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.

Dans cette thèse, les interactions magnétoélectriques et optomagnétiques transmises par les contraintes dans les structures ferroélectriques/ferromagnétiques sont étudiées. Nous montrons que la dynamique des déformations du Pb(ZrxTi1-x)O3 aboutit à la manipulation électrique sous-coercitive de multi-états ferroélastiques rémanents. La mesure par une jauge résistive de ces états, ainsi que l'écriture et l'effacement électriques et le stockage ferroélastique, sont démontrés. La configuration des contraintes de matériaux ferroélectriques créée électriquement, permet de modifier l'anisotropie magnétique d'une couche ferromagnétique. Ce phénomène est utilisé pour contrôler le champ magnétique coercitif des composants magnétostrictifs des vannes de spin au moyen des déformations. L’irradiation lumineuse est également utilisée pour entraîner une photostriction rémanente dans le BiFeO3. Cette déformation rémanente est transférée à une couche ferromagnétique et permet un contrôle optique de la coercivité magnétique. Nous montrons comment les états magnétiques peuvent être écrits au moyen de la lumière et effacés par un champ électrique. / In this thesis, the strain-mediated magnetoelectric and optomagnetic interactions in ferroelectric/ferromagnetic structures are studied. The strain dynamics in Pb(ZrxTi1-x)O3 is shown to result in the sub-coercive electrical manipulation of its remanent ferroelastic multi-states. The resistive readout of these states provided by the strain gauge layers, together with the electrically-triggered ferroelastic writing, storage, and erasing, are demonstrated. These strain configurations created by electric fields in ferroelectrics can effectively impact the magnetic anisotropy of a ferromagnetic adlayer. This phenomenon is shown to control the magnetic coercive field of the magnetostrictive components of spin valves via the strain. Light irradiation is shown to result in remanent photostriction effect (photo-driven deformation) in BiFeO3. Such optically-induced remanent deformations can be transferred to a ferromagnetic adlayer and result in the optical control of the magnetic coercive force. It is shown here how magnetic states can be written by light and erased by an electric field.

Dans cette thèse de doctorat nous présentons une étude théorique de deux types d'instabilitésferroélectriques: celles apparaissant dans des géométries confinés et celles induites par le magnétismedans dans composés massifs de structure perovskite. Dans une première partie nous abordons leproblème des instabilités ferroélectriques apparaissant dans des nanotubes et des nanocoquillesoù nous développons un modèle théorique phénoménologique approprié à ces structures. Nousétudions comment l'émergence de la polarisation est affectée par (i) l'épaisseur des nanostructures,(ii) par la réponse diélectrique des matériaux environant la couche ferroélectrique et (iii) les conditionsaux interfaces. Nous observons un effet de taille finie topologique qui peut promouvoirune compétition inhabituelle entre deux types de distribution de la polarization, irrotationel eten vortex, dans la limite des très petites épaisseurs. Dans une deuxième partie nous utilisons descalculs ab-initio à base de la théorie de la fonctionnelle de la densité pour étudier les instabilitésferroélectriques des perovskites manganites à base de terres rares (RMnO3). A partir de ces calculsnous prédisons qu'il est possible d'induire une transition de phase sous pression dans EuMnO3 lefaisant transiter d'un ordre antiferromagnétique de type A isolant vers un ordre ferromagnétiquemétallique sous pression. Ce type de transition n'avait jamais été reporté précédemment dans lesmatériaux RMnO3. Nous étendons ensuite cette analyse à l'étude des effets de strain épitaxial dansles films minces de TbMnO3 et EuMnO3. Nos résultats montrent que le diagramme de phase souscontrainte d'épitaxie est bien plus riche que celui sous pression hydrostatique. Nous trouvons queles types antiferromagnétiques E-AFM et E*-AFM sont stabilisés dans le cas de TbMnO3, où letype E*-AFM est une phase métallique polaire. Dans le cas de EuMnO3, nous trouvons une phaseantiferromagnétique de type E qui n'a pas été observée sous pression hydrostatique. / In this thesis, we present a theoretical study of two types of ferroic instabilities: the ferroelectric instability in novel confined geometries and magnetic instabilities controlled by the distortion of the underlying crystal lattice. On the one hand, we consider in detail the ferroelectric instability, specifically, in the nanotubes and the spherical nanoshells and develop a phenomenological theory for describing such an instability. We determine how the emergence of polarization is affected bythe thickness of the nanoparticle, the dielectric properties of the surrounding media and the interfacial boundary conditions. We finnd an intriguing topological finite-size effect that can promote an unexpected competition between two different types of distribution of polarization - irrotational and vortex-like - in the ultra-thin limit. One the other hand, we employ a different formalism to investigate the structural, electronic and magnetic properties of the rare-earth manganites. Specifically,we conduct a theoretical investigation from first-principles calculations. First, we predict a pressure-induced A-AFM insulator to FM metal transition on EuMnO3 under hydrostatic pressure, that is unprecedented in the multiferroic rare-earth manganites RMnO3. This investigation is extended to the study to the epitaxial strain effects on both EuMnO3 and TbMnO3 thin films. We show that epitaxial strain generates a much richer phase diagram compared to hydrostatic pressure. We predict novel magnetically-induced insulator { metal and polar { non-polar transitions. More specifically, we find that both the multiferroic E-AFM order and the polar metallic E*-AFM state are stabilized in TbMnO3 by means of epitaxial strain. In the contrast, we find a novel epitaxial-strain-induced multiferroic E-AFM state in EuMnO3 that cannot be obtained by means of just hydrostatic pressure.

Les propriétés physiques intriguantes présentées dans certains oxydes complexes de métaux de transition sont attractives non seulement pour la recherche fondamentale, mais pour les applications, par exemple, la supraconductivité, la magnétorésistance colossale, la multiferroïcité. La forte corrélation électronique est à l'origine de ces comportements.La thèse contribue à la fois à l'effort expérimental de détermination de la structure électronique des systèmes fortement corrélés et à l'analyse critique des modèles théoriques les décrivant. Expérimentalement, le travail a porté sur la diffusion inélastique résonante des rayons X pour déterminer les excitations électroniques de basse énergie de l'état fondamental (excitations d-d, transfert de charge, etc.). Les expériences ont été effectuées sur la ligne de lumière SEXTANTS du synchrotron SOLEIL (France), en utilisant le spectromètre AERHA avec un haut pouvoir résolvant. L'analyse des résulats a été focalisée sur la détermination du champ cristallin des métaux de transition en jeu en utilisant la théorie des multiplets avec champ cristallin.Deux systèmes ont été étudiés: les composés multiferroïques RMnO3 (R = Tb, Dy) et un isolant de Mott ferromagnétique Lu2V2O7. / The intriguing physical properties presented in some complex oxides of transition metals draw attention not only in fundamental research but also in applications, for instance, superconductivity, colossal magnetoresistance, multiferroicity. The strong electronic correlation is at the origin of these behaviours. The thesis is a contribution to both the experimental effort to determine the electronic structure of strongly correlated systems and the critical assesment of the theoretical models describing them. Experimentally, the work of is devoted to the investigations of the low-energy excitations (d-d excitations, charge transfer, …) of the ground state by resonant inelastic x-ray scattering. The experiments have been performed on the SEXTANTS beamline of SOLEIL synchrotron (France) using the high resolving power AERHA spectrometer. The analysis of the data has been focused on the determination of the the crystal field of the transition metal involved using the crystal field multiplets theory.Two systems have been investigated: the multiferroics RMnO3 (R = Tb, Dy) and the ferromagnetic Mott insulator Lu2V2O7.

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.

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

Crystal growth and characterization of few multifunctional materials with perovskite (ABX3) structure are discussed in this thesis. Efforts were made to modify the magnetic and electric behaviour of these materials by selective tuning of A, B and X components. Structural, magnetic and dielectric characterization are detailed in various chapters for doped (A and B site) rare-earth manganites and organometallic compounds with different (Chloride or formate) anions. The relevant aspects of crystal structure and its relationship with ordered ground states are discussed in the introductory chapter. A detailed review of prominent theories pertaining to magnetic and ferroelectric ordering in the literature is provided. Growth of various inorganic compounds by solid-state reaction and floating zone method as well as use of solvothermal techniques for growing organometallic compounds are discussed. Material preparation, optimization of crystal growth processes and results of characterization are addressed in various chapters. The effect of Yttrium doping on structural, magnetic and dielectric properties of rare-earth manganites (RMnO3 where R = Nd, Pr) has been investigated. Neutron diffraction studies (Pr compounds) confirm A-type antiferromagnetic structure and fall in transition temperature as the Yttrium doping level increases. Diffraction experiments in conjunction with dc magnetization and ac susceptibility studies reveal magnetic frustration in excess Yttrium dopedcompounds. When mutliglass properties of 50% B-site doped Nd2NiMnO6 were investigated, evidence of re-entrant cluster glass phase was seen probably due to presence of anti-site disorder. The relaxor-like dielectric behaviour arises from crossover of relaxation time in grain and grain boundary regions. Multiferroic behaviour of the organometallic compound (C2H5NH3)2CuCl4 as well as the ferroelectric transition were investigated in detail. The role of Hydrogen bond ordering in driving structural transitions is elucidated by low temperature dielectric and Raman studies in (C2H5NH3)2CdCl4. It was found possible to tune the magnetic and ferroelectric properties in metal formate compounds (general formula AB(HCOO)3) by selectively choosing organic cations [(CH3)2NH2+; C(NH3)3+] and transition metal ion [B = Mn, Co and Cu]. The nature of magnetic ordering and transition temperature could be altered by the transition metal ion. The effect of reorientation of organic cations which leads to ferroelectric nature is discussed using dielectric and pyroelectric data. Significant results are summarized in the chapter outlining general conclusions. Future prospects of work based on these observations are also provided. The conclusions are corroborated by detailed analysis of experimental data.