Type of presentation: Poster

MS-12-P-3531 Atomic Structure and Properties of Charged Domain Walls in Ferroelectrics

Li L.1, Gao P.1, Nelson C. T.1, Jokisaari J. R.1, Zhang Y.1, Kim S. J.1, Melville A.2, Adamo C.2, Schlom D. G.2, Pan X. Q.1
1Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109, United States, 2Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853, United States
panx@umich.edu

A ferroelectric domain wall can become electronically active, as a result of charged domain walls (CDWs) with a “head-to-head” or “tail-to-tail” polarization configuration. Such domain walls carrying net bound charge can have distinct properties from uncharged domain walls, such as a metallic conductivity. In this work, we show that CDWs in the rhombohedral-like (R-like, Fig. 1a left) BiFeO3 thin films possess a tetragonal-like (T-like, Fig. 1a right) crystal structure; and the bound charge at the CDW also induces the formation of nano-domains with novel polarization states and unconventional domain walls in nearby regions.

Figure 1b shows a diffraction contrast TEM image taken from a cross-sectional specimen of a 20 nm thick (001)P BiFeO3 film grown epitaxially on an insulating (110)O TbScO3 substrate (P denotes pseudocubic and O denotes orthorhombic), in which triangular 109° (vertical)/180° (inclined) domain wall junctions can be observed. Above the junction near the free surface of the BiFeO3 film, a 71° CDW is observed. The CDW is studied by high angle annular dark field (HAADF) imaging using the TEAM0.5 instrument with a point-to-point resolution of 0.5 Å. The HAADF image is processed to obtain mapping of the lattice parameter and the atomic displacement of Fe cations from the center of four Bi neighbors (DFB). The electric polarization is proportional to -DFB. Figure 2a shows the spatial distribution of -DFB overlaid on the HAADF image. A CDW with “head-to-head” polarization configuration is clearly seen above the triangular junction. Interestingly, the polarization rotates gradually from <111> directions beside the CDW to the out-of-plane orientation at the CDW. The lattice parameter mapping (Fig. 2b) also shows a local increase of the c/a ratio at the CDW. These results suggest the formation of a T-like structure at the CDW, surrounded by the regular R-like phase. The T-like CDW also leads to changes in polarization. As seen in Fig. 2a and b, below the T-like CDW, a nano-domain with a pseudocubic structure, with an in-plane oriented polarization, occurs. As a result, unconventional inclined CDW are observed. For sufficiently thin films, for example, 5 nm thick film as shown in Figure 3a and b, CDWs traverse the full thickness of the film.

In summary, we have found stable charged domain walls (CDWs) in BiFeO3 thin films. These CDWs possess crystal structures, ferroelectric polarization states, and properties different from the bulk film due to local charge compensation and polarization rotation. These CDWs can provide metallic conduction channels in the film, since the accumulation of compensating free charge that screen the bound charge at the CDW can in principle intriguer an insulator-metal transition.

the authors gratefully acknowledge the financial support through DOE grant DoE/BES DE-FG02-07ER46416

Fig. 1: (a) Atomic models of the rhombohedral-like and tetragonal-like structures of BiFeO3. (b) Cross sectional dark field TEM image of domain patterns of 20 nm thick BiFeO3 film on TbScO3 substrate, with the corresponding schematic domain configuration shown below.

Fig. 2: (a) Plot of the -DFB vectors overlaid on HAADF image of a 109°/180° domain wall junction near the free surface of the 20 nm thick BiFeO3 film. (b) The corresponding color map of the c/a ratios. The polarization orientation and bound charge are indicated schematically.

Fig. 3: (a) Plot of -DFB vectors and (b) c/a ratio color map of a 5 nm thick BiFeO3 film grown on TbScO3 substrate. The polarization orientation and bound charge are indicated schematically.