Title: Magnetic and magnetoelectric devices for communication and energy harvesting applications
Abstract: Magnetic materials have a long history of usage and play an important role in modern instruments or devices like motor, electrical generator, and transformer, etc. With the development of solid-state physics, the magnetic properties and theories beyond the magnetic static states have been studied and utilized including the magnetostatic wave propagation, magnetization oscillation and spintronics, which enable a wide variety of new applications especially for high frequency integrated electronics and magnetic storage. Magnetoelectrics (ME) refers to the coupling between the magnetism and electricity. The ME coupling effect provides a new methodology to mediate and monitor the magnetism in the magnetic material, and hence brings numbers of new potential features to the magnetic devices. The ME coupling can be divided into two categories, namely direct ME effect (DME) where the electric polarization is controlled by the magnetic field and converse ME effect (CME) where the magnetization is controlled by the electric field. In this dissertation, on-chip band stop filters (BSF) working in C band, on-chip hard magnets for energy harvesting applications and ME heterostructures resonate at very low frequency (VLF) designed for broadband communications and internet of things (IOT) sensor application are introduced and discussed. In the first part, an integrated BSF with magnetic multilayer films on silicon substrate is reported. The device performs a self-biasing band stop characteristic at C-band by taking the advantage from the ferromagnetic resonance (FMR) absorption and shape anisotropy of a slim FeGaB/Al2O3 multilayer ribbon. The magnetic properties of the multilayer material were characterized. The transmission, reflection characteristic and power handling capability of the BSF were measured. A maximum 70% tenability on the center frequency of the stop band has been achieved by applying a 400 Oe magnetic field. BSFs with single FeGaB layer are also fabricated for performance comparison. The second part focuses on the research of on-chip hard magnet fabrication and test. Two methodologies were adopted for on-chip powder based hard magnet fabrication, the spin-casted SmCo5 magnet possessed a coercive field as large as 15.5 kOe, and for dry-packed NdFeB magnets, by incorporating deep reactive ion etched silicon mold and compression tool, the process delivered samples with a remanence of 0.42 T and an energy product of 38 kJ/m3. A series of metal molding NdFeB permanent magnets have been fabricated to investigate the optimized hot-pressing fabrication conditions. In the third part, a novel modulation scheme implemented on VLF ME antenna called non-linear antenna modulation (NAM) is proposed and tested. Aiming to overcome the drawback brought by a typically narrow operational bandwidth of electrically small antennas, NAM enables a broad band communication which is explicitly shown in the distortion analysis of up-converted digital signals. The potential of VLF ME heterostructure serves as a passive vibration IOT sensor node is also explored, it has been shown that a vibration limit of detection of 4 μg is achieved with button magnet bias and wireless power transfer efficiency as high as 94% was observed. The potential application of the compact sensor node includes structural health monitoring (SHM), human activity monitoring and instrument malfunctioning diagnosis.--Author's abstract