ECE PhD Dissertation Defense: Leili Hayati

PhD Dissertation Defense: Ceramic Magnetic Wires at Wireless Communication Frequencies

Leili Hayati

Location: Online

Abstract: Ferrite magnetic devices play an important role in modern wireless telecommunication systems. They generally require permanent magnets in order to magnetically polarize the ferrite material component used in these devices. The permanent magnets are bulky and take up most of the size and weight of a magnetic circuit. The aim of this research is to do away with permanent magnet bias circuits as utilized in circulators and ferrite planar devices, especially in wireless communication systems operating below 2 GHz. Recently, ferromagnetic nanowires (NWs) have been embedded into porous templates, are used to design various microwave magnetic and electronics devices. The main advantage of magnetic NWs is that in zero magnetic field, the microwave absorption frequency can be easily tuned over a large range of frequencies. Clearly, the metallic nature of the magnetic NWs contributed to the high loss. It is expected that insulating magnetic NWs will improve the insertion loss sufficiently to produce viable ferrite devices at wireless communication frequencies below 2GHz and at higher frequencies. There are no pure insulating magnetic materials. However, there are ferrites that are nearly insulating and are ferrimagnetic. Their saturation magnetization is much lower than the metallic ferromagnetic counterpart. This is a desirable property for magnetic device operating below 2 GHz. Of all the ferrite materials yttrium iron garnet (YIG) exhibits the lowest FMR linewidth ever measured and low saturation magnetization. In this work, an array of high-purity YIG NWs embedded in a porous silicon membrane, were synthesized using sol-gel method and the magnetic properties of the pure YIG Nanoparticles and the composite substrate were characterized by utilizing vibrating sample magnetometer (VSM) technique. From the ferromagnetic resonance (FMR) spectra, it has been found that the measurements are characterized by a uniaxial magnetic anisotropy energy due to the high aspect ratio of the NWs. Based on the magnetic parameters of the composite substrate and characterizing YIG NWs, a coplanar waveguide was designed by HFSS software. By applying a small external magnetic field and changing the internal magnetic H field by ±8%, the phase of S21 parameter shifts up to 30̊ degrees near 1.7GHz.