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Probing the Structure and Dynamics of Ferroelectric Domain Walls with Atomic Resolution in Real-Time

Seminar Group: 


Professor Xiaoqing Pan


Department of Materials Science and Engineering
University of Michigan-Ann Arbor, Ann Arbor


Friday, February 1, 2013 - 4:00pm


ESB 1001


Prof. Susanne Stemmer

The ferroelectric switching occurs through the nucleation and growth of favorably oriented domains and is mediated by defects and interfaces. Dislocations, for example, are known to destroy ferroelectric order; neighboring grains and interfaces subject the ferroelectric to localized strain, electric fields, or the screening of electric fields. Thus, it is critical to understand how the ferroelectric domain forms, grows, and interacts with structural defects. In this talk I will show that the atomic scale polarization map in ferroelectrics can be determined using aberration-corrected TEM images owing to the large atomic displacements responsible for the dipole moment. This study reveals how interfaces in complex multidomain geometries lead to the formation of polarization vortices with electric flux closure domains. Using aberration-corrected transmission electron microscopy (TEM) in combination with a customized in situ scanning probing holder the kinetics and dynamics of ferroelectric switching is followed at millisecond temporal and subangstrom spatial resolution in an epitaxial bilayer of an antiferromagnetic ferroelectric (BiFeO3) on a ferromagnetic electrode (La0.7Sr0.3MnO3). We observe localized nucleation events at the electrode interface, domain wall pinning on point defects, and the formation of metastable ferroelectric states localized to the ferroelectric and ferromagnetic interface. These studies show how defects and interfaces impede full ferroelectric switching of a thin film. Using the similar techniques the dynamics of ferroelectric switching in a PbZr0.2Ti0.8O3 (PZT) film, which is a key material for nonvolatile ferroelectric memories, was also studied. It was found that 180° polarization switching initially forms domain walls along unstable planes due to the inhomogenous electric field from the small switching electrode. After removal of the external field, they tend to relax to low energy orientations. In sufficiently small domains this process results in complete backswitching. These findings suggest that even thermo dynamically favored domain orientations are still subject to retention loss, which must be mitigated by overcoming a critical domain size.