Domain walls are attracting significant interest in the field of (multi-)ferroic materials owing to their intriguing functional properties. The domain walls represent natural interfaces that can exhibit substantially different properties than the surrounding bulk due to their low symmetry and unusual electrostatics. The fact, that these interfaces can be moved, erased, and positioned at will is of strong technological interest and highly relevant for the design of domain-wall based nanoelectronic devices. Charged and neutral domain walls in ferroelectrics are of special interest, as they can show diverse electronic behavior ranging from highly conductive to strongly insulating states. Prior to implementation, however, further knowledge is required to generate walls with controllable output and power in order to emulate electronic nano-components such as diodes, transistors and gates.
The scope of this thesis is to present novel strategies to characterize and manipulate functional domain walls in complex ferroelectric oxides. Advanced microscopy studies are realized by combining state-of-the-art imaging techniques, including scanning probe microscopy (SPM) and cathode-lens microscopy (CLM).
Using hexagonal manganites (RMnO3) as model system, we demonstrate chemical impurity doping as a promising tool to engineer and improve the properties of functional domain walls. In p-type semiconducting ErMnO3, chemical doping is applied to increase the current densities at charged domain walls by two orders of magnitude and tune from p-type to n-type dominated screening and transport behavior. Moreover, we demonstrate reversible electric-field control of the electronic transport at the charged domain walls, switching between resistive and conductive domain-wall states.
Aside from the charged walls, we perform a comprehensive analysis of neutral domain walls in ErMnO3 and their functionality. Under adequate boundary conditions, these walls exhibit currents, which can be influenced by thermal annealing in oxygen atmosphere. We further find that the walls facilitate AC-to-DC conversion, emulating the functionality of classical diodes. The rectifying properties, including the practical frequency regime and magnitude of the output, are controlled via the conductivity of the adjacent domains.
Many of the aforementioned discoveries were enabled by the development of new experimental approaches in terms of SPM, including the application of electrostatic force microscopy (EFM) and frequency-dependent domain-wall measurements, as well as pioneering CLM experiments. In particular, x-ray photoemission electron microscopy (X-PEEM) was applied to access local domain-wall physics contact-free and with reduced data acquisition times, and the potential of low-energy electron microscopy (LEEM) was demonstrated to improve spatial resolution to a few nanometers.
This work thus provides novel insight into the physical properties of functional domain walls, demonstrates new advanced characterization techniques, and highlights novel opportunities for the design of future domain-wall based devices
