%0 Journal Article
%K Transmission electron microscopy
%K Scanning electron microscopy
%K Iron compounds
%K Polarization
%K Bismuth compounds
%K Ferroelectricity
%K Ferroelectric polarization
%K Bismuth ferrite
%K High resolution transmission electron microscopy
%K Scanning transmission electron microscopy
%K Doping (additives)
%K Semiconductor doping
%K Differential phase contrast
%K Differential phase contrast imaging
%K Functional properties
%K Periodic distortions
%K Polarization gradients
%A M Campanini
%A R Erni
%A C.-H Yang
%A Ramamoorthy Ramesh
%A M.D Rossell
%B Nano Letters
%D 2018
%G eng
%I American Chemical Society
%P 717-724
%R 10.1021/acs.nanolett.7b03817
%T Periodic Giant Polarization Gradients in Doped BiFeO3 Thin Films
%V 18
%X The ultimate challenge for the development of new multiferroics with enhanced properties lies in achieving nanoscale control of the coupling between different ordering parameters. In oxide-based multiferroics, substitutional cation dopants offer the unparalleled possibility to modify both the electric and magnetic properties at a local scale. Herein it is demonstrated the formation of a dopant-controlled polar pattern in BiFeO3 leading to the spontaneous instauration of periodic polarization waves. In particular, nonpolar Ca-doped rich regions act as spacers between consecutive dopant-depleted regions displaying coupled ferroelectric states. This alternation of layers with different ferroelectric state creates a novel vertical polar structure exhibiting giant polarization gradients as large as 70 μC cm-2 across 30 Å thick domains. The drastic change in the polar state of the film is visualized using high-resolution differential phase-contrast imaging able to map changes in ferroelectric polarization at atomic scale. Furthermore, a periodic distortion in the Fe - O - Fe bonding angle suggests a local variation in the magnetic ordering. The findings provide a new insight into the role of doping and reveal hitherto unexplored means to tailor the functional properties of multiferroics by doping engineering. © 2018 American Chemical Society.