Date of Graduation

12-2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Microelectronics-Photonics (PhD)

Degree Level

Graduate

Department

Graduate School

Advisor

Laurent Bellaiche

Committee Member

Hameed A. Naseem

Second Committee Member

Paul C. Millett

Third Committee Member

Gregory J. Salamo

Fourth Committee Member

Rick Wise

Keywords

bismuth ferrite, magnetoelectric effect, molecular dynamics, Monte Carlo, multiferroics, phase transition

Abstract

Since the silicon industrial revolution in the 1950s, a lot of effort was dedicated to the research and development activities focused on material and solid-state sciences. As a result, several cutting-edge technologies are emerging including the applications of functional materials in the design and enhancement of novel devices such as sensors, highly capable data storage media, actuators, transducers, and several other types of electronic tools. In the last two decades, a class of functional materials known as multiferroics has captured significant attention because of providing a huge potential for new designs due to possessing multiple ferroic order parameters at the same time. More specifically, magnetoelectrics as a category of multiferroic materials, own cross-coupled functionalities such as responding to the application of external magnetic fields by altering their electrical properties together with their magnetic characteristics. Although such exciting functionalities award a variety of options to design innovative devices, deepening our understanding of the physics behind multiferroicity and magnetoelectric effects is still needed. That is the driving force behind a surge of research on this class of materials and the priority that is given to them in many research institutes.

The work presented in this dissertation is focused on investigating the properties of multiferroics in both static and dynamical regimes. In the static case, the response of bismuth ferrite as a prototypic lead-free representative of multiferroics with room-temperature-enabled functionalities was researched when the strained material was subjected to the application of electric field and/or mechanical stress. The investigations were carried out by adopting a state-of-the-art effective Hamiltonian method within the framework of Monte Carlo atomistic simulations. The predictions made by simulations were then tested under experimental situations. The results not only showed the agreement between theoretical predictions and experimental outcomes, but also revealed the capability of multiferroic materials for the deterministic control of phase ratio under the dictated conditions which paves the way toward the design of novel technological devices based on changing the phase population.

Although the static properties of multiferroics have been heavily studied during recent years, exploring their dynamical characteristics is almost an uncharted territory. That is why more attention is given to the dynamical properties of multiferroics in this work. In the dynamical case, the electrical polarization, magnetization, and strain properties of bismuth ferrite were studied under the application of time-dependent magnetic fields with various frequencies using the effective Hamiltonian approach which is adopted for molecular dynamics atomistic simulations. Moreover, the magnetoelectric response of bismuth ferrite was evaluated within a dynamical regime. The results demonstrated the presence of resonances in the dynamical magnetoelectric response of multiferroics because of the emergence of a new quasiparticle introduced in this work as electroacoustic magnons. An analytical model was developed to explain the origin of such strain-mediated quasiparticles that was then verified by atomistic simulations. The results open opportunities to reach strikingly larger magnetoelectric responses by tuning the frequency of the applied magnetic fields as well as playing with the shape and size of multiferroics to be utilized toward the development of low-consumption and highly efficient sensors and storage media.

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