The magnetoelectric (ME) effect, i.e., the induction of magnetization by an applied electric field (E) or a polarization by an applied magnetic field (H), is of great interest to researchers due to its potential applications in magnetic sensors. Moreover, the ME effect in laminate composites is known to be much higher than in single phase and particulate composites due to combination of the magnetostrictive and piezoelectric effects in the individual layers. Given that the highest ME coefficient have been found in Metglas/piezo-fiber laminate composites, this study was designed to investigate and enhance the magnetoelectric (ME) effect in Metglas/piezo-fiber laminate composites, as well as develop their potential for magnetic sensor applications. To initiate this investigation, a theoretical model was derived to analyze the thickness effect of the magnetostrictive, piezoelectric, epoxy and Kapton layers on the ME coefficient. As a result, the importance of the coupling effect by epoxy layers was revealed. I used spin-coating, vacuum bagging, hot pressing, and screen printing techniques to decrease the thickness of the epoxy layer in order to maintain homogeneity, and to obtain good repeatability of the 16 ME laminates fabricated at one time. This protocol resulted in a more efficient way to induce self-stress to Metglas/PZT laminates, which is essential for increasing the ME coefficient. With an enhanced ME effect in the Metglas/piezo-fiber laminates, magnetic field sensitivity could then be increased. An ME sensor unit, which consisted of a Metglas/PMN-PT laminate and a low noise charge amplifier, had a magnetic field sensitivity of 10 pT/Hz0.5 in a well-shielded environment. Stacking four of these ME laminates could further increase the signal-to-noise (SNR) ratio. I studied the optimized distance between a pair of Metglas/PZT ME laminates. A stack of up to four ME sensors was constructed to decrease the equivalent magnetic noise. The magnetic field sensitivity was effectively enhanced compared to a single laminate. Finally, a number of four Metglas/PZT sensor units array was constructed to further increase the sensitivity. ME laminate composites operated in passive mode have typically required an external magnetic bias field in order to maximize the value of the piezomagnetic coefficient, which has many drawbacks. I studied the ME effect in an Ni/Metglas/PZT laminate at zero bias field by utilizing the remnant magnetization between the Ni and Metglas layers. To further enhance this effect, annealed Metglas was bonded on the Metglas/PZT laminate since it is known that hard-soft ferromagnetic bilayers generate built-in magnetic field in these Metglas layers. As a result, giant αME values could be achieved at a zero bias field at low frequency range or at electromechanical resonance (EMR). The sensor unit consisting of self-biased ME laminate arrays is considerably smaller compared to a unit that uses magnet-biased ME laminates. Introducing the converse ME effect and nonlinear ME effect in Metglas/piezo-fiber laminates affords a variety of potential applications. Therefore, I theoretically and experimentally studied converse ME effects in laminates with longitudinally magnetized and longitudinally poled, or (L-L) mode. The optimum structure for producing the maximum effect was obtained for Metglas/PZT laminates. Additionally, the optimum structure and materials for enhancing the nonlinear ME effect in Metglas/PZT laminates are reviewed herein. In particular, this study revealed that modulating the EMR in laminates with high-Q piezo-fibers could enhance the SNR. The stress effect on nonlinear ME effect is also discussed—namely that magnetic field sensitivities can be enhanced by this modulation-demodulation technique. / Ph. D.

Piezoelectric material based sensors are widely used in applications such as automobiles, aircraft, and industrial systems. In past decade, attention has been focused on synthesizing composites that can provide multifunctional properties, i.e., same material exhibits two or more properties. In this group of composites, magnetoelectric materials are particularly interesting as they provide the opportunity of coupling magnetic and electric field. Another class of composite materials that are being actively pursued is piezoresistive materials. Piezoresistivity refers to change in resistance with applied stress and these materials are promising for enhancing the sensitivity of current generation pressure sensors based on silicon. In this study, we focus on two composites systems: ferrite / Terfenol-D / nickel — lead zirconate titanate (magnetoelectric); and lanthanum strontium manganate (LSMO) — carbon nanotube (CNT) – silicon carbonitride (SiCN) (piezoresistive). Recently, Islam et al. have reported a magnetic field sensor based on a piezoelectric transformer with a ring- dot electrode pattern. In this thesis, this design was further investigated by synthesizing Terfenol-D / PZT laminate. The fabricated sensor design consists of a ring-dot piezoelectric transformer laminated to a magnetostrictive disc and its working principle is as follows: When a constant voltage is applied to the ring section of the piezoelectric layer at resonance, a stress is induced in the dot section. Then, if an external magnetic object is introduced in the vicinity of the dot section, the effective elastic stiffness is increased, altering the resonance frequency (fr). The variation of resonance frequency and magnitude of output voltage with applied magnetic field was characterized and analyzed to determine the sensitivity. The sensor showed a shift of ~1.36Hz/Oe over the frequency range of 137.4<fr<144.2 kHz with increasing magnetic bias from 1<Hdc<6kOe. Next, in order to overcome the need of magnetic DC bias in current magnetoelectric composites, a metal – ceramic core-shell composite structure was investigated. Metal-ceramic composite particles were synthesized at room temperature and their magnetic properties were investigated. The particles constitute a core-shell structure where the core is nickel-metal, while the shell is manganese zinc ferrite (MZF). Coprecipitation was used for synthesis of MZF nanoparticles comprising the shell, whereas nickel was synthesized by hydrazine assisted reduction of nickel ions in aqueous media. A core shell structure was then obtained by hetero-coagulation to form a shell of MZF around the nickel particles. Electron microscopy and x-ray diffraction confirmed nickel cores coated by MZF shells. Magnetization studies of MZF nano-particles revealed that they were not super-paramagnetic at room temperature, as expected for such particle sizes of 20nm in size. Sintered composites of metal-ceramic particles core-shell exhibited a magnetostriction of 5ppm. Lastly, the thesis investigates the piezoresistive properties of LSMO – CNT – SiCN composites that were synthesized by the conventional ceramic sintering technique. Recent investigations have shown that CNTs and SiCN have high piezoresistive coefficient. DSC/TGA results showed that pure CNTs decompose at temperatures of ~600°C, however, SiCN was found to sustain the sintering temperature of 1300°C. Thus, LSMO – SiCN composites were used for the final analysis. A fractional resistivity change of 4% was found for LSMO — 12.5 vol% SiCN composites which is much higher compared to that of unmodified LSMO. / Master of Science

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.

Cette thèse présente la modélisation de l’effet magnéto-électrique dans les matériaux composites par la méthode des éléments finis. Les matériaux composites magnéto-électriques sont la combinaison de matériaux piézoélectriques et magnétostrictifs. Les lois de comportement ont été établies en associant les lois de comportement piézoélectrique et magnétostrictive. Le modèle piézoélectrique a été supposée linéaire, contrairement au magnétostrictif qui est non-linéaire. Afin de modéliser des dispositifs dans lequel il y a coexistence d’un champ statique et d’un champ dynamique de faible amplitude, nous avons proposé une étape de linéarisation des lois de comportement. Cette étape consiste à déterminer le point de fonctionnement fixé par le champ statique pour ensuite calculer la variation autour de ce point associée au champ dynamique. Les deux lois de comportement ont ensuite été intégrées dans un code éléments finis 2D. Le code de calcul éléments finis a ensuite été exploité pour différents dispositifs déjà mis en ?uvre expérimentalement dans la littérature. La première application est une inductance variable contrôlée par un champ électrique. Malgré une méconnaissance de certaines valeurs des propriétés des matériaux, le calcul numérique et les résultats expérimentaux sont en bon accord d’un point de vue qualitatif. Les travaux nous ont permis de modéliser des capteurs de champ magnétique. Ces capteurs ont pour but de détecter précisément un champ magnétique statique dans le plan de travail. La comparaison des résultats numériques et expérimentaux a montré à nouveau une bonne concordance qualitative. Quelques améliorations de la structure du dispositif ont été proposées et évaluées à l’aide du modèle développé. / This thesis deals with the modelling of magnetoelectric effect in composite materials using finite element method. The magnetoelectric composite materials result from the combination of piezoelectric and magnetostrictive materials. The magnetoelectric constitutive laws were established by combining piezoelectric and magnetostrictive constitutive laws. The piezoelectric behaviour is assumed to be linear. Unlike the piezoelectric material, the magnetostrictive behaviour is nonlinear. In order to model the smart devices with the coexistence of static and low amplitude dynamic field, a linearization of constitutive laws is proposed. This step is to determine the polarisation point given by static field, then calculate the variation around this point associated with dynamic field. The static and linearized constitutive laws are then integrated in a 2D finite element code using Galerkin method.The finite element program is then used for modeling different devices in experimental. The first application is a tunable inductor controlled by a electric field. The numerical results are closed to experimental results despite unknown material properties. The model is then implemented in the case of magnetic sensor. This sensor is to detect accurately the static magnetic field in working plane. The comparison between numerical and experimental results shows again good qualitative agreement. Some improvements of sensor structure are purposed thanks to the developed model.

Materiais que apresentam um acoplamento entre as propriedades elétricas e magnéticas, conhecidos como magnetoelétricos, estão despertando a atenção da comunidade acadêmica nos últimos anos principalmente devido ao seu grande potencial tecnológico no desenvolvimento de materiais multifuncionais. Sendo um maior esforço concentrado na obtenção de novos materiais multiferróicos, que são aqueles que possuem duas ou mais ordens ferróicas, em especial os que apresentam ferroeletricidade e ferromagnetismo. Este trabalho tem como objetivo uma investigação das propriedades dielétricas e magnéticas de um grupo de materiais ainda pouco estudado a fim de se compreender os mecanismos responsáveis pelo efeito magnetoelétrico, de forma a contribuir com a pesquisa nessa área. As amostras estudadas são os óxidos de titânio e cromo Nd(1-x)A(x)CrTiO5 (A=Y, La) e GdCrTiO5, todos na forma de cerâmicas policristalinas monofásicas. Foram discutidas de que forma a diluição da subrede magnética do neodímio afeta o caráter magnetoelétrico observado nesse material e qual a importância de cada uma das subredes (Nd3+/Gd3+ e Cr3+) nas propriedades dielétricas e magnéticas. Verificamos que o efeito magnetoelétrico é sensível a variações na rede do Nd3+. Entretanto, nossos resultados no composto GdCrTiO5 indicam que as subredes magnéticas são pouco acopladas, o que indica que o ion Cr3+ talvez seja o responsável por dirigir o efeito magnetoelétrico nesta família de materiais. / The coupling of the electric and magnetic properties in the magnetoelectric compounds is attracting a great deal of interest in the scientific community due to the huge potential for application based on new multifunctional materials. Of special attention are the so-called multiferroics, materials that simultaneously show ferromagnetism and ferroelectricity. The main purpose of this work is the experimental investigation of the dielectric and magnetic properties of a relatively less known class of materials with the aim to identify the origin of the magnetoelectric coupling. We study the single phase polycrystalline compounds Nd(1-x)A(x)CrTiO5 (A=Y, La) and GdCrTiO5 and discuss the effect of dilution of the Nd3+ ions and the importance of each magnetic sublattice on the samples behavior. We find that the magnetoelectric effect is susceptible to changes in the Nd3+ site. At the same time, our results indicate that the two magnetic sublattice (Nd3+/Gd3+ and Cr3+) are only weakly coupled indicating that the Cr3+ magnetic sublattice might be responsible for driving the magnetoelectric effect in this family of compounds.

A busca por dispositivos que possuam uma estrutura micrométrica ou nanomé- trica, tem sido um tópico relevante de pesquisa, pois esse tipo de estrutura permite o estudo de efeitos ainda em análise e com várias questões em aberto, como por exemplo, efeito magnetoelétrico, o qual pode estar atrelado à um efeito de superfície. Estes tipos de efeitos podem surgir em dispositivos elétricos de modo a alterarem suas proprieda- des. Neste trabalho, desenvolveu-se uma heteroestrutura capacitiva composta por dois ele- trodos metálicos (Al na base e Ag no topo) com Al2O3 nanoporoso como separador dielétrico. O NAM foi obtido a partir de uma folha de Al de alta pureza por meio do processo de anodização em duas etapas. Esta técnica permite que, naturalmente, um dos planos da folha original permaneça inalterado, preservando uma fina camada de Al. Antes de revestir o plano oposto com Ag (via pulverização catódica), os na- nofios (Nanofios(Nf)) foram eletrodepositados no arranjo poroso hexagonal (poros de aproximadamente 50nm de diâmetro e 3m de comprimento). A heteroestrutura foi caracterizada por difração de raios X e por Microscopia Eletrônica de Varredura. As medições de capacitância versus tensão para frequências entre 1KHz e 5MHz foram realizadas utilizando uma estação de sonda Cascade Microtech controlada por um ana- lisador B1500A e foram obtidas capacitâncias por unidade de área planar de cerca de 50 nF para esta nanoestrutura. A presença de Nf no capacitor permite estudar o com- portamento da capacitância em função da magnetização deles. Os testes de capacitân- cia foram realizados deixando os Nf em estados magnéticos bem definidos. Primeiro, mediu-se a capacitância com os fios no estado desmagnetizado e, em seguida, com eles em estado remanescente após a saturação magnética no plano. Por fim, mediu-se a capacitância com os fios no estado remanescente após a saturação magnética ao longo do eixo do Nf. Após essas medidas, constatou-se o aumento na capacitância quando a magnetização do Nf aumenta. Este efeito magnetoelétrico é uma evidência experimen- tal do acoplamento de interface entre um dielétrico e um metal polarizado por spin. / The search for devices that have a micrometric or nanometric structure, has been a relevant research topic, since this type of structure allows the study of effects still under analysis and with several open questions, such as, for example, magnetoelectric effect, which can be tied to a surface effect. These types of effects can arise in electrical devices in order to change their properties. In this work, we have developed a capacitive heterostructure consisting of two metal electrodes (Al at the base and Ag at the top) with nanoporous Al2O3 as the dielec- tric separator. The NAM was obtained starting from a high-purity Al sheet, using the two-step anodization process. This technique allows one of the planes of the origi- nal sheet to remain unaltered naturally, preserving a thin layer of Al. Before coating the opposite plane with Ag (via sputtering), nano-wires (NW) were electrodeposited in the hexagonal pore array (pores of approximately 60nm of diameter and 10m of length). The heterostructure was characterized by X-ray diffraction. Measurements of capacitance versus voltage for frequencies between 1KHz e 5MHz were performed using a Cascade Microtech probe station controlled by a B1500A Analyzer and there were obtained capacitances per unit of planar area of around 50 nF for this nanos- tructure. The presence of NW in the capacitor allows the study of the behavior of the capacitance as a function of the magnetization of the NW. The capacitance tests were performed by leaving the NW in well-defined magnetic states. First, the wires were measured in the demagnetized state, than with them in a remanent state after the in-plane magnetic saturation. Finally, measures of capacitance were taken when the wires were in the remanent state after the magnetic saturation along the axis of the NW. There was seen the increase in capacitance when the magnetization of the NW increases. This magnetoelectric effect is an experimental evidence of the interface cou- pling between a dielectric and a spin-polarized metal.

Coupling between the electric field, magnetic field, and strain of composite materials is achieved when electro-elastic (piezoelectric) and magneto-elastic (piezomagnetic) particles are joined by an elastic matrix. Although the matrix is neither piezoelectric nor piezomagnetic, the strain field in the matrix couples the E field of the piezoelectric phase to the B field of the piezomagnetic phase. This three-phase electro-magneto-elastic composite should have greater ductility and formability than a two-phase composite in which E and B are coupled by directly bonding two ceramic materials with no compliant matrix. A finite element analysis and homogenization of a representative volume element is performed to determine the effective electric, magnetic, mechanical, and coupled-field properties of an elastic (epoxy) matrix reinforced with piezoelectric and piezomagnetic fibers as functions of the phase volume fractions, the fiber (or particle) shapes, the fiber arrangements in the unit cell, and the fiber material properties with special emphasis on the symmetry properties of the fibers and the poling directions of the piezoelectric and piezomagnetic fibers. The effective magnetoelectric moduli of this three-phase composite are, however, less than the effective magnetoelectric coefficients of a two-phase piezoelectric/piezomagnetic composite, because the epoxy matrix is not stiff enough to transfer significant strains between the piezomagnetic and piezoelectric fibers.

Coupling between the electric field, magnetic field, and strain of composite materials is achieved when electro-elastic (piezoelectric) and magneto-elastic (piezomagnetic) particles are joined by an elastic matrix. Although the matrix is neither piezoelectric nor piezomagnetic, the strain field in the matrix couples the E field of the piezoelectric phase to the B field of the piezomagnetic phase. This three-phase electro-magneto-elastic composite should have greater ductility and formability than a two-phase composite in which E and B are coupled by directly bonding two ceramic materials with no compliant matrix. A finite element analysis and homogenization of a representative volume element is performed to determine the effective electric, magnetic, mechanical, and coupled-field properties of an elastic (epoxy) matrix reinforced with piezoelectric and piezomagnetic fibers as functions of the phase volume fractions, the fiber (or particle) shapes, the fiber arrangements in the unit cell, and the fiber material properties with special emphasis on the symmetry properties of the fibers and the poling directions of the piezoelectric and piezomagnetic fibers. The effective magnetoelectric moduli of this three-phase composite are, however, less than the effective magnetoelectric coefficients of a two-phase piezoelectric/piezomagnetic composite, because the epoxy matrix is not stiff enough to transfer significant strains between the piezomagnetic and piezoelectric fibers.

Materiais que apresentam um acoplamento entre as propriedades elétricas e magnéticas, conhecidos como magnetoelétricos, estão despertando a atenção da comunidade acadêmica nos últimos anos principalmente devido ao seu grande potencial tecnológico no desenvolvimento de materiais multifuncionais. Sendo um maior esforço concentrado na obtenção de novos materiais multiferróicos, que são aqueles que possuem duas ou mais ordens ferróicas, em especial os que apresentam ferroeletricidade e ferromagnetismo. Este trabalho tem como objetivo uma investigação das propriedades dielétricas e magnéticas de um grupo de materiais ainda pouco estudado a fim de se compreender os mecanismos responsáveis pelo efeito magnetoelétrico, de forma a contribuir com a pesquisa nessa área. As amostras estudadas são os óxidos de titânio e cromo Nd(1-x)A(x)CrTiO5 (A=Y, La) e GdCrTiO5, todos na forma de cerâmicas policristalinas monofásicas. Foram discutidas de que forma a diluição da subrede magnética do neodímio afeta o caráter magnetoelétrico observado nesse material e qual a importância de cada uma das subredes (Nd3+/Gd3+ e Cr3+) nas propriedades dielétricas e magnéticas. Verificamos que o efeito magnetoelétrico é sensível a variações na rede do Nd3+. Entretanto, nossos resultados no composto GdCrTiO5 indicam que as subredes magnéticas são pouco acopladas, o que indica que o ion Cr3+ talvez seja o responsável por dirigir o efeito magnetoelétrico nesta família de materiais. / The coupling of the electric and magnetic properties in the magnetoelectric compounds is attracting a great deal of interest in the scientific community due to the huge potential for application based on new multifunctional materials. Of special attention are the so-called multiferroics, materials that simultaneously show ferromagnetism and ferroelectricity. The main purpose of this work is the experimental investigation of the dielectric and magnetic properties of a relatively less known class of materials with the aim to identify the origin of the magnetoelectric coupling. We study the single phase polycrystalline compounds Nd(1-x)A(x)CrTiO5 (A=Y, La) and GdCrTiO5 and discuss the effect of dilution of the Nd3+ ions and the importance of each magnetic sublattice on the samples behavior. We find that the magnetoelectric effect is susceptible to changes in the Nd3+ site. At the same time, our results indicate that the two magnetic sublattice (Nd3+/Gd3+ and Cr3+) are only weakly coupled indicating that the Cr3+ magnetic sublattice might be responsible for driving the magnetoelectric effect in this family of compounds.

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