PhD Dissertation Defense: Huaihao Chen

PhD Dissertation Defense: Integrated RF Devices Based on Magnetoelectric Coupling

Huaihao Chen

Location: Zoom Meeting

Abstract: The magnetoelectric (ME) coupling effect is a coupling behavior between the magnetic properties and electric properties in a single-phase crystal or a composite structure. At present, ME composite is more widely used in practical application than the single-phase crystal due to the larger coupling coefficient and higher working temperature. Based on the coupling direction, there are two types of ME coupling: the direct coupling by using magnetic field to control the electric polarization; the converse coupling by using electric field to control the magnetization. With the strong coupling effect and the low power consumption, ME coupling becomes more and more attractive in RF devices design, including magnetic sensor, ME memory, energy harvester and so on.
In this dissertation, the integrated inductor and ME antenna based on ME coupling are reported, including the design, simulation, fabrication and test of the devices. The performances, advantages and issues of these devices are discussed, and some improvements are applied for a better performance.
The first part of this thesis is the integrated high-Q and RF tunable inductors. In this part, the 1-D laminated iron core inductor model, choosing of magnetic material for the inductor core and the tuning principle based on converse ME coupling is explained firstly. Then, the fabrication process flow is described. The practical high-Q inductor shows a constant inductance of ~1.4 nH in a wide frequency range from DC to 3 GHz, with a peak Q-factor of 32.7, after magnetic annealing. By attaching a PMN-PT slab to the device, a tunable inductor is realized. Inductance tunabilites are achieved under both magnetic field (69.2%) and electric field (191%), which is higher than most of the reported inductors.
The second part is the design of ME antenna for biomedical implants. FeGaB/AlN heterostructure is chosen as the ME resonator of the antenna. The operation frequency is the acoustic resonance, decided by the width of the resonator. This ME antenna has a ~1000 times smaller volume than conventional antennas, due to the smaller wavelength of acoustic wave than electromagnetic wave. After fabrication, the measured S11 resonance peak matches with the simulation, and the antenna gain of -37.1 dBi is calculated by a gain comparison method. The modified Butterworth-Van Dyke (mBVD) model is used to calculate the Q-factor (114) and electromechanical coupling coefficient (kt2, 0.98%). The radiation pattern and polarization are measured, proving that this ME antenna performs as a magnetic dipole. Finally, the input impedance matching is optimized with array structure. The third part is the performance improvement of FeGaB thin film by minimizing the mechanical stress from the film deposition, by controlling the deposition pressure and magnetic annealing. The film deposited under 2 mTorr pressure shows the lowest stress and best magnetic properties of coercive field, saturation magnetization, magnetic damping and magnetostriction. And the magnetic annealing shows an improvement on the FeGaB film. This research helps to improve the performance of devices based on FeGaB thin film.