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.

The main challenge in the design of vibration energy harvesters is the optimization of energy outcome relative to the applied excitation to reach a higher efficiency in spite of the weakness of ambient energy sources. One promising principle of vibration converters is magnetoelectricity due to the outstanding properties of magnetostrictive and piezoelectric laminate composites, which provide interesting possibilities to harvest energy from low amplitude and low frequency vibration with relatively high energy outcome. For these devices, ensuring high deformations in the magnetostricive layers, improvement of the magnto-mechanical and the electro-mechanical couplings are highly required for the optimization of the energy outcome. This thesis primarily aims to develop a model based harvester design for magnetoelectric (ME) converters. Based on a comprehensive understanding of the complex energy flow in magnetoelectric transducers, several design parameters are investigated. For instance, magnetostriction in a Terfenol-D plate is investigated by means of atomic force microscopy under similar conditions as within magnetoelectric transducers. A novel measurement approach was successfully developed to detect the evolution of magnetic domains and measure deformations in a Terfenol-D plate in response to externally non-uniform applied magnetic fields. Furthermore, a finite element model is developed to predict the induced voltage in the ME transducer as a response to the magnet’s displacement, corrected based on atomic force microscopy measurements, and used for the design of the harvester. The presented three- dimensional model takes into consideration the nonlinear behaviour of the magnetostrictive and piezoelectric materials. Additionally, three novel converters having different magnetic circuits are designed and analysed analytically based on Lindstedt-Poincaré method. The effects of the structure parameters, such as the nonlinear magnetic forces, the magnetic field distribution and the resonance frequency are discussed, and the electric output performances of the three designed converters are evaluated. In order to improve both mechanical and electrical coupling between the piezoelectric and the magnetostrictive layers, a bonding technique at room temperature is proposed which uses conductive polymer nanocomposites. Two magnetoelectric transducers are fabricated based on this technique having 1 wt.% and 2 wt.% concentration of multiwalled carbon nanotubes in epoxy resin. Another magnetoelectric transducer is fabricated by a classical technique for comparison purposes. In order to validate the design, a series of demonstrators are designed and fabricated according to the simulation and optimization results. The proposed design is composed by a cantilever beam, a magnetic circuit with several magnet arrangements and a magnetoelectric transducer, which is formed by a piezoelectric PMNT plate bonded to two magnetostrictive Terfenol-D layers. In this design, external vibrations are converted to magnetic field changes acting on the magnetostrictive layers leading to deformations, which are transmitted directly to the piezoelectric layer. The converters are tested under harmonic excitations and real vibration profiles reproduced by an artificial vibration source. Different parameters were investigated experimentally including the magnetic forces between the transducer and the magnetic circuit and the used bonding technique. Tuning the resonance frequency of the ME converter is also addressed using a simple screw/nut system, which allows to control the relative position and therefore the magnetic forces between the magnetic circuit and the transducer. The magnetoelectric transducer bonded with 2 wt.% concentration of multiwalled carbon nanotubes shows better output performances than the two other ME transducers under similar excitations. A maximum power output of 2.42 mW is reached under 1 mm applied vibration at 40 Hz. This performance presents an improvement of minimum 20 % of the reached energy outcome by other magnetoelectric vibration converters using single ME transducer at comparable applied excitations. / Die größte Herausforderung bei der Konstruktion von Vibrations-Energiewandlern ist die Optimierung der gewonnenen Energie im Verhältnis zur angewandten Anregung, um trotz schwacher Umgebungsenergiequellen einen hohen Wirkungsgrad zu erreichen. Ein vielversprechendes Prinzip von Vibrationswandlern ist die Magnetoelektrizität aufgrund der hervorragenden Eigenschaften von magnetostriktiven und piezoelektrischen Verbundwerkstoffen, die interessante Möglichkeiten bieten, Energie aus niederfrequenten Schwingungen mit kleinen Amplituden zu gewinnen. Bei diesen Wandlern ist die Sicherstellung hoher Verformungen in den magnetostriktiven Schichten, die Verbesserung der magnetisch-mechanischen und der elektromechanischen Kopplungen für die Optimierung des Energieertrages sehr wichtig. Diese Arbeit zielt in erster Linie auf die Entwicklung eines modellbasierten Entwurfs für magnetoelektrische (ME) Wandler ab. Basierend auf einem umfassenden Verständnis des komplexen Energieflusses in magnetoelektrischen Wandlern werden mehrere Entwurfsparameter untersucht. So wird beispielsweise die Magnetostriktion in einer Terfenol-D-Platte mittels Rasterkraftmikroskopie unter ähnlichen Bedingungen untersucht wie in magnetoelektrischen Wandlern. Dabei wurde eine neuartige Messmethode erfolgreich entwickelt, um die Entwicklung von magnetischen Domänen zu erfassen und die Deformation in einer Terfenol-D-Platte als Reaktion auf extern ungleichmäßig angelegte Magnetfelder zu messen. Darüber hinaus wird ein Finite-Elemente-Modell entwickelt, um die induzierte Spannung im ME-Wandler als Reaktion auf die Verschiebung des Magneten vorherzusagen, welches auf der Grundlage von Atomkraftmikroskopie Messungen korrigiert und für den Entwurf des Energiewandlers verwendet wird. Das vorgestellte dreidimensionale Modell berücksichtigt das nichtlineare Verhalten der magnetostriktiven und piezoelektrischen Materialien. Zusätzlich werden drei neuartige Wandler mit unterschiedlichen Magnetkreisen nach dem Lindstedt-Poincaré Verfahren konzipiert und analytisch analysiert. Die Auswirkungen der Strukturparameter, wie die nichtlinearen Magnetkräfte, die Magnetfeldverteilung und die Resonanzfrequenz, werden diskutiert und die elektrischen Ausgangsleistungen der drei ausgelegten Wandler ausgewertet. Um die mechanische und elektrische Kopplung zwischen der piezoelektrischen und der magnetostriktiven Schicht zu verbessern, wird eine bei Raumtemperatur prozessierbare Verbindungstechnik vorgeschlagen, bei der leitfähige Nanokomposite verwendet werden. Zwei magnetoelektrische Wandler werden basierend auf dieser Technik mit einer Konzentration von 1 wt.% und 2 wt.% an mehrwandigen Kohlenstoff-Nanoröhren in Epoxidharz hergestellt. Ein weiterer magnetoelektrischer Wandler wurde zu Vergleichszwecken mit einer klassischen Technik hergestellt. Für die Validierung des Entwurfes wird eine Reihe von Demonstratoren entsprechend den Simulations- und Optimierungsergebnissen konstruiert und gefertigt. Der vorgeschlagene Entwurf besteht aus einem Trägerbalken, einem Magnetkreis mit mehreren Magnetanordnungen und einem magnetoelektrischen Wandler, der aus einer piezoelektrischen PMNT-Platte besteht, die mit zwei magnetostriktiven Terfenol-D-Schichten verbunden ist. Bei dieser Konstruktion werden externe Schwingungen in Magnetfeldänderungen umgewandelt, die auf die magnetostriktiven Schichten wirken und zu Verformungen führen, die direkt auf die piezoelektrische Schicht übertragen werden. Die Wandler werden unter harmonischen Anregungen und mit realen Schwingungsprofilen getestet, die von einer künstlichen Schwingungsquelle reproduziert werden. Verschiedene Parameter wurden experimentell untersucht, darunter die magnetischen Kräfte zwischen dem Wandler und dem Magnetkreis sowie die verwendete Verbindungstechnik. Die Abstimmung der Resonanzfrequenz des ME-Wandlers erfolgt ebenfalls über ein einfaches Schrauben-Mutter-System, das es ermöglicht, die relative Position und damit die magnetischen Kräfte zwischen Magnetkreis und Wandler zu steuern. Der magnetoelektrische Wandler, der mit einer Konzentration von 2 wt.% mehrwandiger Kohlenstoff-Nanoröhrchen verbunden ist, zeigt bessere Ausgangsleistungen als die beiden anderen ME-Wandler bei ähnlichen Anregungen. Eine maximale Ausgangsleistung von 2,42 mW wird bei 1 mm angelegter Vibration bei 40 Hz erreicht. Diese Leistung stellt eine Verbesserung von mindestens 20 % im Vergleich zu anderen magnetoelektrischen Schwingungsumrichtern dar, welche mit einem einzigen ME-Wandler bei vergleichbaren Anregungen getestet werden.

Les récentes découvertes sur les phénomènes hyperfréquences et nonlinéaires dans les structures minces ferromagnétiques actives planaires ont fait émerger un grand nombre de nouvelles études et applications pratiques prometteuses. La conversion de l'énergie magnétoélastique peut être beaucoup plus efficace à proximité de la transition de réorientation de spin (TRS). Les structures minces ferromagnétiques actives planaires fournissent un grand nombre de caractéristiques haute fréquence uniques : par exemple, les conditions pour l’effet Doppler anomal peuvent être satisfaites. Les cristaux magnoniques représentent également un domaine prometteur pour les futures investigations.Dans le présent travail nous avons établi la description théorique de la propagation des ondes hyperfréquences et non-linéaires dans les structures minces ferromagnétiques actives planaires de compositions différentes. Il a été démontré expérimentalement et théoriquement que les vibrations basse fréquence d’un cantilever peuvent être amplifiées quand la résonance ferromagnétique est excitée par un champ électromagnétique HF à proximité de la TRS. En outre, l'effet de la démodulation magnétoélastique peut être complété par un effet magnétoélectrique nonlinéaire. La possibilité de l'apparition de l'effet Doppler anomal lors de la propagation d'une onde de surface magnétostatique dans une structure ferrite-diélectrique-métal, dans une certaine plage de paramètres du système, est démontrée. La dispersion d'une onde magnétostatique de surface se propageant dans un film dont l'épaisseur varie linéairement, et possédant une structure périodique sous la forme de bandes parallèles gravées, a été calculée / Recently discovered investigations on the high frequency and nonlinear phenomena in thin active ferromagnetic planar structures showed a great number of new studies and promising practical applications. The magnetoelastic energy conversion can be much more efficient in the vicinity of spin reorientation transition (SRT). The thin active ferromagnetic planar structures provide a lot of unique high frequency features: for instance, the anomalous Doppler effect conditions can be satisfied. The magnon crystals are also an actual area for the further investigation of the domain.In the present work we derived the theoretical description for the high frequency and non-linear waves propagation in thin planar ferromagnetic structures with different compositions. It was demonstrated experimentally and theoretically that LF vibrations of the cantilever can be amplified when FMR is excited by HF electromagnetic field near SRT. Moreover the magnetoelastic demodulation effect can be supplemented with nonlinear magnetoelectric effect. The possibility of the occurrence of the anomalous Doppler effect during propagation of an MSSW in an FDM structure in a certain range of system parameters is substantiated. The dispersion of a surface magnetostatic wave propagating in a film, whose thickness varies linearly, with a periodic structure in the form of parallel etched strips was calculated. As it was clearly demonstrated these works are of great interest for the new studies and practical applications

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.

Conselho Nacional de Desenvolvimento Científico e Tecnológico / Hybrid nanostructures which integrate two or more technologically interesting physical properties are fundamental for developing new generations of electronic devices. Exhibiting at least two coupled ferroic orders, multiferroics are an outstanding class of multifunctional materials. Compounds which present coupling between ferromagnetism and ferroelectricity are specially interesting. Although natural multiferroics are rarely found, the possibility of obtaining strain-mediated magnetoelectric coupling in composite structures, by integrating magnetostrictive and piezoelectric layers, paves the way to control electric properties by applying magnetic field or to the electric control of magnetism. Nevertheless, most scientific efforts have been on monophasic compounds or bulk composites. Considering the incorporation of magnetoelectric nanostructures in devices, expanding the scope of the magnetoelectric effect and targetting it to different kinds of applications is needed. Besides new characterization techniques, seeking new alternative materials to the lead-based piezoelectrics or oxide-based magnetostrictives is necessary. Recently, a few works using semiconductors such as ZnO and AlN, or amorphous magnetic alloys such as those based on Co, Fe and Ni, have been reported. In spite of not presenting remarkable piezoelectric and magnetostrictive effects, the features of such materials are promising for high frequency applications, for instance. Considering these issues, four independent surveys are presented. Firstly, the origin of the coupling, latest advances and current scenario of the field are reviewed. Then magnetostriction measurements in thin films are addressed by employing a direct technique based on the cantilever-capacitance method. The goals are to study magnetoelastic properties of some materials whose magnetostriction are not found very often in literature, and to check the reliability of this technique for investigating thin films. In this sense, measurements of some amorphous magnetic alloys mainly based on Co, Fe and Ni are performed. Most samples presents larger magnetoelastic response for magnetic field applied along the magnetization easy axis, as opposed to the theoretically expected. Two investigations on aluminum nitride thin films are reported. Firstly, the growth of AlN films onto several different substrates and buffer layers is studied. Films grown onto glass and polyimide show excellent structural properties for eletromechanical systems and flexible electronics applications. Samples with low residual stress on silicon substrates, suitable for incorporating in existing technologies, are obtained. Secondly, bilayers composed by AlN and ferromagnetic films are investigated. In addition to the structural and morphological properties of the AlN films which are checked, the magnetic characterization of the structures also contributes to design multilayers for exploring the magnetoelectric effect. Finally, problems involving electric fields in scanning probe microscopies are adressed. Surface images of AlN piezoelectric films are systematically acquired. Among other major observations, the possibility of getting reliable piezoresponse images of strongly polarized areas as well as of visualizing ferroelastic domains, is demonstrated. Furthermore, a new microscopy for investigating a sample s ferro and piezoelectric properties is proposed, exploring the direct piezoelectric effect. By utilizing acoustic excitation and electrical detection, the potency of this technique is illustrated with measurements on quartz and AlN surfaces. / Nanoestruturas híbridas, integrando duas ou mais propriedades físicas de grande interesse tecnológico, são fundamentais para o desenvolvimento de novas gerações de dispositivos eletrônicos. Uma classe interessante de materiais multifuncionais são os multiferróicos, que exibem pelo menos duas ordens ferróicas acopladas. Dentre eles, os que apresentam acoplamento entre ferromagnetismo e ferroeletricidade despertam interesse especial. Apesar de serem raros de ocorrer naturalmente, a possibilidade de gerar efeito magnetoelétrico em estruturas compósitas, intermediado pela deformação elástica entre camadas magnetostrictivas e piezoelétricas, abre caminho para que seja possível controlar propriedades elétricas aplicando-se campo magnético, ou propriedades magnéticas aplicando-se campo elétrico. Todavia, a maior parte das pesquisas atuais ainda envolve compostos monofásicos ou compósitos em forma massiva. Tendo em vista a incorporação de nanoestruturas magnetoelétricas em dispositivos, é fundamental ampliar a abrangência do efeito magnetoelétrico e direcioná-lo para diferentes tipos de aplicações. Para isto, além de novas técnicas de caracterização, é necessário buscar-se materiais alternativos aos tradicionais piezoelétricos baseados em chumbo e magnetostrictivos baseados em óxidos. Recentemente tem-se encontrado trabalhos pontuais onde são utilizados piezoelétricos semicondutores como ZnO e AlN, e ligas magnéticas amorfas como as baseadas em Co, Fe e Ni. Mesmo sem apresentar efeitos piezoelétrico e magnetostrictivo com magnitudes notáveis, as características destes materiais são promissoras para aplicações envolvendo altas frequências, por exemplo. Neste necessário, são apresentados quatro estudos independentes entre si. Primeiramente, é realizada uma revisão sobre a origem do acoplamento, os últimos avanços e o panorama atual das pesquisas na área. Em seguida, através de uma técnica direta baseada no método do cantiléver-capacitância, aborda-se o problema das medidas de magnetostricção em amostras na forma de filmes finos. Os objetivos são estudar as propriedades magnetoelásticas em alguns materiais que não são frequentemente abordados pela literatura, e avaliar a potencialidade da técnica para a análise de filmes finos. Para isto, são realizadas medidas principalmente em ligas ferromagnéticas amorfas baseadas em Co, Fe e Ni. Para a maioria das amostras analisadas, a resposta magnetoelástica é maior quando o campo magnético é aplicado na direção do eixo de fácil magnetização, de forma contrária à esperada teoricamente. São apresentadas duas investigações envolvendo filmes finos de nitreto de alumínio. Primeiro é estudado o crescimento de filmes de AlN sobre vários substratos e camadas semente. Filmes crescidos sobre vidro e poliimida apresentam excelentes propriedades estruturais para aplicações em sistemas eletromecânicos e eletrônica flexível. Amostras obtidas com baixos valores de tensão residual, sobre substratos de silício, são interessantes para incorporação em tecnologias existentes. Segundo, são investigadas bicamadas de AlN com filmes ferromagnéticos. Além das propriedades estruturais e morfológicas dos filmes de AlN, a análise das características magnéticas das estruturas contribui para o design de multicamadas que exploram o efeito magnetoelétrico. Finalmente, são abordados problemas em medidas de microscopias de varredura por sonda envolvendo campos elétricos. Imagens da superfície de filmes piezoelétricos de AlN foram coletadas sistematicamente. Entre outras observações importantes, demonstra-se que é possível adquirir imagens confiáveis de piezo-resposta em regiões fortemente polarizadas, e visualizar a formação de domínios ferroelásticos. Também é proposta uma nova técnica de microscopia, para investigar as propriedades ferro e piezoelétricas de uma amostra, explorando o efeito piezoelétrico direto. Utilizando excitação acústica e detecção elétrica, o potencial da nova técnica é demonstrado com imagens de superfícies cristalinas de quartzo e AlN.

Over the last two decades, great progress has been made in the understanding of multiferroic materials, ones where multiple long-range orders simultaneously exist. However, much of the research has focused on bulk systems. If these materials are to be incorporated into devices, they would not be in bulk form, but would be miniaturized, such as in nanoparticle form. Accordingly, a better understanding of multiferroic nanoparticles is necessary. This manuscript examines the multiferroic phase diagram of multiferroic nanoparticles related to system size and surface-induced magnetic anisotropy. There is a particular focus on bismuth ferrite, the room-temperature antiferromagnetic-ferroelectric multiferroic. Theoretical results will be presented which show that at certain sizes, a bistability develops in the cycloidal wavevector. This implies bistability in the ferroelectric and magnetic moments of the nanoparticles. This novel magnetoelectric bistability may be of use in the creation of an electrically-written, magnetically-read memory element. / Graduate