Structural domain walls in polar hexagonal manganites

The domain structure in the multiferroic hexagonal manganites, h-RMnO3 (R=Sc, Y, Dy-Lu), is currently intensely investigated, motivated by the observation of nanoscale electrical conductivity at the domain walls and intriguing sixfold topological defects at their meeting points, as well as reports of coupling between ferroelectricity, magnetism and structural antiphase domains. The detailed structure of the domain walls, as well as the origin of such couplings, however, was previously unknown. Here we use first-principles electronic structure calculations to elucidate the structure and properties of the structural domain walls in the hexagonal manganites. Our results allow us to explain why ferroelectric domain walls are always simultaneously antiphase walls, propose a mechanism for ferroelectric switching through domain-wall motion, suggest a structure for the observed sixfold topological defects and predict a topological protection of experimentally observed stripe domains in hexagonal manganites.