Author ORCID Identifier

https://orcid.org/0000-0002-5074-9461

Defense Date

2024

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Electrical & Computer Engineering

First Advisor

Supriyo Bandyopadhyay

Abstract

Device miniaturization is a prerequisite for modern day electronics. Spintronics offers immense potential in energy efficient data storage, solid states devices, ultrafast computing and other electronic applications because of their low power consumption, non-volatile nature and unique activation mechanism. The conventional nano-antennas used for wireless communication, biomedical and wearable devices and IoT applications are limited by the traditional Harrington limit. As we attempt to miniaturize the antenna size beyond its emitted wavelength (also called “subwavelength” antenna), its gain and efficiency plummet. To overcome this challenge, the researchers have proposed magnetoelectric antennas. The ultra-thin film based magnetoelastic antennas suffer from eddy current loss and their working frequency depends on ferromagnetic resonance. In this dissertation, we have proposed microwave frequency spin wave nano-antennas based on two different working principles. Our proposed antennas have few advantages over the previously proposed thin film based antennas. Firstly, our antennas consist of an array of tiny magnetostrictive nanomagnets instead of thin film to suppress the eddy current loss (there is no continuity path for eddy current loop because of the gap between the nanomagnets and a large energy is needed to form tiny loops within nanomagnets). Secondly, our antennas can radiate at all frequencies and most efficiently at resonance frequencies. The first nano-antenna that we have proposed is enabled by tripartite phonon-magnon-photon coupling. The surface acoustic wave (phonon) launched in the piezoelectric substrate interacts with the spin wave (magnon) confined in the nanomagnets via phonon-magnon coupling and then the magnetization precession or spin wave emits electromagnetic (EM) wave (photon) in the surrounding medium via magnon-photon coupling. This antenna showed very high gain and efficiency of 35.6% at 5 GHz and 54.4% at 14 GHz frequency exceeding the Harrington limit by at least two orders of magnitude assuming the antenna acts as a point source and ignoring interference effect. Interestingly, our nano-antenna exhibited anisotropic or directive radiation patterns instead of omni-directional patterns because of the intrinsic anisotropy in shape and dipole coupling of the nanomagnets. The second and third nano-antenna that we have proposed operate based on spin-orbit torque induced from the heavy metal or topological insulator layer underneath the nanomagnets and the resulting magnetization precession of the nanomagnets radiates EM waves via magnon-photon coupling. The radiation patterns once again reveal the anisotropic behavior and beam steering capability of the nano-antennas. Finally, we have presented a spintronic frequency comb which originates from the phonon-plasmon-magnon (acousto-plasmo-magnon) coupling. Here, the phonon comes from the femtosecond laser pulse, the plasmon comes from the aluminum thin film and the magnon comes from the nanodots placed on the top of the aluminum layer. This frequency comb spans over two octaves which is difficult to produce using an optical frequency comb (hardly spans over one octave) and is quite robust and field independent and can only be observed for the plasmonic sample, not the non-plasmonic one (without aluminum layer). This type of frequency comb can find applications in precision measurements such as atomic clocks, GPS/navigation and molecular fingerprinting.

Rights

© Raisa Fabiha

Is Part Of

VCU University Archives

Is Part Of

VCU Theses and Dissertations

Date of Submission

12-8-2024

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