Multiferroics have attracted lots of research interest due to their potential in numerous multifunctional applications. The multiferroic materials could simultaneously exhibit two or more ferroic order parameters, and the coupling effects between ferroelectricity and ferromagnetism are named as magnetoelectric (ME) effect. Recently, with the development of thin film growth techniques, the multiferroics magnetoelectric composite heterostructures exhibit a very promising future prospects. This dissertation focused on the design, fabrication and characterization of new multiferroics magnetoelectric composite heterostructures. First, based on the specific phase architectures in BFO-CFO self-assembled thin films grown on variously oriented STO substrates and the epitaxial film growth knowledge, I designed two kinds of new film heterostructures: (i) I utilized self-assembled BFO nanopillars in a BFO-CFO two phase layer on (111) STO as a seed layer on which to deposit a secondary top BiFeO3 layer. The growth mechanism and multiferroic properties of these new heterostructures were investigated. (ii) I demonstrated the formation of a new quasi-(0-3) heterostructure by alternately growing (2-2) and (1-3) layers within the film. I proposed a new concept to overcome limitations of both the (2-2) and (1-3) phase connectivities and identified an indirect ME effect by the switching the characteristics of the piezoresponse for the new heterostructure. Second, for the option for candidates thin film materials with a high piezoelectric coefficient, which is a critical factor for ME composite films, I utilized the simple compositional BaSn0.11Ti0.89O3 bulk ceramic material as a target to grow films with the large piezoelectric properties. The grown high qualify lead-free epitaxial thin films had a chemical constituent similar to the reported giant piezoelectric ceramics near the MPB and with the QP. Both coherent and incoherent regions were observed in the interface and a larger piezoelectric coefficient d33 was achieved in this film. Finally, with respect to their characteristics and potential, I redirected from two-dimensional thin film materials to one-dimensional nanowire materials. By utilizing vertically aligned templates, I fabricated a new type of coaxial two-phase composite nanowires. Multiferroic properties of these new one-dimensional materials have been investigated. All these multiferroics magnetoelectric composite herterostructures would provide lots of potential in applications. / PHD

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Cette thèse expérimentale a pour motivation l'étude des propriétés dynamiques des composés multiferroïques : les langasites de fer Ba3NbFe3Si2O14 et Ba3TaFe3Si2O14 et le manganite hexagonal ErMnO3. Ces investigations ont été réalisées grâce à l'utilisation de deux techniques expérimentales complémentaires : les spectroscopies FIR et THz et la diffusion de neutrons (polarisés et non polarisés). Dans les langasites de fer, des excitations magnéto-électriques ont été observées. Celles-ci ont été interprétées comme des modes de rotation atomique excités par la composante magnétique de l'onde électromagnétique. D'autre part, des modulations fines de la structure magnétique ont également été mises en évidence dans le composé Ba3NbFe3Si2O14 par l'observation de faits expérimentaux incompatibles avec la structure magnétique publiée avant cette étude. Il s'agissait de satellites magnétiques interdits et d'ordre supérieurs ainsi qu'une extinction dans les ondes de spin. La structure magnétique déduite de cette étude correspond à une modulation en accordéon des hélices de spin et à une perte structurale de l'axe de rotation d'ordre 3. Ces modulations sont compatibles avec les excitations magnéto-électriques et la multiferroïcité récemment mise en évidence dans ce composé. En parallèle, les propriétés magnéto-électriques dynamiques du manganite hexagonal ErMnO3 ont été étudiées. Un électromagnon a été observé correspondant à un magnon du Mn excité par la composante électrique de l'onde électromagnétique. Par comparaison avec le composé YMnO3, cet électromagnon a été interprété comme résultant d'une hybridation entre une excitation de champ cristallin de l'Er active électriquement et un magnon standard du Mn. / This experimental thesis is motivated by the study of the dynamical properties of multiferroic compounds : the iron langasites Ba3NbFe3Si2O14 and Ba3TaFe3Si2O14 and the hexagonal manganite ErMnO3. These investigations were performed using two complementary experimental techniques : FIR and THz spectroscopy and neutron scattering (with polarized and unpolarized neutrons). In the iron langasites, magnetoelectric excitations were observed. These excitations have been interpreted as atomic rotation modes excited by the magnetic component of the electromagnetic wave. On the other hand, weak modulations of the magnetic structure were also evidence in the Ba3NbFe3Si2O14 compound by observing experimental evidence inconsistent with the published magnetic structure. These evidence were forbidden and higher order magnetic satellites as well as an extinction in the spin-waves spectrum. The magnetic order deduced from this study presents a bunched modulation of the helix and the structural loss of the 3-fold axis. These modulations are compatible with the magnetoelectric excitations and the multiferroicity recently evidenced in this compound. In parallel, the magnetoelectric dynamical properties of the hexagonal manganite ErMnO3 have been investigated. An electromagnon has been observed corresponding to a Mn magnon excited by the electric component of the electromagnetic wave. From comparison with the YMnO3 compound, this electromagnon was interpreted as a hybrid mode between an electricallyactive Er crystal field excitation and a Mn magnon.

Cette thèse expérimentale a permis d'étudier différents aspects des composés multiferroïques et magnétoélectriques et notamment l'influence des propriétés magnétiques sur les propriétés diélectriques et magnétoélectriques dans deux familles de composé : le trisulfure de phosphore de manganèse MnPS3 et les langasites au fer. Nous nous sommes aussi intéressés en détail aux propriétés de chiralité magnétique dynamique du composé Ba3NbFe3Si2O14 de cette dernière famille.Les langasites au fer présentent une structure cristalline non centrosymétrique et chirale. Les mesures macroscopiques d'aimantation et les expériences de diffraction de neutrons, y compris polarisés avec analyse de polarisation, ont permis de mettre en évidence une double chiralité magnétique (triangulaire et hélicoïdale). Les signatures de cette chiralité statique sont également observées dans les excitations magnétiques, notamment sous la forme d'une section efficace non nulle associée aux corrélations dynamiques antisymétriques. Ces résultats ont été interprétés à l'aide de calculs d'onde de spins conduits dans une approche linéaire. Enfin différentes échelles d'énergie ont été mises en évidence dans les fluctuations paramagnétiques avec en particulier des fluctuations magnétiques associées aux corrélations antisymétriques.Il a été mis en évidence dans le trisulfure de manganèse MnPS3 un ordre magnétique antiferromagnétique également associé à un moment toroïdal macroscopique non nul ainsi qu'un couplage magnétoélectrique non diagonal non nul. Ce couplage a été mis en évidence à l'aide d'une expérience de diffraction de neutrons polarisés avec une analyse de polarisation sphérique ce qui a permis de \og jouer\fg{} avec les domaines antiferromagnétiques de MnPS3 à l'aide de refroidissements au passage de la température de Néel sous des champs magnétique et électrique croisés. / This Phd report shows several aspects of multiferroic and magnetoelectric properties and especially the effect of magnetic properties on dielectric and magnetoelectric behavior of two families of compounds : the manganese trisulfure phosphorus MnPS3 and the iron langasite.We also present in details the magnetic dynamical chirality properties of the compound Ba3NbFe3Si2O14 of the langasite family.Iron langasites show a non centrosymetric and a chiral crystal structure. The macroscopic magnetization measurements and neutron scattering experiments, including the use of polarized neutrons and polarization analysis, have allowed to enlighten a double magnetic chirality (helical and triangular). This chirality has a signature in the magnetic excitations and notably by non zero cross sections associated to dynamical antisymmetric correlations of spins. Those results are interpreted by spin-wave calculations in a linear approach. At last, different energy scales appear in the paramagnetic fluctuations with, in particular, magnetic fluctuations associated to antisymmetric correlations.The MnPS3 manganese trisulfure compound exhibits an antiferromagnetic order with an non zero macroscopic toroidization and a non zero non diagonal magnetoelectric coupling. This coupling has been enlightened by spherical neutron polarimetry experiment. We played with antiferromagnetic domains by cooling the sample through its Néel température under crossed magnetic and electric fields.

La manipulation de parois de domaine magnétique – qui séparent des régions d’aimantation uniforme dans les matériaux – est associée à des enjeux à la fois fondamentaux et technologiques. De nombreux travaux portent sur l’utilisation de champs magnétiques et de courants électriques pour leur déplacement. Cependant, des préoccupations particulières – notamment la dissipation d’énergie - motivent la recherche d’alternatives. Parmi les solutions potentielles, le couplage magnétoélectrique par l’intermédiaire de contraintes mécaniques dans des hétérostructures magnétoélastique/piézoélectrique paraît prometteur. Dans cette thèse, il est montré que l’association d’un champ magnétique de biais et de contraintes mécaniques uniformes peut engendrer le déplacement unidirectionnel d’une paroi de domaine transverse dans des nanostructures à anisotropie uniaxiale. Les considérations statiques et dynamiques de ce phénomène sont étudiées par le biais de procédures numériques ad hoc simulant le couplage mécanique entre substrat de PMN-PT de coupe 011 générant des contraintes, et nanostructures multicouches magnétoélastiques TbCo2/FeCo. Le design du profil de section des nanostructures permet de moduler la réponse du système, par exemple pour contrôler la position de parois confinées. La dynamique du système se distingue des régimes habituels de par la forme de la paroi de domaine. L’atteinte de régimes permanents dans des nanorubans montre que des vitesses comparables aux autres techniques sont obtenues, pour une dissipation d’énergie beaucoup plus faible. Des travaux expérimentaux ont permis de mettre au point un process de fabrication sur PMN-PT et d’explorer l’effet magnétoélectrique / The manipulation of magnetic domain walls – that separate regions of uniform magnetization – is associated with both fundamental and technological research interests. A large part of the literature on domain wall motion deals with the use of magnetic fields and electric currents. However, several concerns – most notably energy dissipation – motivates the search for alternatives. Among potential candidates, the mechanical stress-mediated magnetoelectric coupling in magnetoelastic/piezoelectric heterostructures seems promising. In this thesis, it is shown that the combination of a bias magnetic field and uniform mechanical stress can induce unidirectional domain wall motion in nanostructures with uniaxial anisotropy. Static and dynamic aspects of this phenomenon are studied by means of ad hoc numerical procedures simulating the mechanical coupling of 011-cut PMN-PT generating the stress, and TbCo2/FeCo multilayers magnetoelastic nanostructures. The design of the cross section profile in nanostructures allows to tailor the response of the system, enabling for instance the control of domain wall position in confined geometries. The associated dynamics stands apart from known regimes because of the shape of the domain wall. The existence of steady-state regimes in nanostripes of constant width shows that velocities comparable to those of other techniques can be obtained, for a fraction of the energy required. Experimental investigations resulted in the development of a successful fabrication process on PMN-PT and the exploration of the magnetoelectric effect

Nanostructuring functional materials can lead to a variety of enhanced intrinsic material properties. In particular, nanowires (NWs) have large surface-to-volume ratio and large aspect ratio (length / diameter), which makes them sensitive to low-amplitude vibrations and have increased flexibility compared to the bulk form of the material. In this thesis, piezoelectric, ferroelectric, ferromagnetic and magnetoelectric (ME) NWs have been explored in the context of vibrational energy harvesting and magnetic energy harvesting and sensing; because of their increased piezoelectric coefficients and ME coupling compared to bulk. Low-temperature, solution-processable and hence scalable fabrication techniques have been used throughout this work. Electrochemical deposition or electrodeposition (ED) in conjunction with nanoporous templates i.e. template-assisted electrodeposition (TAED) have been used to grow piezoelectric zinc oxide (ZnO) and ferromagnetic nickel (Ni) NWs and three template-wetting based techniques have been used to grow ferroelectric poly(vinylidene fluoride trifluoroethylene) (P(VDF-TrFE)) NWs and nanotubes (NTs). Both techniques have been optimised and subsequently combined to synthesise core-shell or (1-1) Ni - P(VDF-TrFE) composite NWs. The structural and crystalline properties of each type of nanostructure has been studied using a variety of techniques including: scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), X-ray diffraction (XRD) and transmission electron microscopy (TEM) and all of the NWs have been shown to be polycrystalline. The energy harvesting performance of vertically aligned ZnO NW arrays embedded in flexible, polycarbonate (PC) templates when incorporated into a flexible nanocomposite nanogenerator (NG), has been tested via periodic impacting and flexing of the NG at different frequencies. The voltage ($V$), current ($I$) and power were recorded during testing and measured across a range of external load resistances. The aligned nature of the embedded NWs ensures good piezoelectric performance across the entire device under impacting, while the PC template ensures mechanical stability and longevity of the device, confirmed by good fatigue performance over 24 hours of continuous testing, which is rarely studied in this field. The power density ($P_\mathrm{d}$) was found to be 151 mW m$^{-3}$ for low-amplitude (0.68 mm) and low-frequency (5 Hz) impacting, resulting in energy conversion efficiencies ($\chi$) and device efficiencies ($\chi$') of $\approx$ 4.2 \% and $\approx$ 3.76 x 10$^{-3}$ \% respectively. The nanoscale or surface piezoelectric charge coefficient ($d_{33}$) was measured to be $\approx$ 12.5 pm V$^{-1}$ on an individual ZnO NW, using a combination of Kelvin probe force microscopy (KPFM) and non--destructive piezoresponse force microscopy (ND-PFM). Both nanoscale and bulk ME measurements have been performed on Ni - P(VDF-TrFE) ME composite (1-1) NWs, nanocomposite (1-3) films and (2-2) laminates. The latter two structures have been fabricated using TAED and ED for the Ni NW and film respectively, in combination with drop-casting and spin-coating for the P(VDF-TrFE) films. The scanning probe microscopy (SPM) measurements used here include atomic force microscopy (AFM), KPFM, magnetic force microscopy (MFM) and piezoresponse force microscopy (PFM) and it has been found that the ME coupling in the (1-1) composites NWs is enhanced compared to the other structures, confirmed by approximating the converse ME coupling coefficient ($\alpha^\mathrm{C}$) of each composite. Additionally, vibrating sample magnetometry (VSM) has been used to confirm the ferromagnetic nature of the Ni phases in the composite structures. ME composite devices based on (2-2) and (1-3) composite materials and have been fabricated and preliminary bulk ME measurements of the ME coupling coefficient ($\alpha^\mathrm{E}$) plus energy harvesting measurements have also been performed as a proof of concept that the nanoscale ME coupling translates to the bulk, to some extent.

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No. of bitstreams: 1 teseDSFV.pdf: 5669668 bytes, checksum: 165ca1333f2345e5e1406b40a2c0548c (MD5) Previous issue date: 2016-04-08 / Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / In the case of composites materials, which presents the magnetoelectric coupling, there are so many questions about the influence of the microstructure on the coupled and uncoupled properties. Thereby, this work proposes the study of the grain size influence, of ferroelectric and magnetic phases, on the magnetic, electric and coupled properties in particulate magnetoelectric composites. In order to study the relationship property/microstructure in volumetric magnetoelectric composites, it was necessary to prepare samples with different average grain sizes, ie, the phases having average grain size in the order of micrometers or nanometers. To perform this study it was used as magnetic phase the CoFe2O4 and, as the ferroelectric phase, the [0,675] Pb(Mg1/3Nb2/3)O3 – [0,325] PbTiO3, mixed in molar ratio 20/80 %, using techniques like micro ball milling (to reduce particle size) and high energy ball milling (to mixture the phases) during the powder syntheses, and Spark Plasma Sintering (SPS) to densify the bulk. Due the presence of secondary phases in sintered composites generated during the application of SPS, a processing protocol was created based on a post sintering thermal treatment, in PbO rich atmosphere, which allowed the reduction of secondary phases without, however, changing the average size of grain. These procedures resulted in composite material suitable for the investigations of the electrical, magnetic and magnetoelectric properties, considering the different microstructures obtained. The magnetoelectric composites showed the self-biased effect arising from the intrinsic strain generated in one phase over the other. It was observed the properties dependence, coupled or uncoupled, with average grain size of both phases. Furthermore, the magnetoelectric composites presented the selfbiased effect that appears due to intrinsic strain generated from one phase over the other. Moreover, the evolution of the magnetoelectric coefficient behavior as a function of temperature and applied magnetic field has been more dependent on the variation of the average grain size of the ferroelectric phase than the ferrimagnetic phase, which can be attributed to the largest amount of ferroelectric phase in the structure compound. / No caso de materiais compósitos, que apresentam o acoplamento magnetoelétrico, existem muitas questões sobre influência da microestrutura nas propriedades independentes e acopladas das fases. Dessa forma, este trabalho propõe o estudo da influência do tamanho médio de grão, tanto da fase ferroelétrica quanto da magnética, nas propriedades elétricas, magnéticas e de acoplamento em compósitos magnetoelétricos particulados. Com o objetivo de estudar a relação propriedade / microestrutura em compósitos magnetoelétricos volumétricos, fez-se necessária a preparação de amostras com tamanho médio de grão distintos, ou seja, as fases possuindo tamanho médio de grão na ordem de micrômetros ou nanômetros. Para a realização deste estudo, foi utilizada como fase magnética o CoFe2O4 e, como fase ferroelétrica, o [0,675] Pb(Mg1/3Nb2/3)O3 – [0,325] PbTiO3, misturados na proporção molar 20/80, utilizando técnicas como micromoagem (para redução de partículas) e moagem de altas energias (para a mistura homogênea das fases) durante a síntese dos pós, e a de Spark Plasma Sintering (SPS) para a densificação dos corpos cerâmicos compósitos. Devido à presença de fases secundárias nos compósitos sinterizados, geradas durante a aplicação de SPS, foi criado um protocolo de processamento baseado em tratamento térmico pós-sinterização, com atmosfera rica em PbO, o qual possibilitou a redução das fases secundárias sem, contudo, alterar os tamanhos médios de grão. Tais procedimentos resultaram em material compósito adequado à realização das investigações das propriedades elétricas, magnéticas e magnetoelétricas, considerando as diferentes microestruturas obtidas. Os compósitos magnetoelétricos apresentaram o efeito self-biased, que aparece devido ao strain intrínseco gerado de uma fase sobre a outra. Foi observada a dependência das propriedades, acopladas ou não, com o tamanho médio de grão de ambas as fases. Além disso, a evolução do comportamento do coeficiente magnetoelétrico em função da temperatura e do campo magnético se mostrou mais dependente da variação do tamanho médio de grão da fase ferroelétrica do que da ferrimagnética, fato que pode ser atribuído à maior quantidade de fase ferroelétrica na estrutura do composto.

The aim of this thesis is to create a system, which draws the graphics with the help of a set laser project and eventually is able to synthetize several coloured rays and thus realise a multicolour drawing. The whole system is controlled by a Raspberry PI microcomputer and the data is sent to it from a PC via an Ethernet cable. The transmission of the projector device was identified and possible ways of improving the device were suggested. The results of this thesis allow creation of a multi-coloured vector picture, transferring it into the device and depiction on the project desktop. The whole device can therefore be used for example for the purpose of a presentation.

Le but de cette thèse est l’obtention de matériaux multiferroïques par l’insertion de molécules organiques dans une matrice magnétique d’hydroxide simple lamellaire. Durant cette thèse a été démontré la faisabilité de la fonctionnalisation d’hydroxides simples lamellaires de cuivre et de cobalt par des molécules possédant des fonctions d’accroche acide phosphonique. Le développement des techniques de pré-fonctionnalisation a permis de fonctionnaliser ces hydroxides simple lamellaire par une variété importantes de molécules (fluorènes, benzènes, thiophènes et complexes de métaux de transition) afin d’apporter une propriété additionnelle au magnétisme de l’hydroxyde. Les premières mesures ont permis de mettre en évidence un couplage entre temperature d’ordre magnétique et anomalie diélectrique. La complexité de la mesure des propriétés diélectriques avec ces échantillons (sur poudre pastillées à froid) ont mis en lumière la nécessité d’obtenir des tailles de cristallites plus importantes. Des efforts sur la taille des cristallites ont donc été effectués et ont permis d’obtenir des monocristaux d’hydroxy- dodecylsulfate de cuivre. / The main goal of this thesis is to obtain multiferroic materials via the intercalation of organic molecules in a magnetic inorganic matrix made of layered simple hydroxide. The possibility to use phosphonic acid as grafting moiety for the functionalization of layered simple hydroxides has been demonstrated during this thesis. Pre-functionalization techniques has allowed the functionalization of layered simple hydroxides of copper and cobalt with a wide variety of molecules (fluorenes, benzenes, thiophenes or transition- metal complexes) to bring an additional property to the magnetic properties of the layered hydroxide. The first measurements revealed a coupling between magnetic ordering temperature and dielectric anomaly. The dielectric properties measurements proved to be difficult with the samples (on cold-pressed pellets) and shown the usefulness of developing new methods to improve the cristallite size. To that end, new syntheses procedures led to the obtention of mono crystals of copper-hydroxidodecylsulfonate allowing to get additional structural informations.

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).