Domain boundary in multi-FERROIC materials

The propagation of magnetic domain walls in a nanowire is already used as a memory device today: each time a domain wall passes a pick-up coil, a signal is emitted and encoded. The same technology does not apparently work in ferroelectric or ferroelastic domain walls because the movement of the wall is much less smooth: it can 'jerk' and emit spontaneously acoustic or electric signals. These signals are unwanted and lead to 'noise' in any device application. This limitation is not a physical necessity, though. Smooth movements are seen in ferroelastic SrTiO3 while most porous materials are almost totally 'jerky' and form avalanches of domain walls which can not be controlled in any device application.

The first aim of the proposal is to investigate the crossover between noisy and silent wall propagation.

Ferroelastic and ferroelectric domain walls (they are often both) have another fantastic property: they can be changed easily by doping, they can be bent, and they can form complex domain patterns which contain much more information than can be encoded in single magnetic domain walls. Examples are superconducting domain walls in WO3, highly conducting and photovoltaic walls in BiFeO3 and polar walls in CaTiO3. All these materials are well known and can be deposited routinely as thin films on appropriate substrates. What is missing is the knowledge of the mechanism by which such walls change their local structure and how this effect changes their mobilities. Walls are very thin (1-10nm) and it is therefore extremely difficult and costly to investigate their structural properties by experimental means. We have done some of the most advanced experimental work in this field but now it has become timely to advance our research theoretically by computer simulation of the relevant domain patterns. The appropriate theory is based on complex Landau-Ginzburg theory with interacting order parameters, the computer simulation of the domain patterns is based on mechanical, non-local models of interacting local state parameters with a large number of interacting 'atoms'. Here a minimum of 1 million particles are required to see boundary effects, propagating kink excitations and mutual jamming of domain walls. We will extend this size to >10 million particles.

Our second aim is hence to derive realistic thermodynamic potentials for interacting domain walls and simulate the pattern formation on a large enough scale to be realistic for possible device applications.

Besides the academic impact (already described) we expect significant industrial impact. Our main partners for the industrial applications of the results in the UK are Dr. Neil Mathur (Materials Sciences) and Prof. Jim Scott FRS (Physics) who have excellent track records for the transfer of academic research into industry. Prof. Marty Gregg at Queens University in Belfast works on ferroelectric devices and has visited our group on a regular base. His work will lead to better applications of memory devices.
The PI has three contact points for potential applications abroad:
1. the company Samco in Kyoto has repeatedly supported the PI with substantial financial investments. The have also helped to support part of the salary of Prof. Scott FRS in my group. This company produces thin film deposition facilities. Their main interest is the production of novel device materials with interfacial properties which are confined enough to be useful for large scale applications. The most promising development here is the array of parallel twin walls for potential photovoltaic applications and similar devices for ferroelectric switching. Our model simulations will help to find optimal solutions for the pattern formation (parallel walls, jamming of crossing walls and the pinning by surfaces).
2. The second company interested in the project is expected to be Samsung where one of our former students (Kim DJ) holds the post of manager of their development laboratory. Again, if the project is successful further collaborations with Samsung are expected to develop. Technical managers of Samsung visit the UK rather often so a special trip to Korea can be avoided. The magnetic racetrack technology is currently further improved at SKKU in Seoul, Korea and additional cooperation with the Korean group may be desirable. A decision will be taken after the second year of the project.
3. A most promising development in photovoltaics is initiated by the US-DoE in Washington DC. The program director is a long-standing collaborator of the PI (R.Ramesh). He will hold this position for another year with responsibilities for small scale energy generation. The concrete work will develop on applications in photovoltaics with Prof. Jan Seidel in Sydney. The PI has published one of the key papers in this field with Jan and a visit to or from Sydney is budgeted in this proposal.

Once we can increase the efficiency of the photovoltaic response to >15% we will be able to negotiate joint work with BP which has already signaled support in principle for the development of alternative energy generation. Previously, I was supported by BP for some of my research and have attracted funding for fluid mechanics applications to the Department of Earth Sciences in Cambridge. Industrial applications overseas are more straightforward because they are based directly on domain boundary technologies rather than power applications.
While these collaborations are based on the personal knowledge of industrial partners, we will certainly endeavor to interact with Science Park companies which are operating outside pharma industry. The EPSRC funded KTA in Cambridge is specialized to engineering-based services which can include domain boundary engineering and some materials sciences. We will make use of the local RSD division working on KTAs. In order to attract UK industry we will organize KTA research conferences related to the materials aspects and the fundamental physics of functional, mobile twin boundaries.