Domain boundary in multi-FERROIC materials

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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.