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.

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.