Dissertations / Theses on the topic 'Multiferroic Materials'

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.

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

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.

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.

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.

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.

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.

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.

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 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

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

Flexible electronics have garnered much interest over the past several decades. Hybrid organic-inorganic materials, such as metal-organic frameworks, offer a unique opportunity to encompass the effective electronic properties of the inorganic material and the flexible nature of the organic with the potential of enhancing other desirable properties, such as the contributing multiferroicity. Using a first principles approach, the goal of this thesis is to serve as a guide for identifying potential ferroelectric and multiferroic metal-organic frameworks. This is done through a screening method of metal-organic frameworks based on their geometry; certain symmetry operators cannot be present in a ferroelectric material. We report the theoretical spontaneous polarization for several dozens of MOFs in which ferroelectricity has not previously been tested, and further we discuss the likelihood that these materials could be engineered to have either increased polarization or added ferromagnetism, the latter of which would lead to multiferroicity.

Magnetic and ferroelectric materials have both had a very important impact in our society, not only because of the fascinating science behind the two phenomena, but also as a result of their use in many technological applications. The coupling and coexistence of these two order parameters within the same material opens the door to exiting new functional devices. Materials where magnetism and ferroelectricity coexist are known as multiferroic materials. In this thesis, muon spectroscopy and other complementary experimental techniques, including neutron scattering and resonant ultrasound spectroscopy, are used to investigate two di↵erent multiferroics. Muon and total neutron scattering studies have been performed on BiFeO3, one of the most studied multiferroic materials. Muon measurements reveal an anomaly in the temperature region of 200 - 220 K with a sudden and abrupt change in the muon’s precession frequency that corresponds to a process of muon di↵usion throughout the entire sample. The pair distribution function, calculated from total neutron scattering experiments on the compound, suggest that a change in the local structure of the material involving the bismuth-oxygen bond, in the same temperature region as the muon di↵usion sets in, is a strong indicative that there is a link between two in terms of the muon di↵usion being triggered by these local changes. Also, an extensive analysis and characterisation of the magnetic and ferroelectric properties of Ba4Dy0.87Nb10O30, an entirely new tetragonal tungsten bronze magnetoelectric material, is given. Neutron scattering and dielectric measurements are used to show that this material becomes ferroelectric below 470 K. We use muon spectroscopy and magnetic susceptibility measurements to investigate the magnetic properties of the material. Muon measurements under an applied electric field indicate that there is a strong coupling between the magnetism and ferroelectricity in the material. Resonant ultrasound spectroscopy is use to investigate whether the source of this coupling could be related to strain e↵ects. Magnetic neutron scattering measurements show that there is no long range ordering in the material.

Multiferroics are currently intensely investigated because the coexistence and coupling of ferroic arrangements brings about new physical effects and, for the few room-temperature examples, interesting prospects for applications in various fields. This interest is illustrated by the recent publication of several articles on multiferroics in high impact reviews over the last five years. The main goal of the thesis was to look for new multiferroics by exploiting overlooked and original polar and magnetic arrangements. We more precisely investigated compounds based on lead iron tungsten PbFe2/3W1/3O3 (PFW) and lead zirconate PbZrO3 (PZO) oxides. PFW displays long- and short-range both polar and magnetic orders (ferroelectric-relaxor and antiferromagnetic-spin-glass) while PZO is antiferroelectric with antiferrodistorsivity (oxygen tilts) and existence of ferroelectric instabilities. Combining various techniques from synthesis to electric, magnetic and structural characterizations, we demonstrated that it is possible to get a multiferroic compound (50%PFW-50%PZO) with coexistence of multiple ferroic and structural arrangements with room temperature properties of practical interest. This work opens new prospects in this rich field of multiferroics in peculiar by using antiferroelectrics.

This thesis aims to propose the feasible approaches to control and manipulate the magnetism in two-dimensional (2D) magnets through magnetoelectric coupling by using density functional theory (DFT) calculations. It is found that the goal can be achieved by building 2D magnetic-ferroelectric heterostructures or seeking for the multiferroic candidates. The effective manipulations of magnetic states are expected to not only provide new insights into the fundamental research, but also pave the solid foundations for the spintronic applications.

Low-dimensional materials with novel electronic and ferroic properties play a crucial role in the development of next-generation nanoscale functional devices and have drawn extensive research attentions. By using state-of-art first-principles calculations, Monte Carlo simulations, and structural revolution algorithms, four new low-dimensional materials were predicted showing promising applications in electronics or spintronics and two strategies were proposed to introduce ferroelectricity and realize strong electromagnetic coupling in experimentally fabricated low-dimensional materials, which may accelerate the discovery and design of novel functional low-dimensional materials.

This thesis concerns and combines the results of experimental studies of magnetocaloric, multiferroic and spin-glass materials, using SQUID magnetometry as the main characteriza-tion technique.  The magnetocaloric effect offers an interesting new technology for cooling and heating applications. The studies of magnetocaloric materials in this thesis are focused on experimen-tal characterization of fundamental magnetic properties of Fe2P-based materials. These are promising magnetocaloric materials with potential industrial use. It is found that the magneto-caloric properties of Fe2P can be optimally tuned by substitution of manganese for iron and silicon for phosphorus. Furthermore, a simple device to measure the magnetocaloric effect in terms of the adiabatic temperature change was constructed.  Materials that simultaneously exhibit different types of ferroic order, for example magnetic and electrical order, are rare in nature. Among these multiferroic materials, those in which the ferroelectricity is magnetically-induced, or vice versa the magnetism is electrically-induced, are intensively studied due to a need for new functionalities in future data storage and logic devices. This thesis presents results on two materials: Co3TeO6 and Ba3NbFe3Si2O14, which belong to the group of magnetically-induced ferroelectrics and exhibit strong coupling be-tween the magnetic and the electrical order parameter. Their ordering properties were studied using magnetic and electrical measurement techniques. The coupling between the magnetic and electronic degrees of freedom was investigated using high-field and low-temperature Raman spectroscopy.  Spin-glass materials exhibit complex magnetism and disorder. The influence of the spin dimensionality on the low and high magnetic field properties of spin glasses was investigated by studying model Heisenberg, XY and Ising spin-glass systems. Significant differences were found between the non-equilibrium dynamics and the hysteresis behavior of Heisenberg systems compared to those of XY and Ising spin glasses.

With the overriding goal of developing functional multiferroic systems with technological potential, this thesis focuses on the role of orbital and charge ordering in coupling magnetism and ferroelectricity in synthetic quadruple perovskites. Using x-ray diffraction as the primary characterisation tool, modulations to crystal ordering have been interpreted in terms of orbital occupation and charge variation. Expanding on previous magnetic structure studies and polarisation measurements, structural analysis of CaMn₇O₁₂ has led to the experimental realisation of a new mechanism for multiferroicity, resulting from a "magneto-orbital helix". Motivated by the idea of tuning multiferroic properties through varying manganese valence, the doped system CaCu_xMn_7-xO₁₂ has been studied. Structural models considering the possibility of domain formation and multiple coexisting modulations have been tested against x-ray diffraction data. Finally, motivated by theoretical predictions of ferroelectric phases and multiferroicity in doped, simple, manganite perovskites, a structural model for the low temperature phase of NaMn₇O₁₂ has been developed, based upon theoretical predictions for orbital ordering and the experimentally determined magnetic structure. This model has been tested against previously measured neutron diffraction data. The importance of understanding crystal formation and domain structures when applying theoretical models has been highlighted, and has prompted the consideration of future work involving viewing and manipulating twin formation.

This essay is intended to be a review of the entire resear h work developed during my PhD s hool, highly based on published papers and enri hed by ba kground introdu tions of the debated topi s. My resear h fo used on several distin t topi s sharing the use of ab-initio density fun tional theory al ulations for studying di erent types of materials aimed to te h- nologi al appli ations. First, it deals with the study of the tunneling magneto-resistan e and interfa e magneto- ele tri e e ts of the SrRuO3/PbTiO3/SrRuO3 multi-ferroi tunnel jun tion (MFTJ) with asymmetri interfa es, by using standard DFT approa hes. A work about V doping of PbTiO3, a proper multiferroi ompound, is also presented. Se ond, an on-going work about a layered perovskite Bi5Ti5O17 is presented. We predi t it as the rst ferroele tri metal: it has a non-zero density of states at the Fermi level, metal- like ondu tivity along with spontaneous polarization in zero eld and a depolarizing eld in a nite system. Third, it reports the investigation of the orrelated material CaCu2Ti4O12 by using dif- ferent density fun tional methods, where the pSIC method, developed by my resear h group, manages to give a orre t interpretation of the four opti al transitions assessed experimen- tally. Fouth, a study about the wide-gap semi ondu tor Ga2O3 is presented. By means of beyond-DFT approa hes su h as Hybrid fun tionals, the famous GW method and the pSIC method, we assess the anisotropy of opti al absorption in phase of Ga2O3, supporting the experimental spe tra. In addition, it presents a further DFT-GGA investigation of the doping of −Ga2O3 with In. We assess its ele troni and strutural properties, maximum In solubility, and the band alignment of interfa e between -Ga2O3 and (Ga1−xInx)2O3. Finally, it reports my ontribution to enhan e the development of the pSIC ode, providing it with two new tools, namely two routines for al ulating the diele tri fun tion and the X-ray absorption ross se tion, whi h aims to obtain a more dire t omparison between theoreti al band stru ture and experimental measure of opti al properties. The ode to al ulate the diele tri fun tion has been used in the gap hara terization of −Ga2O3. Some test appli ations are shown, where XAS routine, implemented to exploit pSIC method, allows us to a urately reprodu e experimental spetra of di erent types of materials.

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.

Multiferroic materials combine two or more ferroic orders such as ferroelectricity, ferromagnetism and ferroelasticity. They have a great interest for the development of the next generations of digital electronic memory devices. Bismuth ferrite has shown to posess both ferroelectric and magnetic order at room temperature, which makes it both scientifically and comercially interesting.The negative impact on the environment by using lead in mass-produced units has made development of lead-free piezoelectric materials for use in sensors, actuators and transducers prioritized and samarium substituted bismuth ferrite has shown to posess a piezoelectric response high enough to be considered for commercial applications. In this project work, 10mol% samarium substituted bismuth ferrite, Bi_{0.9}Sm_{0.1}FeO_{3}, powders have been synthesized using a wet-chemical method and calcined to a range of temperatures to achieve a range of nanocrystalline sizes in the powders. These powders has been investigated using X-ray diffraction (XRD), Rietveld refinement and differential scanning calorimetry (DSC). The powders have also been milled, pressed and sintered to pellets for ferroelectric and piezoelectric testing. The crystal structure was found to belong to the R3c group as for bismuth ferrite, BiFeO_{3}, but the unit cell geometry is changed slightly, and the ferroelectric polarizability is lowered in the case of samarium substitution. This fact is interesting regarding piezoelectricity by thereby increasing the ability to achieve a higher piezoelectric response. No results were achieved from ferroelectric and piezoelectric measurement due to the conductivity found in the sintered pellets were too high, probably due to secondary sillenite phases found in the material. There has been found a finite size effect of a decrease in unit cell volume and tetragonality, of which the decrease in volume is contradictory to the expectations. There is also clear finite size effects on the decrease of cooperative displacement of cations, a measure for the polarization, and for the lowering of the ferroelectric phase transition temperature. The finite size effect for the ferroelectric phase transition has not been reported before. The magnetic phase transition temperature also show a decrease as a clear finite size effect. A linear relationship between electric polarization and magnetic ordering has also been observed in this project. The results from this project contributes to the understanding of material properties of BiFeO_{3} and derived compositions by an investigation of substitution effects and finite size effects. The results presented here motivates further research on these materials.

Multiferroic materials exhibit unique properties such as simultaneous existence of two or more of coupled ferroic order parameters (ferromagnetism, ferroelectricity, ferroelasticity or their anti-ferroic counterparts) in a single material. Recent years have seen a huge research interest in multiferroic materials for their potential application as high density non-volatile memory devices. However, the scarcity of these materials in single phase and the weak coupling of their ferroic components have directed the research towards multiferroic heterostructures. These systems operate by coupling the magnetic and electric properties of two materials, generally a ferromagnetic material and a ferroelectric material via strain. In this work, horizontal heterostructures of composite multiferroic materials were grown and characterized using pulsed laser ablation technique. Alternate magnetic and ferroelectric layers of cobalt ferrite and lead zirconium titanate, respectively, were fabricated and the coupling effect was studied by X-ray stress analysis. It was observed that the interfacial stress played an important role in the coupling effect between the phases. Doped zinc oxide (ZnO) heterostructures were also studied where the ferromagnetic phase was a layer of manganese doped ZnO and the ferroelectric phase was a layer of vanadium doped ZnO. For the first time, a clear evidence of possible room temperature magneto-elastic coupling was observed in these heterostructures. This work provides new insight into the stress mediated coupling mechanisms in composite multiferroics.

L'avenir de la spintronique repose sur le développement de matériaux ayant des propriétés magnétiques remarquables. L'objectif de cette thèse est de comprendre la physique des deux matériaux fonctionnels proposés pour des applications spintroniques qui utilisent des simulations de la densité fonctionnelle.Nous nous sommes intéressés dans une première partie au ferrite de gallium pour lequel il a été montré que les propriétés dépendaient de la concentration de fer.Les spectres optiques ont été calculés et comparés aux spectres expérimentaux suggérant des niveaux élevés de désordre. Dans la deuxième partie, nous avons montré une polarisation de spin à l’interface hybride formée entre la phthalocyanine de manganèse et la surface de cobalt,en accord avec les expériences de photoémission.La formation de la spinterface a été expliquée par différents mécanismes d'hybridation dans chaque canal de spin.Cette polarisation de spin est coordonnée avec des moments magnétiques induits sur les sites moléculaires
The future of the spintronics technology requires developing functional materials with remarkable magnetic properties. The aim of this thesis is to understand the physics of functional materials proposed for spintronic applications using ab-initio density functional simulations. We investigated the properties of two different functional materials. We first studied the magnetoelectric gallium ferrite GFO. The dependence of the different properties on the iron concentration has been demonstrated and discussed. The optical spectra were calculated and compared to the experimental once suggesting high levels of iron disorder. In the second part, we demonstrated a highly spin polarized hybrid interface formed between manganese phthalocyanine and cobalt surface in agreement with photoemission experiments. The formation of this spinterface was described by different hybridization mechanisms in each spin channel. This high spin polarization is coordinated with induced magnetic moments on the molecular sites

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.

The integration of sensors, actuators, and control systems is an ongoing process in a wide range of applications covering automotive, medical, military, and consumer electronic markets. Four major families of ceramic and metallic actuators are under development: piezoelectrics, electrostrictors, magnetostrictors, and shape-memory alloys. All of these materials undergo at least two phase transformations with coupled thermodynamic order parameters. These transformations lead to complex domain wall behaviors, which are driven by electric fields (ferroelectrics), magnetic fields (ferromagnetics), or mechanical stress (ferroelastics) as they transform from nonferroic to ferroic states, contributing to the sensing and actuating capabilities. This research focuses on two multiferroic crystals, Pb(Mg1/3Nb2/3)O3-PbTiO3 and Fe-Ga, which are characterized by the co-existence and coupling of ferroelectric polarization and ferroelastic strain, or ferro-magnetization and ferroelastic strain. These materials break the conventional boundary between piezoelectric and electrostrictors, or magnetostrictors and shape-memory alloys. Upon applying field or in a poled condition, they yield not only a large strain but also a large strain over field ratio, which is desired and much benefits for advanced actuator and sensor applications. In this thesis, particular attention has been given to understand the structure-property relationships of these two types of materials from atomic to the nano/macro scale. X-ray and neutron diffraction were used to obtain the lattice structure and phase transformation characteristics. Piezoresponse and magnetic force microscopy were performed to establish the dependence of domain configurations on composition, thermal history and applied fields. It has been found that polar nano regions (PNRs) make significant contributions to the enhanced electromechanical properties of PMN-x%PT crystals via assisting intermediate phase transformation. With increasing PT concentration, an evolution of PNRï  PND (polar nano domains)-> micron-domains-> macro-domains was found. In addition, a domain hierarchy was observed for the compositions near a morphotropic phase boundary (MPB) on various length scales ranging from nanometer to millimeter. The existence of a domain hierarchy down to the nm scale fulfills the requirement of low domain wall energy, which is necessary for polarization rotation. Thus, upon applying an E-field along <001> direction(s) in a composition near the MPB, low symmetry phase transitions (monoclinic or orthorhombic) can easily be induced. For PMN-30%PT, a complete E-T (electric field vs temperature) diagram has been established. As for Fe-x at.% Ga alloys, short-range Ga-pairs serve as both magnetic and magnetoelastic defects, coupling magnetic domains with bulk elastic strain, and contributing to enhanced magnetostriction. Such short-range ordering was evidenced by a clear 2theta peak broadening on neutron scattering profiles near A2-DO3 phase boundary. In addition, a strong degree of preferred [100] orientation was found in the magnetic domains of Fe-12 at.%Ga and Fe-20 at.%Ga alloys with the A2 or A2+DO3 structures, which clearly indicates a deviation from cubic symmetry; however, no domain alignment was found in Fe-25 at.%Ga with the DO3 structure. Furthermore, an increasing degree of domain fluctuations was found during magnetization rotation, which may be related to short-range Ga-pairs cluster with a large local anisotropy constant, due to a lower-symmetry structure.
Ph. D.

Doutoramento em Engenharia Física
O presente trabalho de doutoramento é um estudo de propriedades físicas e aspectos estruturais de filmes de óxidos e heteroestruturas multiferróicas, englobando técnicas de caracterização do nível macroscópico ao microscópico. O objectivo principal é a compreensão de novas heteroestruturas epitaxiais multifuncionais e as suas interfaces para junções de túnel magnetoelétricas e filtros de spin. Os principais materiais em estudo foram manganitas à base de La dopadas com iões divalentes (ba, Sr), apresentando efeito magnetoelétrico, sendo preparadas em diferentes substratos e diferentes técnicas de crescimento, optimizadas para epitaxia e qualidade de interface. O estudo combinado de propriedades eléctricas e magnéticas permitiu estabelecer as condições necessárias para a aplicação dos materiais multiferróicos em estudo, por técnicas experimentais apresentadas neste trabalho. O trabalho consistiu no estudo sistemático de microestrutura de filmes finos de La0:7Sr0:3MnO3 em substratos de SrTiO3, preparados por pulsed laser deposition, o filme fino de La0:9Ba0:1MnO3 e a heteroestrutura La0:9Ba0:1MnO3/BaTiO3/La0:9Ba0:1MnO3 em substrato de Al2O3, e filme fino de La0:9Ba0:1MnO3, BaTiO3 e heteroestrutura de La0:9Ba0:1MnO3/BaTiO3/La0:9Ba0:1MnO3 em substrato de Si, preparado por RF magnetron sputtering. A caracterização estrutural das amostras foi feita principalmente por difracção de raio-X (XRD) convencional e de alta resolução e Microscopia de Transmissão de Alta Resolução (HRTEM). A composição química foi analisada por Electron Dispersion Spectroscopy (EDS), Rutherford backscattering spectroscopy (RBS) e energy filtered transmission electron microscopy (EFTEM). As medidas de magnetização forram realizada com a um magnetómetro superconducting quantum interference device (SQUID). A análise da topografia e efeitos locais foi realizada por microscopia de varimento de ponta usando microscopia da Força Atómica (AFM) e de resposta piezoeléctronica (PFM). Os resultados mostram claramente uma evolução da microestrutura dos filmes finos de La0:7Sr0:3MnO3, á medida que aumenta a sua espessura, passando de uma estrutura policristalina no filme mais fino (13.5 nm) a colunar inclinado (45 nm e 200 nm), a uma estrutura ramificada no filme mais espesso (320 nm). A alteração na estrutura do filme é devida à tensão pelo substrato e deformação da estrutura nas etapas iniciais de crescimento, onde se detectaram fronteiras anti-phase e maclas. A evolução da estrutura modificou as propriedades magnéticas dos filmes a baixa temperatura (abaixo da temperatura de transição estrutural do substrato de SrTiO3), mostrando magnetização em excesso e defeito, para espessuras abaixo e acima de 100 nm, respectivamente. Análises STEM-EELS e EFTEM mostraram a diferença em composição elementar dos filmes perto das fronteiras e na interface com o substrato.No âmbito do plano de trabalhos de doutoramento, o segundo substrato consiste em estudar as propriedades físicas e estruturais de filmes finos de La0:9Ba0:1MnO3 e heteroestruturas La0:9Ba0:1MnO3/BaTiO3/La0:9Ba0:1MnO3 em substratos de Al2O3, revelando estruturas altamente orientadas. A razão La/Ba do filme e heteroestrutura é drasticamente diferente do alvo providenciado, La0:7Ba0:3MnO3, como provado por XRD, RBS e transições de fase magnéticas. As propriedades magnéticas e eléctricas das estruturas mostraram uma forte dependência na cristalinidade do filme e da heteroestrutura. A parte final do trabalho é dedicada aos filmes de La0:9Ba0:1MnO3, BaTiO3 e a heteroestrutura de La0:9Ba0:1MnO3/BaTiO3/La0:9Ba0:1MnO3 em substrato de Si, que em comparação com as estruturas em substrato de ALO, provaram o efeito da cristalinidade nas propriedades magnéticas, eléctricas e de magneto-resistência do filme e heteroestrutura. Foi mostrado que um grau superior de cristalinidade leva a uma mais elevada magnetização, reduzindo a resistividade das estruturas. Pela primeira vez, um estudo de deformação de topografia por aplicação de uma tensão dc externa foi feito num filme fino de BaTiO3 em Si, usando uma técnica de poling num microscópio de força piezoresponse. Os resultados mostraram a capacidade de uma modificação controlada da superfície, por aplicação de uma voltagem externa nointervalo 14V < Vapp < 20V. Abaixo destes valores, não se observou alguma deformação na topografia, enquanto acima deste intervalo, a 30V, a superfície foi completamente danificada. A mudança topográfica produzida mostrou estabilidade no tempo, onde após a aplicação de 20V, a área modificada alcançou 83% da altura as-poled ( 9 nm) em 90 minutos, a 7,4 nm. A resposta assimétrica de piezoresponse da área poled foi associada à existência de um campo eléctrico interno na amostra, que foi também provado através de medidas de espectroscopia de switching no filme fino. A heteroestrutura no substrato de Si mostraram o mesmo fenómeno que a mono-camada de BaTiO3, onde o arranjo de heteroestrutura realça o efeito de voltagem aplicada na topografia. Aplicando 10V, a estrutura da superfície foi alterada na heteroestrutura e houve uma modificação visível da camada de BaTiO3, alterando também a topografia da camada superior de La0:9Ba0:1MnO3.
This present PhD work made a study of structural aspects and physical properties of the oxide films and multiferroic heterostructures, encompassing the techniques from macroscopic level to microscopic description. The understanding of novel multifunctional epitaxial heterostructures and their interfaces for magneto-electrically driven tunnel junctions and spin-filters is the central objective. The main materials in study were La based doped manganites with magnetoelectric effect prepared on different substrates and growth conditions, optimized for epitaxy and interface quality. The combined study of electric and magnetic properties allowed us examining the conditions required for application of the studied multiferroic materials and experimental techniques are presented in this work. The work consists of three main substrates, a systematic study of microstructure of La0:7Sr0:3MnO3 thin films on SrTiO3 substrate, prepared by pulsed laser deposition, the La0:9Ba0:1MnO3 thin film and La0:9Ba0:1MnO3/BaTiO3/La0:9Ba0:1MnO3 heterostructure on Al2O3 substrate, and the La0:9Ba0:1MnO3 thin film, BaTiO3 and La0:9Ba0:1MnO3/BaTiO3/La0:9Ba0:1MnO3 heterostructure on Si substrate, prepared by RF magnetron sputtering. Main structural characterization of samples was performed by conventional and high resolution X-Ray Diffraction (XRD), High Resolution Transmission Electron Microscopy (HRTEM); chemical composition was determined by Electron Dispersion Spectroscopy (EDS), Rutherford Backscattering Spectroscopy (RBS) and Energy Filtered Transmission Electron Microscopy (EFTEM); Magnetization measurements done with a Superconducting Quantum Interface Device (SQUID) magnetometer. Surface probing of topography and local effects was performed, using Atomic Force (AFM) and Piezo-Response (PFM) Microscopy. Results clearly showed that there is an evolution in the microstructure of the La0:7Sr0:3MnO3 thin films, by increasing their thickness, changing from polycrystalline structure in the thinnest film (13.5 nm) to tilted columnar structure(45 nm and 200 nm) and to a branched structure in the thickest film (320 nm). The change in the structure of the film is due to the strain from the substrate and deformation of the structure in the early stages of the growth, where anti-phase boundaries and twinning were detected. The evolution of the structure modified the low temperature (below structural phase transition of SrTiO3 substrate) magnetic properties of the films, showing in-excess and in-defect magnetization, below and above 100 nm thickness, respectively. Also, STEM-EELS and EFTEM analysis showed the difference in the elemental composition of the films near the boundaries and interface with the substrate.In the scope of the PhD work plan, the second substrate consists of studying the structural and physical properties of La0:9Ba0:1MnO3 thin film and La0:9Ba0:1MnO3/BaTiO3/La0:9Ba0:1MnO3 heterostructure on Al2O3 substrate, where they showed highly oriented structure. The La/Ba ratio of the single layer film and heterostructure is drastically different from the target, La0:7Ba0.3MnO3, proven by XRD, RBS, and magnetic phase transitions. The magnetic and electrical properties of the structures showed strong dependence on the crystallinity of the samples. The final part of the work is devoted to the La0:9Ba0:1MnO3 and BaTiO3 thin films and La0:9Ba0:1MnO3/BaTiO3/La0:9Ba0:1MnO3 heterostructure on Si substrate, which in comparison with the structures on Al2O3 substrate, highlights the influence of crystallinity on magnetic, ferro-electrical and magnetoresistance properties of the film and heterostructure. It is shown that higher degree of crystallinity leads to higher magnetization and lowers the resistivity. For the first time, a study of the topography deformation by applying a dcexternal voltage was done on BaTiO3 thin film on Si, using a poling technique in a piezoresponse force microscope. The results show the ability of controlled modification of the surface, by applying an external voltage/electric field in the range of 14V< Vapp<20V. Below this range, no deformation is observed on the topography, and above this interval, at 30V, the surface is completely damaged. The produced topographical change show stabilization in respect to time, where after applying 20V, the modified area reaches its 83% of the as-poled height ( 9nm) in 90 minutes, to 7.4 nm. The asymmetrical response in the piezoresponse of the poled area is related to the existence of an internal built-in electric field in the sample, which is also confirmed by performing switching spectroscopy measurements on the single layer. The heterostructure on the Si substrate shows the same phenomena, as the BTO single layer, where the heterostructure arrangement enhances the applied voltage effect on the topography. With applying 10V, the structure of the surface changes in the heterostructure and a visible modification of BaTiO3 layer, changing also the topography of La0:9Ba0:1MnO3 top layer is observed.

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

Les matériaux caloriques à l'état solide, qui subissent un changement de température adiabatique ou un changement d'entropie isothermal lorsque certains stimuli externes (champ électrique, champ magnétique, contrainte ou pression mécanique) est appliquée ou retirée, sont prometteurs pour la réfrigération à l'état solide, comme alternative aux dispositifs de refroidissement conventionnels inventé il y a cent ans qui utilisent des gaz dangereux. Compte tenu des améliorations des systèmes de réfrigération à compression de vapeur approchant très vite de leur limite d'efficacité théorique, en plus des préoccupations environnementales accrues, il y a eu récemment une recrudescence de la recherche mondiale pour de nouvelles solutions de réfrigération plus économiques et respectueuses de l'environnement. Les caloriques les plus importants sont les matériaux "ferroiquement" ordonnés (ferroélectriques, ferroélastiques et ferromagnétique / antiferromagnétique) qui présentent souvent des effets caloriques géants près de leurs transitions ferroïques. Dans cette thèse, nous présentons nos résultats théoriques et expérimentaux sur l'effet électrocalorique, élastocalorique, barocalorique et magnétocalorique dans différents matériaux ferroïques. Nos résultats montrent que tous ces effets caloriques peuvent donner des solutions de réfrigération prometteuses avec un faible impact environnemental. Nous abordons les ferroélectriques qui apparaissent comme matériaux idéaux permettant à la fois des réponses électrocaloriques, élastocaloriques et barocaloriques géantes près de la température ambiante. Pour la première fois, nous mettons en évidence un effet électrocalorique négatif dans des films minces antiferroélectriques et nous proposons un nouveau mécanisme pour comprendre la réponse calorique dans antiferroiques en général incluant antiferroélectrique et antiferromagentique. Par ailleurs, pour la première fois en utilisant une caméra infra-rouge, nous effectuons la mesure résolue spatialement sur l'effet électrocalorique dans des condensateurs multicouches, l'un des systèmes les plus étudiés considérés comme le prototype électrocalorique le plus prometteur. Nos résultats fournissent la première preuve expérimentale directe sur le flux de chaleur électrocalorique à la fois temporellement et spatialement dans un dispositif électrocalorique spécifique. En outre, pour la première fois, nous concevons un cycle de réfrigération multicalorique combinant effet électrocalorique avec des effets élastocaloriques / magnétocaloriques via des matériaux ferroélectriques. Nous avons réalisé ce cycle mutlicalorique pour résoudre un problème réel et de longue date, à savoir une grande hystérésis magnétique qui a empêché l'utilisation pourtant prometteuse de FeRh découvert il y a près de 26 ans en tant que matériau magnétocalorique. Nous espérons que cette thèse fournira non seulement des connaissances utiles pour comprendre fondamentalement l'effet calorique à l'état solide dans les matériaux ferroïques et ce qui est véritablement mesuré, mais pourra aussi servir de guide pratique pour exploiter et développer les ferrocalorics vers la conception de dispositifs appropriés
Solid-state caloric materials, which undergo an adiabatic temperature change or isothermal entropy change when some external stimulus (electric field, magnetic field, stress and pressure) is applied or withdrawn, are promising for solid-state refrigeration, as an alternative to hazardous gases used in conventional cooling devices invented a hundred years ago. Given that the highly refined vapor-compression refrigeration systems asymptotically approach their theoretical efficiency limit in addition to the concern on environment, there has been a recent upsurge in worldwide search for new refrigeration solution which is economical and environmentally friendly. The most prominent calorics are ferroically ordered materials (ferroelectric, ferroelastic and ferromagnetic/antiferromagentic) that often exhibit giant caloric effects near their ferroic transitions. In this thesis, we present our theoretical and experimental results on electrocaloric effect, elastocaloric effect, barocaloric effect and magnetocaloric effect in different ferroic materials. Our findings show that all these caloric effects may appear promising with low environmental impact. We address ferroelectrics emerging as ideal materials which permit both giant elastocaloric, electrocaloric and barocaloric responses near room temperature. For the first time, we find a large negative electrocaloric effect in antiferroelectric thin films and we propose a new mechanism to understand the caloric response in antiferroics including antiferroelectric and antiferromagentic. In addition, for the first time using Infra-red camera we carry out spatially-resolved measurement on electrocaloric effect in multilayer capacitors, one of the most studied systems which are regarded as the most promising electrocaloric prototype. Our findings provide the first direct experimental evidence on the electrocaloric heat flux both temporally and spatially in a specific electrocaloric device. Moreover, for the first time, we design a multicaloric refrigeration cycle combining electrocaloric effect with elastocaloric/magentocaloric effects bridged by ferroelectric materials. We realized such mutlicaloric cycle to solve a real and longstanding problem, i.e., a large hysteresis that impeded reversibility in an otherwise promising magnetocaloric material FeRh discovered almost 26 years ago. We hope that this thesis will not only provide a useful background to fundamentally understand the solid-state caloric effect in ferroics and what we are really measuring, but also may act as a practical guide to exploit and develop ferrocalorics towards design of suitable devices

L'effet magnétoélectrique direct est défini par la modification de la polarisation électrique à partir de l'application d'un champ magnétique. Bien que cet effet existe de manière intrinsèque dans certains matériaux, nous étudions ici l'effet extrinsèque, où l'effet magnétoélectrique résulte d'un couplage intermédiaire entre deux phases distinctes. Dans ce cas, l'idée la plus répandue est de lier mécaniquement (par un collage) un matériau piézoélectrique à un matériau magnétostrictif. Ainsi, en appliquant un champ magnétique, le matériau magnétostrictif se déforme, transmet une contrainte au matériau piézoélectrique qui voit sa polarisation changer. Dans cette thèse nous nous intéressons à deux types de composites magnétoélectriques laminaires. Ceux employant les ferrites magnétostrictifs doux (ferrite de nickel) et ceux qui utilisent les ferrites semi-durs (ferrite de cobalt). Pour chacun des composites, on s'intéresse à optimiser l'effet magnétoélectrique en mettant en avant les paramètres qui l'influencent majoritairement. De ce fait, nous traitons différents aspects tels que l'influence de l'effet démagnétisant dans les multicouches, de la fraction volumique des composites, des phases secondaires, de la magnétostriction dynamique, de l'anisotropie magnétique uniaxe, et enfin de la fréquence et de l'amplitude du champ d'excitation magnétique sur l'effet magnétoélectrique. Grâce à la compréhension de ces phénomènes, nous avons pu optimiser le couplage magnétoélectrique de manière à développer un capteur de courant présentant des caractéristiques comparables aux capteurs de courant actuellement commercialisés et qui utilisent d'autres technologies (effet Hall, transformateur de courant)
The direct magnetoelectric effect is defined by the modification of the electric polarization induced by a magnetic field. Although this effect exists intrinsically in some materials, here we study the extrinsic effect, where the magnetoelectric effect results from an intermediate coupling between two distinct phases. In this case, the most common idea is to mechanically couple (by gluing) a piezoelectric material to a magnetostrictive material. Thus, by applying a magnetic field, the magnetostrictive material is deformed and transmits a stress to the piezoelectric material which makes its polarization change.In this thesis, we are interested in two types of laminar magnetoelectric composites: those using soft magnetostrictive ferrites (nickel ferrite) and those using semi-hard ferrites (cobalt ferrite). For each composites, we want to optimize the magnetoelectric effect by highlighting the parameters that mainly influence this coupling. As a result, we deal with different aspects such as the influence of the demagnetizing effect in multilayers, the volume fraction in the composites, the secondary phases, the dynamic magnetostriction, the uniaxial magnetic anisotropy, and finally the frequency and the amplitude of the magnetic exciting field on the magnetoelectric effect. Thanks to the understanding of the physical phenomena involved and the optimization of the resulting magnetoelectric coupling, we have been able to develop a current sensor with characteristics comparable to currently marketed current sensors that use other technologies (Hall effect, current transformer)

Multiferroics (MF) have attracted much research attention due to the coexistence of ferroelectric and magnetic ordering as well as magnetoelectric (ME) coupling. At present there are very few room temperature single phase MF except BiFeO3. Multiferroic properties of Aurivillius compound Bi5FeTi3O15 were reported at 80 K. The at 80 K. The at 80 K. The at 80 K. The at 80 K. The magnetization of Bi5FeTi3O15 was significantly improved by substituting parts of Fe cations by Co cations. Bi5FeTi3O15 showed ferromagnetic order above room temperature. The magnetic cations Fe/Co in B-site contribute to the both ferroelectric and ferromagnetic properties, which could possibly induce strong magnetoelectric effect. Aurivillius materials are layered structured materials with formula (Bi (Bi2O2)2+ (A m-1BmO3m+1 )2-. The polarization of Aurivillius materials is mainly in a-b plane . High dense and textured ceramics were fabricated by a two-step spark plasma sintering (SPS) method to improve the polarization of ceramics. The multiferroic properties of of Aurivillius materials with different octahedral layers (m=2, 3, 4 and 5) were investigated. All these materials showed ferroelectric and ferromagnetic order at room temperature except Bi 3Nb 1.1251.1251.1251.1251.125Fe 0.1250.1250.1250.1250.125Co 0.1250.1250.1250.1250.125Ti 0.750.750.750.75O9 (m = 2). (m = 2). (m = 2). (m = 2). (m = 2). Bi 3.253.253.253.25La 0.750.750.750.75Nb 0.250.250.250.25Fe 0.1250.1250.1250.1250.125Co 0.1250.1250.1250.1250.125Ti 2.52.52.5O12 (m = 3) was identified to be single phase. (m = 3) was identified to be single phase. Although a small amount of secondary phase (CoFe (CoFe2O4/Co /Co2FeOFeOFeO4) were found in Bi 4.254.254.254.25La 0.750.750.750.75Fe 0.50.50.5Co 0.50.50.5Ti 3O15 (m = 4) and Bi 5.25.25.25La 0.750.750.750.75FeCoTiFeCoTiFeCoTiFeCoTi FeCoTi3O18 (m = 5), ), the intrinsic multiferroicity of the main Aurivillius phase was confirmed by the magnetic controlled ferroelectric domain switching. Clear ME couplings were observed in these materials.

Ce travail de thèse est principalement axé sur le développement d’une nouvelle méthode de manipulation de vortex d’Abrikosov individuels dans les supraconducteurs de type II. Cette méthode, rapide, efficace et précise, est basée sur l’optique en champ lointain et repose sur l’échauffement local du supraconducteur sous l’action d’un faisceau laser focalisé. Elle apporte une excellente alternative aux techniques existantes de manipulation de vortex, toutes basées sur l’utilisation de sondes locales, et donc intrinsèquement lentes et difficiles à mettre en oeuvre dans un environnement cryogénique. La combinaison de cette méthode à une technique d’imagerie magnéto-optique performante permet de déplacer des vortex individuels avec un taux de réussite de 100% et sur de grandes échelles limitées uniquement par le champ de l’objectif de microscope. Les vitesses de manipulation atteintes sont élevées, de l’ordre de 10 mm.s-1, mais encore limitées par l’instrumentation utilisée et loin des limites fondamentales offertes par cette méthode, estimées au km.s-1. La méthode de manipulation optique permet aussi de mesurer la distribution des forces de piégeage de chaque vortex d’un échantillon. En utilisant des puissances de chauffage laser permettant de dépasser localement la température critique, nous avons également pu étudier la pénétration des vortex à l’interface entre une zone normale et une zone supraconductrice.Durant ces travaux, nous avons aussi eu l’opportunité de mettre en évidence, par spectroscopie de molécules uniques, l’effet flexomagnétoélectrique dans un matériau multiferroïque, en employant un supraconducteur de type I comme générateur de champ magnétique inhomogène. Enfin, nous proposons à la fin de ce mémoire un concept de jonction Josephson créée tout optiquement, et dont les propriétés seraient contrôlables en temps réel par laser
This thesis focuses on the development of a new manipulation technique to handle single Abrikosov vortices in type II superconductors. This fast, efficient and precise method is based on far field optics and rests on the local temperature elevation produced by a focused laser beam. It brings an excellent alternative to the existing techniques which are all based on local probes and thus heavy to implement in a cryogenic environment. The combination of this method with an efficient magneto-optical imaging system allows us to manipulate single vortices with a 100% rate of success on a large scale only limited by the field of view of the microscope objective. Manipulation speeds are high, of the order of 10 mm.s-1, but still limited by our setup and far from the fundamental limits offered by this technique, estimated to the km.s-1. This manipulation technique also allows to measure the pinning force of any single vortex in a superconducting sample. By using a high enough laser power which locally pushes the temperature above the critical temperature, we could also study the vortex penetration at the interface between normal and superconducting areas.In the course of this work, we also evidenced, with single molecule spectroscopy, the flexomagnetoelectric effect in a multiferoic material, by using a type I superconductor as a source of inhomogeneous magnetic field. Finally, we propose at the end of the manuscript the new concept of an optically created Josephson junctions, whose properties could be controlled in real time just with a laser beam

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.

In this thesis multiferroic materials are investigated through a number of different experimental techniques, particularly p,+SR, neutron scattering and impedance spec- troscopy. The magnetic and dielectric properties of materials, which form multiferroic states through different mechanisms, are explored. Inelastic neutron scattering studies have been made of LuMn03 and LuFe204. Large reciprocal space maps have been made of LuFe204 and an initial spin wave dispersion found. The exchange parameters have been estimated from the data and are shown to support ab initio calculations. The spin wave dispersion of LuMn03 has been mapped throughout the Brillouin zone and the exchange parameters extracted by comparison with a spin-wave model. Com- parisons have then been made with other members of the hexagonal manganite family. The analysis is supported by thermal expansion and magnetization studies. The first experiment on a multiferroic combining the p,+SR technique and applied electric fields was performed as a proof of principle experiment on HoMn03. The effect of the elec- tric field on the magnetism has been detected and calculations performed to try and identify muon stopping sites. The family of rare earth chromites have been studied through magnetization and heat capacity experiments as well as p,+SR. The vary- ing entropy and energy level splitting within the family is examined and the debate as to whether they are multiferroic is addressed. An instrument to study dielectric properties at cryogenic temperatures through impedance spectroscopy was designed and constructed. Multiferroics investigated in this thesis using neutron scattering and p,+SR were then studied with this apparatus to look for coupling between magnetic and ferroelectric order parameters. A dielectric anomaly associated with the magnetic transition temperature was observed in LuFe204 which is evidence of the coupling between the order parameters.

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.

Les matériaux multiferroïques sont des matériaux dans lesquels des ordres magnétiques, électriques et élastiques peuvent coexister dans une même phase. Ces ordres peuvent être couplés entre eux et l’étude de ces couplages permet de mieux comprendre les mécanismes à l’œuvre dans ces matériaux. Cette thèse porte sur l’étude de différents composés multiferroïques par spectroscopie Raman. Dans la ferrite de bismuth (BiFeO₃), l'effet de la contrainte sur le magnétisme, aussi bien sur les films minces (par contrainte épitaxiale) que le bulk (par pression hydrostatique) est étudié en détail. Cette thèse présente également une étude des excitations hybrides magnéto-électriques (électromagnons) dans les composés de type II à forte polarisation ferroélectrique comme CaMn₇O₁₂ et TbMnO₃. En outre, les modes de phonons ainsi que les excitations de basses énergies ont été étudiés (notamment sous champ magnétique) dans des composés au magnétisme frustré comme h-YMnO₃, h-YbMnO₃ et dans le langasite de fer au niobium
Multiferroics are materials in which magnetic, electric and elastic orders can coexist in the same phase. These orders can be coupled to each other and their study of high interest to understand the mecanisms at stake in the multiferroic materials. This PhD thesis has been focused on the study of several multiferroic compounds by the mean of Raman spectroscopy. In bismuth ferrite (BiFeO₃), the effect of strain on the magnetic order, both on thin films (epitaxial strain) and single crystals (hydrostatic pressure), has been thoroughly investigated. This thesis also focuses on the study of hybrid magneto-electric excitations (electromagnons) in type II multiferroic compounds with strong ferroelectric polarizations such as CaMn₇O₁₂ and TbMnO₃. Furthermore, phonons modes and of low energy excitations have been measured and studied (especially under magnetic field) in compounds with frustrated magnetic orders such as h-YMnO₃, h-YbMnO₃ and in the niobium iron langasite (Ba₃NbFe₃Si₂O₁₄)

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.

Dans cette thèse on étude les oxydes de manganèse NaMn7O12 et LaMn7O12, qu'appartiennent à la structure de pérovskite quadruple sous les points de vue de la multiferroïcité, du couplage magnétoélectrique, et aussi des anomalies structurelles dynamiques en NaMn7O12. Ces études concernent l'existence, les mécanismes d'établissement de la multiferroïcité et le couplage entre les ordres ferroïques en ces matériaux. Les forces motrices des anomalies structurels dynamiques en NaMn7O12 sont aussi étudiés au sens de clarifier les mécanismes de déformation structurel et quel est son importance ao établissement des ordres ferroïques. Manipulations de IXS et DXS révèlent anomalies dynamiques dans la transition structurel du NaMn7O12. Un phonon amollit avec le même vecteur d’onde de la modulation structurel sont révélé lié à pics diffuses de rayons-x qui ont un vecteur d’onde diffèrent, indiquant une compétition entre différents distorsions et que la distorsion engendré par distorsions du type Jahn-Teller est finalement dominant. Une transition ferroélectrique a été trouvé simultanément à anomalies dans la chaleur spécifique et dans la magnétisation du NaMn7O12. Les courbes d’hystérésis magnétique sont bien plus larges dans la phase ferroélectrique. Ces propriétés sont expliquées dans le cadre d’un modelé de ordonnément de frontières de domaine antiferromagnétique. Au LaMn7O12, une transition ferroélectrique a été trouvé à la température de transition antiferromagnétique du site B. Malgré les péculiarité de chaque système, ses propriétés structurels et magnétiques permettent d’affirmer qui sa ferroélectricité est d’origine magnétique travers le mécanisme de striction d’échange
In this work we study the quadruple perovskite structure manganese oxides NaMn7O12 and LaMn7O12 under the point of view of multiferroicity, magnetoelectric coupling, and also structure anomalies on NaMn7O12. These studies range from the existence to the mechanism of the establishment of multiferroicity and coupling between ferroic orders in these materials. The driving force of the dynamic structural anomalies in NaMn7O12 are also studied with the goal of clarifying the mechanisms of structural distortion and how they relate to the onset of ferroic orders. The results of IXS and DXS experiments show interesting dynamical anomalies in NaMn7O12's structural transition, a softened phonon at the structural modulation wavevector that was shown to be related to diffuse x-ray peaks with a different wavevector point in the direction of competing distortions and that the Jahn-Teller mechanism driven distortion overcome its competitor. A ferroelectric transition was found simultaneous to anomalies in specific heat and magnetization in NaMn7O12. Magnetic hysteresis loops are quite wider in the ferroelectric phase. These properties are explained in function of a magnetoelectricity mediated antiferromagnetic domain wall ordering model. As for LaMn7O12, a ferroelectric transition at the B site antiferromagnetic transition temperature. Besides the particularities of each system, structural and magnetic properties imply that their ferroelectricity is of magnetic origin through the exchange striction mechanism

The structure and magnetism of selected compounds of the pnictides iron based superconductors with chemical formula LnO{1-x}FeAsFx (Ln = La,Sm and Ce), commonly known as 1111, and of rare earth iron borates RFe3(BO3)4 (R = Tb, Gd, Nd and Y), were studied by means of hard x-ray diffraction. For the 1111 pnictides compounds, Rietveld refinement of powder x-ray diffraction measurements at room temperature reveals, that the ionic substitution of O by F has no effect on the structure of the FeAs layers of tetrahedra, whereas the major changes takes place in the LnO layer. These changes are reflected as a shrinkage of the crystal lattice, specially in the c direction. Additionally, a study of the temperature dependent structure of the Sm and Ce-1111 compounds was performed and an estimation of the the structural transition temperature was obtained. The results of the structural measurements, combined with electrical resistivity and µSR, were used to construct the Sm and Ce-1111 phase diagrams. These phase diagrams are characterized by two regions, consisting of a spin density wave (SDW) state and a superconducting state, which are sharply separated upon doping. Considering the different Ln ion, upon F doping the transition temperatures are more efficiently suppressed in Ce-1111 as compared to Sm-1111. More intriguingly, for the Ce case, a coexistence region between static magnetism and superconductivity without an orthorhombic distortion has been observed. Further analysis of the width of the Bragg peaks reveals strong lattice fluctuations towards phase transitions, which are reflected in magnetic and transport properties. Moreover, a strong damping of the lattice fluctuations is observed at Tc for superconducting Sm-1111 samples, giving experimental evidence of competing orders towards phase transitions in the iron pnictides. Regarding the iron borates, non-resonant x-ray scattering studies have shown several new diffraction features, from the appearance of additional reflections that violate the reflection conditions for the low temperature crystal structure, to the emerging of commensurate superlattice peaks that appear below TN. A detailed analysis of the structure factors and q dependencies of the earlier reflections, demonstrate their magnetic nature. Additional resonant x-ray magnetic scattering experiments on NdFe3(BO3)4 were performed at the Nd L2,3 and Fe K edges. The results show that the magnetization behavior is different for the Nd and for the Fe sublattices. Moreover, we find that the magnetization of the Nd sublattice is induced by the Fe magnetization. The temperature dependent measurements also show a commensurate to incommensurate transition where the magnetic structure changes from a commensurate collinear structure, where both Nd and Fe moments align in the hexagonal basal plane, to an incommensurate spin helix structure that propagates along c. When a magnetic field is applied, the spin helix is destroyed and a collinear structure is formed where the moments align in a direction perpendicular to the applied magnetic field. Moreover, the critical field at which the spin helix is destroyed is the same field at which the magnetic induced electric polarization is maximum, thus, showing that the spin helix is not at the origin of the electric polarization.

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.

There is great interest in the development of new polar dielectric ceramics and multiferroic materials with new and improved properties. A family of tetragonal tungsten bronze (TTB) relaxors of composition Ba₆M³⁺Nb₉O₃₀ (M³⁺ = Ga³⁺, Sc³⁺ and In³⁺, and also their solid solutions) were studied in an attempt to understand their dielectric properties to enable design of novel polar TTB materials. A combination of electrical measurements (dielectric and impedance spectroscopy) and powder diffraction (X-ray and neutron) studies as a function of temperature was employed for characterising the dynamic dipole response in these materials. The effect of B-site doping on fundamental dipolar relaxation parameters were investigated by independently fitting the dielectric permittivity to the Vogel-Fulcher (VF) model, and the dielectric loss to Universal Dielectric Response (UDR) and Arrhenius models. These studies showed an increase in the characteristic dipole freezing temperature (T[subscript(f)]) with increase B-cation radius. Crystallographic data indicated a corresponding maximum in tetragonal strain at T[subscript(f)], consistent with the slowing and eventual freezing of dipoles. In addition, the B1 crystallographic site was shown to be most active in terms of the dipolar response. A more in-depth analysis of the relaxor behaviour of these materials revealed that, with the stepwise increase in the ionic radius of the M³⁺ cation on the B-site within the Sc-In solid solution series, the Vogel-Fulcher curves (lnf vs. T[subscript(m)]) are displaced to higher temperatures, while the degree of relaxor behaviour (frequency dependence) increases. Unfortunately, additional features appear in the dielectric spectroscopy data, dramatically affecting the Vogel-Fulcher fitting parameters. A parametric study of the reproducibility of acquisition and analysis of dielectric data was therefore carried out. The applicability of the Vogel-Fulcher expression to fit dielectric permittivity data was investigated, from the simple unrestricted (“free”) fit to a wider range of imposed values for the VF relaxation parameters that fit with high accuracy the experimental data. The reproducibility of the dielectric data and the relaxation parameters obtained by VF fitting were shown to be highly sensitive to the thermal history of samples and also the conditions during dielectric data acquisition (i.e., heating/cooling rate). In contrast, UDR analysis of the dielectric loss data provided far more reproducible results, and to an extent was able to partially deconvolute the additional relaxation processes present in these materials. The exact nature of these additional relaxations is not yet fully understood. It was concluded application of the Vogel-Fulcher model should be undertaken with great care. The UDR model may represent a feasible alternative to the evaluation of fundamental relaxation parameters, and a step forward towards the understanding of the dielectric processes in tetragonal tungsten bronzes.

Transition metal fluorides represent an important family of complex solids displaying a variety of different properties and interesting phenomena. Despite their remarkable behaviour, these classes of materials have not received much attention and the rationalization of their behaviour is still lacking a systematic approach. This thesis aims to contribute to the field by examining previously unknown or understudied complex fluorides. The compounds were selected for their intriguing physical properties that range from superconductivity to multiferroism. The discovery of superconductivity in the iron pnictides sparked new interest in materials with layered ZrCuSiAs-type structure. Herein the properties of one of these systems, namely SrFeAsF, will be discussed. We have found that it behaves as a poor metal and undergoes a tetragonal (P4/nmm) to orthorhombic (Cmma) structural transition at T = 180 K, accompanied by a spin density wave in magnetic susceptibility and electrical resistivity. Below T < 150 K, the Fe moments order in antiferromagnetic spin-stripes. Electron doping with La3+ is a successful route to obtain superconducting phases, with maximum Tc = 27 K (x = 0.2). The isostructural AeMnPnF series (Ae = Sr, Ba; Pn = P, As, Sb) was also investigated to elucidate the influence of transition metal d-electrons and size effects of Ae and Pn on the physical properties. The isoelectronic replacement of Ae and Pn leads to a significant distortion in the tetragonal building blocks. All d5 Mn fluorides investigated here are insulating antiferromagnets with TN ~ 350 K. Due to the coexistence of electronic and magnetic ordering, the tetragonal tungsten bronze (TTB) materials KxM2+ xM3+ 1-xF3 (x = 0.4 – 0.6; M = transition metal) are potential multiferroics. The type of structural distortion adopted by these systems is strongly dependant on the M2+/M3+ ratio. For instance, our high-resolution diffraction study on K0.5Mn0.5Cr0.5F3 has revealed a small orthorhombic distortion, which indicates full chemical order of Mn2+ and Cr3+ on all crystallographic sites. K0.5Mn0.5Cr0.5F3 remains orthorhombic Ccc2 on cooling through the ferromagnetic transition at TN = 23 K. On heating, the structure is acentric up to T = 373 K, where a change to tetragonal P42/mbc symmetry marks the transition from ferroelectric (polar) to paraelectric (apolar) states. High-pressure diffraction experiments have shown that the Ccc2 structure is robust upon pressurization with anisotropic axial compressibility up to the maximum pressure applied p = 18 GPa. The crystal structure of related mixed-valence TTB fluoride K0.6Cr2+ 0.6Cr3+ 0.4F3 is influenced by the presence of Jahn-Teller active Cr2+. The structural analysis described here revealed the presence of a small polar monoclinic distortion (P112) providing a clear signature of full charge order (CO). On heating, the gradual loss of CO leads to two consecutive structural phase transitions to orthorhombic (Pba2, T = 423 K) and then tetragonal (P42/mbc, T = 823 K) lattices, the latter is the signature of the ferro- to paraelectric transition. Below T = 150 K, increased X-ray exposure time leads to CO-melting and the stabilization of a new, charge-disordered orthorhombic phase (Cmm2), with a phenomenology similar to the CO manganites. In highpressure diffraction experiments, a further transition to tetragonal P4bm symmetry is found at p = 6 GPa. The magnetic susceptibility points towards a complex spin arrangement, with two transitions at TN = 33 K and 6 K. The results presented herein show the richness of the structural, electronic and magnetic phase diagrams of transition metal fluorides and clearly demonstrate that systematic studies on these systems will greatly enhance our current understanding of the underlying mechanisms of important phenomena such as superconductivity and ferroelectricity.

Doutoramento em Ciência e Engenharia de Materiais
The comprehensive study on the coupling of magnetism, electrical polarization and the crystalline lattice with the off-stoichiometric effects in self-doped multiferroic hexagonal h-LuMnxO3±δ (0.92≤x≤1.12) ceramic oxides was carried out for the PhD work. There is a complex coupling of the three ferroic degrees. The cancelation of the magnetic moments of ions in the antiferromagnetic order, electric polarization with specific vortex/antivortex topology and lattice properties have pushed researchers to find out ways to disclose the underlying physics and chemistry of magneto-electric and magneto-elastic couplings of h-RMnO3 multiferroic materials. In this research work, self-doping of Lu-sites or Mn-sites of h-LuMnxO3±δ ceramics prepared via solid state route was done to pave a way for deeper understanding of the antiferromagnetic transition, the weak ferromagnetism often reported in the same crystalline lattices and the ferroelectric properties coupled to the imposed lattice changes. Accordingly to the aim of the PhD thesis, the objectives set for the sintering study in the first chapter on experimental results were two. First, study of sintering off-stoichiometric samples within conditions reported in the bibliography and also extracted from the phase diagrams of the LuMnxO3±δ, with a multiple firings ending with a last high temperature step at 1300ºC for 24 hours. Second, explore longer annealing times of up to 240 hours at the fixed temperature of 1300 ºC in a search for improving the properties of the solid solution under study. All series of LuMnxO3±δ ceramics for each annealing time were characterized to tentatively build a framework enabling comparison of measured properties with results of others available in literature. XRD and Rietveld refinement of data give the evolution the lattice parameters as a function to x. Shrinkage of the lattice parameters with increasing x values was observed, the stability limit of the solid solution being determined by analysis of lattice parameters. The evolution of grain size and presence of secondary phases have been investigated by means of TEM, SEM, EDS and EBSD techniques. The dependencies of grain growth and regression of secondary phases on composition x and time were further characterized. Magnetic susceptibility of samples and magnetic irreversibility were extensively examined in the present work. The dependency of magnetic susceptibility, Neel ordering transition and important magnetic parameters are determined and compared to observation in other multiferroics in the following chapter of the thesis. As a tool of high sensitivity to detect minor traces of the secondary phase hausmannite, magnetic measurements are suggested for cross-checking of phase diagrams. Difficulty of previous studies on interpreting the magnetic anomaly below 43 K in h-RMnO3 oxides was discussed and assigned to the Mn3O4 phase, with supported of the electron microscopy. Magneto-electric coupling where AFM ordering is coupled to dielectric polarization is investigated as a function of x and of sintering condition via frequency and temperature dependent complex dielectric constant measurements in the final chapter of the thesis. Within the limits of solid solubility, the crystalline lattice of off-stoichiometric ceramics was shown to preserve the magneto-electric coupling at TN. It represents the first research work on magneto-electric coupling modified by vacancy doping to author’s knowledge. Studied lattices would reveal distortions at the atomic scale imposed by local changes of x dependent on sintering conditions which were widely inspected by using TEM/STEM methods, complemented with EDS and EELS spectroscopy all together to provide comprehensive information on cross coupling of distortions, inhomogeneity and electronic structure assembled and discussed in a specific chapter. Internal interfaces inside crystalline grains were examined. Qualitative explanations of the measured magnetic and ferroelectric properties were established in relation to observed nanoscale features of h-LuMnxO3±δ ceramics. Ferroelectric domains and topological defects are displayed both in TEM and AFM/PFM images, the later technique being used to look at size, distribution and switching of ferroelectric domains influenced by vacancy doping at the micron scale bridging to complementary TEM studies on the atomic structure of ferroelectric domains. In support to experimental study, DFT simulations using Wien2K code have been carried out in order to interpret the results of EELS spectra of O K-edge and to obtain information on the cation hybridization to oxygen ions. The L3,2 edges of Mn is used to access the oxidation state of the Mn ions inside crystalline grains. In addition, rehybridization driven ferroelectricity is also evaluated by comparing the partial density of states of the orbitals of all ions of the samples, also the polarization was calculated and correlated to the off-stoichiometric effect.
No trabalho de investigação para o doutoramento foi realizado um estudo abrangente dos efeitos do desvio estequiométrico sobre o comportamento magnético e o acoplamento do magnetismo com a polarização elétrica e a rede cristalina do óxido cerâmico multiferróico de rede hexagonal do LuMnxO3±δ auto-dopado no intervalo de composição (0.92≤x≤1.12). Existe um acoplamento complexo dos diferentes ordenamentos ferróicos. O cancelamento dos momentos magnéticos dos iões Mn no sistema antiferromagnético, a polarização elétrica com topologia específica de vórtice/antivórtice conduziram os investigadores a propor modelos para a física e química subjacentes ao acoplamento magneto-elétrico e magnetoelásticas dos materiais multiferróicos de h- RMnO3. Neste trabalho optou-se pela auto-dopagem das posições R e do Mn da rede cristalina dos cerâmicos h-LuMnxO3±δ preparados por reacção no estado sólido como caminho para uma compreensão mais completa da transição antiferromagnética, do ferromagnetismo fraco frequentemente observado nas mesmas redes cristalinas e das propriedades dieléctricas e ferroeléctricas associadas a alterações impostas à rede pela auto-dopagem. Em linha com o propósito traçado para a tese de doutoramento, foram dois os objectivos definidos para o estudo de sinterização no primeiro capítulo sobre resultados experimentais. Primeiro foi feito o estudo de sinterização das amostras com desvio de estequiometria em correspondência com condições do processamento relatadas na bibliografia e conformes com os diagramas de fases do LuMnO3, tendo-se optado por diversas etapas de reacção a temperatura elevada terminando com um último passo a 1300ºC durante 24 horas. Numa segunda parte, explorou-se o efeito do aumento do tempo de recozimento até às 240 horas à mesma temperatura fixa de 1300 ºC para melhorar as propriedades sob estudo da solução sólida. Caracterizaram-se todas as séries para cada tempo de recozimento dos cerâmicos de h-LuMnxO3±δ na tentativa de construir um referencial que permitisse a comparação das propriedades medidas nas composições com desvio de estequiometria com os resultados de outros estudos da bibliografia. A difracção de raios-X (DRX) e o refinamento de Rietveld dão a evolução dos parâmetros de rede em função de x tendo-se observado uma retracção nos parâmetros de rede com o aumento dos valores de x e identificado em primeira aproximação o limite de estabilidade da solução sólida pela análise dos mesmos parâmetros de rede. As evoluções do tamanho do grão e a da quantidade de fases residuais detectadas foram investigadas por técnicas de TEM, SEM, EDS e EBSD. Foram ainda caracterizadas as dependências do crescimento de grão e da diminuição de teor em fases secundárias em função da composição x e tempo de processo. A susceptibilidade magnética das amostras e a irreversibilidade magnética são caracterizadas e avaliadas extensivamente no capítulo seguinte da tese. Foram determinadas as dependências da susceptibilidade magnética, da temperatura de Néel da transição antiferromagnética e dos parâmetros principais do comportamento magnético e comparadas com observações feitas em outros multiferróicos. Como ferramenta de alta sensibilidade para detectar vestígios menores da fase magnética hausmannite, sugere-se que as medidas magnéticas podem ser úteis na confirmação dos diagramas de fase dos óxidos RMnO3. São discutidas dificuldades patentes em estudos anteriores sobre as interpretações dadas à anomalia magnética nos óxidos h-RMnO3 abaixo de 43 K sendo esta atribuída à fase Mn3O4 com suporte na microscopia eletrónica. No capítulo final da tese é investigado o acoplamento magneto-eléctrico com o ordenamento AFM acoplado às fronteiras de domínio da polarização dielétrica e com a determinação da constante dielétrica complexa em função da frequência e temperatura. Dentro do limite de estabilidade da solução sólida do h-LuMnxO3±δ, o acoplamento magneto-eléctrico em TN continua a observar-se em todo o intervalo do desvio estequiométrico. Tanto quanto o autor tem conhecimento este estudo representa o primeiro trabalho de investigação sobre acoplamento magneto-eléctrico modificado por auto-dopagem com lacunas nas redes de h-RMnO3. A observação das redes cristalinas revela distorções à escala atómica criadas pelas variações locais da razão x na composição do LuMnxO3±δ, dependentes das condições de sinterização as quais foram analisadas em detalhe por TEM/STEM, complementado com EDS e espectroscopia EELS por forma a obterem-se informações mais completas sobre relações cruzadas entre distorção, falta de homogeneidade composicional e estrutura eletrónica. Estes resultados encontram-se reunidos e são discutidos num capítulo específico da tese. Foram encontradas interfaces internas nas secções finas TEM dos grãos cristalinos de várias tipologias. São propostas explanações qualitativas para as propriedades magnéticas e ferroelétricas medidas correlacionando-as com as observações feitas à nano-escala nos cerâmicos de LuMnxO3±δ. Os domínios ferroelétricos e defeitos topológicos estão presentes tanto em imagens de TEM como de AFM/PFM. Esta última técnica é utilizada para a caracterização do tamanho, distribuição e comutação de polaridade dos domínios ferroelétricos na escala do mícron em relação com potenciais efeitos da dopagem por lacuna e fazendo a ponte com os estudos TEM sobre a estrutura atómica dos mesmos domínios ferroelétricos. Como suporte ao estudo experimental, foram feitas simulações DFT usando o código Wien2K a fim de interpretar os espectros de EELS da banda K do oxigénio e para obter informação sobre a hibridização dos catiões com o oxigénio. As linhas das bandas L3,2 do Mn nos espectros de EELS são utilizadas para estabelecer o estado de oxidação dos iões Mn no interior dos grãos cristalinos. Além disso, a ferroelectricidade induzida por re-hibridização é também avaliada por comparação com a densidade parcial de estados dos orbitais do conjunto de iões da estrutura cristalina e a polarização elétrica é computada e correlacionada com efeitos do desvio estequiométrico.