RF magnetoelectric microsystems

Abstract

Multiferroic materials are the materials that inherently exhibit two or more ferroic properties, such as ferroelectricity, ferromagnetism and ferroelasticity, etc. Magnetoelectric (ME) materials with coupled magnetization and electric polarization have attracted intense interests recently due to the realization of strong ME coupling and their key roles in ME applications. Since the revival of thin-film ME heterostructures with giant ME coefficients, a variety of multifunctional ME devices, such as sensors, inductors, filters, antennas etc. have been developed. Exciting progress has been made on novel ME materials and devices because of their high-performance ME coupling. In this dissertation, we will first show the properties of magnetostrictive (FeGaC and SmFe) and piezoelectric (ZnO) thin-film materials that are necessary for realizing strong ME coupling. A systematic investigation of the soft magnetism, the change of modulus of elasticity with magnetization ($\Delta$E effect), and microwave properties was carried out on FeGaC and SmFe thin films. We successfully developed the magnetostrictive FeGaC thin films with low coercive field of less than 1 Oe, high saturation magnetization, narrow ferromagnetic resonance (FMR) linewidth, and an ultra-low Gilbert damping constant of 0.0027. A record high piezomagnetic coefficient of 9.71 ppm/Oe, high saturation magnetostriction constant of 81.2 ppm, and large $\Delta$E effect of -120 GPa at 500 nm were achieved. ZnO films with high c-axis crystal orientation was also achieved by carefully optimizing the sputtering process parameters. These properties make them attractive materials for magnetoelectric and other voltage tunable RF/microwave device applications. After presenting the magnetostrictive and piezoelectric thin films and their static and dynamic properties, we introduce the radio frequency (RF) ME microsystems. Mechanically driven antennas have been demonstrated to be the most effective method to miniaturize antennas compared to state-of-the-art compact antennas.The ME antennas based on a released magnetostrictive/piezoelectric heterostructure rely on electromechanical resonance instead of electromagnetic wave resonance, which results in an antenna size as small as one-thousandth of an electromagnetic wavelength. Due to the strong ME coupling in thin-film ME heterostructures, we proposed the ultra-compact MEMS ME antennas and improved their performance by using anchor designs, array structure, and SMR structure. These miniaturized robust ME antennas can be implemented in numerous real-world applications such as internet of things, wearable and bio-implantable devices, smart phones, wireless communication systems, etc. The ME antennas, with an overall dimension of 700 μm × 700 μm (L × W), were designed to operate at a resonant frequency of $\approx$ 2 GHz and experimentally demonstrated a gain of -18.85 dBi. Furthermore, we demonstrated highly sensitive integrated RF giant magnetoimpedance (GMI) sensors based on amplitude and phase sensitive mechanisms. The amplitude and phase magnetic noise levels were demonstrated to be 810 $pT/\sqrt{Hz}$ at 1000 Hz and 100 $pT/\sqrt{Hz}$, respectively.--Author's abstract