Heterostructured material systems devoid of ferroic components are presumednot to display ordering associated with ferroelectricity. In heterostructurescomposed of transition metal oxides, however, the disruption introduced by aninterface can affect the balance of the competing interactions among electronicspins, charges and orbitals. This has led to the emergence of properties absentin the original building blocks of a heterostructure, including metallicity,magnetism and superconductivity. Here we report the discovery offerroelectricity in artificial tri-layer superlattices consisting solely ofnon-ferroelectric NdMnO3/SrMnO3/LaMnO3 layers. Ferroelectricity was observedbelow 40 K exhibiting strong tunability by superlattice periodicity.Furthermore, magnetoelectric coupling resulted in 150% magnetic modulation ofthe polarization. Density functional calculations indicate that broken spaceinversion symmetry and mixed valency, because of cationic asymmetry andinterfacial polar discontinuity, respectively, give rise to the observedbehavior. Our results demonstrate the engineering of asymmetric layeredstructures with emergent ferroelectric and magnetic field tunable functionsdistinct from that of normal devices, for which the components are typicallyferroelectrics.
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