Tunable RF Circuits using Iron-Cobalt Ferromagnetic Nanoparticles
Authors:   , Malallah Yaaqoub Y. , Daryoush Afshin S. 
Published: July 2021

Modern communication systems are evolving very quickly and tunable Radio Frequency (RF) components are needed to utilize the spectrum more effectively. Recent developments in small size and tunable microwave/RF components are attractive for communication and radar applications for industrial, civilian, and military purposes. Different frequency tuning methods of electrical, mechanical, optical, acoustic, and magnetic are employed for dual frequency of operation. New manufacturing methods are being continuously improved with great interest in additive manufacturing. This PhD thesis work focuses on design and frequency tuning of passive RF circuits using magnetic flux control of ferroic nanomaterials fabricated using additive manufacturing. In particular, the focus of the thesis is using nanoparticles of ferromagnetic Iron-Cobalt (Fe60Co40) and Cobalt ferrites (CoFe2O4) considered for their electromagnetic properties. Compared to ferroelectric materials, the tuning control circuit of ferro/ferrimagnetic materials can be designed without adding parasitic effects. The extracted electromagnetic properties of the realized composites are refraction index of nFeCo=4 and nCoFeO=2, magnetization saturation of MFeCo=198 emu/g and MCoFeO=46.8 emu/g, loss tangents of tan[delta]FeCo<0.25 and tan[delta]CoFeO<0.09, and coercivity of HFeCo=158 Oe and HCoFeO=900 Oe; which makes them viable at microwave frequencies and many RF applications. The FeCo composites were synthesized and fabricated on FR4 substrate using subtractive manufacturing through milling machine and additive manufacturing using squeegee printing. The approximate electromagnetic properties of FeCo were extracted over a broadband of 1-10GHz using perturbation theory applied to microstrip transmission lines. Then these initial estimates were further refined using annular ring resonance structure for accurate extraction over a narrowband frequency of 1-4 GHz, which is applicable for Wi-Fi application. Full electromagnetic field modeling of annular ring resonator was also developed to best curve fit the initial seed values from perturbation technique to the measured 2-port scattering parameters. My realized extraction procedure employs the Finite Element Method (FEM) based electromagnetic field modeling using the commercially available numerical modeling (e.g., COMSOL Multiphysics and Ansoft HFSS) and experimental measurements of the S-parameters using 8722ES Network Analyzer. The extraction is performed for baseline annular ring design and FeCo nanoparticle based composite substrate with and without applied magnetic fields from 0-1500G. The extracted results of the FeCo range in permeability from ur=15 and tan[delta]u=0.25 to ur=10 and tan[delta]u=0.1 with applied 1.5kG magnetic field. Accurate extraction with anisotropic behavior was used to realize tunable antennas, tunable impedance matching circuits, and tunable 3rd order annular ring and hairpin filters at different frequency bands. The fabricated annular ring 3rd order filter resulted in 200MHz (8%) tuning when 1.5kG magnetic field was applied at Wi-Fi band of 2.4GHz, while the tunable antenna resulted in 100MHz tuning with the same applied field. Furthermore, additive manufacturing is used to resolve some of the fabrication challenges encountered with subtractive methods. Characterization is performed using annular ring resonators for Polylactic Acid with the extracted [epsilon]r=1.9 and tan[delta][epsilon]=0.02 material that is used for additive manufacturing along with magnetic CoFeO with ur=2.3 and tan[delta]u=0.09 nanoparticles. 3D printed magnetic filament material is used to design tunable phase shifter with about 16° phase shift with applied magnetic field operating at C-band frequencies. Additional analysis using CoFeO is provided for tunable periodic structures with about 8x magnification compared the nonperiodic case. Meanwhile, a bandstop with 1.25GHz bandwidth exists at 6.25GHz. Moreover, at 5.74GHz there is about 40° phase shift for the periodic structure compared to the non-periodic baseline case since the wave is slower. Future applications of this magnetically tunable circuits are considered for tunable idler, periodic structures, impedance transformers, and frequency selective surfaces.