  Heterostructured material systems devoid of ferroic components are presumed
not to display ordering associated with ferroelectricity. In heterostructures
composed of transition metal oxides, however, the disruption introduced by an
interface can affect the balance of the competing interactions among electronic
spins, charges and orbitals. This has led to the emergence of properties absent
in the original building blocks of a heterostructure, including metallicity,
magnetism and superconductivity. Here we report the discovery of
ferroelectricity in artificial tri-layer superlattices consisting solely of
non-ferroelectric NdMnO3/SrMnO3/LaMnO3 layers. Ferroelectricity was observed
below 40 K exhibiting strong tunability by superlattice periodicity.
Furthermore, magnetoelectric coupling resulted in 150% magnetic modulation of
the polarization. Density functional calculations indicate that broken space
inversion symmetry and mixed valency, because of cationic asymmetry and
interfacial polar discontinuity, respectively, give rise to the observed
behavior. Our results demonstrate the engineering of asymmetric layered
structures with emergent ferroelectric and magnetic field tunable functions
distinct from that of normal devices, for which the components are typically
ferroelectrics.
