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Interface-induced multiferroism by design in complex oxide superlattices

Significance Developments in synthesis and characterizing artificially structured materials have greatly advanced the possibility to explore new states of matter in material science. Recent discoveries show that new quantum states can be achieved at heterointerfaces with various electric and mechanical boundary conditions. It remains an open question of how to design ultrathin layers with properties inaccessible in bulk phases that are amenable to technological applications. In this work, we grow heterostructures with extremely high-quality interfaces shown by state-of-the-art atomically resolved electron microscopy and spectroscopy. This combination allows us to identify an interface-induced structure that stabilizes ferromagnetism. Coupled with theory, we provide a conceptually useful recipe to design low-dimensional materials with unique functionalities, in line with the loop “make, measure, model.” Interfaces between materials present unique opportunities for the discovery of intriguing quantum phenomena. Here, we explore the possibility that, in the case of superlattices, if one of the layers is made ultrathin, unexpected properties can be induced between the two bracketing interfaces. We pursue this objective by combining advanced growth and characterization techniques with theoretical calculations. Using prototype La2/3Sr1/3MnO3 (LSMO)/BaTiO3 (BTO) superlattices, we observe a structural evolution in the LSMO layers as a function of thickness. Atomic-resolution EM and spectroscopy reveal an unusual polar structure phase in ultrathin LSMO at a critical thickness caused by interfacing with the adjacent BTO layers, which is confirmed by first principles calculations. Most important is the fact that this polar phase is accompanied by reemergent ferromagnetism, making this system a potential candidate for ultrathin ferroelectrics with ferromagnetic ordering. Monte Carlo simulations illustrate the important role of spin–lattice coupling in LSMO. These results open up a conceptually intriguing recipe for developing functional ultrathin materials via interface-induced spin–lattice coupling.