Hexagonal Ferrite-Ferroelectric Core-Shell Nanofibers, Field-Assisted Assembly of Superstructures and Studies on Magnetoelectric Interactions

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Nontechnical Abstract: Nano-materials "by design" can be capable of conversion of a magnetic field into an electric field or vice versa. There are very few materials in nature capable of such field conversion and most of them have either poor efficiency or can function only at low temperatures. This project focuses on the fundamental physics of engineered nanomaterials with two phases. One is ferromagnetic that deforms in an applied magnetic field and the other a ferroelectric that converts this deformation to produce an electric field. Since the conversion is critically dependent on the surface contact between the two phases, the research focuses on coaxial nanowires with a much higher surface area-to-their volume ratio than bulk composites. The coaxial fibers are synthesized by a technique called electrospinning and characterized in terms of field conversion efficiency and sensitivity. These composites are ideal for use as ultrasensitive, miniature magnetic field sensors and arrays for imaging applications such as magneto-cardiography (MCG) and magneto-encephalography (MEG) that are powerful tools for diagnosis of disorders associated with the heart and brain. The nanomaterials also have the potential for applications in signal processing and energy harvesting. Other significant impacts of the research are human resources development at all levels in advanced materials synthesis and characterization. The PI and Co-PI plan to recruit undergraduate science and engineering majors for participation in the research. High school students, women and minorities in particular, are to be recruited to participate in the research. Technical Abstract: Proposed research focuses on fundamental physics and synthesis of coaxial nanowires of ferrimagnetic hexagonal ferrites and ferroelectric lead zirconate titanate (PZT), assembly of nanostructures into superstructures with the aid of magnetic and/or electric fields, and studies on the intrinsic nature of coupling between magnetic and electric subsystems. The primary focus is on fibers with Y- or W-type hexagonal ferrite due to their high anisotropy field and self-magnetic bias characteristics that will give rise to strong magneto-electric (ME) coupling without the need for an external bias magnetic field. Specific tasks are as follows. (i) Synthesis of core-shell fibers with (Ni, Zn) Y-type or (Co, Zn) W-type hexaferrites, with the choice of Zn-substituted ferrites aimed at control of magnetic order parameters for ME studies at low frequencies and at resonance modes over a wide frequency range, from 1Hz to 110 GHz. (ii) Transmission electron microscopy (TEM) and scanning probe microscopy (SMM) of nanocomposites and arrays, where Lorenz-TEM and SMM studies are planned for imaging the fibers and interfaces in terms of magnetic, ferroelectric, and electromagnetic parameters. (iii) Investigations of ME effects on individual nano-wires and assemblies under local mechanical, electrical and magnetic excitations to establish correlations among ME coupling strengths, symmetry/connectivity of composites, and field-directed assembly parameters, and optimize the morphology of the nanocomposites to achieve a maximum ME response. (iv) Modeling of field directed assembly and ME interactions in individual nanocomposites and assemblies for comparison with data. Overall, the research not only enhances fundamental understanding of ME effects in nanomaterials, but is also useful for a new family of sensors and signal processing and energy devices.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.