Magnetoelectric Nanocomposites for Flexible Electronics

Abstract

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.