Exploration of topological defects in ferroelectrics

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Ferroelectrics (FEs) are materials which exhibit spontaneous polarisation below a certain temperature, and have two or more stable polarisation states, reversibly accessible by application of external fields. Previous applications have involved domains -regions of the FE with a consistent sense of polarisation- as the functional part of the material. Recent focus has switched to the domain walls (DWs), the nanoscale membrane-like structures that separate domains. DWs have been shown to exhibit properties distinct from the domains surrounding them. This project aims to investigate two areas of this evolving field: domain wall conduction and FE skyrmions.Enhanced electrical conductivity at DWs in an otherwise insulating bulk(1) has now been demonstrated in many materials. Also, since the polarisation is reversible with the application of external fields, DWs can be moved and controlled, even created and removed, giving rise to the idea of a new kind of reconfigurable circuitry, where the domain wall is a mobile electrical connection(2). Conduction varies, but characterising the conduction is key to realising domain wall nanoelectronics, and is one of the main aims of the project. Work is under way to obtain a measurement for the carrier mobility of conducting DWs in lithium niobate (LN) by measuring the geometric magnetoresistance- a change in resistivity upon application of a magnetic field. This effect arises due to the unique conical geometry of the DWs that can be written in LN thin films(1), making a novel measurement of mobility in domain walls possible. Furthermore, plans are in place to make some of the first direct measurements of the electronic band structure specifically at conducting DWs. Using a large facility X-Ray source to excite carriers locally at the DWs, energy and momentum of carriers can be measured. These parameters comprise the electronic band structure, giving insight into the mechanism behind the enhanced conductivity.Another aspect of the project is to investigate a FE analogy to the magnetic skyrmion. A magnetic skyrmion is a confined spin pattern, described as being 'topologically protected', meaning it cannot be continuously deformed and removed, and is somewhat stable. Their promise comes from their nanometer size, making them ideal for high density data storage applications, such as the recently envisioned racetrack memory(3). An electric counterpart, where polarisation replaces spin, has been theoretically predicted(4) and observed(5) in complex structures, but the general case is yet to be seen(4). If found, FE skyrmions could be controlled by fields in a similar way to the DWs mentioned above, and thus hold promise as functional elements(4). This project seeks to use focussed ion beams to cut thin slices of FE lead titanate, write a domain pattern which is expected to be skyrmionic, and use atomic resolution transmission electron microscopy (in collaboration with University of Limerick), to verify this. If observed, we could move forward into using novel atomic force microscopy methods to write, move and control these polarisation patterns with applied fields, investigating their response and transport properties, exploring their physical origin and ultimately characterising them for potential application.1. Schröder, Mathias, et al. "Conducting domain walls in lithium niobate single crystals." Advanced Functional Materials 22.18 (2012): 3936-3944.2. McQuaid, Raymond GP, et al. "Injection and controlled motion of conducting domain walls in improper ferroelectric Cu-Cl boracite." Nature communications 8 (2017): 15105.3. Tomasello, Riccardo, et al. "A strategy for the design of skyrmion racetrack memories." Scientific reports 4 (2014): 6784.4. Gonçalves, MA Pereira, et al. "Theoretical guidelines to create and tune electric skyrmion bubbles." Science advances 5.2 (2019):eaau7023.5. Das, Sujit, et al. "Observation of room-temperature polar skyrmions." Nature 568.7752 (2019): 368.