Manipulation of domain wall mobility by oxygen vacancy ordering in multiferroic YMnO₃

The mobility of the ferroelectric domain phases and the local conductivity of ferroelectric domain walls in multiferroic YMnO₃ crystals grown in air and reduced atmosphere were studied by piezoresponse force microscopy (PFM), tip-enhanced Raman spectroscopy (TERS) and conductive atomic force microscopy (CAFM). Oxygen vacancies were found to reduce the strength of 4d-2p (Y³⁺-O^2-) hybridization and structural trimerization distortion, leading to the disappearance of the six wedge-shaped ferroelectric domain phases in oxygen deficient YMnO_3-δ crystals. We observed anisotropic domain wall motion such that the wedge-shaped domain configuration joined at one point could be changed to the stripe domain configuration by applying high electric fields in oxygen deficient YMnO_3-δ single crystals. The local conductivity of the domain walls increased significantly in poled YMnO_3-δ single crystals. The straight conductive domain walls in YMnO_3-δ, instead of the twisted insulating ones in the stoichiometric crystal, are induced by the ordered oxygen vacancies which are verified by TERS measurements.