Ferroelectric, Ferroelastic and Multiferroic Domain Walls: a New Horizon in Nanoscale Functional Materials

Some functional materials, such as ferroelectrics, contain membrane or sheet structures called "domain walls". For decades, domain walls were dismissed as being minor microstructural components of little significance. It is now clear that nothing could be further from the truth. Domain walls often, in fact, have unique functional properties that are completely different from the domains that they surround: they can be conductors or superconductors when the rest of the material is insulating; they can display magnetic order in non-magnetic crystals and they can possess aligned electrical dipoles when the matrix surrounding them is non-polar. In effect, domain walls represent a new class of sheet-like nanoscale functional material.

Gaining a basic understanding of the behaviour of such a new family of sheet materials, which already shows a very wide gamut of properties, is certainly worthwhile, but domain walls offer so much more: uniquely, they are spatially mobile, can be controllably shunted from point to point, and can be spontaneously created, or made to disappear. This unique "now-you-see-it, now-you-don't" dynamic property could radically alter the way in which we think about the integration of functional materials into devices and the way in which device functionality is enabled: functionally active domain walls themselves could be introduced or removed as the primary mechanism in device operation. As a simple example, a new form of transistor could readily be envisaged where switching between the "ON" and "OFF" states is achieved through the injection and annihilation respectively of conducting domain wall channels connecting the source and drain electrodes. Multiple controlled domain wall injection events (resulting from sequential pulses in electrical bias between source and drain, for example) could create a series of different resistance states, depending on the number of conducting walls introduced. Thus a new kind of memristor device could be created.
Possibilities for future domain wall-based applications are tantalising. However, relevant research is still at an early stage; a great deal needs to be done to establish the basic physics of the functional behavior of domain walls and strategies need to be developed to allow their reliable deployment with nanoscale precision. Only then can the potential for domain wall based devices be properly assessed.

In this Critical Mass Grant, we seek to harness the collaborative effort of a number of world-class UK-based academic teams (in Cambridge, St. Andrews, Warwick and Belfast) to explore novel functionally active ferroelectric, ferroelastic and multiferroic domain walls. Together, we will:
(i) Generate badly needed new and fundamental insight into the properties of known functionally active domain wall systems;
(ii) Perform smart searches for new functionally active domain wall systems;
(iii) Demonstrate simple electronic and thermal devices (transistors, memristors and smart heat transfer chips) in which domain wall properties are the key to device performance and hence assess the potential for wider domain wall-based applications.

Within the Economy Sector of Impact:
The focus of the programme is on fundamental blue-skies research. Nevertheless, we wish to start to assess the potential for new disruptive technology based on domain walls as the active device elements and will therefore make a few different simple domain-wall based demonstrator devices as part of our research (such as domain wall transistors and nanoheat flow controllers). We have allocated roles for the senior professorial investigators, both of whom are Fellows of the Royal Society, to act as "Applications Champions"; they will guide the scientific endeavour where possible to have end-user relevance. Industrial members of the Advisory Board (initially Seagate Technology and IBM Zurich) will inform and guide this aspect of our work and wider engagement will be sought using institutional exploitation offices and by holding end-user engagement workshops.

Within the Knowledge Sector of Impact:
The proposed programme tackles a topic at the very cutting edge of global research; we will be undertaking an exploration of genuinely new science, establishing previously undiscovered information for the first time. The scientific advances are expected to be extremely exciting academically and have a major influence on the progression of a significant sector of functional materials science. The applicants already have a strong track record in inspiring and shaping global research themes and we believe that the results from the proposed programme will enhance this scientific influence. We will ensure impact within the "knowledge sector" through dissemination at conferences, meetings and in high visibility journal forums. We will also hold open workshops and invite interaction with relevant academics beyond the investigator group. A number of new techniques will also be developed during the research to allow the proposed technically challenging nanoscience to be realised and these could be rolled out to help with other scientific research well beyond the scope of the current proposal.

Within the People Sector of Impact:
The Critical Mass Grant has a significant element of training and professional development associated with it for the expected PDRAs and PhD's. Around 12 researchers in all will be introduced to a coherent, coordinated cutting edge programme, developing their research skills, experience and publication track records under expectations of the very highest academic standards (as set by the permanent academics involved). These trained researcher / problem solvers will be a tremendous asset to academia, private or public sectors within the UK.

Within the Society Sector of Impact:
This research programme aims to develop key scientific knowledge which has the potential to eventually lead to revolutionary device applications, which could have knock-on consequences for quality of life and potentially for health and for the environment. Ferroelectrics are renowned for the low energy input associated with switching, in comparison with ferromagnetism, and so devices operating using ferroelectric domain walls could consume relatively little energy in their operation (this was always one of the major advantages for FeRAM non-volatile memory). Applications in heat flow control using domain wall physics could clearly also have environmental impact. As a group of academics, we are well integrated into outreach programmes; we foresee that some of the research questions posed and discoveries made will be shared through such outreach with the general public, helping young people to be enthused about STEM subjects.