This chapter introduces research on functional domain boundaries. Ever since the discovery of superconducting twin boundaries in the 1990s, highly conducting, polar, photovoltaic, magnetic, and so on, domain boundaries have been discovered while the same bulk material displays none of these properties. Domain boundaries constitute planar templates for device applications with thicknesses of ca. 1 nm. Domains within domains are then the next step in miniaturization with Bloch lines within domain walls and Bloch points between Bloch lines. In the overwhelming majority of cases, the geometrical template for the functional domain boundaries stems from the ferroelastic domain structure, while antiphase boundaries are equally potential template providers. Complex structures are a particular case because they add vortices and skyrmions to the template topology. Correlations between such sub-structures maintain features like polarity and piezoelectricity in randomized samples where structural averages would not allow macroscopic polar effects. The dynamics of the change of functionality is often much faster than the speed with which twin boundaries move. The novel information carrier is the kink inside twin walls, which moves with supersonic speed.
GENERATION AND ANNIHILATION OF VERTICAL BLOCH LINES IN STRIP DOMAIN WALLS
