Research On Device-based Magnetoelectric Antenna With Acoustic Excitation

Magnetoelectric heterojunction materials produce magnetoelectric coupling effects by means of strain-mediated coupling mechanisms,and are widely used in antennas,sensors,random access memories,energy harvesters,and tunable radio frequency devices（inductors,filters,and phase shifters）,thus becoming a hot research topic.Piezoelectric/magnetostrictive composite films have outstanding magnetoelectric coupling coefficients in the ultra-high frequency band due to good interlayer coupling,which has led to the application in device-based magnetoelectric antennas excited by acoustic waves.Unlike the traditional electric antennas,the magnetoelectric antennas drive the magnetic moment to generate electromagnetic radiation by means of the mechanical stress field of piezoelectric acoustic resonance,so their dimensions are comparable to the acoustic wave wavelength.Due to the difference of four to five orders of magnitude between acoustic wave wavelength and electromagnetic wave wavelength at the same frequency,the magnetoelectric antennas offer significant miniaturization advantage over the electric antenna,and can exert significant radiation gain advantage under the same size.The principle of acoustic resonance induced magnetic dipole radiation also gives the magnetoelectric antenna without external matching network and ground plane mirror current attenuation radiation advantage.However,due to the complex multi-physics coupling radiation mechanism,device-based magnetoelectric antennas in the UHF band currently have low gain（-18 d Bi）,radiation efficiency（0.45%）,and fractional bandwidth（about1%）,which cannot be meet needs in practical application scenarios.Therefore,this thesis mainly constructs a device-based magnetoelectric antenna excited by bulk acoustic waves based on Al（Sc）N/Fe Ga（B）magnetoelectric heterojunction,and conducts optimization explorations on electromagnetic radiation and bandwidth enhancement.The specific research content as follows:（1）The piezoelectric effect and magnetoelastic coupling of the magnetoelectric heterojunction were described,and the radiation model of the magnetoelectric antenna was established.The coupled bulk acoustic wave and electromagnetic wave propagating along the c-axis are derived in the piezoelectric material Al N,and the mechanism that the magnetic moment is equivalent to a magnetic dipole is revealed in the micromagnetic module that solves the Landau-Lifshitz-Gilbert equation.The dynamic radiation characteristics of the magnetoelectric antenna were determined using the finite element method（FEM）coupled piezoelectric,magnetoelastic coupling,and electromagnetic radiation modules.（2）Deduce the key parameters of Al N/Fe Ga magnetoelectric heterojunction,establish the finite element model of the magnetoelectric and inverse magnetoelectric effects,and characterize the（35）E effect and the magneto-mechanical coupling factor（k_H）.The effects of parameters such as direct magnetoelectric coefficient(α_DME),converse magnetoelectric coefficient(α_CME),k _H and（35）E effect on the radiation in magnetoelectric heterojunction were described.Based on the FEM analysis,the strain-mediated coupling effect in the magnetoelectric heterojunction was revealed,and the strongestα_DME was achieved in the resonant state.While the optimization of theα_DME by the DC bias magnetic field was realized,α_DME is close to 3 k V/cm·Oe at a field strength of 150 Oe.The bulk acoustic wave excited（35）E tester shows the in-plane and out-of-plane anisotropic（35）E effect in the Fe Ga film,the maximum Young’s modulus change is 500 MPa,and the k _H is up to 0.08.（3）A longitudinal-wave magnetoelectric antenna based on Al N/Fe Ga magnetoelectric heterojunction was constructed,Mason and FEM models were developed for design optimization,and the MEMS fabrication and characterization were completed.The standing wave and impedance characteristics of the magnetoelectric antenna were simulated in the Mason model,the FBW_{-10d B} bandwidth is 1.05%@2.45 GHz,and the Q_s and Q_pare 22 and 72,respectively.The FEM simulation reveals the time-varying stress field,radiated magnetic field and electric field corresponding to the acoustic resonance of the magnetoelectric antenna,where the radiation field pattern is consistent with the simulation results of time-domain finite difference method and the decay of the magnetic field H_r with distance agrees with the trend of the small loop antenna（equivalent magnetic dipole）.The simulated bandwidth and radiation efficiency of the magnetoelectric antenna reach 1.05%@2.45 GHz and 0.359%,respectively,which are closer to the measured bandwidth（0.78%）and radiation efficiency（0.355%）,and the radiation enhancement effect（radiation gain increased by 12.7 d B compared to the highest reported value）is verified in the magnetoelectric antenna actuated by asymmetric electric field.（4）The magnetoelectric antenna with floating potential structure was constructed to optimize the radiation gain,and the radiation enhancement mechanism was verified and explained by combining FEM simulation and experiments.The floating potential magnetoelectric antenna based on asymmetric electric field excitation can realize both electromagnetic radiation enhancement and impedance matching.Compared with the grounded structure,the floating potential structure with the same size（18×22 mm²）has a gain enhancement of about 20 d B,reaching 1.5 d Bi.The FEM simulation reveals that the radiation enhancement originates from the floating potential induced electric dipole radiation and the magnetic dipole radiation excited by the coupling of three bulk acoustic modes,which also leads to a positive correlation of the electromagnetic radiation enhancement effect with the antenna plane size.The electric dipole radiation comes from the conduction current induced by the piezoelectric displacement current in the bottom electrode,and the acoustic resonance state makes the equivalent electric dipole produce effective radiation power,which is exactly the difference between the floating potential and the grounded magnetoelectric antennas,indicating the plausibility of the equivalent electric dipole.The radiation efficiency of the floating potential magnetoelectric antenna（size 1×4 mm²）excited by high-overtone acoustic is 3%in microwave anechoic chamber,the gain and directivity coefficient are-18.8 d Bi and 2.7,respectively.（5）Two mechanisms of multi-mode acoustic wave coupling and longitudinal wave resonance cascading were proposed to realize the optimization of the bandwidth of the magnetoelectric antenna.The multi-mode acoustic wave coupling excitation magnetoelectric antenna utilizes tiny-frequency-shifted multi-resonance regions in equivalent parallel connection to achieve a fractional bandwidth of 2.7%@0.8 GHz with a size of only 1/536 of the wavelength of the working electromagnetic wave,and the correspondence between the standing wave bandwidth and radiation bandwidth in the magnetoelectric antenna is elucidated;the cascaded longitudinal wave excited magnetoelectric antenna achieves a fractional bandwidth of 5.35%@2.45 GHz with the size of 1/88 of the wavelength of the working electromagnetic wave,and the dependence of the bandwidth and impedance matching on the mechanical loss resistance is also simultaneously demonstrated.