Date of Graduation

8-2024

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Physics (PhD)

Degree Level

Graduate

Department

Physics

Advisor/Mentor

Bellaiche, Laurent

Committee Member

Oliver, William F. III

Second Committee Member

Hu, Jin

Third Committee Member

Churchill, Hugh O.H.

Keywords

Effective Hamiltonian; Ferroelectricity; Frustrated magnets; Phase transition; Polar topological defects; Two-dimensional magnetism

Abstract

Ferroic matter are of great importance as they provide a platform to extend the understanding of many-body physics and also for their amazing functional properties relevant for technological advancements. The work in this dissertation is dedicated to two of such ferroic materials. First is CrGeTe3 (CGT), a two dimensional (2D) magnet discovered in 2017. The discovery of two-dimensional magnetism has invigorated the research community as their low dimensionality offers a unique avenue to understand and investigate the origin of magnetism in these atomically thin systems and harvest diverse application such as information processing, storage and transfer using their magnetic spins. Here, we investigated the magnetic properties of 2D-CGT using density functional theory calculations to extract the parameters of interactions between the spins in this system as well as to manipulates these parameter by using strain as the tuning knob. The parameters clearly reveal that 2D-CGT is a ferromagnet (FM) in its unstrained state and can be transitioned into an antiferromagnet (AFM) via the application of compressive uniaxial strain. Further, we developed an effective Hamiltonian (��!"") using the parameters and preformed finite temperature simulation on this system employing Monte Carlo technique. Our simulations revealed that the FM-AFM transition is bordered by a magnetically frustrated region. The frustrated magnetic systems, due to many competing interaction, are in themselves an exciting research field where the inability of the system to form an ordered state can lead to the realization of exotic states. We present a magnetic phase diagram and demonstrate that strain engineering is a viable method to tune 2D magnets and harness interesting properties in them, specifically to deterministically design frustrated magnetic states in this case. Second is ultrathin films of ferroelectric PbZr0.4Ti0.6O3 (PZT). Ultrathin ferroelectric films in the quasi-two-dimensional limit have shown tremendous promise owing to the novel emergent properties that originate from their confined geometry. The interfacial effects play a vital role in these automictically thin polar films- the depolarizing field that arises from the surface electric dipoles forces them to reorganize themselves leading to the emergence of exotic phases in these materials such as the polar vortex, electric skyrmions, merons, bubbles and many more. The latter phases are a means to deepen the fundamental understanding of these complex systems and are promising for technological advancement. For example, these systems have potential for energy storage, ultra-high density information storage and ultrafast information processing. Here, we employ state-of-the-art first principles derived ��!"" for PZT and implement them in Monte Carlo (MC) and Molecular Dynamics (MD) simulations to explore its properties. It was found that epitaxially strained ultrathin PZT films host many exotic topological states that emerge out of the competition between long range and short range interactions between the local electric dipoles. Here, we studied the stability regime and control over various topological defects that occur in this polar system. We found thickness, stress, screening of depolarizing field and electric field can all be used as control parameters to fine-tune the stability and transition of the topological states. Our simulations were tested experimentally and were found to be in good agreement with the experimental outcomes. We also present that the topological phase transitions in PZT can be understood in terms of a unique residual depolarizing field conserving mechanism of the system. Polar vortex is a commonly observed state in the ultrathin ferroelectrics and is the precursor to the aforementioned exotic topological defects. Here, we also explored the dynamics of the vortex state in PZT, by employing MD simulations to obtain the phonon spectra, which unveiled the origin of the vortex state. We found that the vortex state results from the condensation of a phonon mode and is associated with a symmetry breaking mechanism. In such light, the vortex state can be understood as a condensed wave pattern, and we extended this understanding to explore a practical application. Specifically a terahertz ac-electric field was applied to the vortex state where we observed a resonant switching behavior of the polar vortex state between its two equivalent orientations. These predictions imply an exciting future in understanding of the dynamics of the topological defects and employing them for novel practical applications.

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