Structure and dynamics of interfaces in ferroic materials

Ferroic materials display a variety of (electrical, mechanical, magnetic, etc.) properties, which make them enormously appealing for technological applications, e.g. in electronic and electromechanical devices. Recently it was found, that the properties of ferroic materials can be tuned by exploiting the unique functionalities of interfaces, e.g. of domain walls or phase fronts. Such interfaces are usually very small objects, i.e. of the order of view nanometres thick (1 nanometre = 0.000001 mm). Nevertheless, they can drastically change the macroscopic (mm scale) behaviour of a material, since in real crystals there can exist millions of such interfaces. Within the present project we will perform a close collaboration between experimental work and theory to reach understanding of the structure, the static properties and the dynamic behaviour of functional interfaces. This became only possible due to the enormous progress made in high resolution local probe techniques. We will apply these methods in close cooperation with renowned groups in Belgium and Germany to perform high resolution transmission electron microscopy, atomic force microscopy and piezoforce microscopy in complex oxided and complement them by macroscopic techniques and theoretical calculations. The innovative aspect of our project is the multi-scale approach, i.e. the aim to link local (0.00000001 mm) and average structures across different length scales (over some nanometres and finally to mm) and to clarify their influence on the macroscopic behaviour of the materials. Many of the expected results will be of importance for novel functional materials with superior properties.

"Ferroic" crystals are materials with a vast range of functional properties. The most prominent ones are probably ferromagnetic or piezoelectric crystals, with sensor-, storage- or actuator-applications, etc. For long time, people have tried hard to grow such crystals in monodomain form, or switch them into monodomain state by application of an external field, if they consisted of a variety of orientational states (domains). Only recently, a change of paradigm took place, since scientists discovered interesting functional properties of the boundary regions (domain walls) which separate different domains. They found out, that such domain walls can carry e.g. an electric polarization even if the surrounding bulk material is non-polar. An interesting question is e.g. if such a domain wall is ferroelectric, i.e. if the polarization can be switched into opposite by application of an external electric field. Since such domain walls are very thin, i.e. only a few nm thick, this would offer an enormous potential for possible nanotechnological applications. In the frame of the present project we have developed a new theoretical method - based on group theoretical arguments - to calculate the properties of domain walls and to find out, if e.g. such domain walls are switchable or not. First results on some selected materials have led to very promising results. Another topic - which is closely related to the first one - was related to the study of some connections between the motion of domain walls and the freezing of glasses and ferroic relaxors. Also here we have made considerable progress. By measuring the time resolved jerky motion of domain walls in some perovskites as a function of temperature and applied static and dynamic stress, we could identify different dynamic phase transitions of the corresponding domain wall systems. The project was carried out in cooperation with scientific institutes in Czech Republic, England, Poland, Luxembourg and Austria. The results have been published in renowned international journals and will serve as a basis for the development of novel functional materials for nanotechnology.