IMF2017

The applications of ferroelectric materials stem from the ordering of polarization and switching of the polarization states by applied bias. A fundamental understanding of the atomic scale mechanism underlying the domain formation and polarization switching is critical for the design of devices. In this talk, the atomic structure and dynamic behaviors of domain walls and polar voritices in multiferroic BiFeO₃ films will be reported. According to in-situ TEM studies, the charged domain walls can be created or erased by applying a bias, and the resistance of the local film strongly depends on the characteristics of these charged domain walls. Furthermore, by mapping the polarization distribution, it was found that a monolayer thick conducting oxide existing on the BiFeO₃ film surface causes a significant increase of local polarization and exotic high-density nano-domains with large strain variations emerge. Finally, small defects in ferroelectric thin films can act as nano-building-blocks for the emergence of novel topological states of polarization ordering, namely, hedgehog/antihedgehog nanodomain arrays in BiFeO₃. The emergent polarization states such as hedgehog/antihedgehog and vortex/ antivortex topologies not only modify the local lattice symmetries and thus induce the coexistence of mixed-phases resembling the morphotropic phase boundary with high piezoelectricity, but also lead to flux-closure vortex structures. Phase-field simulations suggest that the observed novel polarization states are formed due to the coupling between the polarization ordering and the charged defects existing in the films. Thus, engineering of defects may provide a new route for developing ferroelectic/multiferroic-based nanodevices.