Abstract
Ferroelectric materials have been utilized in a broad range of electronic, optical, and electromechanical applications and hold promise for the design of future high-density nonvolatile memories and multifunctional nano-devices. The applications of ferroelectric materials stem from the functional structures of domains and domain walls and the ability to switch them by applying an electric field. A fundamental understanding of the atomic scale mechanism underlying the domain formation and the domain switching, therefore, is critical for the design of devices. In this work, I will present a systematic study of domain structures and polarization switching processes in multiferroic BiFeO3 thin films using in situ transmission electron microscopy (TEM) with the atomic resolution. We found that the charged domain walls can be created or erased in ferroelectric thin films by applying a bias, and the resistance of the local film depends strongly on the characteristics of the charged domain walls. By mapping the polarization on high resolution TEM image, we found that a monolayer thick conducting oxide existing on the surface of a BiFeO3 film causes a significant increase of local polarization and an emergence of exotic high-density nano-domains with large strain variations. Furthermore, I will also show that small defects in a ferroelectric thin film can act as nano-building-blocks for creating structures with novel electrical polarization topologies, namely, hedgehog/antihedgehog nanodomain arrays in the prototypical multiferroic BiFeO3. Atomic-scale scanning transmission electron microscopy imaging reveals the existence of exotic polarization rotation patterns around these defects resembling hedgehog/antihedgehog topologies, which can not only cause local changes in lattice symmetries leading to mixed-phase structures resembling the morphotropic phase boundary with high piezoelectricity, but also lead to flux-closure vortex structures. Phase-field simulations indicate the observed polarization configurations are formed to charged states at the defects. Engineering defects thus may provide a new route for developing ferroelectic/multiferroic-based nanodevices.
Coffee and tea will be served 20 minutes prior to the seminar.
Anyone interested is welcome to attend.