TANMS Demonstrates Electrically Small Multiferroic Antennas

Achievement date: 
2017
Outcome/accomplishment: 

The NSF-funded Nanosystems Engineering Research Center (NERC) for Translational Applications of Nanoscale Multiferroic Systems (TANMS), headquartered at the University of California in Los Angeles (UCLA), has analytically shown that an electrically small multiferroic antenna is superior to a conventional compact antenna of similar size. The researchers demonstrated that electrically small multiferroic antennas (operating at cell phone frequencies of about 2 gigahertz (GHz)) rely on acoustic resonance, which reduces antenna dimensions from electromagnetic wavelengths to sizes more comparable with acoustic wavelengths, which are 10-100 times smaller.

Impact/benefits: 

Electrically small multiferroic antennas represent a paradigm shift for the antenna community in that they replace mechanically driven structures with electrically driven structures. TANMS new multiferroic antennas provide both receiving and transmit mechanisms with higher quality factor, compact size, low loss, and CMOS compatibility over conventional compact antennas. By changing the design and geometry, multiferroic antennas can flexibly operate over greater frequency ranges from low megahertz (MHz) to GHz (with the upper bound dictated by ferromagnetic resonance.) Also, because electrically small multiferroic antennas rely on acoustic wave resonance, they can increase radiation and sensitivity in both transmit and receive functions of much smaller antennas (less than λ0/10). As a result, electrically small multiferroic antennas represent a transformative change for future antenna design applications in areas spanning: internet of things (IoT), smart phones, satellites, and miniature radars.

Explanation/Background: 

State-of-the-art compact antennas rely on electromagnetic wave resonance, which produces antenna sizes comparable to the electromagnetic (EM) wavelength λ0. Reducing the antenna size below this value dramatically decreases the radiation energy with practical antennas sizes being limited by Chu limit, or larger than ~ λ0 /10. Specifically, these size reductions may represent up to 10-5 smaller antenna compared to conventional electromagnetic antenna in the future.

Multiferroic devices can be categorized into several groups based on the mechanisms employed for their control. Direct multiferroic coupling or magnetic field control of electrical polarization has been used in ultrasensitive magnetometers, energy harvesters, and other devices; whereas, converse magnetoelectric (ME) coupling or E-field control of magnetization switching has been used in spintronics, including ME Random Access Memory (MERAM). The sensing and receiving process of electromagnetic waves in multiferroic antennas corresponds to the direct ME coupling previously demonstrated in magnetometers. During the receiving process, these multiferroic antennas are sensitive to radio frequency (RF) magnetic field and produce a piezoelectric voltage output through strain-mediated coupling at mechanical resonance frequency. Conversely, the transmitting process of electromagnetic waves in multiferroic antennas corresponds to the converse ME coupling demonstrated in magnetic memory devices. During the transmitting process, these multiferroic antennas produce an oscillating mechanical strain under an alternating current (AC) voltage input, which is then transferred to the magnetic layer through strain-mediated ME coupling. The mechanical oscillation in the magnetic layer induces a magnetization oscillation or a magnetic current that radiates electromagnetic waves. Therefore, these multiferroic antennas operate at their acoustic resonance, which is 5 orders of magnitude smaller than the EM wavelength at the same frequency.

Bulk acoustic wave resonator (BAW) multiferroic antennas tested at TANMS consist of a magnetoelastic FeGaB thin film deposited onto a piezoelectric AlN thin-film. A dynamic voltage applied to the piezoelectric material produces a thickness extensional mode oscillation at 2.53 GHz, which is within antenna operating frequency. The mechanical oscillations generated by the piezoelectric create mechanical strains in the FeGaB magnetoelastic film, producing dynamic magnetic flux changes that transmit electromagnetic waves. Conversely, an incoming electromagnetic wave at 2.53 GHz produces magnetic changes in the FeGaB film, generating oscillating stresses that dynamically strain the piezoelectric. This time varying strain in the piezoelectric produces output voltages representing the receive function. Both transmit and receive functions have been experimentally confirmed. The multiferroic BAW antenna shows a gain of -18 dBi at the resonance frequency of 2.53 GHz with a device having an active area of φ200 µm or λ0/593. This represents a 1-2 order size reduction compared to state-of-the-art compact antennas.