Atomic structures and dynamic behaviors of domain walls in ferroelectric thin films
Authors: Pan Xiaoqing , Li Linze 
Published: December 2016

Ferroelectric materials are characterized by a spontaneous electric polarization that can be reoriented between different orientations by an applied electric field. The ability to form and manipulate domains with different polarization orientations at the nanometer scale is key to the use of ferroelectric materials for devices such as nonvolatile memories. The ferroelectric switching occurs through the nucleation and growth of favorably oriented domains and is strongly mediated by defects and interfaces. Thus, it is critical to understand how the ferroelectric domain forms, grows, and interacts with defects. Here, we demonstrate the ability of studying the atomic structure and dynamic behaviors of ferroelectric domain walls using the state‐of‐the‐art atomic‐resolution scanning transmission electron microscopy (STEM) and
in situ
transmission electron microscopy (TEM) techniques.

Figure 1a shows a high‐angle annular dark‐field (HAADF) image of a BiFeO
3
thin film in [100] pseudocubic orientation. With a quantitative analysis of this image, the displacement (

D
FB

) vector was determined, which is measured by the displacement of the Fe position (which is located in the center of an oxygen octahedron) from the center of its four Bi neighbors. The vector

D
FB

as the manifestation of the ferroelectric polarization in BiFeO
3
, points toward the center of the negative oxygen ions (Fig. 1b), and thus can be used to determine the polarization vector in the image plane. Using the polarization mapping technique based on HAADF imaging, we found that a localized vortex domain structure can be formed at the termination of 109° domain walls at the BiFeO
3
/TbScO
3
interface. While in thicker (20 nm) BiFeO
3
films (Fig. 1c), the vortex is accompanied by triangular domains consist of a mirrored pair of inclined 180° domain walls in conjunction with the previously existing 109° domain wall; no obvious triangular nano‐domain patterns are observed in thinner (5 nm) films (Fig. 1d), and instead, the polarization vectors rotate smoothly to form a vortex core. These observations indicate that the configuration of the spontaneous flux‐closure domains strongly depends on the size of the system, suggesting a potential method to tune the polarization vortex structures for practical devices.

Figure 2 shows
in situ
switching of domains in a 20 nm thick BiFeO
3
films grown on TbScO
3
substrate by applying a bias between a W probe and an epitaxial La
0.7
Sr
0.3
MnO
3
bottom electrode. A charged domain wall (CDW) can be created by applying a bias (Fig. 2a). The initial stable structure (Fig. 2b) contained 109° and 180° domain walls separated by 10 nm at the substrate interface. The onset of domain wall motion occurred at a critical bias of 1.7 V (Fig. 2c). As the bias increases, the two 109° and 180° domain walls moved toward each other until they intersected (Fig. 2d). Then further shrinkage of the triangular domain led to upward motion of the triangular domain tip and resulted in the formation and elongation of a CDW (Fig. 2e,f). After the bias was removed, a stable state of the CDW remained (Fig. 2g). Such switching processes involving the evolution of CDWs result in significant changes in the local resistance of the film, suggesting the CDW may play an important role in future memory devices.