Scanning probe-based tomography and stress-mediated functionalities in ferroelectrics

Abstract

Ferroelectric materials have attracted attention for decades due to their potential for applications such as lowpowered, high-density data storage, efficient actuators, high-k dielectrics, and reconfigurable domain wall-based nanoelectronics. These functionalities arise from the property of spontaneous polarisation, switchable through an electric field. Besides electric field, functional properties in ferroelectric materials can be controlled through stress or mechanical boundary conditions. For instance, its phase-transition temperature, coercive bias and domain microstructure have been shown to alter under various stress regimes. The advancement of scanning probe microscopy has allowed for in situ studies of stress-induced behavior in ferroelectrics, where high stress can be applied through atomic probes while simultaneously capturing responses. Parallelly, significant effort is directed towards exploring the use of these atomic probes to machine nanostructures on material surfaces by applying stress beyond the fracture point, which reveals properties hidden within the material—a technique known as Tomographic-Atomic Force Microscopy (T-AFM). This thesis investigates the application of stress across all three regimes—elastic, plastic, and beyond fracture—in three different classes of ferroelectric materials. First, in relaxor strontium barium niobate, the stress in the elastic regime enables more efficient ferroelectric switching under an oscillating electric field. Second, in mixed-phase bismuth ferrite, plastic stress induces stable phase transitions, and the associated currents are measured. Finally, T-AFM is employed in hexagonal manganites to map domain microstructure, domain wall charge states, and current paths in the third dimension. These studies offer valuable insights into the fundamental properties and stress responses of these material systems, paving the way for their application in future nano-electronics.

Thesis is embargoed until 31 December 2025.

Date of Award	Dec 2024
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Tezpur University
Supervisor	Amit Kumar (Supervisor), Marty Gregg (Supervisor) & Shyamal Kumar Das (Supervisor)