Insulating interlocked ferroelectric and structural antiphase domain walls in multiferroic YMnO3

Hexagonal YMnO3 shows a unique improper ferroelectricity induced by structural trimerization. Extensive research on this system is primarily due to its candidacy for ferroelectric memory as well as the intriguing coexistence of ferroelectricity and magnetism. Despite this research, the true ferroelectric domain structure and its relationship with structural domains have never been revealed. Using transmission electron microscopy and conductive atomic force microscopy, we observed an intriguing conductive 'cloverleaf' pattern of six domains emerging from one point-all distinctly characterized by polarization orientation and structural antiphase relationships. In addition, we discovered that the ferroelectric domain walls and structural antiphase boundaries are mutually locked and this strong locking results in incomplete poling even when large electric fields are applied. Furthermore, the locked walls are found to be insulating, which seems consistent with the surprising result that the ferroelectric state is more conducting than the paraelectric state. These fascinating results reveal the rich physics of the hexagonal system with a truly semiconducting bandgap where structural trimerization, ferroelectricity, magnetism and charge conduction are intricately coupled.