Interface reconstruction with emerging charge ordering in hexagonal manganite.

Multiferroic materials, which simultaneously have multiple orderings, hold promise for use in the next generation of memory devices. We report a novel self-assembled MnO double layer forming at the interface between a multiferroic YMnO3 film and a c-Al2O3 substrate. The crystal structures and the valence states of this MnO double layer were studied by atomically resolved scanning transmission electron microscopy and spectroscopy, as well as density functional theory (DFT) calculations. A new type of charge ordering has been identified within this MnO layer, which also contributes to a polarization along the [001] direction. DFT calculations further establish the occurrence of multiple couplings between charge and lattice in this novel double layer, in addition to the polarization in nearby YMnO3 single layer. The interface reconstruction reported here creates a new playground for emergent physics, such as giant ferroelectricity and strong magnetoelectric coupling, in manganite systems.