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This work proposes to use magnetic material as the substrate of planar antennas to overcome the platform effect caused by the conducting ground plane. The upper bound of the radiation efficiency of an electric-current-driven low-profile antenna is theoretically derived, which is inversely proportional to the Gilbert damping factor of the magnetic material. Meanwhile, the improvement of radiation due to the use of magnetic material is demonstrated by a three-dimensional (3D) multiphysics and multiscale time-domain model. The simulation results match the theoretical derivation, showing 25% radiation efficiency from a planar antenna backed by a FeGaB thin film with 2.56 um thickness. Furthermore, for conductive ferromagnetic materials, it is shown that the eddy current loss can be well suppressed by laminating the thin film into multiple layers. The radiation efficiency of the modeled antenna with a conductive ferromagnetic substrate is improved from 2.2% to 11.8% by dividing the substrate into 10 layers, with a ferromagnetic material fill factor of 93%.

The magnetoelectric properties of exchange-coupled Ni/CoFeB-based composite multiferroic microstructures are investigated. The strength and sign of the magnetoelastic effect are found to be strongly correlated with the ratio between the thicknesses of two magnetostrictive materials. In cases where the thickness ratio deviates significantly from one, the magnetoelastic behavior of the multiferroic microstructures is dominated by the thicker layer, which contributes more strongly to the observed magnetoelastic effect. More symmetric structures with a thickness ratio equal to one show an emergent interfacial behavior which cannot be accounted for simply by summing up the magnetoelastic effects occurring in the two constituent layers. This aspect is clearly visible in the case of ultrathin bilayers, where the exchange coupling drastically affects the magnetic behavior of the Ni layer, making the Ni/CoFeB bilayer a promising next-generation synthetic magnetic system entirely. This study demonstrates the richness and high tunability of composite multiferroic systems based on coupled magnetic bilayers compared to their single magnetic layer counterparts. Furthermore, because of the compatibility of CoFeB with present magnetic tunnel junction-based spintronic technologies, the reported findings are expected to be of great interest for the development of ultralow-power magnetoelectric memory devices.

An unconditionally stable finite-difference time-domain (FDTD) algorithm is proposed to predict and understand the complex response in strain-mediated multiferroic radio frequency devices, such as antennas. A system of three coupled sets of governing equations is solved simultaneously: 1) Maxwell's equations for electromagnetic (EM) wave propagation; 2) Landau–Lifshitz–Gilbert equation for magnetic spin response; and 3) Newton's law for acoustic behavior. The formulation of this algorithm is elaborated on in detail followed by a demonstration of its capability through the simulation of a 1.3- μ m-thick bulk-acoustic-wave (BAW)-based strain-mediated multiferroic antenna. Analysis of the far-field radiation efficiency of this antenna as a function of magnetic dc bias demonstrates the importance of aligning the ferromagnetic resonance (FMR) and the mechanical BAW resonance to enhance EM radiation performance. Results also show that reducing the magnetic loss, or in other words, reducing the FMR linewidth, represents the dominating feature to achieve higher radiation efficiencies in these antennas.

In this work, we investigate magneto-acoustic attenuation in thin film multiferroic Lamb wave delay lines. By leveraging magneto-acoustic interactions, multiferroics have potential to realize passive chip-scale alternatives to bulky ferrite devices. For the first time, magnetic field dependence of magneto-acoustic interactions in multiferroic Lamb wave devices is characterized. Multiferroic heterostructures of aluminum nitride and cobalt iron boron are fabricated into Lamb wave delay lines operating at 7.492 GHz to study the effect of strain nonuniformity on the multiferroic coupling. The attenuation of the Lamb waves is characterized as a function of the magnitude and angle of an applied in-plane bias magnetic field. It is found that the bias magnitude for peak attenuation is a strong function of angle, indicating that it is due to coupling between the Lamb waves and spin wave modes. This is in contrast with current models of attenuation in multiferroic SAW delay lines, where the uniform surface strains couple to ferromagnetic resonance, which has no angular dependence on the in-plane bias field magnitude. These results are the first steps towards passive chip-scale alternatives to ferrite devices utilizing Lamb wave devices, which have better coupling and scalability than their SAW counterparts. [2020-0114]

Arbitrary patterning of micro-objects in liquid is crucial to many biomedical applications. Among conventional methodologies, acoustic approaches provide superior biocompatibility but are intrinsically limited to producing periodic patterns at low resolution due to the nature of standing waves and the coupling between fluid and structure vibrations. This work demonstrates a near-field acoustic platform capable of synthesizing high resolution, complex and non-periodic energy potential wells. A thin and viscoelastic membrane is utilized to modulate the acoustic wavefront on a deep, sub-wavelength scale by suppressing the structural vibration selectively on the platform. Using 3 MHz excitation (λ ∼ 500 μm in water), we have experimentally validated such a concept by realizing patterning of microparticles and cells with a line resolution of 50 μm (one tenth of the wavelength). Furthermore, massively parallel patterning across a 3 × 3 mm2 area has been achieved. This new acoustic wavefront modulation mechanism is powerful for manufacturing complex biologic products.

In this paper we explore parametric amplification on the Lamb wave acoustic platform for the purpose of developing nonlinear and non-reciprocal devices. The strength of the acoustic wave platform over the electromagnetic platform is its low loss and short wavelength at radio frequency which enables devices with a small footprint and high quality factor. Compared to surface acoustic waves, Lamb waves exhibit much higher phase velocity which support higher frequency operation with fabrication tolerance. To realize parametric amplification, the intrinsic nonlinear stiffness of aluminum nitride is used to couple acoustic waves at different frequencies. Novel Lamb wave resonator structures and non-reciprocal devices implementation have been proven in theory and simulation. Experiment of Lamb wave transducers demonstrates parametric effects and proves nonlinearity in AlN thin films.

Lamb wave devices were fabricated and tested to study parametric mixing through a nonlinear elastic modulus. A carrier wave was used to drive the time-variation of the elastic modulus in a traveling wave fashion. The signal wave is launched in the same direction as the carrier wave. The nonlinearity results in the signal to be mixed up and down with the carrier signal generating intermodulation terms. Through optimization, next generation RF components that are 5 orders in magnitude smaller than the current state of the art can be designed, freeing up real estate for larger batteries and additional sensors.

In this Letter, we explore a non-degenerate phase independent parametric quality factor (Q)-enhancement technique for aluminum nitride (AlN) Lamb wave resonators. Unlike other active Q-enhancement techniques which require precise phase control of the electronic feedback loop, this technique is implemented by parametrically pumping AlN material stiffness to realize a negative resistance seen at the signal path. The negative resistance is dependent on the nonlinear material modulation and multi-resonance coupling in the device. A nonlinear circuit model is developed to simulate the parametric coupling of each resonance and extract the nonlinearity of AlN from experimental data. With proper pump frequency and pump power, the device quality factor is boosted in both simulation and experiment. The demonstrated Q-enhancement method is simple to implement and can be applied to other types of resonators that have nonlinear behavior and support multi-resonance operation.

Software-defined radio (SDR) requires multi-use or reconfigurable radio-frequency front ends, with filters capable of dynamically shifting their operating frequencies. Current approaches to tunable filters typically require prohibitively large electric fields to achieve large tuning. This study utilizes a multiferroic approach, with an electric field straining magnetoelastic elements to tune the system's magnetic resonance. Furthermore, dipole coupling of the elements yields mode splitting, for additional resonance peaks that depend on element spacing and applied strain. This could lead to innovative types of rf filters for SDR, where communication frequencies are rapidly changing.

Frequency conversion is an essential tool in modern communication devices. Traditionally, frequency conversion is achieved through parametric coupling via nonlinear inductors or capacitors whose reactance is modulated by a carrier wave. In this study, nonlinear acoustic Lamb wave devices are explored for simultaneous signal filtering and frequency conversion. Three sets of interdigitated transducers are fabricated on a piezoelectric aluminum nitride (AlN) thin film to provide a carrier wave, a low-power signal wave, and to receive a frequency converted mixed wave. Two devices are fabricated and tested to demonstrate frequency upconversion and downconversion by utilizing mechanical nonlinearity of AlN, and the results are compared to a nonlinear circuit model. The nonlinear circuit model is used to link experimentally observed phenomenon to the acoustic material's intrinsic nonlinearity. The nonlinearity of AlN reaches a maximum of 2.8% with a carrier wave power at 28 dBm. An analytical model is used to predict device performance along with physical dimensions. These analytical results show that nonlinear acoustic mixers and filters can approach sub-millimeter sizes, which is orders-of-magnitude smaller than conventional structures using nonlinear inductors and capacitors.

This paper experimentally demonstrates the operating principles of a near-field multiferroic antenna. The antenna uses a piezoelectric lead-zirconate-titanate stack to apply a time varying strain to a magnetoelastic iron gallium Fe80.77Ga19.23 (FeGa) rod. The voltage induced strain controls the net magnetization of the FeGa rod to generate oscillating magnetic fields in free space surrounding the antenna. Direct experimental measurements of the mechanical force, strain, magnetic field, and magnetic flux inside the FeGa rod were collected and show a strong correlation with the dynamic magnetic field measured in free space. Additionally, an analytical dipole model is used to demonstrate that the free space signal originates from the changing magnetization in the FeGa rod, providing a clear multiferroic antenna proof-of-concept demonstration. Finally, analysis is provided to highlight the importance of device geometry and demagnetization fields when optimizing the antenna response.

Wireless multiferroics is an emerging field taking advantage of energy efficient multiferroic coupling to design and develop ultra-conformal, electrically-small antennas. To date, all work on MEMS scale multiferroic devices utilize linear multiferroic coupling. In this work, we present the first demonstration of frequency doubling through nonlinear multiferroics in MEMS resonators. Multiferroic composite cantilevers are fabricated and tested, and it is shown that these resonators can be wirelessly driven to mechanical resonance with magnetic fields at half of the resonant frequency by utilizing perpendicular magnetic poling. This is a potential method for low noise measurement of wireless signals, a problem that plagues all electrically-small wireless devices, which would revolutionize the emerging field of wireless multiferroic devices.

A novel electrically-small bulk acoustic wave (BAW)-mediated multiferroic antenna is proposed based on transverse shear waves instead of the longitudinal ones used in the previously proposed multiferroic antenna [1]. Two in-plane orthogonal pairs of electrodes provide independent excitation of the orthogonal transverse shear modes, enabling the first investigation of polarization control in a BAW-mediated multiferroic antenna. Using this approach, polarization can be controlled simply by adjusting the phase difference between the input RF signals of the two electrode pairs. The proposed design concept is verified numerically via COMSOL simulations and a series of post-processing steps in MATLAB. By assuming ideal input RF signals with equal amplitude and 90° phase difference, broadband circularly polarized radiation is achieved in the sub-GHz band.

© 2017 Author(s). Enhancing the magnetoelectric coupling in a strain-mediated multiferroic composite structure plays a vital role in controlling magnetism by electric fields. An enhancement of magnetoelastic coupling between ferroelectric single crystal (011)-cut [Pb(Mg1/3Nb2/3)O3](1-x)-[PbTiO3]x(PMN-PT, x≈ 0.30) and ferromagnetic polycrystalline Ni thin film through an interposed benzocyclobutene polymer thin film is reported. A nearly twofold increase in sensitivity of remanent magnetization in the Ni thin film to an applied electric field is observed. This observation suggests a viable method of improving the magnetoelectric response in these composite multiferroic systems.