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

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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.