"In-situ TEM Studies of Structures and Dynamic Switching Dynamics in Ferroelectrics and Multiferroics"

 

As advances in aberration-corrected transmission electron microscopy (TEM) have enabled the determination of the three-dimensional structure of materials with sub-angstrom resolution, the recent development of in situ TEM in combination with scanning probe techniques allows us to study the dynamic behaviors of nanostructures under applied fields or stress while the atomic structure is imaged directly.  We use these advanced TEM techniques to study the atomic structure and energies of domain walls in ferroelectric (PbZr0.2Ti0.8O3, PZT) and multiferroic BiFeO₃.  I will show that the atomic scale polarization map can be quantitatively determined using aberration-corrected TEM images owing to the large atomic displacements responsible for the dipole moment. 

 

Our studies revealed that interfaces in complex multidomain geometries lead to the formation of polarization vortices with electric flux closure domains.  I will also show that the domain wall width and energy depend strongly on the tilt and rotation of the oxygen octahedra across the domain walls.  Using aberration-corrected transmission electron microscopy in combination with in situ scanning probe, the kinetics and dynamics of ferroelectric switching are followed at millisecond temporal and sub-angstrom spatial resolution in ferroelectric thin films.  We observed 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. It was also found that 180° polarization switching in PZT initially forms domain walls along unstable planes due to the inhomogeneous 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 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.

---