential for application based on new multifunctional materials. Of special attention are the so-called multiferroics, materials that simultaneously show ferromagnetism and ferroelectricity. The main purpose of this work is the experimental investigation of the dielectric and magnetic properties of a relatively less known class of materials with the aim to identify the origin of the magnetoelectric coupling. We study the single phase polycrystalline compounds Nd(1-x)A(x)CrTiO5 (A=Y, La) and GdCrTiO5 and discuss the effect of dilution of the Nd3+ ions and the importance of each magnetic sublattice on the samples behavior. We find that the magnetoelectric effect is susceptible to changes in the Nd3+ site. At the same time, our results indicate that the two magnetic sublattice (Nd3+/Gd3+ and Cr3+) are only weakly coupled indicating that the Cr3+ magnetic sublattice might be responsible for driving the magnetoelectric effect in this family of compounds.

Au cours des dernières décennies, la recherche de nouveaux matériaux multiferroïques nanostructurés avec des propriétés optimisées a conduit à l'élaboration d'une grande variété de modèles théoriques et des approches de simulation. Allant des modèles ab initio capables de décrire les propriétés à la température nulle des composés artificiels monocristallins à des approximations phénoménologiques pour la description des composites à la mésoscopique, ces recherches ont soulevé la question fondamentale de la relation entre la géométrie de la structure des systèmes hétérogènes et les propriétés des leurs transitions de phase. Cependant, malgré des progrès significatifs en la matière,cette question n'a pas encore été élucidée et les relations entre les modèles à différentes échelles ne sont pas entièrement distingués. La présente étude est consacrée à lier l’ensemble des modèles décrivant les matériaux nanocomposites multiferroïques à différentes échelles. Tout d'abord, nous présentons un développement méthodologique de l'approche Hamiltonien effectif couramment utilisé pour étudier les transitions de phase structurales. Les modifications introduites permettent d'étendre cette méthode pour prédire les propriétés à la température finie des systèmes hétérogènes. Le modèle construit est ensuite utilisé pour étudier les propriétés des nanostructures et solutions solides (BiFeO3)(BaTiO3). Recourant à des simulations Monte-Carlo, nous montrons que notre modèle fournit des résultats qui sont en ligne avec les observations expérimentales récentes et qu’il permet de prédire théoriquement les propriétés d'une large gamme de systèmes avec différentes géométries composites. La deuxième partie de l'étude consiste en l'application de la théorie de Ginzburg-Landau des transitions de phase à l’étude des propriétés des multicouches ferroélectriques et ferromagnétiques avec des interfaces épitaxiales. Plus précisément, nous décrivons théoriquement l’effet magnétoélectrique exhibé par les hétérostructures autonomes Pb(Zr0.5 Ti0.5) O3-FeGaB et BaTiO3-FeGaB. Enfin, nous montrons que la géométrie multicouche d'un nanocomposite ferroélectrique et ferromagnétique ouvre la voie à une amélioration radicale du signal de charge de sortie
During past decades, the search for new nanostructured multiferroic materials with optimized properties has lead to the development of a vast variety of theoretical models and simulation approaches. Spreading from first principles based models able to describe zero-temperature properties of artificial single crystal compounds to phenomenological approximations for composites with mesoscale morphology, these investigations have raised the fundamental question of how the geometry of the structure affects the properties of phase transitions exhibited by heterogeneous systems. However, despite significant progress, the answer to this question still lacks clarity and the bridge connecting models at different scales is not fully constructed. The current study is devoted to linking together models of multiferroic nanocomposite materials applicable at different scales. First, we present a methodological development of effective Hamiltonian approach commonly used to study structural phase transitions. The introduced modifications allow to extend this widely used method to predict finite-temperature properties of compositionally heterogeneous systems. The constructed model is then used to study properties of (BiFeO3)(BaTiO3) nanostructures and solid-solutions. Resorting to Monte-Carlo simulations, we show that our model provides results that are in-line with recent experimental observations and allows to theoretically predict properties of a wide range of systems with different composite geometries. The second part of the study consists inapplication of Landau theory of phase transitions to investigate the properties of ferroelectric-ferromagnetic multilayerswith epitaxial interfaces. Specifically, we theoretically describe the strain-mediated direct ME effect exhibited byfree-standing Pb(Zr0.5 Ti0.5 )O3 -FeGaB and BaTiO3 -FeGaB heterostructures. Finally, we show that the multilayer geometry of a ferroelectric-ferromagnetic nanocomposite opens the way for a drastic enhancement of the output charge signal

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2015.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references.
Complex oxide thin films offer unique functionalities which can potentially extend the utility of current storage, processing and optical isolator technologies. In this thesis, we present three categories of studies on complex oxide growth using pulsed laser deposition (PLD) and structural, magnetic, magneto-optical and ferroelectric characterization. We first focused on enhancing integrated magneto-optical isolator performance by improving the growth method of magneto-optical Ce1Y2Fe5O12 (Ce:YIG) films. The spectral and substrate orientation dependence of the magneto-optical figure of merit of epitaxial Ce: YIG on GGG substrates show very high magneto-optical figure of merit (379-400° dB-1 at [lambda] = 1550 nm for all substrate orientations). The thermal budgets of Ce: YIG growth on ShN4 (2 high temperature PLD steps and a rapid thermal anneal, RTA), silicon-on-insulator substrates (a high and a low temperature PLD step and a RTA) and optical resonator chips (one PLD step, one RTA, YIG seed layer from the top) were progressively reduced to achieve improved integrated optical isolators with low insertion loss of 7.4 ± 1.8 dB and an isolation ratio of 13.0 ± 2.2 dB. We demonstrated that the ferrimagnetic insulator YIG thin films (Y3Fe5O12) epitaxially grown on GGG substrates achieve ultralow Gilbert damping of spin waves ([alpha] = 2.2-7 x 10-4 ), which enable em-long in-plane propagation of spin waves. This demonstration enables researchers to fabricate near-dissipationless magnon-based logic computers. Finally, we present a substitutionally-doped perovksite, STCo30 (Sr Ti0.70 CO0.30 O3-[delta]) integrated on Si, STO (100), and on Nb:STO substrates. This perovskite oxide has been found to exhibit ferroelectricity and magnetism at room temperature. Experimental results on magnetism, ferroelectricity and structure were reproduced using density functional theory simulations.
by Mehmet Cengiz Onbaş̧lı.
Ph. D.

Notre étude porte sur des matériaux à effets relativistes importants. L'hamiltonien semi-relativiste, couplé aux équations de Maxwell (EM), permet de déduire les sources de courant et de densité, incluant des termes de second ordre (polarisations de spin et de Darwin). Différents modèles sont développés par expansion des EM. L'étude ab initio montre que (1) le désordre atomique peut produire le ferrimagnétisme (FM) dans GaFeO3 (GFO) multiferroïque, (2) les états 3d Fe des octaèdres déformés ont une levée de dégénérescence tétraédrique (théorie du champ cristallin), (3) la polarisation électrique concorde avec l'expérience, (4) le mécanisme magnétoélectrique (ME) direct est insuffisant pour expliquer le ME observé. Pour Cr2O3, le calcul de l'état massif sous contraintes biaxiales n'explique pas son FM, la taille de l'échantillon ou l'excès d'oxygène pourrait de fait être important. Enfin, nous avons développé le XAS et le XMCD dans le code VASP et calculé ces spectres pour GFO
We studied the physics of materials where relativistic effects are important. We first coupled the semi-relativistic Hamiltonian with the Maxwell's equations, and derived the current and density sources, which included second-order terms like the spin and Darwin polarizations. Different models were developed, by expanding the Maxwell's equations. We then performed ab initio studies to explain (1) site disorders as the origin of ferrimagnetism in multiferroic GaFeO3 (GFO), (2) crystal-field theory where the Fe 3d states at the deformed octahedra have tetrahedral splittings, (3) the electric polarization as a function of temperature, and (4) the insufficiency of the direct magnetoelectric (ME) mechanism to explain observed ME behavior. For Cr2O3, bulk calculations for different biaxial strains failed to explain its ferromagnetism, indicating that size or excess-O effects might be important. Finally, we implemented XAS and XMCD in VASP and computed these spectra for GFO

Well known multiferroic compounds TbMnO3 and DyMnO3 show a strong coupling between antiferromagnetism and ferroelectricity. Magnetic susceptibility data for both compounds have been taken, showing large magnetic anisotropy, and providing strong evidence for the existence of crystal field effects up to 150 K above TN (∼40 K). Further evidence for crystal field effects is given by a measurement of the magnetic heat capacity of TbMnO3. A new group of multiferroic compounds, of the form Sm1−xYxMnO3, were discovered as part of this work. The parent compound, SmMnO3, is not ferroelectric and exhibits commensurate A-type antiferromagnetic order below TN∼58 K. By doping with Y on the Sm site, significant changes are seen in the magnetic properties, with a feature seen in the susceptibility data at ∼24 K for x = 0.4, 0.5 corresponding with a peak in the dielectric constant and the onset of an electric polarisation (∼275 μC m−2 for Sm0.5Y0.5MnO3) along the crystallographic c-axis. The magnetoelectric coupling seen in Sm0.6Y0.4MnO3 and Sm0.5Y0.5MnO3 is linked to alterations in the local structure of the Mn-O octahedra, and in particular to the Mn-O-Mn bond angle, which can be tuned by Y doping. The zero-field electric properties of Sm1−xYxMnO3 (x = 0.4, 0.5) are very similar to those of TbMnO3, but show markedly different magnetic field dependence. Neutron diffraction and X-ray resonant scattering experiments were performed on Sm1−xYxMnO3 (x = 0, 0.4, 0.5) in order to attempt to determine the magnetic structure of the Y doped compounds. Evidence was found for a sinusoidally modulated collinear order of the Mn moments along the b-axis below TN1∼47 K, and cycloidal Mn order in the b-c plane below TN2∼24 K. This magnetic model is similar to that of TbMnO3, and it is proposed that the difference in the magnetic field dependence of the electric properties seen in TbMnO3 is due to coupling of the Mn moments with the strongly anisotropic Tb moments. The studies of Y doping were extended to the GdMnO3 system. Preliminary measurements show a similar magnetoelectric coupling in polycrystalline Gd1−xYxMnO3 (x = 0.1-0.4). Further research has been carried out on this group of compounds as a consequence of this work.

The demands from electronic devices have always been to be portable, fast, non-volatile, more intelligent and to consume low energy. One way towards this goal is to introduce multifunctionality of materials in devices. Ferromagnetism and ferroelectricity are two order parameters that can be coupled in a limited number of multiferroics and their coexistence implies the control over magnetisation and polarisation with both electric and magnetic fields. Similar properties were observed at ferromagnetic/ferroelectric thin film interfaces and attracted attention, since high quality thin film devices can be easily obtained nowadays through monitoring in real time of their structural and physical properties. This effect was observed also in tunnel junction configurations, devices which are formed from metallic electrodes separated by a very thin insulating barrier. By combining a barrier with various ferroelectric order parameters (ferroelectric, antiferroelectric, ferrielectric) and ferromagnetic electrodes, multi-field controlled multi-state non-volatile memory devices can be obtained. Tunnelling processes, especially in junctions containing d orbital elements are not completely understood and need deeper investigation. In this thesis, multiferroic tunnel junctions with La0:7Sr0:3MnO3/PbTiO3/Co structure are shown to be functional down to 3 unit cells. Moreover, the domain structure is shown to change with thickness, going through complex patterns including toroidal flux closure structures. The fabrication and working principle of the novel antiferroelectric tunnel junctions are reported for the first time using La0:7Sr0:3MnO3/PbZrO3/Co structures. Both investigated systems exhibit a multiferroic interface characterised by a magnetoelectric coupling which can be tailored by switching the ferroelectric polarisation.

Photoferroelectrics, which is defined as the interaction of ferroelectric materials with light, has attracted renewed attention recently and emerged as a topic of both fundamental interest and technological importance. It not only provides potential applications in sensors and photovoltaic devices but also offers a fertile playground to gain insight into the physics of ferroelectricity. As a prominent example, the bulk photovoltaic effect manifested in the ferroelectric materials under illumination gives rise to an anomalous open-circuit photovoltage exceeding the bandgap as well as a light polarisation-dependent photocurrent, offering an alternative approach to boost the solar energy conversion efficiency. Although it has been established for decades, the field is still in its fancy and many fundamental issues remain to be resolved to fully exploit its potential. In the first part of this thesis, we focus on the photoelectric processes in the bulk photovoltaic effect of bismuth ferrite to unravel respectively the essential role of the sub-bandgap levels, its correlation with ferroelectric polarization and role of domain walls in conduction of photovoltaic current. Results demonstrate the sub-bandgap levels is at the electronic origin of the bulk photovoltaic effect in bismuth ferrite. The activity of the sub-bandgap levels in the photoelectric processes can be effectively utilized to tailor the ferroelectric photovoltaic performance. Also, contrary to the common intuition, we prove the independence of the bulk photovoltaic effect on the ferroelectric polarization. We also found that the ferroelectric domain walls can facilitate the conduction and collection of the photocurrent originated in the bulk photovoltaic effect despite its adverse effect on the photovoltage. Inspired by the abundant phenomena in the photoferroelectric field, we explored the light-induced reversible manipulation of the ferroelectric polarization in a deterministic way. This interesting issue is successfully addressed in this thesis by utilizing a combination of the bulk photovoltaic effect and a nanoscale electrode. The collection of photocurrent by an atomic force microscope tip generates a giant electric field locally, enabling ferroelectric switching. By tuning the direction of the photocurrent via either illumination areas or light polarization, the ferroelectric polarization can be reversibly controlled. At the last part of the thesis, we creatively generalised the bulk photovoltaic effect, which was originally constrained to the non-centrosymmetric materials, to a universal effect allowed in all the semiconductors irrespective of their symmetry by the mediation of the flexoelectric effect. This new photovoltaic effect, termed as flexo-photovoltaic effect, may offer a new mechanism to enhance solar cell efficiency. The research works studied in this thesis not only provide fundamental insights into the interactions of ferroelectrics with light but also largely expand the scope of photoferroelectrics into centrosymmetric materials.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2014.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 133-150).
To respond to the growing demand for smart and connected devices, such as smartphones, tablet PCs arid other mobile hardware, while meeting the needs for increased power efficiency, miniaturization and reduced manufacturing costs, new material solutions need to be considered. These should address the shortcomings of incumbent semiconductor-based technologies which provide a limited number of functionalities, suffer from high power consumption and heat dissipation, and whose conventional planar processing is increasingly complex and resource-intensive. Potential replacement materials include complex oxides, which exhibit interesting physical phenomena such as superconductivity, colossal magnetoresistance and multiferroicity. New functionalities are especially found at interfaces between two oxides, including emergent electronic states like two-dimensional electron gases, enhanced ionic transport and magnetoelectric coupling, among many other. In this this thesis, we focus on self-assembled oxide nanocomposites, which elegantly organize into vertical nanostructures via spontaneous phase-separation, naturally forming numerous such heterointerfaces. These provide a rich playground for studying interfacial effects, which could be used in future devices, and the self-assembly promises cheap arid high throughput manufacturing providing it can be integrated into useful architectures. BiFeO₃-CoFe₂O₄ self-assembled nanocomposites, in particular, have been studied for the magnetoelectric coupling that takes place between the ferrimagnetic spinel phase, which forms discrete vertical pillars, arid the ferroelectric perovskite phase, which forms a matrix that surrounds the spinel pillars. Here, after an in-depth study of the mechanisms responsible for the formation of this self-assembled nanostructure, we develop a templating method enabling the precise control over the morphology of the film, resulting in useful structures for potential devices like magnetoelectric memories and logic devices. To study the structural, magnetic and electrical properties of our samples, a set of experimental and theoretical methods is developed, adapted to the unique requirements of these thin film nanostructures with iicron-scale ordering. Using finite element analysis and micromagnetic modeling, the effect of the strain-mediated magnetoelectic coupling on the magnetic switching of the CoFe₂O₄ nanopillars is predicted. Scanning Probe Microscopy is also used to characterize the local ferroelectric and magnetic behavior, and observe, for the first time in these templated composites, electrically-induced magnetic switching of the pillar magnetization. The tools and methods developed in this thesis could pave the way towards a wider use of templated self-assembly to leverage the promising properties of oxide heterointerfaces and enable their use in future devices with low manufacturing costs.
by Nicolas M. Airmon.
Ph. D.

Les matériaux multiferroïques magnétoélectriques (ME) présentant simultanément des propriétés couplées de ferromagnétisme et de ferroélectricité suscitent beaucoup d’attention, non seulement pour leurs applications (comme la RAM magnétique de nouvelle génération), mais aussi pour la compréhension de la physique relative à ce couplage. Le but de cette thèse était de découvrir et de caractériser de nouveaux composés potentiellement multiferroïques, d’où le choix des trirutiles inverses. Ce manuscrit présente l'étude détaillée du trirutile inverse Mn2TeO6 et quelques résultats sur la série substituée au chrome Mn2-xCrxTeO6. Les composés Mn2TeO6 et substitués sont préparés par réaction à l'état solide à relativement basse température (<700°C). De nombreuses expériences de diffraction de neutrons et de rayons-X ont été réalisées pour étudier les structures cristallines et magnétiques en fonction de la température (de 700°C à 1.5K), ces données ont été analysées en lien avec les caractérisations des propriétés magnétiques et électriques. Mn2TeO6 s’est révélé un matériau complexe et riche en transitions en fonction de la température. En température décroissante, Mn2TeO6 présente d’abord une transition d’une structure tétragonale (P42/mnm) à une double maille monoclinique (P21/c) vers 400°C due à l'effet Jahn-Teller. Une seconde transition très hystérétique apparait à plus basse température, avec la coexistence de deux phases monocliniques entre 45 et 100K. Des transitions magnétiques sont également observées par des mesures de susceptibilité magnétique et de diffraction neutronique. La structure cristalline à température ambiante met en évidence un ordre orbitalaire, dû au manganèse trivalent, complexe et inédit avec une alternance d’octaèdres MnO6 allongés et aplatis décrivant des chevrons. L’impact de l’effet Jahn-Teller induit par le manganèse trivalent est confirmé dans les composés substitués au Cr (avec x ≥ 0,15) qui conservent la structure quadratique sur toute la gamme de température
Magnetoelectric (ME) multiferroic materials, which present simultaneously two coupled properties between ferromagnetism and ferroelectricity, have attracted much attention recently, not only owing to their application perspectives, e.g., next-generation magnetic RAM, but also for the rich physics associated with the understanding of this coupling. Inverse trirutiles are of particular interest here since ME properties have been reported in this family of compounds. This manuscript presents the study of inverse trirutile Mn2TeO6 and its Cr-substitution series Mn2-xCrxTeO6. Mn2TeO6 and Cr-substituted series were prepared by solid state reaction at relatively low-temperature (< 700°C). Thanks to an extensive use of different techniques performed in a large temperature range (1.5K to 700°C), encompassing synchrotron, neutron and electron diffraction experiments combined with physical properties measurements, the very complex behaviour of Mn2TeO6 was revealed. A structural transition at 400°C from tetragonal (P42/mnm) to monoclinic (P21/c) is observed first, and related to a cooperative Jahn-Teller effect. Further cooling the sample, a hysteretic structural transition is observed spanning more than 50K, which leads to the coexistence of two monoclinic phases. A series of magnetic transitions are also observed between 48K and 22K, with magnetization, heat capacity measurement and neutron diffraction. Cr-substituted (x ≥ 0.15) samples crystallize in the tetragonal phase, implying the suppression of the cooperative Jahn-Teller effect, and involving a simpler, though short–range, magnetic order

Les nouveaux défis de la microélectronique depuis déjà deux décennies nécessitent le développement de matériaux et dispositifs bien au-delà des exigences de compatibilité avec la technologie silicium. Parmi ces défis figurent la miniaturisation des éléments de circuit intégrés et l’intégration 3D, pour surmonter le goulot d’étranglement de von Neumann, ainsi que l’élaboration de dispositifs neuromorphiques à base de memristors. Les fonctionnalités supplémentaires, offertes par la mise en œuvre de matériaux ferroélectriques et magnétiques, permettraient de réaliser des progrès significatifs dans tous les domaines évoqués. Dans cette thèse, nous étudions l’hétérostructure tri-couche multiferroïque (c’est-à-dire ferroélectrique et ferromagnétique simultanément) La0.7Sr0.3MnO3 / BaTiO3 / La0.7Sr0.3MnO3. Nous étudions en détail les propriétés structurales, chimiques et magnétiques des couches minces fabriquées ainsi que leurs interfaces pour établir les fondements nécessaires à la compréhension des expériences de transport électronique sur les dispositifs finaux. Dans les échantillons avec une barrière épaisse de BaTiO3, nous observons une commutation de résistance stochastique et une magnéto- résistance à champ faible plutôt complexe. Dans les dispositifs à barrière mince, nous démontrons la commutation de résistance quasi-analogique (sur de multiples niveaux de résistance) et mettons en évidence une forte dynamique temporelle, liée à la polarisation et aux lacunes d’oxygène. Le comportement de notre memristor s’avère reproduire idéalement celui des synapses biologiques, en particulier concernant les effets de second ordre intervenant dans la transmission des impulsions neuronales et qui proviennent de la dynamique transitoire d’ions Ca2+. Via la mise en place d’expériences adaptées, nous mettons en exergue plusieurs réponses synaptiques de second ordre dans notre memristor multiferroïque. Nos résultats suggèrent un nouveau concept physique de memristor de second ordre pour l’émulation bioréaliste des synapses et constituent une première étape dans la mise en œuvre matérielle de réseaux de neurones artificiels
New challenges in microelectronics already for two decades have been requiring the development in materials and device concepts way beyond conventional Si scaling. Among these challenges are integral circuit elements’ miniaturization and 3D integration, overcoming the von Neumann bottleneck and elaboration of memristor-based neuromorphic hardware. Additional functionalities, offered with implementation of ferroelectric and magnetic materials, would allow to achieve significant progress in all the mentioned fields. In this thesis, we study the tri-layer multiferroic (i.e., ferroelectric and ferromagnetic simultaneously) heterostructures La0.7Sr0.3MnO3 / BaTiO3 / La0.7Sr0.3MnO3. We thoroughly investigate the structural, chemical and magnetic properties of the fabricated thin layers and interfaces between them to establish the foundation for further electronic transport experiments. In thick BaTiO3 barrier samples, we observe stochastic resistance switching and a rather complex low-field magnetoresistance. In thin barrier devices, we demonstrate the multilevel quasi-analog resistance switching that shows a strong temporal dynamic, in relation with polarization and oxygen vacancies. This behavior is found to be emulating very accurately that of the biological synapses, with the emphasis on the so called second order effects, which are originating from the transient dynamics of Ca2+ ions, mediating the neural pulse transmission. Via appropriate experiments, we demonstrate the operation of several second order synaptic functions in our multiferroic memristors. Our findings suggest the new physical concept of second order memristor for biorealistic emulation of synapses and make a step toward the hardware implementation of artificial neural networks

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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.

Multiferroic materials, in which two or more ferroic ordering take place in the same phase, have driven major interest in the last few years, not only due to the possibility of exploring novel physical properties in those materials, but also the implications that such properties show in novel technological applications. From those materials, the especially interesting are those in which the ferromagnetic (FM) and ferroelectric (FE) ordering take place, due to their direct application in magnetodielectric devices. In the field of multiferroic materials such materials could play an important role in a new generation of none volatile magnetic random access memories (M RAM), in which a sufficiently strong magnetodielectric coupling could allow for the modification of the magnetic state, not only with a magnetic field, but with an electric field. This fact would allow for a dramatic reduction in energy consumption and would promote the further technological integration (the major commercial drawback of MRAMs), due to the fact that an electric field, contrary to the magnetic field, can be applied locally. Additionally, such multiferroic materials could prove useful in magnetic tunnel junctions, in which the ferroelectric and ferromagnetic nature would allow them to codify four resistive states, instead of the traditional two states of ferroelectric or ferromagnetic junctions, allowing for the implementation of a generation of four state memories. The materials with perovskite structure, ABB"03 (A=Rare Earth, Bismuth, Lead and Yttrium), bring a broad spectrum of possibilities when it comes to design of multifunctional materials. This is due to the wide variety of A, B, B" cations that are compatible with such structure. However, in the case of R(NiMn)03, such oxides have been poorly studied and many detailed studies, both in bulk and thin films are needed. The cation selection of B and B' seems to transform the paramagnetic ordering (PM) into FM below room temperature. The multiferroicity of these materials is typically provided by the A cation of the perovskite formula, which can be Bi or Pd, in order to create a Type 1 multiferroic. In this type of materials, i.e: Bi2NiM n06, the ferroelectricity and ferromagnetism arose by separate mechanisms, the FE is provided by the A cation, with so called long pair electrons, which are free electrons in the valence band that do not participate in any chemical reaction in the compound, while the Ni2+(d8) and (M n4+) (d3) provides the FM. However, even though the materials are multiferroic, their magnetodielectric coupling, crucial for future industrial applications, is weak, due to the different mechanisms that provide their FM and FE ordering. On the other hand, the FE induction by geometrical distortion of the perovskite lattice, for example in YM n03, is an interesting case since rotations of the M nO6 octahedrons promote an important structural change, in which the oxygen atoms move closer to the Y and, due to a large dipole interaction, generate a stable FE state. Moreover, the deformation of the unit cell generates a weak spin canting on the Mn cations, that can be promoted by Li doping or lattice distortions. This behavior could prove useful in the R(NiM n)06 family, which shows strong FM . This thesis is devoted to the study of R(Ni0.5M n0.5)03 (Y,Sm, Nd and Pr) and Bi(Fe0.5M n0.5)06 grown in thin films by pulsed laser deposition technique. Firstly, this thesis focuses on the growth and characterization of thin films of Y(Ni0.5M n0.5)03 (YNM 0) on strontium titanate substrates SrTiO3(001) (STO). The influence of the deposition parameters, such as temperature, fluence and ablation frequency, on the morphology and crystalline quality of the films is investigated. The study reveals that the YNMO films grown on STO(001,011 and 111) substrates are epitaxial and that their crystalline quality and epitaxial relationship are similar to those of the YMO compound. In particular, it is observed that a single out of plane domain is the norm for all the substrate orientations, while there are various in-plane domains. Moreover, chemical composition studies reveal Ti diffusion from the substrate to the YNMO film when STO(111) substrates are used. Once the growth conditions of YNMO are optimized, the magnetic and dielectric properties are studied. All the films show a paramagnetic to ferromagnetic transition at a temperature around 95K, with a magnetic moment of YNMO(001) = 4.35µB/f.u, YNMO(100) = 4,4 µB/f.u and YNMO(101) = 3,7µB/f.u, confirming the ferromagnetic nature of the samples. The dielectric characterization reveal a FE ordering on the YNMO films, and what is more, the existence of a dielectric anisotropy on the films, that is characterized by the absence of ferroelectric response on YNMO samples deposited on STO(001), while YNMO samples on STO(111) show a strong FE response. This anisotropy could be explained, according to recent theoretical studies, in the improper origin of the observed ferroelectriciy. The coexistence of FM and FE response shows in a conclusive manner the multiferroic nature of the YNMO compound. Secondly, studies similar to those previously presented are performed for thin films of R(Ni0.5Mn0.5)O3 (Sm, Nd and Pr) compounds grown on STO(001). In this case the deposition temperature turns out to play a crucial role on the epitaxial growth of all the studied compounds. It is shown that the ratio between the b/a lattice parameters influences the epitaxial growth of the films, being the decisive factor between single or multi domain films. All the samples show a PM to FM transition at temperatures around 190K Finally, films of Bi(Fe0.5Mn0.5)O6 have been grown on STO(001) substrates. The films are epitaxial and grow under epitaxial strain. Samples show a FM behavior at room temperature with a weak signal of 7,42 emu/cm3 and 0,4 µB/f.u(Fe-Mn). The dielectrical characterization shows the influence of external magnetic fields on the dielectric properties of the film above room temperature.
Los materiales multiferroicos, en los que dos o más ordenes ferroicos tienen lugar en la misma fase, ha despertado gran interés en los últimos años debido, no solo al hecho de explorar nuevas propiedades físicas en los materiales, sino también a las implicaciones de las nuevas propiedades funcionales en las aplicaciones tecnológicas. De dichos materiales resultan especialmente interesantes aquellos que presentan un orden ferroeléctrico (FE) y ferromagnético (FM) debido a su aplicación directa en dispositivos magnetoelectrónicos. En este ámbito los materiales multiferroicos podrían tener una gran relevancia en una nueva generación de memorias magnéticas RAM (MRAM) de control eléctrico, no volátiles, en las que, si el acoplamiento magnetoeléctrico es suficientemente grande, se podría modificar el estado magnético no con un campo magnético sino con un campo eléctrico. Este hecho permitiría una reducción radical en el consumo de potencia y favorecería a su vez una mayor integración (la principal desventaja de las MRAMs para competir en el mercado), ya que el campo eléctrico, a diferencia del campo magnético, puede aplicarse de forma muy localizada. Por otro lado, dichos materiales multiferroicos podrían emplearse en una nueva generación de uniones túnel, en las que el carácter ferroeléctrico y ferromagnético permitiría codificar información en cuatro estados resistivos en lugar de en dos, como viene siendo hasta ahora en las convencionales uniones túnel magnéticas o ferroeléctricas, dando lugar a una nueva generación de memorias de cuatro estados. Los materiales con estructura perovskita, ABB '03, (A=Tierra Rara, Bismuto, Plomo e Ytrio) ofrecen una gran versatilidad a la hora de diseñar materiales funcionales debido a la gran variedad de cationes A, B y B' compatibles con tal estructura. Sin embargo en el caso de R(NiMn)03, estos óxidos han sido poco estudiados y muchos carecen de estudios detallados tanto en forma másica como en capa fina. Esta selección de cationes en la posición B y B' parece transformar la estructura perovskita la cual típicamente presenta un ordenamiento paramagnético (PM) en FM a temperaturas inferiores a la ambiente. El carácter multiferroico de estos materiales es típicamente aportado por el catión A en la formula perovskita, el cual puede ser un átomo de Bi, o Pb, para crear un multiferroico tipo 1. En los materiales de este tipo, por ejemplo el Bi2NiMnO6, la ferroelectricidad y el ferromagnetismo provienen de fuentes diferentes, el carácter FE es aportado por el catión A con -lone pairs electrons-, los cuales son electrones libres en la banda de valencia que no participan en las reacciones químicas del compuesto, mientras la combinación Ni2+ (d8) and Mn4+ (d3) aporta el FM. Pese al carácter multiferroico de estos materiales su acoplamiento magnetoelectrico, indispensable para sus aplicaciones industriales futuras, es débil, puesto que su FE y FM provienen de efectos independientes. Por otra parte la inducción de FE por distorsiones geométricas de la celda perovskitas, como es el caso de YMnO3 (YMO), es un caso interesante de considerar ya que la rotación de los octaedros Mn05 genera un cambio estructural importante, en el cual los oxígenos se desplazan a una posición más cercana al Y, esto sumado a una larga interacción de los dipolos conduce al material a un estado FE estable. Además la deformación de la celda genera un débil FM en este material, el cual proviene un pequeño giro en los espines del Mn ya sea debido a un dopaje con Li o por la deformación de la celda. Este comportamiento podría resultar interesante en la familia de perovskitas R(NiMn)03 las cuales presentan un fuerte FM. Esta tesis está dedicada al estudio de la perovskitas R(Ni0.5Mn0.5)O3 (Y, Sm, Nd y Pr) y Bi(Fe0.5Mn0.5)O6 crecidas en capa fina usando la técnica de depósito mediante ablación por láser pulsado. En primer lugar, esta tesis se centra en el crecimiento y caracterización de capas finas del compuesto Y(Ni0.5Mn0.5)O3 (YNMO) sobre substratos de titanato de estroncio, SrTiO3(001) (STO). Se estudia la influencia de los parámetros de depósito tales como temperatura, fluencia y frecuencia de ablación sobre la morfología y la calidad cristalina de las capas obtenidas. El estudio pone de manifiesto que las capas de YNMO crecidas sobre substratos de STO(001,011 y 111) son epitaxiales de YNMO y que la calidad cristalina y las relaciones epitaxiales entre la capa y el substrato son semejantes a las obtenidas en el compuesto YMO. En particular se observa un único dominio cristalino fuera del plano independientemente de la orientación del sustrato, mientras que dentro del plano se presentan varios dominios cristalinos. Por otra parte, los estudios de composición química revelan una difusión de Ti desde el sustrato hacía la capa de YMNO cuando se utilizan substratos STO(111).. Una vez optimizadas las condiciones de crecimiento del compuesto YNMO, se estudian sus propiedades magnéticas y dieléctricas. Todas las capas presentan una transición de fase paramagnetica a ferromagnética a una temperatura alrededor de 95K con un momento magnético de YNMO(001)= 4.35µB/f.u, YNMO(100) = 4,4 µB/f.u and YNMO(101) = 3,7µB/f.u, confirmando el carácter ferromagnético de las muestras. La caracterización dieléctrica revela el carácter FE de las capas de YNMO y lo que es más interesante, la existencia de anisotropía dieléctrica en las capas, ésta se pone de manifiesto en la ausencia de respuesta FE en capas YNMO sobre STO(001) que contrasta con la fuerte respuesta de las capas de YNMO sobre STO(111). Esta anisotropía puede tener su origen, a la luz de los recientes estudios teóricos, en el carácter impropio de la ferroelectricidad observada, a la luz de recientes estudios teóricos. La coexistencia de FM y FE muestra de manera conclusiva el carácter multiferroico del compuesto YNMO. En segundo lugar se han realizado estudios similares a los anteriores para el caso de capas finas de los compuestos del tipo R(Ni0.5Mn0.5)O3 (Sm, Nd y Pr) crecidas en STO(001). En este caso la influencia de la temperatura de depósito resulta ser un factor importante para la obtención, en todos los compuestos estudiados, de crecimiento epitaxial. Se observa que el cociente b/a entre las constantes red juega un factor importante en la epitaxia de las capas, siendo este cociente un factor determinante en el crecimiento mono-dominio o multi-dominio de las capas. Todas las muestras presentan transiciones PM a FM a temperaturas alrededor de 190K. Por último, se han crecido y estudiado capas finas del compuesto Bi(Fe0.5Mn0.5)O6 depositadas sobre STO(001). Las capas obtenidas son epitaxiales y crecen sometidas a estrés inducido por el substrato. Presentan comportamiento FM a temperatura ambiente pero con una débil señal de 7,42 emu/cm3 y 0,4 µB/f.u(Fe-Mn). La caracterización dieléctrica pone de manifiesto la influencia, a temperaturas superiores a la ambiente, de la presencia de campo magnético sobre las propiedades dieléctricas.

In this report, 10 at% lanthanum was introduced to the perovskite structure of bismuth ferrite at the expense of bismuth through a modified Pechini wet chemical method. Green body powders were calcined at several temperatures, and the lattice parameters for each size were found by Rietveld refinement of X-ray powder diffraction data. The parameters displayed strong size-dependent properties, and the rhombohedral distortion from the cubic structure was reduced by decreased particle size. With decreasing crystallite size, the Néel temperature decreased and the cooperative cationic displacement (s - t) showed size dependence at crystallite sizes below 30 nm. A size-dependency was also observed in the Curie temperature.

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

Mestrado em Engenharia de Materiais
Materiais magnetoelétricos e multiferróicos que exibem simultaneamente propriedades ferroelétricas e ferromagnéticas, têm suscitado um crescente interesse na comunidade científica para o desenvolvimento de materiais multifuncionais. Neste trabalho pretendeu-se fabricar materiais cerâmicos multiferroicos e realizar um estudo do acoplamento entre propriedades magnéticas e as propriedades piezoelétricas/ferroelétricas dos compósitos. Assim, foram realizados estudos sobre a síntese das hexaferrites (BaFe12O19, SrFe12O19, Ba3Co2Fe24O41 e Sr3Co2Fe24O41), com diferentes temperaturas de reacção e diferentes métodos de preparação: reação de estado sólido, co-precipitação, sol-gel e combustão de citratos. De seguida, foram preparados compósitos de hexaferrites e piezoelétricos, nomeadamente BaTiO3 e K0.5Na0.5NbO3 (KNN), foram realizadas as prensagens: uniaxial e isostática a frio. A densidade relativa obtida após a sinterização foi 85% da densidade teórica nos compósitos com BaTiO3 e 81% nos compósitos com KNN. Foram efetuadas medições magnéticas, nomeadamente por microscópia de força magnética e magnetometria por vibração de amostra, e medições piezoelétricas, por microscópia de força piezoelétrica. Em alguns compósitos foi detetada a inter-difusão de átomos do bário e estrôncio produzindo várias fases secundárias diminuindo o efeito piezoelétrico. As medições magnéticas mostraram que os compósitos com ferrites duras (BaFe12O19 e SrFe12O19) formaram um ciclo de histerese com maior área do que os compósitos com ferrites macias (Ba3Co2Fe24O41 e Sr3Co2Fe24O41). Alguns compósitos demonstram acoplamento magnetoelétrico significativo.
Magnetoelectric and multiferroic materials that exhibit both ferroelectric and ferromagnetic properties, have raised great interest in the scientific community for the development of multifunctional materials. In this work we intended to fabricate multiferroic ceramics and to study the coupling between magnetic properties and piezoeletric/ferroelectric properties of composite materials. Synthesis of hexaferrites (BaFe12O19, SrFe12O19, Ba3Co2Fe24O41 and Sr3Co2Fe24O41), was undertaken with different reacting temperatures and different preparation methods: solid state reaction, coprecipitation, sol-gel and citrate. Also we performed analysis by XRD and determined the phases of each material. We prepared composites of hexaferrites and piezoelectric phases (BaTiO3, KNN) with uniaxial pressing and cold isotactic pressing. BaTiO3 composites have shown a relative density of 85% compared with the theoretical density and the KNN composites had a maximum density of 81%. We performed magnetic (MFM and VSM), and piezoelectric (PFM) measurements, and it was verified that in some composites we observed diffusion of atoms between barium and strontium sources producing new phases lowering the piezoelectric effect. In the magnetic analysis it also was verified that the hard ferrites formed hysteresis loops with greater area than the soft ferrites. It was found that some composites demonstrate capable magnetoelectric coupling.

Thesis (M.S.) -- University of Maryland, College Park, 2004.
Thesis research directed by: Dept. of Materials Science and Engineering. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

Thesis (Ph. D.) -- University of Maryland, College Park, 2008.
Thesis research directed by: Dept. of Materials Science and Engineering. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

Multiferroic materials, in which both ferroelectric and (anti)ferromagnetic orders coexist in the same phase, have received much interest in the last few years. The possibility of these two ferroic orders being coupled allows new functionalities in these materials as controlling the magnetisation by an electric field or, conversely, controlling the polarisation by a magnetic field. The fulfilment of this magnetoelectric coupling is not only interesting in terms of fundamental research but it would also pave the way for designing novel magnetoelectric applications. For this latter purpose, ferromagnetic multiferroics would have greater advantages over the antiferromagnetic ones because of the net magnetisation. However, it is the antiferromagnetic order which prevails in multiferroic materials. Bi-based perovskite and double-perovskite oxides, BiBO3 and Bi2BB’O6, respectively, where B and B’ are magnetic transition metal ions present an excellent starting point to investigate new ferromagnetic ferroelectric materials. In these compounds ferroelectricity arises from the stereochemical activity of Bi3+ cations. Conversely, magnetism is driven by the superexchange interaction between the magnetic ions through the adjacent oxygen ions (B – O – B). In particular, to date, BiMnO3 and Bi2NiMnO6 systems are the only reported ferromagnetic Bi-based perovskite oxides. Hence, both systems are investigated in the work of this thesis in thin films . First of all, this thesis addresses the synthesis of these compounds. In this process, three main hindrances were met. Firstly, these Bi-based compounds are highly metastable, which implies that they are only possible to be synthesised in bulk under extreme conditions, i.e. under high temperatures and high pressures. The strategy used to circumvent the required high pressures consisted of replacing the mechanical pressure by the epitaxial stress in thin films. For this purpose these Bi-based compounds were grown by pulsed laser deposition (PLD) onto single-crystal (001)-oriented SrTiO3 substrates. Secondly, Bi is a highly volatile element and consequently the synthesis temperature was not a free deposition parameter, forcing the use of low synthesis temperatures in order to prevent non-stoichiometric films or even the no-formation of the compound when the Bi-deficiency was too large. Yet the general metastable character of these compounds demands the use of high temperatures to the synthesis process. These two antagonistic requirements were tried to be balanced by using 10% Bi-rich PLD targets in the case of BiMnO3 system and by partial replacement of Bi3+ cations by La3+ cations (by 10%) in the case of Bi2NiMnO6 system. In the latter approach, La-doping gives rise to a slightly reduced unit cell volume, exerting the socalled chemical pressure which contributes to prevent Bi3+ cations from desorption during the growth process. Thirdly, both in the ternary Bi – Mn – O and quaternary Bi – Ni – Mn – O systems a strong multiphase formation tendency was found, especially in the former, in which apart from the desired BiMnO3 and Bi2NiMnO6 compounds, different parasitic oxide phases appeared in the grown films. As a consequence of all these facts the single-phase stabilisation of either BiMnO3 or (Bi0.9La0.1)2NiMnO6 was greatly hampered and only possible to be achieved under a narrow window of deposition conditions. Once the deposition conditions for single-phase stabilisation of the Bi-based compounds are controlled, structural characterisation proves that both BiMnO3 and (Bi0.9La0.1)2NiMnO6 grow fully coherent on SrTiO3 substrates, thus adopting as the inplane lattice parameter that of the cubic substrate and subsequently a tetragonal-like structure. Importantly enough for the magnetic properties, (Bi0.9La0.1)2NiMnO6 thin films are found to display long-range B-site order and the Ni2+/Mn4+ electronic configuration, which is the required condition for a long-range ferromagnetism. Indeed, ferromagnetic behaviour is recorded but with a reduced Curie temperature probably due to the epitaxial strain of the substrate. Instead, BiMnO3 thin films are found to exhibit similar Curie temperature to that of bulk specimens. Two-dimensional growth mode is obtained for (Bi0.9La0.1)2NiMnO6 thin films, attaining very low rough surface, whereas BiMnO3 thin films were in all cases displaying a clear three-dimensional growth mode, yielding rougher surface morphology. Finally, in order to study the dielectric/resistive, magnetoelectric and ferroelectric properties parallel-plate capacitors were fabricated using single-crystal (001)-oriented Nb doped SrTiO3 substrates as bottom electrode and sputtered Pt as top electrodes. In (Bi0.9La0.1)2NiMnO6 thin films ferroelectric domains switching current is measured, which allows conclusively stating that (Bi,La)2NiMnO6 compounds are indeed ferroelectric up to at least 10% La content. By structural characterisation the ferroelectric transition temperature is inferred to be around 450 K. The second part of this bloc is devoted to study the dielectric properties and the possible magnetoelectric coupling of these compounds. In this work both the dielectric response and the magnetoelectric response was assessed by impedance spectroscopy, the latter using magnetic fields while recording the impedance response, with the final aim of observing any deviation of the dielectric permittivity of these compounds either in the vicinity of the ferromagnetic transition temperature or when applying a magnetic field. Either phenomenon would indicate magnetoelectric coupling. Special attention is given to the conventional artefacts these measurements often produce when performed on dielectric thin films, causing misleading interpretations, like apparent colossal dielectric constants and/or apparent large magnetoelectric couplings. Following these precautions the intrinsic dielectric and magnetoelectric response of BiMnO3 and (Bi0.9La0.1)2NiMnO6 thin films are extracted. Despite the fact that BiMnO3 dielectric data shows clear magnetoelectric signs, results points to a weak magnetoelectric coupling, which is especially emphasised in (Bi0.9La0.1)2NiMnO6 thin films, probably driven by the fact that magnetism and ferroelectricity arise by two independent mechanisms in these Bibased compounds.
Los materiales multiferroicos, en los cuales coexisten en la misma fase un ordenamiento ferroeléctrico y magnético, han recibido mucho interés en los últimos años. La posibilidad de que estén acoplados los dos órdenes ferroicos permite nuevas funcionalidades en estos materiales como el control eléctrico de la magnetización o, por el contrario, el control magnético de la polarización. La realización de dicho acoplamiento magnetoeléctrico no solo sería interesante en términos de investigación básica, sino que abriría camino para el diseño de nuevas aplicaciones magnetoeléctricas, especialmente en el campo de la spintrónica, como filtros de spin o uniones túneles magnéticas controladas mediante campos eléctricos en lugar de campos magnéticos y por lo tanto promoviendo una nueva generación de dispositivos de almacenamiento de alta densidad y bajo consumo. Para este último propósito, los multiferroicos que poseen un ordenamiento ferromagnético tendrían mayores ventajas que aquellos antiferromagnéticos debido a que los primeros mostrarían magnetización neta y por lo tanto permitirían un control más fácil del estado magnético. No obstante, es el orden antiferromagnético el que prevalece en estos materiales. Por eso es necesario la búsqueda de nuevos materiales que sean ferromagnéticos y ferroeléctricos. Los óxidos en estructura perovskita y doble perovskita basados en Bi, BiBO3 y Bi2BB’O6, respectivamente, donde B y B’ son iones magnéticos de metales de transición (es decir, con la capa electrónica externa d parcialmente ocupada), presentan un excelente punto de partida para investigar nuevos materiales ferromagnéticos y ferroeléctricos. En primer lugar, esta tesis aborda el problema de sintetizar estos compuestos. En este proceso se topó con tres principales obstáculos. Primero, estos compuestos basados en Bi son altamente metaestables, lo que implica que en su forma masiva sólo se pueden sintetizar bajo condiciones extremas: altas temperaturas y altas presiones (del orden de los GPa). Segundo, Bi es un elemento altamente volátil y por consiguiente la temperatura de síntesis de estos compuestos no fue un parámetro de crecimiento libre. Tercero, tanto en el sistema ternario Bi – Mn – O como cuaternario Bi – Ni – Mn – O se encontró una fuerte tendencia multifásica, especialmente en el primero, en los cuales, aparte de los compuestos deseados BiMnO3 y Bi2NiMnO6, se forman diferentes fases parásitas de óxidos como Mn3O4, Bi2O3 y MnO2 en el primer caso y NiO en el segundo. Como consecuencia de todos estos factores la estabilización monofásica de tanto BiMnO3 como (Bi0.9La0.1)2NiMnO6 fue dificultada en gran medida y solo se pudo conseguir bajo una ventana estrecha de condiciones de crecimiento. Especialmente crítico fue la temperatura de depósito, la cual sólo permitía una ventana de 10ºC alrededor de 630ºC y 620ºC para la síntesis de BiMnO3 y (Bi0.9La0.1)2NiMnO6, respectivamente.

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

Multiferroic thin-film heterostructures have attracted a great deal of attention due to the increasing demand for novel energy-efficient micro/nano-electronic devices. Both single phase multiferroic materials like BiFeO3 (BFO) thin films, and strain-mediated magnetoelectric (ME) nanocomposites, have the potential to fulfill a number of functional requirements in actual applications—principally, direct control of magnetization by the application of an electric field (E) and vice-versa. From the perspective of material science, however, it is essential to develop a fuller understanding of the complex fabrication-structure-property triangle relationship for these multiferroic thin films. Pulsed laser deposition (PLD) was used in this study to fabricate diverse epitaxial thin film heterostructures on top of single crystal substrates. The crystal structure, phase transition processes (amongst nanodomain distributions, dielectric phases, magnetic spin states, etc.), and various ME-related properties were characterized under different E or temperature environments. Resulting data enabled us to determine the structure-property relationships for a range of multiferroic systems. First, BFO-based heterostructures were studied. Epitaxial BFO thin films were deposited on top of (001)-oriented Pb(Mg1/3Nb2/3)O3-30PbTiO3 (PMN-30PT) single crystal substrates. The strain states of BFO and crystal structural phases were tunable by E applied on the PMN-30PT via both the in-plane and out-of-plane modes. The strain-mediated antiferromagnetic state changes of BFO were also studied using neutron diffraction spectroscopy under E. Then, CoFe2O4(CFO)/tetragonal BFO nanocomposites were successfully fabricated on top of (001)-oriented LaAlO3 single crystal substrates. The surface morphology, crystal structure, magnetic properties, and ME effects were evaluated and compared with CFO/rhombohedral BFO nanocomposites. To enhance the performance of ME heterostructures with PMN-PT substrates, PMN-30PT single crystals with nanograted electrodes were also studied, which evidenced an enhancement in piezoelectric properties and dielectric constant by 36.7% and 38.3%, respectively. X-ray diffraction reciprocal space mapping (RSM) was used to monitor E-induced changes in the apparent symmetry and domain distribution of near-surface regions for the nanograted PMN-30PT crystals. Finally, in order to add antiferroelectric thin films to the family of strain-mediated multiferroic nanocomposites, epitaxial antiferroelectric thin films were prepared. Epitaxial (Pb0.98La0.02)(Zr0.95Ti0.05)O3 (PLZT) thin films were deposited on differently oriented SrTiO3 single crystal substrates. A thickness dependent incommensurate/commensurate antiferroelectric-to-ferroelectric phase transition was identified. The crystal structure, phase transition characteristics and pathways, and energy storage behaviors from room temperature to 250 ℃ were studied, enabling a more systematic understanding of PLZT-based AFE epitaxial thin films. To summarize, a range of epitaxial thin films were prepared using PLD, whose crystal structures and multiferroic properties were related through the strain. Accordingly, properties such as dielectricity, antiferroelectricity, and antiferromagnetism could be adjusted by E. This study sheds further light on the potential for designing desirable strain-mediated multiferroic nano-/micro-devices in the future.
Doctor of Philosophy
As a general definition, the class of materials known as multiferroics possess more than one ferroic order parameter. Multiferroic thin-film heterostructures have attracted a great deal of attention due to the increasing demand for novel energy-efficient micro/nano-electronic devices. Both single phase multiferroic materials like BiFeO3 (BFO) thin films and strain-mediated magnetoelectric (ME) nanocomposites show significant potential for use in next-generation devices due to the fact that one can control magnetic properties via the application of an electric field (E) and vice-versa. From the perspective of material science, however, it is essential to develop a fuller understanding of the complex fabrication-structure-property triangle relationship for these multiferroic thin films. In this study, diverse epitaxial thin film heterostructures were fabricated on top of single crystal substrates. The crystal structure, phase transition processes (amongst nanodomain distributions, dielectric phases, magnetic spin states, etc.), and various ME-related properties were characterized under different E or temperature environments. Resulting data enabled us to determine the structure-property relationships for a range of multiferroic systems. First, BFO-based heterostructures were studied. Epitaxial BFO thin films were deposited on top of (001)-oriented Pb(Mg1/3Nb2/3)O3-30PbTiO3 (PMN-30PT) single crystal substrates. The strain states of BFO and crystal structural phases were tunable by E applied on the PMN-30PT via both the in-plane and out-of-plane modes. The strain-mediated antiferromagnetic state changes of BFO were studied using powerful neutron diffraction spectroscopy under E. Then, CoFe2O4(CFO)/tetragonal BFO nanocomposites were successfully fabricated on top of (001)-oriented LaAlO3 single crystal substrates. The surface morphology, crystal structure, magnetic properties, and ME effects were discussed and compared with CFO/rhombohedral BFO nanocomposites. To enhance the performance of ME heterostructures with PMN-PT substrates, PMN-PT single crystals with nanograted electrodes were also studied, which evidenced an enhancement in piezoelectric properties and dielectric constant by 36.7% and 38.3%, respectively. X-ray diffraction reciprocal space mapping (RSM) technique was used to monitor E-induced changes in the apparent symmetry and domain distribution of near-surface regions for nanograted PMN-PT crystals. Finally, in order to add antiferroelectric thin films to the family of strain-mediated multiferroic nanocomposites, epitaxial antiferroelectric thin films were prepared. Epitaxial (Pb0.98La0.02)(Zr0.95Ti0.05)O3 (PLZT) thin films were deposited on differently oriented SrTiO3 substrates. A thickness dependent incommensurate antiferroelectric-to-ferroelectric phase transition was identified. The crystal structure, phase transition characteristics and pathways, and energy storage behaviors from room temperature to 250 ℃ were studied, enabling a more systematic understanding of PLZT-based AFE epitaxial thin films. To summarize, a range of epitaxial perovskite thin films were prepared, whose crystal structures and multiferroic properties were related through the strain. Accordingly, the properties such as dielectricity, antiferroelectricity, and antiferromagnetism could be adjusted by E. This study sheds further light on the potential for designing desirable strain-mediated multiferroic nano-/micro-devices in the future.

Mestrado em Engenharia de Materiais
An enormous contribution in the scientific community of material engineering is being made by the exceptionally rapid evolution of the field of multifunctional materials. Multiferroics combine simultaneously at least two of the three ferroic properties: ferroelectricity, ferromagnetism and ferroelasticity. Magnetoelectric multiferroics’ ability of magnetic field manipulation via electric fields or vice versa can be extremely promising for information storage applications, leading to thinner, as well as flexible devices, with significantly high energetic efficiencies and elevated capacities. The aim of this work is the preparation and characterization of bismuth ferrite porous thin films, having as further objective to be able to serve as matrices for future functionalization. The strategy of this work consists of: a) dense film preparation with varying deposition velocities, b) porous film preparation with varying solution template quantities, inorganic precursor concentration and deposition velocities. Annealing temperature studies were also required, for the obtainment of the desired properties and control of microstructure. The methodologies for the film preparation in use were: a) sol-gel process, b) Evaporation Induced Self-Assembly (EISA), for the induction of porosity, and c) dip-coating technique. A series of dense films with varying deposition velocities were produced, serving as means of comparison for the porous thin films. Increasing the sol-gel deposition velocity led to increasing thickness. Piezoresponse Force Microscopy (PFM) characterization was conducted, revealing the expected ferroelectric domains. By the same technique, local piezoelectric hysteresis loops were obtained, showing increase of polarization saturation with increasing thickness. Lastly, magnetic moment measurements were carried out by the use of Superconducting Quantum Interference Device (SQUID), presenting decrease of remnant magnetization with increasing thickness. Varying template concentration was introduced in order to obtain a homogenous porous network. Homogeneity and lack of cracks in the films were successfully achieved, by decreasing solution template mass, for a given solution concentration. Thermal treatment studies revealed loss of porous network ordering at elevated annealing temperatures, required for the obtainment of crystallization and enhanced multiferroic properties. Local piezoelectric hysteresis loops showed increase of the effective piezoelectric coefficient with increasing thickness. SQUID characterization presented increasing remnant magnetization with increasing porosity. Lastly, increasing inorganic precursors concentration resulted in better control of porosity order and increase in the piezoelectric coefficient.
Uma enorme contribuição na comunidade científica da Engenharia de Materiais tem sido feita pela evolução excecionalmente rápida no âmbito dos materiais multifuncionais. Os multiferróicos combinam simultaneamente pelo menos duas das três propriedades ferróicas: ferroeletricidade, ferromagnetismo e ferroelasticidade. Os multiferróicos magnetoelétricos que permitem a manipulação do campo magnético através do campo elétrico e vice versa são extremamente promissores para aplicações de armazenamento de informação, levando a dispositivos mais finos e flexíveis com eficiência energética significativamente mais alta e elevadas capacidades. O objetivo deste trabalho é a preparação e caracterização de filmes porosos de ferrite de bismuto, com vista a serem capazes a uma futura funcionalização. A estratégia deste trabalho consiste: a) preparação de filme denso variando a velocidade de deposição, b) preparação de filme poroso variando o template da solução concentração do precursor inorgânico, e velocidades de deposição. Os estudos sobre temperatura de calcinação são também necessários, para a obtenção das propriedades requeridas e o controlo da microestrutura. As metodologias para a preparação dos filmes foram: a) sol-gel, b) Evaporation Induced Self-Assembly, para a indução da porosidade, e c) dip-coating. Foi preparada uma série de filmes densos variando a velocidade de deposição, servindo como meio de comparação para os filmes porosos. Aumento da velocidade de deposição resulta em aumento da espessura dos filmes. Foi utilizada a caracterização por piezoresponse force microscopy (PFM), revelando domínios ferroelétricos como esperado. Pela mesma técnica, foram obtidas curvas de histerese piezoelétricas locais mostrando o aumento da saturação da polarização com o aumento da espessura. Por fim, as medidas dos momentos magnéticos foram obtidos através do Superconducting Quantum Interference Device (SQUID), apresentando uma diminuição da magnetização remanescente com o aumento da espessura. A variação da concentração do template foi introduzida de modo a obter uma porosidade homogénea. A homogeneidade e ausência de fissuras nos filmes foi conseguida com sucesso pela diminuição da massa do template da solução, para uma determinada concentração da solução. Os estudos do tratamento térmico revelou a perda da porosidade ordenada para temperaturas mais elevadas, necessárias para a obtenção da cristalização e melhoria das propriedades multiferróicas. As curvas de histerese piezoelétrica local mostraram um aumento do coeficiente efetivo piezoelétrico com o aumento da espessura. A caracterização por SQUID apresentou um aumento da magnetização remanescente com o aumento da porosidade. Por fim, o aumento da concentração dos precursores inorgânicos resulta em um melhor controlo da ordem da porosidade e aumento do coeficiente piezoelétrico.

Nanomagnetic logic, incorporating logic bits in the magnetization orientations of single-domain nanomagnets, has garnered attention as an alternative to transistor-based logic due to its non-volatility and unprecedented energy-efficiency. The energy efficiency of this scheme is determined by the method used to flip the magnetization orientations of the nanomagnets in response to one or more inputs and produce the desired output. Unfortunately, the large dissipative losses that occur when nanomagnets are switched with a magnetic field or spin-transfer-torque inhibit the promised energy-efficiency. Another technique offering superior energy efficiency, “straintronics”, involves the application of a voltage to a piezoelectric layer to generate a strain which is transferred to an elastically coupled magnetrostrictive layer, causing magnetization rotation. The functionality of this scheme can be enhanced further by introducing magnetocrystalline anisotropy in the magnetostrictive layer, thereby generating four stable magnetization states (instead of the two stable directions produced by shape anisotropy in ellipsoidal nanomagnets). Numerical simulations were performed to implement a low-power universal logic gate (NOR) using such 4-state magnetostrictive/piezoelectric nanomagnets (Ni/PZT) by clocking the piezoelectric layer with a small electrostatic potential (~0.2 V) to switch the magnetization of the magnetic layer. Unidirectional and reliable logic propagation in this system was also demonstrated theoretically. Besides doubling the logic density (4-state versus 2-state) for logic applications, these four-state nanomagnets can be exploited for higher order applications such as image reconstruction and recognition in the presence of noise, associative memory and neuromorphic computing. Experimental work in strain-based switching has been limited to magnets that are multi-domain or magnets where strain moves domain walls. In this work, we also demonstrate strain-based switching in 2-state single-domain ellipsoidal magnetostrictive nanomagnets of lateral dimensions ~200 nm fabricated on a piezoelectric substrate (PMN-PT) and studied using Magnetic Force Microscopy (MFM). A nanomagnetic Boolean NOT gate and unidirectional bit information propagation through a finite chain of dipole-coupled nanomagnets are also shown through strain-based "clocking". This is the first experimental demonstration of strain-based switching in nanomagnets and clocking of nanomagnetic logic (Boolean NOT gate), as well as logic propagation in an array of nanomagnets.

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

Recently, the use of an electric field (E-field) to control the magnetic properties of thin magnetic films has drawn intensive interest due to their important potential applications such as magnetoelectric random access memory (MERAM) devices and magnetoelectric (ME) sensor. In this thesis, the work first includes a study of the strain-mediated ME coupling strength manipulation by either changing ferromagnetic layer thickness (30-100 nm) or inserting a thin Ti buffer layer (0-10 nm). A large remanence ratio (Mr/Ms) tunability of 95% has been demonstrated in the 65 nm CoFe/PMN-PT heterostructure, corresponding to a giant ME constant (α) of 2.5 × 10-6 s/m, when an external E-field of 9 kV/cm was applied. Also, a record high remanence ratio (Mr/Ms) tunability of 100% has been demonstrated in the 50 nm CoFe/8 nm Ti/PMN-PT heterostructure, corresponding to a large ME constant α of 2.1 × 10-6 s/m, when the E-field of 16 kV/cm was applied. Furthermore, the E-field induced magnetic response was repeatable and quick even after 30 repeats were made. Secondly, a study of non-volatile magnetization change has been demonstrated in the 65 nm CoFe/24 nm Metglas/PMN-PT. In this heterostructure, the E-field created two new non-volatile remanence states, although the as-grown magnetic anisotropy was altered permanently, when the E-field between -6 kV/cm to +6 kV/cm was applied. Based on giant magnetoresistance (GMR) or anisotropic magnetoresistance (AMR), the MERAM memory cell was proposed for the fast, low-power and high-density information storage.

Multiferroics which combine two or more order parameters of ferroelectricity, ferromagnetism and ferroelasticity, have drawn great interests in the past few years due to their promising potential of application in sensors, transducers, spintronics and multistate memories. Coupling between the ferroelectricity and ferromagnetism renders the induction of an electric polarization P upon applying a magnetic field, or the induction of a magnetization M upon applying an electric field which is called magnetoelectric coupling effect. There are single phase multiferroics which simultaneously possess ferroelectricity and magnetism in nature. However, these natural multiferroics only exhibit weak magnetoelectric coupling effect at very low temperature which hinders the practical applications. An alternative and more promising choice is to fabricate multiferroic composites. In the multiferroic composite systems, large magnetoelectric coupling effects can be produced indirectly from the strain-mediated interaction even at room temperature and great design flexibility can be obtained. In the present study, two types of multiferroic composite nanostructures are investigated: the vertical heteroepitaxial multiferroic thin films and film-on-substrate heterogeneous bilayers with incorporation of various influences, such as film thickness, misfit strains and flexoelectricity. Since the first fabrication of vertical epitaxial multiferroic nanostructures, great scientific interests have been attracted for the potential large magnetoelectric effects arising from the relaxed substrate constraint and large interfacial area between the ferroelectric and ferromagnetic phases. A three dimensional phase field model is devised to precisely describe the complex strain state of this nanostructure. The simulation results demonstrate that both film thickness and misfit strains are important in determining the magnitude of magnetoelectric effect. Due to the strong strain-mediated magnetoelectric coupling effect in film-on-substrate system with a ferromagnetic thin film directly growing on a thick ferroelectric substrate, precision electric control of local ferromagnetism, i.e. ferromagnetic domain pattern and domain wall properties, are achievable. The results show that the domain pattern of the ferroelectric substrate can be fully transferred onto the as-deposited ferromagnetic thin film. High stability of the magnetic domain is observed when the system is subjected to an external magnetic field. Under an applied electric field, the transferred domain pattern in magnetic film can be either maintained or erased depending on the direction of applied electric field. Moreover, when a pulse of in-plane electric field is applied, the magnetic domain wall motion can be observed in concurrence with the ferroelectric domain wall motion. With the decrease of material size, some effects that can be neglected in bulk materials may play an important role on the overall properties of material, such as flexoelectric effects which describe the induction of polarization from strain gradient. A two dimensional phase field model is adopted to study the influence of flexoelectric effects on the epitaxial ferroelectric films. A thermodynamic phenomenological model is then utilized to analyze the influence of flexoelectric effects on magnetic field induced electric polarization in the multiferroic nanocomposite bilayers. By decreasing the film thickness, the induced polarization from flexoelectric effects becomes more and more dominant and finally overcomes the electrostrictive induced polarization which is dominant when film thickness is large.
published_or_final_version
Mechanical Engineering
Doctoral
Doctor of Philosophy

Developing a greater understanding of multiferroic materials, particularly those in which a strong coupling is exhibited between magnetic and electrical orderings, is of great importance if potential applications are to be realised. This thesis reports new experimental findings on several multiferroics using the techniques of X-ray and neutron diffraction together with nonlinear optical experiments. Spherical neutron polarimetry measurements on RbFe(MoO)₂ show how this system's chiral magnetic structure can be controlled by an external electric field. Consideration is given to the axial distortion that the crystal structure makes, and the effect that this has on the stabilised magnetic structures. A ferroaxial coupling is invoked to explain, from a symmetry point of view, the spin driven multiferroicity in this proper screw system. The charge ordering in YbFe₂O₄ is examined by a detailed imaging of reciprocal space measured by elastic X-ray diffraction. Continuous helices of scattering are observed above the three-dimensional ordering transition temperature, whereas the intensity is concentrated onto separated maxima below this. The low temperature data are modelled using a simple oxygen displacement pattern, generalised to an incommensurate structure. The observed incommensurability implies that YbFe₂O₄ cannot be truly ferroelectric. The low field magnetic structures of a Y-type hexaferrite Ba_0.5Sr_1.5Zn₂Fe₁₂O₂₂ are observed in a resonant soft X-ray diffraction study. In zero field the system is helimagnetic, and with small applied fields peaks corresponding to a new phase appear. Energy calculations are used to suggest a suitable magnetic structure for the new phase and to show how this relates to the known commensurate phases that are present in low fields. Finally, an experimental setup designed to measure second harmonic generation from non-centrosymmetric crystals is presented, along with static measurements on the multiferroic system MnWO₄. An optical pump / second harmonic probe study is then undertaken, with the result that a pump induced enhancement in the efficiency of the second harmonic generation is observed.

Les matériaux multiferroïques présentent plusieurs ordres ferroïques, i.e. ferromagnétiques, ferroélectriques et/ou ferroélastiques. Le couplage entre ses ordres ferroïques permet de contrôler les propriétés magnétiques en appliquant un champ électrique et vice versa. Afin d’utiliser leur multifonctionnalités dans des nouvelles applications, ce couplage doit être efficace à température ambiante. Cette thèse étudie les systèmes couplant artificiellement ensemble une phase ferromagnétique / magnétostrictive à une phase ferroélectrique / piézoélectrique. Le couplage entre ces deux phases est appelée magnétoélectrique (ME). Le premier chapitre décrit l’état de l’art de ce couplage ME dans différentes structures composites multiferroïques. Tandis que les techniques de caractérisation et les outils de simulation micromagnétiques utilisées sont présentées dans le deuxième chapitre. Dans le troisième chapitre, une hétéro-structure type film magnétostrictif/substrat flexible/actuateur piézoélectrique (FeCuNbSiB/Kapton/PE) a été étudiée. Les domaines magnétiques du FeCuNbSiB ainsi que leur orientation sont contrôlées en appliquant un champ électrique et étudiées par microscopie locale (MFM). Le quatrième chapitre étudie un composite incluant des nanoparticules magnétostrictives dans une matrice piézoélectrique flexible (polymère PVDF). L’effet des inclusions (nanoparticules) sur la réponse piézoélectrique locale du PVDF est étudiée par microscopie de piézoréponse (PFM). Symétriquement, l’influence de la matrice piézoélectrique sur les propriétés magnétiques des nanoparticules est analysée. Dans le dernier chapitre, l’optimisation des propriétés magnétiques statiques d’un ensemble de nanoparticules anisotropes (nanofils de cobalt) est étudiées sous l’influence de leur structure, de leur forme et de leurs interactions. Cette étude expérimentale est corroborée par les simulations et vise des nouveaux composites incluant ces nanoparticules anisotropes dans une matrice piézoélectrique flexible
Multiferroic materials present several ferroic orders, i.e. ferromagnetic, ferroelectric and/or ferroelastic. The coupling between these ferroic orders allow the control of the magnetic properties by applying an electric field and vice versa. In order to use their multifunctionality in new applications, this coupling must be efficient at room temperature. This thesis concentrates on materials artificially coupling together a ferromagnetic/ magnetostrictive phase with a ferroelectric/piezoelectric one. The coupling between these two phases is called magnetoelectric (ME). The first chapter describes the state of the art of this ME coupling for different multiferroic composite structures. Characterization techniques and micromagnetic simulation tools are presented in the second chapter. In the third chapter, a hetero-structure given by a magnetostrictive film/flexible substrate/piezoelectric actuator (FeCuNbSiB/Kapton/PE) is studied. The magnetic domains of FeCuNbSiB as well as their orientation are controlled by applying an electric field and studied by local microscopy (MFM). The fourth chapter focuses on a nanocomposite material including magnetostrictive nanoparticles in a flexible piezoelectric matrix (PVDF polymer). The effect of these inclusions (nanoparticles) on the local piezoelectric response of the PVDF is studied by piezoeponse microscopy (PFM). Symmetrically, the influence of the piezoelectric matrix on the static magnetic properties of the nanoparticles is analyzed. In the last chapter, the optimization of the magnetic properties of a set of anisotropic nanoparticles (cobalt nanowires) is studied as fonction of their structure, shape and mutual interactions. This experimental study is corroborated by simulations and targets new composites ME materials including the anisotropic nanoparticles in a flexible piezoelectric matrix

This thesis presents optical characterization of the static and dynamic magnetic interactions in ferromagnetic and multiferroic heterostructures with time-resolved and interface-specific optical techniques. The focus of the thesis is on elucidating the underlying physics of key physical parameters and novel approaches, crucial to the performance of magnetic recording and spintronic devices.;First, time-resolved magneto-optical Kerr effect (TRMOKE) is applied to investigate the spin dynamics in L10 ordered FePt thin films, where perpendicular magnetic anisotropy Ku and intrinsic Gilbert damping alpha0 are determined. Furthermore, the quadratic dependence of Ku and alpha0 on spin-orbit coupling strength xi is demonstrated, where xi is continuously controlled through chemical substitution of Pt with Pd element. In addition, a linear correlation between alpha0 and electron scattering rate 1/T e is experimentally observed through modulating the anti-site disorder c in the L10 ordered structure. The results elucidate the basic physics of magnetic anisotropy and Gilbert damping, and facilitate the design and fabrication of new magnetic alloys with large perpendicular magnetic anisotropy and tailored damping properties.;Second, ultrafast excitation of coherent spin precession is demonstrated in Fe/CoO heterostructures and La0.67Ca0.33MnO 3 thin films using TRMOKE technique. In the Fe/CoO thin films, Instant non-thermal ferromagnet (FM) -- antiferromagnet (AFM) exchange torque on Fe magnetization through ultrafast photo-excited charge transfer possesses in the CoO layer is experimentally demonstrated at room temperature. The efficiency of spin precession excitation is significantly higher and the recovery is notably faster than the demagnetization procedure. In the La0.67Ca 0.33MnO3 thin films, pronounced spin precessions are observed in a geometry with negligible canting of the magnetization, indicating that the transient exchange field is generated by the emergent AFM interactions due to charge transfer and modification of the kinetic energy of eg electrons under optical excitation. The results will help promoting the development of novel device concepts for ultrafast spin manipulation.;Last, the interfacial spin state of the multiferroic heterostructure PbZr0.52Ti0.4803/La0.67Sr0.33MnO 3 and its dependence on ferroelectric polarization is investigated with interface specific magnetization induced second harmonic generation (MSHG). The spin alignment of Mn ions in the first unit cell layer at the heterointerface can be tuned from FM to AFM exchange coupled, while the bulk magnetization remains unchanged as probed with MOKE. The discovery provides new insights into the basic physics of interfacial magneto-electric (ME) coupling.

Les multiferroïques présentent la rare propriété de posséder simultanément un ordre magnétique et un ordre ferroélectrique qui interagissent via le couplage magnétoélectrique. Les caractéristiques des multiferroïques font ressortir de nouveaux phénomènes physiques et offrent des possibilités de nouvelles fonctions de l'appareil. BiFeO3 et RMnO3 hexagonaux sont deux multiferroïques selon la plupart des enquêtes. Dans ce travail, nous avons étudié les excitations de spin et réseau dans les RMnO3 hexagonaux (R = Yb, Y, Ho) et BiFeO3 films sur des substrats différents. Nos mesures de RMnO3 hexagonaux indiquent le couplage des excitations de phonons et des ondes de spin. En outre, les résultats montrent clairement que le R-Mn et l'interaction R-R interplan au long de l'axe c sont importants pour la structure magnétique de la h-RMnO3. Dans le BiFeO3 film, nous avons observé qu'une grande contrainte épitaxiale peut détruire la modulation cycloïdale et les non colinéaires commandes sont stables à faible contrainte. Une phase de mélange de deux ordres magnétiques est également détectée dans le BiFeO3 film. Dans l'état faible contrainte, un spin cycloïdal nouveau avec un vecteur d'onde de propagation le long de [110] est prédit et observé expérimentalement. Nos résultats ont des implications profondes pour la mise en oeuvre de BiFeO3 films dans les appareils magnonic et spintronique
Multiferroic materials present the rare properties that simultaneously exist magnetic and ferroelectric orders and interaction between them. These features in mutliferroics bring out novel physical phenomena and offer possibilities for new device functions. BiFeO3 and hexagonal RMnO3 are two most investigated multiferroics. In this work, we have studied the lattice and spin excitations in hexagonal RMnO3 single crystals (R=Yb, Y, Ho) and BiFe03thin films on different substrates. Our measurements of hexagonal RMnO3 single crystals indicate the coupling of lattice and spin excitations. Moreover, the results clearly show the R-Mn and R-R interplane interaction along c-axis also play an important role in the magnetic structure of h-RMn03. In the case of BiFe03thin films, we have observed that high epitaxial strain can destroy the bulk-like cycloidal modulation and non-collinear orders are stable at low strain. A mixture phase of both magnetic orders is also detected in BiFe03thin films. In the lower¬strain state, a new cycloidal spin structure with a propagation wavevector along [110] is predicted and experimentally observed. Our findings have profound implications for the implementation of BiFeO3 films in magnonic and spintronic devices. Lndeed, our Raman scattering measurements reveal that strain can completely quench high energy magnon modes, offerin exciting possibilities for BiFe03-based magnonic devices

The effects of strain on the magnetic phases in perovskites are of interest in the highly active research field of multiferroics. A Monte Carlo program is written to investigate the influence of strain on the low– temperature magnetic phase diagram of the manganese perovskites, RMnO3, where R is a cation in the lanthanide series. A Metropolis simulation scheme is implemented together with parallel tempering to perform computations in a two–dimensional geometry using a conventional nearest–neighbor and next–nearest–neighbor Heisenberg Hamiltonian, extended to include spin–lattice couplings and single–ion anisotropies. The latter two are important to account for structural distortions such as octahedral tilting and the Jahn–Teller effect. It is shown that even weak single–ion anisotropies render incommensurability in the otherwise structurally commensurate E–type ordering, and that the Dzyaloshinskii–Moriya interaction, in combination with single–ion anisotropies, is crucial for the stabilization of previously experimentally observed incommensurate spin spirals. Simulations performed to account for strain in the crystallographic ab–plane show that tensile strain may improve stability of E–type ordering for R elements with small atomic radii and that compressive strain drives the magnetic ordering toward the incommensurate spiral states.
Spänningsinverkan på de magnetiska faserna i perovskiter är av intresse inom den just nu högaktiva forskningen om multiferroiska material. Ett Monte Carlo-program har skrivits för att undersöka effekterna av spän- ning på de magnetiska lågtemperaturfaserna i multiferroiska manganitpe- rovskiter, RMnO3, där R är en katjon i lantanoidserien. En kombination av Metropolisalgoritmen och parallelltemperering har använts för att utföra beräkningar i tvådimensionell geometri med en konventionell Heisenberghamiltonian, utökad till att även inkludera spinn–gitterkopplingar och enkeljonsanisotropier. De senare har visats vara viktiga för att ta i beaktande den strukturella distortion i materialet som följer av t.ex. syreoktahederförskjutning och Jahn–Tellereffekten. Det visas att även svaga anisotropier orsakar inkommensurabilitet i den i övrigt kommensurabla E–typsfasen, och att Dzyaloshinskii-Moriyainteraktionen, i kombination med anisotropitermerna, är avgörande för att kunna stabilisera de sedan tidigare experimentellt bekräftade inkommensurabla spinnspiralsfaserna. Simuleringar som modellerar spänning i materialets kristallografiska ab–plan visar att dragspänning kan förbättra stabiliteten hos E–typsfasen för R–atomer med liten radie och att tryckspänning leder den magnetiska ordningen mot inkommensurabla spiraltillstånd.

The anisotropy of the direct magnetoelectric effect in textured nickel ferrite/lead zirconate titanate strain mediated bilayer composites has been studied. The magnetic layers of these samples have been crystallographically textured in planes of the form {100}, {110} and {111}. In this study, it is shown that the optimum bias field and the maximum magnetoelectric coupling signal can be controlled by changing the alignment of the applied magnetic field with respect to the magnetocrystalline anisotropy directions. It is also shown that the product of the optimum bias field and the maximum magnetoelectric coupling signal are proportional to the theoretical saturation magnetostriction. The samples have been magnetically characterised using a recommissioned and developed biaxial vibrating sample magnetometer, capable of detecting the component of a sample’s magnetic moment in 2 perpendicular directions and thus determining the net magnetic moment vector of the sample. Coupled with sample rotation this allows insight into the magnetic anisotropy of the sample, which has been compared with a micromagnetic model. A specialist magnetoelectric coupling rig has also been developed to allow application of DC and AC magnetic fields to a sample simultaneously. As part of the magnetic anisotropy study, a modified torque magnetometry method has been developed to enhance the identification of the anisotropy directions in magnetically soft samples, as well as a method by which torque magnetometry can be approximated using the in-field direction component of magnetisation as measured using a standard vibrating sample magnetometer.

Nas últimas décadas, o consumo de dispositivos eletrônicos e a alta demanda por armazenamento de dados tem mostrado grandes oportunidades para a criação de novas tecnologias que garantam as necessidades mundiais na área de computação e desenvolvimento. Alguns materiais multiferroicos tem sido amplamente estudados e o BiFeO3, considerado o único material multiferroico em temperatura ambiente, ganhou destaque como candidato para produção de dispositivos lógicos e de memória. O uso de técnicas de crescimento como a deposição por laser pulsado permitiu a produção de filmes finos de BiFeO3 com elevado controle de qualidade. Heteroestruturas de filmes multiferroicos de BiFeO3 e LaBiFeO3 foram crescidas com diferentes espessuras sobre substratos de SrTiO3(100), DyScO3(110) e SrTiO3/Si(100) para avaliação e teste de suas propriedades elétricas e magnéticas. Filmes ferromagnéticos de Co0,9Fe0,1 foram depositados por sputtering sobre os filmes multiferroicos para avaliação da interação interfacial entre ordenamentos magnéticos. Técnicas como fotolitografia foram utilizadas para padronização de microdispositivos gravados sobre as amostras. Tanto os filmes finos de BiFeO3 como os de LaBiFeO3 foram crescidos epitaxialmente sobre os substratos já cobertos com uma camada buffer de SrRuO3 usado como contato elétrico inferior. A estrutura cristalina romboédrica das ferritas de bismuto foi confirmada pelos dados de difração de raios X, bem como a manutenção de tensão estrutural causada pela rede cristalina do substrato para amostras de 20 nm. Os valores de coeficiente do tensor piezelétrico d33 foram da ordem de 0,15 V (∼ 60 kV.cm-2) para amostras com 20 nm de espessura enquanto que os valores de voltagem coerciva para as análises de histerese elétrica foram da ordem de 0,5 V para as mesmas amostras. A relação de coercividade elétrica com a espessura corresponde ao perfil encontrado na literatura pela relação E≈d-2/3. As amostras de CoFe/BFO e CoFe/LBFO depositadas em diferentes substratos apresentam acoplamento interfacial entre ordenamento ferromagnético e antiferromagnético com momento ferromagnético de rede.
For the last few decades, the consumption of electronic devices and the high demand for data storage have shown great opportunities to create modern technologies that assure the worldwide needs in computing and development. Some multiferroic materials have been extensively studied and BiFeO3, considered the only multiferroic material at room temperature, has received attention as a candidate to produce logic and memory devices. The use of growth techniques such as pulsed laser deposition allowed the production of thin films of BiFeO3 with high quality control. Multiferroic film heterostructures of BiFeO3 and LaBiFeO3 were grown with different thicknesses on SrTiO3 (100), DyScO3 (110) and SrTiO3/Si (100) substrates to evaluate and test their electrical and magnetic properties. The allow Co0.9Fe0.1 ferromagnetic films were deposited by sputtering on the multiferroic films to evaluate the interfacial interaction between magnetic ordering. Techniques such as photolithography were used to pattern microdevices on the samples. Both the BiFeO3 and LaBiFeO3 thin films were grown epitaxially on the substrates already covered with a SrRuO3 buffer layer used as the lower electrical contact. The rhombohedral crystalline structure of the bismuth ferrites was confirmed by the X-ray diffraction data as well as the strain maintenance caused by the crystal lattice of the substrate for 20 nm samples. The coefficient values of the piezoelectric tensor d33 were around 0.15 V (∼ 60 kV.cm-2) for 20 nm thick samples whereas the coercive voltage values for the electrical hysteresis analyzes were about 0.5 V for the same samples. The relation between electric coercivity and the thickness corresponds to the profile found in the literature by the relation E≈d-2/3. The samples of CoFe/BFO and CoFe/LBFO deposited in different substrates present interfacial coupling between ferromagnetic and antiferromagnetic arrangement with net ferromagnetic moment.

Recently, multiferroics-defined as materials with coexistence of at least two of the ferroelectric, ferroelastic and ferromagnetic effects-have attracted enormous research activities. In this thesis, the structure and properties of multiferrioic BiFeO3-x%PbTiO3 and Fe-x%Ga crystalline solutions were investigated. First, the results show that modified BiFeO3-PbTiO3 based ceramics have significantly enhanced multiferroic properties, relative to BiFeO3 single crystals. The data reveal: (i) a dramatic increase in the induced polarization; and (ii) the establishment of a remnant magnetization by a breaking of the translational invariance of a long-period cycloidal spin structure, via substituent effects. In addition, temperature dependent magnetic permeability investigations of BiFeO3-xPbTiO3 crystalline solutions have shown that aliovalent La substitution results in a significant increase in the permeability. Second, room temperature high-resolution neutron and x-ray diffraction studies have been performed on Fe-x%Ga crystals for 12Master of Science

Thesis (Ph. D.) -- University of Maryland, College Park, 2004.
Thesis research directed by: Material Science and Engineering. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

Thin film multiferroic oxides with co-existing ferroelectric and ferromagnetic ordering have attracted much interest in recent years, partly as a result of the enhancements achieved through the adoption of strained thin film geometries. This thesis presents work on two such thin film oxides; lanthanide substituted BiFeO₃ and Fe substituted PbTiO₃. Coherent magnons and acoustic phonons were impulsively excited and probed in thin films of the room temperature multiferroic Bi_1-x-yDy_xLa_yFeO₃ using femtosecond laser pulses. The elastic moduli of rhombohedral, tetragonal and rare-earth doped BiFeO₃ were determined from acoustic mode frequencies in conjunction with spectroscopic ellipsometry. A weak ferromagnetic order, induced alternately by magnetization in the growth direction or by tetragonality, created a magnon oscillation at 75 GHz, indicative of a Dzyaloshinskii-Moriya interaction energy of 0.31 meV. Bulk crystals and thin films of PbTi_1-xFe_xO₃ (PTFO) are multiferroic, exhibiting ferroelectricity and ferromagnetism at room temperature. Here we report that the Ruddlesden-Popper phase Pb_n+1(Ti_1-xFe_x)_nO_3n+1 forms spontaneously during pulsed laser deposition of PTFO on LaAlO₃ substrates. High-resolution transmission electron microscopy, x-ray difraction and x-ray photoemission spectroscopy were utilised to perform a structural and ompositional analysis, demonstrating that n≃8 and x≃0.33. The complex dielectric function of the films was determined from far-infrared to ultraviolet energies using a combination of terahertz time-domain spectroscopy, Fourier transform spectroscopy, and spectroscopic ellipsometry. The simultaneous Raman and infrared activity of phonon modes, and the observation of second harmonic generation, establishes a non-centrosymmetric point group for Pb_n+1(Ti_0.67Fe_0.33)_nO_3n+1-δ consistent with ferroelectricity. No evidence of macroscopic ferromagnetism was found in SQUID magnetometry. The ultrafast optical response exhibited coherent magnon oscillations compatible with local magnetic order, and additionally was used to study photocarrier cooling on picosecond timescales. An optical gap smaller than that of BiFeO₃ and long photocarrier lifetimes may make this system interesting as a ferroelectric photovoltaic.

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

This thesis presents the results of investigations of the synthesis, structure and physical properties of multiferroic materials. Multiferroics are materials in which two or all three of ferroelectricity, ferromagnetism and ferroelasticity occur in the same phase. Such materials have the potential applications of their parent materials, as well as new ones because of the interaction between the order parameters. The thesis is organized into four sections. Section 1 gives an overview of multiferroics, explaining the origin of mul-tiferroicity , occurrence of magnetoelectric coupling, their possible technological ap-plications and the challenges involved. Section 2 gives the scope of the investigations. The specific objectives of the present research on yttrium chromite, heavy rare earth chromites, solid solutions of yttrium chromite, rare earth manganites doped with alkaline earth metals, charge-ordered rare earth ferrites and indium manganite are outlined. Experimental aspects of the work carried out are discussed in section 3. It gives details of the experimental set up and the basic operation principles of various structural and physical characterizations of the materials prepared. In section 4, results of the investigations are discussed. Magnetic and di-electric properties of yttrium chromite (YCrO3), heavy rare earth chromites and YCr1-xMnxO3 are reported in section 4.1. These materials show canted antiferro-magnetic behavior below the Nel transition temperatures and dielectric transitions at high temperatures. Role of local non-centrosymmetry is discussed based on high-resolution neutron powder diffraction data. In 4.2 we discuss the results of charge-ordered rare earth ferrites which show good magnetoelectric effect. Magnetic, dielectric and magneto-dielectric properties of YCr1-xMnxO3 (Ln = rare earth) are discussed in 4.3. These materials show magneto-dielectric effect. In 4.4 we discuss the near normal incidence far infrared reflectivity spectra of a single crystal of TbMnO3, in the spectral range of 50cm−1 to 700 cm−1 from 10 K to 300 K. Finally in 4.5, magnetic and dielectric properties of bulk and thin films of indium manganite are discussed.