Self-poling with oxygen off-stoichiometry in ferroelectric hexagonal manganites

Topological vortices with complex domains and domain walls exist in hexagonal manganites, which undergo a structural transition accompanying ferroelectric polarization and trimerization. We have systematically studied the origin of two different types (type-I and type-II) of vortex domains with controlled oxygen stoichiometry and found that the evolution between type-I and type-II vortex domains and the presence of charged walls between two different trimerization antiphase domains result from an oxygen vacancy gradient. We have also discovered a rare phenomenon of vortex core split, which appears to stem from the instability of charged ferroelectric walls between domains with the same trimerization antiphase.