Le contrôle de la polarisation et de l’aimantation par le biais de champs magnétiques et électriques respectifs font des systèmes magnétoélectriques des candidats prometteurs à de nombreuses applications, parmi lesquelles les dispositifs micro-ondes, les dispositifs de stockage de données à haute densité, etc. L’élaboration d’hétérostructures toujours plus innovantes reste un défi majeur dans le but d’optimiser les densités d’interfaces entre les phases ferroélectriques et ferromagnétiques,et ainsi promouvoir les interactions de couplage mécaniques. Au cours de ce projet de thèse, deux stratégies sont mises en oeuvre pour la conception des matériaux : i) une structure coeur-écorce de type (1-1) composée de nanofils ferromagnétiques (1) dans des nanotubes ferroélectriques (1) àl’intérieur d’une membrane nanoporeuse tridimensionnelle auto supportée etii) une structure en couche mince de type (1-3) constituée de nanofils ferromagnétiques (1) supportés sur un substrat rigide et encapsulés dans une matrice ferroélectrique (3). / Controlling polarization or magnetization by an applied magneticand electric field respectively make magnetoelectric systems promisingcandidates for applications in microwave devices, high density data storagedevices, etc. Designing innovative magnetoelectric heterostructures is thus achallenge to optimize interface density between both ferroelectric andferromagnetic phases, and promote mechanical coupling interactions. In thisthesis project, two strategies are followed for material design: i) 1-1 coreshellstructure with ferromagnetic nanowires (1) inside ferroelectricnanotubes in a self-supported tridimensionnal porous template (1) and ii) 1-3structure where ferromagnetic nanowires (1) are supported on a substrateand embedded in a ferroelectric matrix (3).

Une sous-classe intéressante de matériaux multiferroïques est celle des composés multiferroïques magnétoélectriques, dans lesquels il existe un couplage entre les paramètres d’ordres ferroïques (magnétique et électrique). De ce point de vue, la classe des matériaux Ga₂₋ₓFeₓO₃ a attiré l’attention des chercheurs. Ces composés sont actuellement connus pour leur température de transition élevée ainsi que pour l’interaction possible entre leurs propriétés ferrimagnétiques et piézoélectriques. Leur structure cristallographique et magnétique est assez complexe, du fait du désordre de substitution interne Fe/Ga. Les oxydes M₂Ga₂Fe₂O₉ (M=In, Sc) appartiennent à cette même famille de matériaux et ont été synthétisés pour la première fois afin d’obtenir une structure cristallographique ordonnée de GaFeO₃. Afin d’étudier les propriétés physiques de ces différents composés, nous avons synthétisé par la méthode de la zone flottante (au four à image), en utilisant différentes conditions de croissance, des monocristaux de composition Ga₂₋ₓFeₓO₃ (x=0.90, 1.00 et 1.10). Nous avons également élaboré des échantillons polycristallins de composés GaFeO₃ faiblement dopés en indium ainsi que le composé M₂Ga₂Fe₂O₉ (M=In, Sc). Nous avons enfin préparé de monocristaux de composition In₂Ga₂Fe₂O₉ par la méthode de croissance en flux. L’affinement Rietveld des diffractogrammes des rayons X et des neutrons nous a permis de montrer que les céramiques de GaFeO₃ faiblement dopées en indium et les monocristaux de Ga₂₋ₓFeₓO₃ cristallisent dans le groupe d’espace Pc2₁n. Les paramètres cristallins et la température de Néel caractéristiques pour les monocristaux de Ga₂₋ₓFeₓO₃ varient de manière linéaire avec la teneur en fer. Les affinements nous ont permis de conclure que la structure de ces composés est caractérisée par un désordre élevée (25% de la quantité du fer se trouve sur les sites natifs du gallium). L’incorporation graduelle de l’indium s’accompagne d’une augmentation du volume de la maille ainsi qu’à une diminution de la température de transition magnétique. Le spectre d’excitations magnétiques mesuré pour les cristaux de Ga₂₋ₓFeₓO₃ nous a permis de mettre en évidence une coexistence de l’ordre ferrimagnétique à longue portée et d’un signal de diffusion diffuse en-dessous de la température de Néel. Ce signal diffus suggère l’existence d’une composante de type verre de spin du fait du désordre interne des sites. L'étude de la variation thermique de la constante diélectrique sur un cristal de GaFeO₃ révèle l’absence d’un couplage magnétoélectrique au sein de ces matériaux. L’affinement Rietveld des diagrammes de diffraction des rayons X et des neutrons mesurés sur les poudres de M₂Ga₂Fe₂O₉ (M=In, Sc) révèle une structure orthorhombique de type Pba2 fortement désordonnée, avec quatre sites cationiques d’occupation mixte. Les données de susceptibilité DC et AC couplées avec les mesures de chaleur spécifique et les spectres Mössbauer indiquent, en-dessous d’une température de Tg ≈ 19 K, l’existence d’un état fondamental de type verre de spin dans ce système. Les mesures du spectre d’excitations magnétiques ont mis en évidence l’absence d’ordre magnétique à longue portée et confirment l’existence d’une transition d’un état paramagnétique vers un état verre de spins. L’existence d’un comportement de type verre de spin dans les systèmes Ga₂₋ₓFeₓO₃ et M₂Ga₂Fe₂O₉ (M=In, Sc) souligne l’importance du désordre interne pour la caractérisation de l'état fondamental magnétique. / An interesting sub-class of the multiferroic materials are the multiferroic magnetoelectrics, in which exists a coupling between the two ferroic order parameters (magnetic and electric). From this viewpoint, the case of the (Ga₂₋ₓFeₓO₃)-class of materials has retained special attention. These compounds are now well-known for their high transition temperature as well as for the potential interaction between their ferrimagnetic and piezoelectric properties. Their crystallographic and magnetic structure are quite complicated, due to the existence of an internal site disorder. M₂Ga₂Fe₂O₉ (M=In, Sc) belongs to the (Ga₂₋ₓFeₓO₃)-class of materials and it was first synthesized in an attempt to obtain an ordered GaFeO₃ crystallographic structure. In order to study the physical properties of these compounds, Ga₂₋ₓFeₓO₃ (x=0.90, 1.00 and 1.10) single crystals have been synthesized by the floating zone method in an infrared image furnace, using different growth conditions. Indium doped GaFeO₃ (up to 10% indium content amount) and M₂Ga₂Fe₂O₉ (M=In, Sc) polycrystalline materials have been prepared by solid state reaction. Also, In₂Ga₂Fe₂O₉ single crystals were prepared by the flux method. The indium doped GaFeO₃ and Ga₂₋ₓFeₓO₃ samples crystallize in the Pc2₁n space group as determined from Rietveld refinement of the X-ray and neutron single crystals and powder patterns. The cell parameters and the Néel temperature (TN) of the Ga₂₋ₓFeₓO₃ single crystals varies linearly with the iron content amount. The occupation factors were calculated by refinement and the results showed a disordered structure (25% of the iron amount is found on the native gallium sites). The gradual incorporation of indium is accompanied by an increase of the cell volume and a decrease of the magnetic transition temperature. The magnetic excitations spectra measured for the Ga₂₋ₓFeₓO₃ single crystals revealed a coexistence of a ferrimagnetic ordering and a diffuse scattering signal below the Néel temperature. The diffuse signal suggests the existence of a spin glass like component due to the internal site disorder. Dielectric investigations showed no temperature dependent anomaly of the dielectric constant for the GaFeO₃ single crystal, suggesting a lack of a magnetoelectric coupling signal in this system. The Rietveld refinement of the X-ray and neutron powder patterns for the M₂Ga₂Fe₂O₉ (M=In, Sc) revealed a highly disordered orthorhombic Pba2 structure, offering four mixed cationic crystallographic sites. DC and AC susceptibility data in conjunction with the heat capacity data and Mössbauer measurements indicated a spin-glass-like behavior in this system, with a freezing temperature near Tf ≈ 19 K. The absence of an long range magnetic ordering and the existence of a transition from a paramagnetic state to a «frozen» state were showcased by the spin dynamics spectra measurements. The evidence for glassy behavior in the Ga₂₋ₓFeₓO₃ and M₂Ga₂Fe₂O₉ (M=In, Sc) systems highlights the importance of site disorder in determining the ground state magnetic properties.

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.