The magnetoelectric (ME) effect, a coupling effect between magnetic and electric orders, has been widely investigated, both from a fundamental science perspective and an applications point of view. Magnetoelectric composites with one piezoelectric phase and one magnetostrictive phase can be magneto-electrically coupled via elastic strain mediation. Bulk magnetoelectric composites have been intensively studied as magnetic sensors due their significant magnetic-to-electric signal transforming efficiency, which promises high magnetic field sensitivity. In contrast, electric field-controlled magnetization in magnetoelectric thin films is more attractive for information recording and novel electrically-tunable microwave magnetic devices. For the present work, we prepared a series of magnetoelectric structures capable of modulating the magnetization with an electric field -- all of which display unprecedented magnetic coercive field tunability. These structures show promise for a number of applications, including magnetic memory and spintronics. First, we generated self-assembled BiFeO3-CoFe2O4 (BFO-CFO) nanostructures of varying architectural structures on differently-oriented perovskite substrates. We were able to control aspect ratio through both thickness control and by manipulating growth thermodynamics. The relationship between magnetic shape and strain anisotropy was systematically analyzed using both in-plane and out-of-plane magnetic easy axis data. The BFO-CFO self-assembled structures may be useful for applications, including longitudinal and perpendicular magnetic memory; additionally they can serve as a prototype for analyzing the magnetoelectric effect-based magnetoresistive random-access memory. BFO-CFO grown on piezoelectric Pb(Mg,Nb)O3-PbTiO3 (PMN-PT) shows a large magnetoelectric coupling coeffcient. Second, we sought to clarify the relationship between ferroelectric/ferroelastic phase transformation and the magnetoelectric effect in CFO films on PMN-PT heterostructures. Elastic strain is an essential component of electro-mechanical-magnetic coupling. Most prior studies that used piezoelectric materials as a strain source assumed that these materials shared a linear relationship (d31 or d33) with the electric field, which is true only with small electric field signals. In contrast, the largest strain is produced during phase transformation in piezoelectric single crystals. In this work, we systematically investigated electric field induced phase transformation in PMN-PT single crystals with different compositions. A signficant finding that emerged from this study is that a large in-plane uniaxial strain can be controlled by an electric field, and this strain can be used to control the magnetic easy axis distribution in the in-plane. The electric field is along the out-of-plane direction, which is perpendicular to the uniaxial strain and the surface of the sample, and thus can be easily incorporated into real device design. Finally, we identified very large magnetic coercive field tunability in the CFO/PMN-PT monolithic structures -- in fact, more than ten times larger than previously reported magnetoelectric heterostructures. We used a <011> oriented PMN-PT substrate, where a large uniaxial strain can be induced by an electric field. Importantly, since the two in-plane directions have the same dimensions, the uniaxial strain can induce a significant magnetic anisotropy distribution change in the two in-plane directions. A unprecedented magnetic coercive field change of up to 580 Oe has been observed, which shows great potential for applications in both magnetic memory and microwave magnetic devices. / Ph. D.

The magnetoelectric (ME) effect is a property that results in power/energy conversion between magnetic and electric forms. Two-phase composites consisting of magnetostrictive and piezoelectric materials have been developed that show remarkable ME voltage/charge coefficients. This extrinsic ME effect is achieved by using mechanical coupling as a medium between the magnetostrictive and piezoelectric phases. As described in this thesis, I investigated the optimization of the material properties of sensors/gradiometers, transmitters, and gyrator applications using ME heterostructures with a multi-push-pull structure. In applications, ME sensors will need to work in an open environment where there will be a mix of magnetic signals and microphonic noises. Prior research has determined that both passive and active mode ME sensors are affected by vibrational noise in the open environment. Therefore, as described herein, an ME gradiometer consisting of a pair of ME sensors working under H-field modulation (active mode) was developed to address the issue of microphonic noise. The common mode rejection ratio of my ME gradiometer was determined to be 74. Gradiometer curves were also measured, which presented the gradiometer outputs as a function of the normalized distance between the magnetic source and the ME gradiometer. Based on resulting data, the proposed ME gradiometer was confirmed to be capable of significant vibration noise rejection. However, this method is not appropriate for rejecting longitudinal vibrations due to the propagation direction being the same as the magnetic field. To resolve this dilemma, a new ME laminate structure was designed that could better reject vibrational noise. Additionally, two different configurations were developed to measure the gradiometer curve. Second, in order to understand how much energy can be wirelessly transmitted by ME laminates within a local area, a portable (area ~ 16 cm^2), a very low-frequency transmitter was developed using ME laminate with Metglas/PZT structure. The proposed strain-driven ME laminate transmitter functions as follows: (a) a piezoelectric layer is first driven by alternating current electric voltage at its electromechanical resonance (EMR) frequency; (b) subsequently, this EMR excites the magnetostrictive layers, giving rise to a magnetization change; (c) in turn, the magnetization oscillations result in oscillating magnetic fluxes, which can be detected through the use of a search coil as a receiver. The prototype measurements revealed an induction transmission capabilities in the near field. Furthermore, the developed prototype evidenced a 10^4 times higher efficiency in the near field over a small-circular loop of the same area, exhibiting its superiority over the class of traditional small antennas. Next, recent efforts in our group resulted in the development of an ME gyrator based on ME heterostructures. Such gyrators facilitate current-to-voltage conversion with high power efficiency. ME gyrators working at their resonance frequency are capable of converting power with an efficiency of > 90 %, which show potential for use in power convertors. Here, we found that the resonance frequency could be tuned through the use of a frequency-modulation technique. Accordingly, this method can be utilized to match the frequency difference between the power supply and the piezoelectric transducer in actual applications, which will increase the power efficiency. Another problematic issue is that the electromechanical coupling factor of piezoelectric transducers is limited by bandwidth. Typically, transducers cannot be impedance matched to a power supply, which significantly reduces power efficiency. Our initial studies have shown that an improved impedance match can be realized by using an ME gyrator to geometrically tune a transducer, which will substantially enhance power efficiency. The last chapter will mainly focus on ME gyrator applications. Designing linear power amplifiers that operate reliably at high frequency is quite challenging, which is mainly due to the fact that the parasitic impedances of their electronic components tend to dominate at higher frequencies, thereby leading to significant power-efficiency loss. Therefore, ME gyrator may play an important role between the power amplifier and the acoustic transducer to reduce the power loss. In this chapter, we achieved the impedance matching between a piezoelectric transducer and a power supply by implementing geometric changes to the gyrator. Both the power efficiency of an individual ME gyrator and a piezoelectric transducer are > 90%. Therefore, the total power efficiency of the ME gyrator and the piezoelectric transducer also approach > 80% when they got connected together. The second aspect of this chapter pertains to resonance-frequency tuning using three method. Since an ME gyrator will be used to achieve impedance matching, the resonance frequency of the ME gyrator and a piezoelectric transducer may not exactly match. This limitation will be overcome through capacitance tuning of the piezoelectric transducer in order to achieve frequency matching. Finally, an equivalent circuit will be developed that connects a piezoelectric transducer with a gyrator, thereby enabling the impedance of the output port of the transducer and the shifted EMR frequency of the transducer to be modified. / Doctor of Philosophy / In my dissertation, I focus on the magnetoelectric (ME) effect is a property that results in power/energy conversion between magnetic and electric forms. Two-phase composites consisting of magnetostrictive and piezoelectric materials have been developed that show remarkable ME voltage/charge coefficients. As described in this dissertation, I investigated the optimization of the material properties of sensors/gradiometers, transmitters, and gyrator applications using ME heterostructures with a multi-push-pull structure. An ME gradiometer consisting of a pair of ME sensors working under H-field modulation (active mode) was developed to address the issue of microphonic noise. The common mode rejection ratio of my ME gradiometer was determined to be 74. Gradiometer curves were also measured, which presented the gradiometer outputs as a function of the normalized distance between the magnetic source and the ME gradiometer. Besides that, a new ME laminate structure was designed that could better reject vibrational noise. Second, in order to understand how much energy can be wirelessly transmitted by ME laminates within a local area, a portable, a very low-frequency transmitter was developed using ME laminate with Metglas/PZT structure. The prototype measurements revealed an induction transmission capability in the near field. Furthermore, the developed prototype evidenced a 10^4 times higher efficiency in the near field over a small-circular loop of the same area, exhibiting its superiority over the class of traditional small antennas. In the last chapter, we achieved the impedance matching between a piezoelectric transducer and a power supply by implementing geometric changes to the gyrator. The total power efficiency of the ME gyrator and the piezoelectric transducer approach > 80% when they got connected together. The second aspect of this chapter pertains to resonance-frequency tuning using three methods. Finally, an equivalent circuit will be developed that connects a piezoelectric transducer with a gyrator, thereby enabling the impedance of the output port of the transducer and the shifted EMR frequency of the transducer to be modified.

Flexibility, low cost, versatility, miniaturization and multi-functionality are key aspects driving research and innovation in many branches of the electronics industry. With many anticipated emerging applications, like wearable, transparent and biocompatible devices, interest among the research community in pursuit for novel multifunctional miniaturized materials have been amplified. In this context, multiferroic polymer-based nanocomposites, possessing both ferroelectricity and ferromagnetism, are highly appealing. Most importantly, these nanocomposites possess tunable ferroelectric and ferromagnetic properties based on the parameters of their constituent materials as well as the magnetoelectric effect, which is the coupling between electric and magnetic properties. This tunability and interaction is a fascinating fundamental research field promising tremendous potential applications in sensors, actuators, data storage and energy harvesting. This dissertation work is devoted to the investigation of a new class of multiferroic polymer-based flexible nanocomposites, which exhibits excellent ferromagnetism and ferroelectricity simultaneously at room temperature, with the goal of understanding and optimizing the origin of their magnetoelectric coupling. The nanocomposites consist of high aspect ratio ferromagnetic nanowires (NWs) embedded inside a ferroelectric co-polymer, poly(vinylindene fluoride-trifluoroethylene), P(VDF-TrFE) matrix. First, electrochemical deposition of ferromagnetic NWs inside anodic aluminum oxide membranes is discussed. Characterization of electrodeposited iron, nickel and highly magnetostrictive iron-gallium alloy NWs was done using XRD, electron and magnetic force microscopy. Second, different nanocomposite films have been fabricated by means of spin coating and drop casting techniques. The effect of incorporation of NWs inside the ferroelectric polymer on its electroactive phase is discussed. The remanent and saturation polarization as well as the coercive field of the ferroelectric phase are slightly affected. Third, effects of NW alignment on the magnetic properties of nanocomposites are discussed. Nanocomposites with aligned NWs showed anisotropic magnetic properties while the ones without showed isotropic properties. Forth and last, the effects of NWs loading, alignment and material on the magnetoelectric properties of the nanocomposites are analyzed. Low NW concentrations are found to promote the electroactive phase of the nanocomposite, whereas high concentrations lower it. Nanocomposites with aligned NWs showed an anisotropic magnetoelectric effect. Higher magnetostrictive NWs exhibited a higher magnetoelectric coupling, demonstrating the advantage of galfenol-based nanocomposites, which are reported in this thesis for the first time.

The magnetoelectric (ME) effect is an electric output in response to an applied magnetic field. In a heterostructure configuration where the two-phases are engineered with close interface contact, a giant electric response to a magnetic field has been found, which is designated as the ME voltage (or charge) coefficient α^ME. This effect is mediated by a mechanical-coupling between magnetostrictive and piezoelectric phases. In this thesis, I concentrate on application study for ME sensors with respect to noise control and rejection, thermal stability, triple-axis sensor design, array imaging, DC and AC magnetic sources detection and active mode ME sensor development, which is important for future ME sensor device applications. / Ph. D.

Magnetoelectric (ME) materials are attracting increasing attention due to the achievable reading/writing source (electric field and magnetic field in most cases), fast response time, and larger storage density. Therefore, nanocomposites featuring both magnetostriction and piezoelectricity were investigated to increase the converse magnetoelectric (CME, α) coefficient. Among all the nanocomposites, vertically/horizontally-integrated heterostructures were investigated; these materials offer intimate lattice contact, lower clamping effect, dramatically enhanced α, easier reading direction, and the potential to be patterned for complicated applications. In the present work, we focused on three principal goals: (a) creating two-phase vertically integrated heterostructures with different ME materials that provide much larger α, and enhanced strain-induced magnetic shape anisotropy compared with the single-phased ME nanomaterials; (b) creating a vertically integrated heterostructure with large α, lower loss, and higher efficiency; and (c) investigating the stable magnetization states that this heterostructure could achieve, and how it can be used in advanced memory devices and logic devices. Firstly, a BiFeO3-CoFe2O4 (BFO-CFO) heterostructure was epitaxially deposited on Pb(Mg1/3Nb2/3) O3-x at%PbTiO3 (PMN-xPT). The resulting PMN-xPT was proven to have a large piezoelectric effect capable of boosting the CME in the heterostructure to create a much higher α. Secondly, a novel material, CuFe2O4 (CuFO), featuring lower coercivity and loss, was chosen to be self-assembled with BFO. This low-loss could increase the efficiency of the ME effect. Also, our findings revealed a much larger α in the vertically integrated heterostructure compared to single-layer CuFO. Accordingly, the self-assembled structure represents a convenient method for increasing the CME in multiferroic materials. Thirdly, the magnetization states for all these vertically integrated heterostructures were studied. Note that vertically integrated heterostructures are typically fabricated using materials with volatile properties. However, these composites have shown a non-volatile nature with a multi-states (N≥4), which is favored for multiple applications such as multi-level-cell. Moreover, several self-assembled heterostructures were created that are conducive to magnetic anisotropy/coercivity manipulation. One such example is Ni0.65Zn0.35Al0.8Fe1.2O4 (NZAFO) with BFO, which forms a self-assembled nanobelt heterostructure that exhibits high induced magnetic shape anisotropy, and is capable of manipulating magnetic coercivity (from 2 Oe to 50 Oe) and magnetic anisotropy directions (both in-plane and out-of-plane). Finally, we deposited a SrRuO3-CoFe2O4 (SRO-CFO) vertically integrated composite thin film on the single crystal substrate PMN-30PT, with a CFO nanopillar and SRO matrix. In such a heterostructure, the SRO would serve as the conductive materials, while CFO offers the insulated property. This unique conductive/insulating heterostructure could be deposited on PMN-PT single crystals, thus mimicking patterned electrodes on the PMN-PT single crystals with enhanced dielectric constant and 33. / Doctor of Philosophy / Multi-ferroic materials, which contain multiple ferroic orders like ferromagnetism/ferroelectricity order, were widely studied nowadays. These orders are coupled together, which could manipulate one order via another one through the coupling. Due to the achievable reading/writing source (electric field and magnetic field in most of the case), fast response time and larger storage density, magnetoelectric (ME) materials aroused most interests to-date. To be used in different applications, such as memory devices and logic devices, a high transfer efficiency, or say a high coupling coefficient, is required. However, single-phase materials have nearly neglectable ME effect. Therefore, a nanocomposite that contents both magnetostriction and piezoelectricity were investigated to increase the converse magnetoelectric (CME, α) coefficient. Amongst all the nanocomposite, a vertically integrated heterostructure was revealed, which has intimate lattice contact, lower clamping effect, dramatically enhancedα, easier reading direction, and potential to be patterned for complicated applications. In this present work, we focused on several different aspects: (a) creating two-phase vertically integrated heterostructure with different ME materials, which provides much larger α, large strain-induced magnetic shape anisotropy comparing with the single-phased ME nanomaterials; (b): creating a vertically integrated heterostructure with large α and lower losses and higher efficiency; (c) investigate the stable magnetization states that this heterostructure could achieve, which shows the potential of being used in advanced memory devices and logic devices. Firstly, in this work, a BiFeO3-CoFe2O4 (BFO-CFO) heterostructure was epitaxially deposited on the Pb(Mg1/3Nb2/3) O3-x at%PbTiO3 (PMN-xPT), which could boost the CME in the heterostructure to create a much higher α. Then, a novel materials CuFe2O4 (CuFO), was chosen to be self-assembled with BFO, which has lower losses and higher efficiency of the ME effect. Secondly, several self-assembled heterostructures were created, such as Ni0.65Zn0.35Al0.8Fe1.2O4 (NZAFO) with BFO, which manipulated the magnetic coercivity (from 2 Oe to 50 Oe) and magnetic anisotropy directions (Both in-plane and out-of-plane). And a heterostructure: SrRuO3 with CFO, created a vertically integrated heterostructure, could be used as patterned electrodes in different applications. Moreover, magnetization states were studied in all these vertically integrated heterostructures. A multi-states (N≥4) was revealed, which was favored by multiple applications such as multi-level-cell or logical devices. Finally, we deposited a SrRuO3-CoFe2O4 (SRO-CFO) vertically integrated composite thin film on the single crystal substrate PMN-30PT, with a CFO nanopillar and SRO matrix. In such a heterostructure, the SRO would serve as the conductive materials, while CFO offers the insulated property. This unique conductive/insulating heterostructure could be deposited on PMN-PT single crystals, thus mimicking patterned electrodes on the PMN-PT single crystals with enhanced dielectric constant and d_33.

Since the turn of the millennium, giant magnetoelectric (ME) effects have been found in laminated composites of piezoelectric and magnetostrictive layers. Compared to ME single phase and two phase particulate composites, laminated composites have much higher ME coefficients and are also readily fabricated. In this thesis, I have investigated ME effect in laminated composites including materials, structures, fundamental properties and devices. Giant permeability Metglas was incorporated in ME laminates. The piezomagnetic coefficient of the Metglas is larger than that of widely used magnetostrictive materials, such as Terfenol-D or nickel ferrite. The experimental results show that Metglas based ME laminates have giant ME voltage coefficients and small required DC magnetic biases. Besides, the laminates have a good directional dependence of the magnetic field: it can only sense the magnetic field along its longitudinal direction. Symmetric bimorph and differential mode magnetoelectric laminates have been designed to reject (decrease) thermal and vibration noise sources, respectively. The mechanism for the noise cancellation capability is that the laminate operates in a bending (or longitudinal) mode, whereas the noise is contained in the other mode. The ME susceptibility (α_me) is the fundamental property that describes the coupling between the polarization and magnetization of a ME media. It is a complex quantity ( ). I discuss the relationship of the ME susceptibility between the magnetic permeability, dielectric permittivity of the materials, and the widely used ME voltage coefficient. The shape of the magnetic layer has a large impact on the giant permeability due to shape demagnetization effects. A long, thin and narrow shape increases the ME voltage coefficient and decreases the required optimum DC bias. The resonance frequency of Terfenol-D/PZT laminates can be continuously tuned by magnetic field over a wide range. This large tunability is due to the large magnetostriction of Terfenol-D. It results in a dramatic increase in the bandwidth over which devices might take advantage of the resonance enhanced ME coefficient. Four device applications have also been studied based on the giant ME effect of laminate composites. (i) ME laminates offer much potential for low-frequency (10⁻² to 10³ Hz) detection of minute magnetic fields (10^-12Tesla or below) in a passive mode of operation. With a wrapped active coil, the Metglas/PZT laminates are also capable of detecting changes of 0.8 nano-Tesla in DC magnetic fields without an applied DC bias. (ii) A geomagnetic field sensor is shown to have high sensitivity to variations in Earth's field of H_DC=0.8nano-Tesla. It could offer potential applications in global positioning. (iii) Under electro-mechanical resonance drive conditions, ME laminates have been shown to have a high gyration effect. These findings indicate the potential existence of a fifth fundamental network element. (iv) A multimodal system has been developed for simultaneously harvesting mechanical vibration and magnetic energies. / Ph. D.

Bismuth ferrite (BiFeO₃) is a magneto-electric material which exhibits simultaneously ferroelectric and antiferromagnetic properties. We have used high-field electron spin resonance (ESR) as a local probe of the magnetic order in the magnetic range of 0-25 Tesla. With increasing magnetic field, an induced transition has been found between incommensurately modulated cycloidal antiferromagnetic and homogeneous magnetized spin state. The data reveal a number of interesting changes with increasing field, including: (i) significant changes in the ESR spectra; (ii) hysteresis in the spectra near the critical field. We have analyzed the changes in the ESR spectra by taking into account the magnetic anisotropy of the crystal and the homogeneous anti-symmetric Dzyaloshinsky-Moria exchange. We have also investigated phase induced transition by epitaxial constraint, and substituent and cystalline solution effects. Variously oriented BiFeO₃ epitaxial thin films have been deposited by pulsed laser deposition. Dramatically enhanced polarization has been found for (001)c, (110)c, and (111)c films, relative to that of BiFeO₃ crystals. The easy axis of spontaneous polarization lies close to (111)c for the variously oriented films. BiFeO₃ films grown on (111)c have a rhombohedral structure, identical to that of single crystals. Whereas, films grown on (110)c or (001)c are explained in terms of an epitaxially-induced transition between cycloidal and homogeneous spin states, via magneto-electric interactions. Finally, lanthanum modified BiFeO₃-xPbTiO₃ crystalline solutions have been found to have a large linear magneto-electric coefficient, ∝p. The value of ∝p (2.5x10⁻⁹ s/m or C/m²-Oe) is ∼10x greater than that of any other material (cg., Cr₂O₃ ∼2.5x10⁻¹⁰ s/m), and many order(s) of magnitude higher than unmodified BiFeO₃ crystals. The data also reveal: (i) that ∝p is due to a linear coupling between polarization and magnetization; and (ii) that ∝p is independent of dc magnetic bias and ac magnetic field. We show that the ME effect is significantly enhanced due to the breaking of the transitional invariance of a long-period spiral spin structure, via randomly distributed charged imperfections. / Master of Science

Oxide interfaces have attracted considerable attention in recent years due to emerging novel properties that do not exist in the corresponding parent compounds. Furthermore, modern atomic-scale growth and probe techniques enable the formation and study of new artificial interface states distinct from the bulk state. A central issue in controlling the novel behavior in oxide heterostructures is to understand how various physical variables (spin, charge, lattice and/or orbital hybridization) interact with each other. In particular, density function theory (DFT) has provided significant insight into underlying physics of materials at the atomic level, giving quantitative results consistent with experiment. In this dissertation using density functional theory methods, we explore the electronic, magnetic and structural properties developed near the interface in SrTiO3/LaAlO3, EuO/LaAlO3, Fe/PbTiO3/Pt, Fe//BaTiO3/Pt and Cs/SrTiO3 heterostructures. We study the interplay between physical interactions, and quantify parameters that determine physical properties of hetetrostructures. These theoretical studies help understanding how physical variables couple with each other and how they determine new properties at oxide interfaces. / text

L'effet magnétoélectrique (ME) est la modification de la polarisation électrique par l'application d'un champ magnétique (effet ME direct), ou bien la modification de l'aimantation magnétique par l'action d'un champ électrique (effet ME inverse).L'utilisation de matériaux composites permet de reproduire de manière extrinsèque cet effet. Le couplage mécanique entre des matériaux magnétostrictifs et piézoélectriques fournit un effet ME extrinsèque plus important à température ambiante que celui fournit intrinsèquement. Nous avons dégagé (théoriquement et expérimentalement) différentes caractéristiques de matériaux nécessaires à l'obtention d'un effet ME important et justifé l'utilisation de ferrite et de PZT dans les composites ME. Nous expliquons dans cette thèse, les méthodes de fabrications des différentes géométries de composites étudiées (empilement de couches ou bien inclusions d'une phase dans l'autre). En particulier, l'utilisation du frittage non conventionnel par Spark Plasma Sintering, pour améliorer le couplage mécanique y est abordée. En nous focalisant sur la géométrie en multicouche, nous avons montré l'importance de facteurs tels que le champ démagnétisant ou encore la symétrie de la structure. Nous présentons un prototype de capteur de courant utilisable en génie électrique. Nous en avons montré sa bonne linéarité et sensibilité, mais aussi ses défauts en terme de bande passante. / The magnetoelctric (ME) response consists in the modification of the electric polarization by an applied magnetic field (direct effect) or the modification of the magnetic polarization by an applied electric field (inverse effect). Intrinsic multiferroics are rather uncommon and the effect is often weak at room temperature. An alternative route to achieve ME effect, consists in using magnetostrictive and piezoelectric materials and coupling the two phases by mechanical stress. We draw (theoretically and experimentally) some material characteristics to achieve an importantME effect, which justify the use of ferrite and PZT. We describe the production process of the two studied connectivity schemes (stack of layers or inclusion of a phase in another). We focus on the sintering by Spark Plasma Sintering as a potential improvement of the mecanical bonding. We devoted a part of our work on multilayer composite and showed the importance of some factors such as the demagnetizing effect or the symmetry of the structure. We introduce a current sensor prototype suitable for electrical engineering application. We showed its good linearity and sensitivity but also some effects of its bandwidth.

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