Engineering Unequal Antipolar Displacement in Ferromagnetic Layered Oxide Heterostructures.

Heterostructure engineering provides a versatile route for tailoring emergent functionalities that are often difficult to realize in single-phase materials. In this work, the focus is on superlattices composed of the insulating and ferromagnetic double perovskites La2NiMnO6 and Sm2NiMnO6, which constitute an intriguing model system. These layered structures are predicted to feature unequal antipolar displacements of the La and Sm ions; when combined with odd periodicity stacking, this structural motif is expected to give rise to polar behavior. The respective superlattices are grown with atomic precision and display robust ferromagnetism, as confirmed by in-house magnetometry and synchrotron measurements. Scanning transmission electron microscopy combined with first-principles calculations confirms the presence of the predicted unequal antipolar displacements, paving the way for the realization of hybrid improper ferroelectricity in such oxide heterostructures.