2D Ferroelectricity and Bloch lines : The hunt begins

Lead Research Organisation: Queen's University of Belfast
Department Name: Sch of Mathematics and Physics


Domain boundary engineering is an emerging field in which surprisingly unique properties related to interfaces and domain boundaries can be exploited for functionality. One such case is the domain boundaries in ferroelastic materials like CaTiO3 [1] where localised two-dimensional ferroelectric ordering has been predicted and observed. It has been suggested that the geometrical considerations for such walls contain electric dipole moments which will lead to ferroelectric centers and other properties mediated via the localised polar ordering. However, establishing comprehensive experimental evidence for such behaviour is a significant challenge and is a central focus of this project. We aim to utilise low temperature SPM techniques and other characterisation methods to identify 2D ferroelectricity in these and analogous systems and then fabricate confined structures (e.g. nano-islands) which will allow bias mediated switching of the polarity at these walls. In addition, novel functionalities made accessible by such interfaces will be evaluated for proof-of-concept applications. As opined by Salje and Scott [2,3], vortices inside such polar domain walls in ferroelastic materials can form ordered arrays resembling Bloch-lines in magnets. These Bloch lines are energetically degenerate where the dipoles are oriented perpendicular to the wall. Such walls can have the functionality that they can reverse in polarity even when the polarity of the twin wall may be non-switchable. This project would also target towards exploring such behaviour in ferroelastic twinned walls.

References :
[1] 'Direct observation of Ferrielectricity at Ferroelastic Domain Boundaries in CaTiO3 by electron Microscopy' S. Aert, S. Turner, R. Delville, D. Schryvers, G. Tendeloo and E. Salje, Advanced Materials, 24, 523-527 (2012).
[2] 'Domain glasses : Twin Planes, Bloch lines, and Bloch Points' E. Salje and M. Carpenter, Physica Status Solidi B 252, 12, 2639-2648 (2015).
[3] 'Ferroelectric Bloch-line switching : A paradigm for memory devices ?' E. Salje and J. Scott, Applied Physics Letters 105, 252904 (2014).


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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509541/1 01/10/2016 30/09/2021
1786464 Studentship EP/N509541/1 01/10/2016 31/03/2020 Joseph Gerald Guy
Description The constant demand for faster computer processing and more efficient data storage requires that we scale our technology down to the micro- and nano-scale regimes. By exploiting their switchable, spontaneous electrical polarisation, materials referred to as 'ferroelectrics' have proven useful throughout this process of miniaturisation during the last thirty years or so. Ferroelectric memories, for example, have been realised that bolster quick speed read/write operation and small switching fields, which is a crucial aspect of quickly storing/retrieving data on your phones/computer/tablets etc. Conventional designs, however, may no longer be optimal as polarisation retention becomes increasingly more difficult on the nano-scale. A new paradigm in small-scale functionality is therefore sought.

There are a growing number of non-traditional materials that could circumvent problems related to miniaturisation; graphene is now a well-established candidate, possessing novel features such as reduced spatial dimensions and enhanced electrical properties that are confined entirely to two dimensions. Many aspects of ferroelectricity, within the context of device integration, would prove invaluable if they could also be realised at reduced dimensions, where there is clear relevance for denser memory application and the use of considerably lower switching voltages. Ferroelectrics, however, are typically exploited for their bulk properties which are due, in part, to (ferroelectric) domains, three dimensional regions within the ferroelectric material consisting of uniformly oriented electrical polarisation. The two-dimensional interface that delineates adjacent domains is called a domain wall, something that was initially thought to be a simple juxtaposition between the more useful piece of the material.

The field of ferroelectrics underwent a dramatic transformation in 2009, however, when peculiar interfacial properties, not present in the bulk, were reported by Seidel et al. within the thin-film ferroelectric bismuth ferrite (BFO). More specifically, they observed electrical conduction localised entirely within domain walls; an astounding feature given that the material is a bulk electrical insulator. Such a unique property was instantly acknowledged for its potential device applications, and we can now readily create nano-sized, mobile conducting channels via the application of a voltage, adding a useful dynamical aspect to the situation. The hunt to discover enhanced functionality at interfaces has proven fruitful over the last ten years or so, leading to important discoveries elsewhere such as interfaces containing defects with a high mobility, two-dimensional electron gases, and multiferroicity.

Controlling the motion, spatial distribution and density of domain walls in ferroelectric materials will become a crucial aspect of future nanodevices in view of their enhanced functionality and reduced dimensionality; traits now firmly grounded in a decade's worth of intense global research. Recently at QUB, McQuaid et al. reported the injection and electric-field-driven motion of electrically charged domain walls in the improper ferroelectric Cu-Cl boracite (Cu3B7O13Cl).

We have since performed subsequent work characterising the electric-field-induced anomalous motion of some of these charged domain walls within Cu-Cl boracite. Unlike conventional ferroelectric switching, specific domain walls appear to move in a way that facilitates the growth of domains with polarisation anti-aligned with the applied electric field. Using multiple scanning probe microscopy (SPM) techniques, these walls are found to possess a head-to-head charged polar configuration consistent with that reported by McQuaid. Such movement could indicate a negative dP/dE and, if observed for all applied fields, may point towards a negative capacitance. This phenomenon was only recently observed for ferroelectric materials adopting unstable configurations as part of a ferroelectric/dielectric heterostructure, whereas single-phase ferroelectric materials exhibiting negative capacitance have yet to be reported.
Exploitation Route Electrical circuits have a tendency to heat up over time due to the flow of current. The ability to dissipate such heat away from the circuit has become increasingly more difficult as things are scaled down to the micro/nano-scale. The heat generated is intimately related to the voltage applied across the constituent components of the circuit (it is proportional to the square of the voltage). By exploiting the negative capacitance of Cu-Cl boracite, assuming it retains this unique property were it integrated into a circuit, one would be able to supply a relatively low-voltage and have it amplified via the negative capacitance component of our circuit, thus reducing the total power consumption and the amount of heat generated.
Sectors Aerospace, Defence and Marine,Digital/Communication/Information Technologies (including Software),Electronics,Energy