Nucleation, growth, and control of ferroelectric-ferroelastic domains in thin polycrystalline films

The unique response of ferroic materials to external excitations facilitates them for diverse technologies, such as nonvolatile memory devices. The primary driving force behind this response is encoded in domain switching. In bulk ferroics, domains switch in a two-step process: nucleation and growth. For ferroelectrics, this can be explained by the Kolmogorov-Avrami-Ishibashi (KAI) model. Nevertheless, it is unclear whether domains remain correlated in finite geometries, as required by the KAI model. Moreover, although ferroelastic domains exist in many ferroelectrics, experimental limitations have hindered the study of their switching mechanisms. This uncertainty limits our understanding of domain switching and controllability, preventing thin-film and polycrystalline ferroelectrics from reaching their full technological potential. Here we used piezoresponse force microscopy to study the switching mechanisms of ferroelectric-ferroelastic domains in thin polycrystalline Pb${}_{0.7}$Zr${}_{0.3}$TiO${}_3$ films at the nanometer scale. We have found that switched biferroic domains can nucleate at multiple sites with a coherence length that may span several grains, and that nucleators merge to form mesoscale domains, in a manner consistent with that expected from the KAI model.

Figure 1

Bundle domains in ferroelectric-ferroelastic materials. (a) Schematics of bundle domains within a crystallite. The dotted lines illustrate boundaries between different domains. (b) EPFM amplitude image demonstrates bundles of periodically alternating parallel ferroelastic striped domains within a 1.2 × 1.2-μm${}^2$ area in a PZT film. (c) Topography of the same area.

Figure 2

Bundle domain nucleation and growth with in-phase PFM. Schematics (left) and in-phase PFM images (right) of the switching process. (a) Native domain distribution demonstrates dominant bundle domain in a large grain (inside the blue line). Inset is topography. (b) Application of external electric field emerged small bundle domains at the grain boundaries (highlighted with yellow lines), while shrinking the original domain (within the blue polygon). (c) Repeating excitation merged the growing small domains, completing the switching of the entire bundle domain (designated by a yellow line). Dashed blue lines follow the bundle domain evolution in the top left grain, demonstrating its tendency to align in parallel to the large grain domains. Dark gray lines denote the grain boundaries. Scale bars are 250 nm.

Figure 3

(a) Topography of area showing grain where reversibility experiments were carried out (positions indicated by spots); (b) PFM image of same area. “$a$” denotes an area where the ferroelastic domains are perpendicular to a grain boundary.

Figure 4

Ferroelectric-ferroelastic domain engineering. In-phase PFM images of the area shown in Fig. 3, after different zones were scanned at a voltage higher than the coercive value. (a) Native domain distribution. (b) After 5C was written. (c) After writing 6C. (d) After writing 4D. (e) After writing 6D. (f) After writing 5D. (g) After writing 4B (at the $a$-$a$ junction). (h) After writing 5A. (i) After writing 4A. (j) After writing 4B (at the $a$-$a$ junction). (k) After writing 5A. (l) After writing the entire area. Arrows indicate the center of the areas that were “written.”