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.

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.

Skyrmions are spin textures with particle character that order themselves into so-called “skyrmion lattices” (SkLs). A skyrmion is topologically nontrivial, which adds stability against external perturbations and attracts tremendous interest from the theoretical side. Since skyrmions can be moved with small electrical currents, they are being discussed for novel spintronic applications, such as racetrack memory. Further interest has been spurred by the discovery of multiferroic compounds that also host SkLs, resulting in additional properties that are highly interesting both for applications and for fundamental research. The scope of this thesis encompasses the investigation of two completely different exemplary SkL-hosting multiferroic systems using a broad set of scanning probe microscopy techniques. These can probe multiple properties on a local scale in real space with a single measurement, examining details not resolved by non-local techniques. In the first part, there is a brief introduction to magnetic skyrmions and scanning probe microscopy with a short review of the theoretical background. The materials of interest and their known properties are then introduced. These are Cu2OSeO3, an insulator exhibiting the emergence of Bloch-type skyrmions as well as type-II multiferroicity, and the lacunar spinel chalcogenides, which were recently found to exhibit multiferroic Néel-type skyrmions pinned to magnetic easy-axes/planes together with type-I multiferroicity originating from a structural Jahn–Teller transition. The second part first presents various scanning probe studies and their results for Cu2OSeO3, where, aside from the magnetic textures of the various magnetic phases, the magnetoelectric effect and the magnetic phase transitions are investigated and described with basic theoretical models. Results show a good correlation between observations and theory, as well as with other experimental methods. Various lacunar spinels are then investigated, mostly GaV4S8 and GaMo4S8. Observation of the structural phase transition leads to the observation of {100}-type domain boundaries compatible with the compatibility critera based on crystal geometry. Furthermore, measurements of the magnetic textures of the different magnetic phases for GaV4S8 are presented and analysed. Results highlight a pinning of the pitch vector to the magnetic hard plane, and that the structural domain boundaries are by necessity magnetic domain boundaries. Analysing the influence of surface anisotropy and structural domain boundaries reveals a strong effect of both on the formation of magnetic patterns in their vicinity. Finally, the magnetoelectric effect of different lacunar spinels is investigated by measuring the surface potential with changing magnetic fields leading to a hysteretic behaviour in all materials.:Abstract/Kurzdarstellung 1 Introduction – Skyrmions meet Multiferroicity 2 Magnetic Skyrmion Lattices 2.1 What is a Skyrmion? 2.2 Formation of Skyrmion Lattices 2.2.1 Basic Considerations 2.2.2 Emergence of Skyrmion Lattices 2.3 General Properties of Skyrmions 2.4 Ways to Observe Skyrmions 3 Scanning Probe Microscopy 3.1 General Aspects 3.2 SPM in Contact Mode 3.2.1 Atomic Force Microscopy 3.2.2 Conductive Atomic Force Microscopy 3.2.3 Piezoresponse Force Microscopy 3.3 SPM in Non-Contact Mode 3.3.1 Atomic Force Microscopy 3.3.2 Kelvin Probe Force Microscopy 3.3.3 Magnetic Force Microscopy 3.4 About Scanning Dissipation Microscopy 3.4.1 Possible Origins of Dissipation 3.4.2 Measuring Dissipation 3.4.3 Mathematical Background 3.5 Experimental Setup 4 Investigated Materials 4.1 Cubic copper(II)-oxo-selenite Cu2O(SeO3) 4.2 Lacunar Spinel Chalcogenides 4.2.1 General Aspects and Materials Chosen 4.2.2 Structural Phase Transition and Expected Piezoresponse 4.2.3 Magnetic Phase Transition 4.2.4 Investigated Crystals 5 Investigations on Cu2OSeO3 5.1 Observing the Different Magnetic Phases 5.1.1 Analysis of Magnetic Textures with Magnetic Force Microscopy 5.1.2 Analysis of Magnetic Textures with Scanning Dissipation Microscopy 5.2 Analysis of the Magnetoelectric Effect 5.2.1 Observing the Magnetoelectric Effect with KPFM 5.2.2 Heuristic Description of the Magnetoelectric Effect 5.3 Analysing the Magnetic Phase Transitions with SPM 5.3.1 Motivation from Theory 5.3.2 Distinguishing the Helical, Conical and Field-Polarised Phases 5.3.3 The Helical–Conical Phase Transition 5.3.4 Passing through the Conical Phase 6 Investigations on GaV4S8 6.1 Observing the Structural Phase Transition 6.1.1 Results from nc-AFM 6.1.2 Results from ct-AFM and PFM 6.2 Observing the Magnetic Phases 6.3 Analysing the Magnetic SDM Images 6.3.1 Theoretical Considerations 6.3.2 Rescaling from the Measured to the Magnetic Hard Plane 6.3.3 Influence of the Surface on the Patterns Observed 6.4 Influence of Structural Domain Walls on Magnetic Patterns 7 Further Investigation on Lacunar Spinels 7.1 Investigations on GaMo4S8 7.1.1 Experimental Results 7.1.2 Theoretical Considerations 7.1.3 Evaluation of the Experimental Data 7.2 Magnetoelectric Effect of Lacunar Spinels 8 Remarks About Magnetic Non-Contact Dissipation 9 Summary and Outlook 9.1 Synopsis 9.2 Outlook – Probing the Future A Permissions For Usage of Content B Some Additional Information on Non-Contact Dissipation C Bonus Images Bibliography Publications Acknowledgements Erklärung / Skyrmionen sind teilchenartige Spintexturen, welche sich in sogenannten Skyrmionengittern anordnen. Jedes Skyrmion besitzt eine topologische Ladung. Dieses Konzept ist von bedeutendem Interesse für die Theorie und führt zu zusätzlicher Stabilität gegen externe Störungen. Da Skyrmionen mit geringen elektrischen Strömen bewegt werden können, sind sie auch Kanditaten für neuartige, spintronische Anwendungen wie den Racetrack-Speicher. Zusätzlich wurden vor einiger Zeit multiferroische Materialien entdeckt, welche ebenso Skyrmionengitter bilden und aufgrund dessen weitere, interessante Eigenschaften besitzen, welche sowohl für Anwendungen als auch für die Grundlagenforschung interessant sind. Inhalt dieser Dissertation ist die Untersuchung zweier verschiedener, exemplarischer multiferroischer Materialien mit Skyrmiongitterphasen mittels verschiedener Rastersondentechniken. Dies erlaubt das gleichzeitige Erfassen mehrerer Parameter auf einer lokalen Skala im Realraum mit einer einzigen Messung und somit die Untersuchung von Details, welche durch nicht-lokale Techniken nicht erfasst werden können. Im ersten Teil wird eine kurze Einleitung über magnetische Skyrmionen und die Rastersondenmikroskopie sowie Abrisse über deren theoretischen Hintergrund gegeben. Im Anschluß werden die untersuchten Materialien und deren Eigenschaften vorgestellt. Das erste System ist Cu2OSeO3, ein Isolator, welcher Bloch-artige Skyrmionengitter formiert und ein Typ-II Multiferroikum ist. Weitere Systeme gehören zur Klasse der lakunären Spinell-Chalkogenide, welche nach neuesten Erkenntnissen multiferroische Néel-artige Skyrmionen formieren, deren Modulationsvektor zur magnetisch harten Achse/Ebene fixiert ist. Ebenso sind diese aufgrund eines strukturellen Jahn-Teller Überganges Typ-I Multiferroika. Im zweiten Teil werden verschiedene Rastersondenuntersuchungen und ihre Ergebnisse präsentiert. Beginnend mit Cu2OSeO3, werden, neben den den magnetische Texturen der verschiedenen magnetischen Phasen, der magnetoelektrische Effekt und der helisch-konische Phasenübergang untersucht sowie mit grundlegenden theoretischen Modellen verglichen. Die Ergebnisse zeigen eine gute Übereinstimmung zwischen den Beobachtungen und der Theorie sowie mit anderen Meßmethoden. Im Anschluß werden verschiedene lakunäre Spinell-Chalkogenide, vor allem GaV4S8 und GaMo4S8, untersucht. Beobachtungen des strukturellen Phasenüberganges ergeben die Formierung von {100}-artigen Domänenwänden, welche mit den Vorhersagen der Kompatibilitätskriterien resultierend aus der Kristallgeometrie übereinstimmen. Des Weiteren werden Messungen der magnetischen Texturen der verschiedenen magnetischen Phasen von GaV4S8 präsentiert sowie analysiert. Die Ergebnisse heben hervor, daß der Modulationsvektor an der magnetisch harten Ebene fixiert ist und daß die strukturellen Domänengrenzen notwendigerweise auch die magnetischen Domänengrenzen sein müssen. Eine Analyse des Einflusses der Oberflächenanisotropie sowie der strukturellen Domänengrenzen zeigt eine starke Wirkung beider auf die Formierung magnetischer Texturen in ihrer Nähe. Schließlich wird der magnetoelektrische Effekt der lakunären Spinell-Chalkogenide durch Messung des Oberflächenpotentiales als Funktion des angelegten Magnetfeldes untersucht. Beobachtungen ergeben ein hysteretisches Verhalten in allen Materialen.:Abstract/Kurzdarstellung 1 Introduction – Skyrmions meet Multiferroicity 2 Magnetic Skyrmion Lattices 2.1 What is a Skyrmion? 2.2 Formation of Skyrmion Lattices 2.2.1 Basic Considerations 2.2.2 Emergence of Skyrmion Lattices 2.3 General Properties of Skyrmions 2.4 Ways to Observe Skyrmions 3 Scanning Probe Microscopy 3.1 General Aspects 3.2 SPM in Contact Mode 3.2.1 Atomic Force Microscopy 3.2.2 Conductive Atomic Force Microscopy 3.2.3 Piezoresponse Force Microscopy 3.3 SPM in Non-Contact Mode 3.3.1 Atomic Force Microscopy 3.3.2 Kelvin Probe Force Microscopy 3.3.3 Magnetic Force Microscopy 3.4 About Scanning Dissipation Microscopy 3.4.1 Possible Origins of Dissipation 3.4.2 Measuring Dissipation 3.4.3 Mathematical Background 3.5 Experimental Setup 4 Investigated Materials 4.1 Cubic copper(II)-oxo-selenite Cu2O(SeO3) 4.2 Lacunar Spinel Chalcogenides 4.2.1 General Aspects and Materials Chosen 4.2.2 Structural Phase Transition and Expected Piezoresponse 4.2.3 Magnetic Phase Transition 4.2.4 Investigated Crystals 5 Investigations on Cu2OSeO3 5.1 Observing the Different Magnetic Phases 5.1.1 Analysis of Magnetic Textures with Magnetic Force Microscopy 5.1.2 Analysis of Magnetic Textures with Scanning Dissipation Microscopy 5.2 Analysis of the Magnetoelectric Effect 5.2.1 Observing the Magnetoelectric Effect with KPFM 5.2.2 Heuristic Description of the Magnetoelectric Effect 5.3 Analysing the Magnetic Phase Transitions with SPM 5.3.1 Motivation from Theory 5.3.2 Distinguishing the Helical, Conical and Field-Polarised Phases 5.3.3 The Helical–Conical Phase Transition 5.3.4 Passing through the Conical Phase 6 Investigations on GaV4S8 6.1 Observing the Structural Phase Transition 6.1.1 Results from nc-AFM 6.1.2 Results from ct-AFM and PFM 6.2 Observing the Magnetic Phases 6.3 Analysing the Magnetic SDM Images 6.3.1 Theoretical Considerations 6.3.2 Rescaling from the Measured to the Magnetic Hard Plane 6.3.3 Influence of the Surface on the Patterns Observed 6.4 Influence of Structural Domain Walls on Magnetic Patterns 7 Further Investigation on Lacunar Spinels 7.1 Investigations on GaMo4S8 7.1.1 Experimental Results 7.1.2 Theoretical Considerations 7.1.3 Evaluation of the Experimental Data 7.2 Magnetoelectric Effect of Lacunar Spinels 8 Remarks About Magnetic Non-Contact Dissipation 9 Summary and Outlook 9.1 Synopsis 9.2 Outlook – Probing the Future A Permissions For Usage of Content B Some Additional Information on Non-Contact Dissipation C Bonus Images Bibliography Publications Acknowledgements Erklärung

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).

This thesis presents the synthesis and investigations of physical and chemical properties of multiferroic materials experimentally as well as theoretically. Multiferroics are materials in which at least two of the three ferroic orders, ferroelectricity, ferromagnetism and ferroelasticity occur in the same phase. Multiferroics, have the potential to be used as a four state as well as cross switchable memory devices. The thesis is organized into seven Chapters. Chapter 1 gives a brief overview of the different facets of multiferroics, explaining the origin of Multiferroicity and magnetoelectric coupling, their possible technological applications and the challenges involved. Chapter 2-4 concerns the experimental aspects and chapter 5-7 concerns the theoretical aspects. Chapter 2 deals with experimental investigations on nanoscale charge-ordered rare earth manganites. It shows with decreasing particle size the ferromagnetic interaction increases and the charge-ordering vanishes down to the lowest sizes. Chapter 3 describes magneto-dielectric, magnetic and ferroelectric properties of hexagonal LuMnO3. It also describes the Raman spectroscopy of this compound through the magnetic and ferroelectric transition temperatures. Chapter 4 deals with the anisotropic multiferroic properties in single crystals of hexagonal ErMnO3. In chapter 5 a brief introduction of density functional theory (DFT) is given. Chapter 6 deals with the magneto-structural changes, spin-phonon couplings and crystal field splittings for the different magnetic orderings LuMnO3. Chapter 7 elucidates the role of Lu d0-ness for the ferroelectricity observed of this compound.

The present research work focuses on the fabrication and characterization of epitaxial heterostructures of 0.7 Pb(Mg1/3N2/3)O3 – 0.3 PbTiO3 (PMN-PT) and La0.6Sr0.4MnO3 (LSMO) using multi target pulsed laser ablation technique. Different heterostructures such as bilayered thin films with different individual layer thickness; symmetric and asymmetric superlattices of different periodicities were fabricated. Roles of individual layer thickness, elastic strain and interfaces between PMN-PT and LSMO layers on different physical properties were studied. An attempt has been made to understand the influence of the charge depleted interface states in addition to the probable strain mediated elastic coupling effect on the observed magneto-dielectric response in these engineered heterostructures. Chapter 1 provides a brief introduction to the multiferroic materials, occurrence of magnetoelectric (ME) coupling in them, their possible technological applications and the challenges involved. A short historical account of the multiferroic research is discussed to emphasize the importance of artificial multiferroics, particularly the engineered thin film heterostructures. Finally the specific objectives of the current research are outlined. Chapter 2 deals with the various experimental studies carried out in this research work. It gives the details of the experimental set up and the basic operation principles of various structural and physical characterizations of the materials prepared. A brief explanation of material fabrication, structural, micro structural and physical property measurements is discussed. Chapter 3 addresses the phase formation, structural and microstructural features of the engineered heterostructures fabricated epitaxially on single crystalline LaAlO3 (100) substrates. A thin layer of LaNiO3 used as the bottom electrode material for electrical characterizations was grown on the bare substrate prior to the fabrication of the PMN-PT/LSMO heterostructures. The structural and microstructural features of different individual layers were also studied by fabricating single layer thin films of the materials. The effects of individual layer thicknesses on the surface roughness, grain size and lattice strain of the heterostructures are discussed. Chapter 4 deals with the ferroelectric studies of the PMN-PT/LSMO epitaxial heterostructures. Polarization hysteresis (P-E), capacitance – voltage (C-V) and pulsed polarization (PUND) measurements were carried out as functions of applied voltage, frequency and delay time to characterize the ferroelectric properties of the heterostructures. All the bilayered heterostructures exhibited robust ferroelectric response and contribution of non – remnant components to their polarization behaviour were observed from the P-E studies. The symmetric superlattices did not exhibit any ferroelectricity due to high leakage current conduction. After optimizing the LSMO and PMN-PT layer thicknesses ferroelectricity was observed in the asymmetric superlattices accompanied by substantial reduction in the leakage current conduction. The P-E loops were found to be asymmetrically shifted along the electric field axis in all the superlattices indicating the presence of dielectric passive layers and strong depolarizing fields at the interfaces between PMN-PT and LSMO. Chapter 5 deals with the ferromagnetic studies of the PMN-PT/LSMO heterostructures. All the heterostructures exhibited ferromagnetic behaviour in the temperature range of 10 K – 300 K with an in plane magnetic easy axis ([100]) compared to the out of plane ([001]) direction. The magnetization behaviour of the bilayers and superlattices as a function of magnetic field strength, temperature and different individual layer thickness of PMN-PT and LSMO are discussed in terms of the oxygen deficiency, magnetic dead layers and lattice strain effects in these engineered epitaxial heterostructures. Chapter 6 addresses the magneto-dielectric response, dielectric properties and ac conduction properties of the engineered biferroic heterostructures. In order to investigate the manifestation of strain mediated ME coupling in these heterostructures their dielectric response as a function of ac electric signal frequency have been studied under different static magnetic fields over a wide range of temperatures. The appearance of magneto-capacitance and its dependence on magnetic field strength and temperature along with the magnetoresistive characteristics of the heterostructures suggested that the charge depleted interfaces between PMN-PT and LSMO can have an effect on the observed dielectric response in addition to the probable strain mediated ME coupling. Dielectric characterization of the heterostructures performed over a wide range of temperature indicated a Maxwell-Wagner type relaxation mechanism. The manifestation of Maxwell-Wagner effect and the very low activation energy of ac conductivity obtained from the ac conduction studies revealed the strong influence of the charge depleted interfaces between PMN-PT and LSMO on the dielectric properties of the heterostructures. Chapter 7 deals with the dc leakage current conduction characteristics of the heterostructures. The leakage current characterization was performed over a wide range of temperature and analyzed in the framework of different models to investigate the leakage mechanism. All the heterostructures were found to obey the power law I∝Vα over the entire range of temperature with different values of α at different applied voltages. The bilayered heterostructures exhibited ohmic conduction in the lower electric field region and space charge limited conduction was observed at higher electric fields. On the other hand the low field dc conduction behaviour of the superlattices could not be attributed unambiguously to a single mechanism. Depending on the superlattice periodicity the low field conduction behaviour was dominated by either Poole-Frenkel (PF) emission or a combined contribution from the PF effect and ohmic conduction. At higher electric fields all the superlattices exhibited space charge limited conduction. Chapter 8 gives the summary and conclusions of the present study and also discusses about the future work that could give more insight into the understanding of the engineered epitaxial biferroic heterostructures.

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 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).

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.

After the prediction of magnetoelectric effect in Cr2O3, in early 1960's, D. Asrov became the first to experimentally verify this phenomenon. After the pioneering work on magnetoelectric materials in 1960's and 1970's, the discovery of large magnetoelectric effect in orthorhombic rare-earth manganite TbMnO3 has revived great interest in magnetoelectric materials, especially during the last decade. Magnetoelectric multiferroics have great potential in applications such as novel memory storage devices and sensors. As a result of extensive theoretical and experimental investigations conducted on rare-earth magnetoelectric manganites, TbMnO3 has become a prototype magnetoelectric multiferroic material. Orthorhombic rare-earth manganites RMnO3 (R = Gd, Tb and Dy) exhibit improper ferroelectricity where the origin of ferroelectricity is purely magnetic in nature. RMnO3 exhibit diverse and complex magnetic interactions and phases. Doped manganites of the type R1-xAxMnO3 (A = Ca, Sr and Ba) present a rich magnetic and electronic phase diagram. The doping concentration, average ion-size and size mismatch (i.e. disor-der) at A-site, all contribute to determine the ground state. A variety of magnetic phases, competing with each other, are responsible for many functional properties like magnetoelectric effect, colossal magnetoresistance (CMR), magnetostriction and magnetocaloric effect (MCE). In this context, studies of magnetoelectric materials are of great relevance from technical as well as fundamental aspects. Notably, complexity of electronic (and magnetic) phases and experimental difficulties in acquiring reliable measurement-data easily are the most concerning issues in establishing a clear understanding of magnetoelectric materials. In the magnetic phase diagram of RMnO3, GdMnO3 lies on the border between A-type antiferromagnetic and cycloidal antiferromagnetic ground states. Cycloidal spin arrangement is responsible for the induction of ferroelectricity in these materials. There are disparate opinions about the ground state of GdMnO3 (whether the ground state is ferroelectric or not). Understanding of the influence of rare-earth magnetic sublattice on magnetism in GdMnO3 (at low temperature) lacks clarity till date. Neutron scattering studies on GdMnO3 due to high absorption cross-section of Gd ion, yield little success in determining the nature of complex magnetic phases in this material. Interestingly, an earlier report on strontium-substituted gadolinium manganite Gd0.5Sr0.5MnO3 demonstrated the spontaneous electric polarization and related magnetoelectric effect. It was hypothesized that the observed ferroelectricity could be improper and electronic in nature. Strontium doping facilitates quenched disorder that leads to interesting magnetic phases and phase transitions. In order to understand the physical properties of gadolinium manganites and to unravel the relationship between them, it is essential to investigate high quality single crystals of these materials. This thesis deals with growth and investigation of several important physical phenomena of gadolinium manganites such as magnetic, electric, magnetoelectric and magnetocaloric properties. The thesis is organized in seven chapters. A brief summary of each chapter follows: Chapter:1 This chapter provides general introduction to magnetoelectric effect and multiferroicity. The term multiferroicity refers to simultaneous existence of magnetic and electric ordering in a single phase material. Magnetoelectric multiferroics have shown great potential for several applications. They exhibit cross coupling between the electronic and magnetic order parameters, hence basics of various magnetic interactions (and magnetism) are brie y discussed in the rst section of the chapter. It is followed by a brief discussion about the principle of magnetoelectric effect. Magnetoelctric coupling is broadly classified into two types namely, direct coupling and indirect coupling. In the former, the emphasis is given on linear magnetoelectric effect. The concept of multiferroicity is introduced in the next section followed by a brief overview and application potential of multiferroics. Further, classi cation scheme of multiferroic materials is discussed. The concept of improper ferroelectricity and description of subcategories namely, magnetic ferroelectric, geometric ferroelectric and electronic ferroelectric are documented. Magnetic ferroelectric category is considered the most relevant; featuring the type of ferroelectric material as GdMnO3 referred in this thesis. The microscopic theory for mechanism of ferroelectricity in spiral antiferromagnets is presented. While brie ng the thermodynamic background of the magnetocaloric effect, indirect estimation of two important characteristics namely, isothermal magnetic entropy change (∆SM ) and adiabatic change in temperature (∆Tad) under the application of magnetic field are dealt with. In the last part of the chapter, motivation and scope of the thesis is discussed. Chapter:2 This chapter outlines various experimental methodologies adopted in this work. It describes the basic principles of various experimental techniques and related experimental apparatuses used. The chapter starts with the synthesis tech-niques used in the preparation of different compounds studied. The principle of oat-zone method, employed for single-crystal growth, is described. Orientation of single crystals was determined using a home-built back- reflection Laue set up. The basics of Laue reflection and indexing procedure for recorded Laue photographs are described. Various physical properties (electric, magnetic, thermal, magnetoelectric and magnetocaloric properties) were studied using commercial as well as home-built experimental apparatuses. Design and working principle of all the experimental tools are outlined in this chapter. Fabrication details, interfacing of measurement instruments and calibration (standardization) of equipment used in this work are described in appropriate sections. Chapter:3 Chapter-3 describes the investigation of various physical properties of high quality single crystals of magnetoelectric multiferroics, GdMnO3. Synthesis of GdMnO3 is carried out using solid state synthesis route. Single phase nature of the material is confirmed by X-ray powder diffraction technique. Single crystals of GdMnO3 are grown in argon ambience using oat-zone method. As grown crystals are oriented with the help of back-reflection Laue method. GdMnO3 exhibits incommensurate collinear antiferromagnetic phase below 42 K and transforms to canted A-type antiferromagnetic phase below 23 K. Magnetic and specific heat studies have revealed very sharp features near the magnetic transitions which also confirm the high quality of the single crystal. dc magnetization studies illustrate the anisotropic behavior in canted A-type antiferromagnetic phase and clarifies the influence of rare-earth magnetic sub-lattice on overall magnetism (at low temperature). Application of magnetic field (above 10 kOe) along `b' axis helps formation of the cycloidal antiferromagnetic phase. Here, spontaneous electric polarization is induced along `a' axis. The temperature variation plot of dielectric constant, ϵa (under ap- plied magnetic field along `b' axis) shows sharp anomalies in the vicinity of magnetic ordering transitions suggesting magnetodielectric effects. Magnetic field tuning of electric polarization establish the magnetoelectric nature of GdMnO3. Magnetocaloric properties of single crystals of GdMnO3 are investigated using magnetic and magnetothermal measurements. The magnitude of the giant magnetocaloric effect observed is compared with that of other rare-earth manganite multiferroics. Magnetocaloric studies shed light on magnetic ordering of rare-earth ion Gd3+. The phenomenon of inverse magnetocaloric effect observed at low temperature and under low fields is possibly linked to the ordering of Gd3+ spins. Complex interactions between the 3d and 4f magnetic sublattices are believed to influence magnetocaloric properties. Chapter:4 The details of synthesis and single crystal growth of Gd0.5Sr0.5MnO3 using oat-zone method are presented in Chapter 4. Single phase nature of the material is veri ed by carrying out powder x-ray diffraction analysis and confirmation of single crystallinity and orientation through back-reflection Laue method. Electric transport studies reveal semiconductor-like nature of Gd0.5Sr0.5MnO3 until the lowest temperature achieved. This is due to charge localization process which occurs concurrently with decrease in temperature. Gd0.5Sr0.5MnO3 exhibits charge-ordered insulator (COI) phase below 90 K (ac-cording to an earlier report). It is found that under application of magnetic field above a critical value, charge ordering melts and the phase transforms to ferromagnetic metallic (FMM) phase. This transformation is first-order in nature with associated CMR (109%). The first-order phase transition (FOPT) occurs between competing COI and FMM phases and manifests as hysteresis across the FOPT. Strontium doping at A-site induces a large size mismatch at A-site resulting in high quenched disorder in Gd0.5Sr0.5MnO3. The disorder plays a significant role in CMR as well as glass-like dynamics within the low-temperature magnetic phase. ac susceptibility studies and dynamic scaling analysis reveal very slow dynamics inside the low-temperature magnetic phase (below 32 K). According to an earlier report, spontaneous electric polarization and magnetoelectric effect were pronounced near FOPT (at 4.5 K and 100 kOe) between COI and FMM phases. It is prudent to investigate FOPT across COI and FMM phases in Gd0.5Sr0.5MnO3 to understand complex magnetic phases present. Thermodynamic limits of the FOPT (in magnetic field - temperature (H-T) plane), such as supercooling and superheating, are experimentally determined from magnetization and magnetotransport measurements. Interestingly, thermomagnetic anomalies such as open hysteresis loops are observed while traversing the FOPT isothermally or isomagnetically in the H-T plane. These anomalies point towards incomplete phase transformation while crossing the FOPT. Phenomenological model of kinetic arrest is invoked to understand these anomalies. The model put for-ward the idea that while cooling across the FOPT, extraction of specific heat is easier than that of latent heat. In other words, phase transformation across FOPT is thermodynamically allowed but kinetics becomes very slow and phase transformation does not occur at the conventional experimental time scale. Magnetization relaxation measurements (at 89 kOe) with field-cooled magnetization protocol reveal that the relaxation time constant rst decreases with temperature and later, increases non-monotonically below 30 K. This qualita-tive behavior indicates glass-like arrest of the FOPT. Further, thermal cycling studies of zero field-cooled (ZFC) and eld-cooled (FC) magnetization indicate that a low temperature phase prepared with ZFC and FC protocols (at 89 kOe) is not at equilibrium. This confirms the kinetic arrest of FOPT and formation of magnetic phase similar to glass. Chapter:5 Chapter-5 deals with the investigation of the effect of an electric field on charge ordered phase in Gd0.5Sr0.5MnO3 single crystals. As discussed in the previous chapter, application of magnetic field above a critical value collapses the charge ordered phase which transforms to FMM phase. In this view, it is interesting to investigate effect of electric field on the charge ordering. There are various reports on doped manganites such as Pr1-xCaxMnO3 (x = 0:3 to 0:4) that claim melting of charge ordering under application of electric field (or current) above a critical value. In this thesis work, current - voltage (I - V) characteristics of Gd0.5Sr0.5MnO3 are studied at various constant temperatures. Preliminary measurements show that the I-V characteristics are highly non-linear and are accompanied by the onset of negative differential resistance (NDR) above a critical current value. However, we suspect a major contribution of Joule heating in realization of the NDR. Continual I - V loop measurements for five loops revealed thermal drag and that the onset of NDR shifts systematically towards high current values until it disappeared in the current window. Two strategies were employed to investigate the role of Joule heating in realization of NDR: 1) monitoring the sample surface temperature during electric transport measurement and 2) reducing of the Joule heating in a controlled manner by using pulsed current I - V measuremenets. By tuning the duty cycle of the current pulses (or in other words, by controlling the Joule heating in the sample), it was feasible to shift the onset of NDR to any desired value of the current. At low magnitude of the duty cycle in the current range upto 40 mA, the NDR phenomenon did not occur. These experiments concluded that the NDR in Gd0.5Sr0.5MnO3 is a consequence of the Joule heating. Chapter:6 `Chapter-6 deals with the thermal and magnetocaloric properties of Gd0.5Sr0.5MnO3 oriented single crystals. Magnetocaloric properties of Gd0.5Sr0.5MnO3 have been studied using magnetic and magnetothermal measurements. Tempera-ture variation of ∆SM is estimated for magnetic field change of 0 - 70 kOe. The eld 70 kOe is well below the critical magnetic eld required for FOPT between COI and FMM phases. Magnetzation - field (M-H) loop shows minimal hysteresis for measurements up to 70 kOe. The minimal hysteresis behavior al-lows one to make fairly accurate estimation of magnetocaloric properties. ∆Tad was separately estimated from specific heat measurements at different magnetic fields. Specific heat studies show the presence of Schottky-like anomaly at low temperature. Chapter:7 Finally, Chapter-7 summarizes various experimental results, analyses and conclusions. A broad outlook of the work in general with future scope of research in this area are outlined in this chapter.