Manipulating topological transformations of polar structures through real-time observation of the dynamic polarization evolution

Abstract

Topological structures based on controllable ferroelectric or ferromagnetic domain configurations offer the opportunity to develop microelectronic devices such as high-density memories. Despite the increasing experimental and theoretical insights into various domain structures (such as polar spirals, polar wave, polar vortex) over the past decade, manipulating the topological transformations of polar structures and comprehensively understanding its underlying mechanism remains lacking. By conducting an in-situ non-contact bias technique, here we systematically investigate the real-time topological transformations of polar structures in PbTiO₃/SrTiO₃ multilayers at an atomic level. The procedure of vortex pair splitting and the transformation from polar vortex to polar wave and out-of-plane polarization are observed step by step. Furthermore, the redistribution of charge in various topological structures has been demonstrated under an external bias. This provides new insights for the symbiosis of polar and charge and offers an opportunity for a new generation of microelectronic devices.

Fig. 1

Structural characterization of the PTO_(n)/STO₍₁₀₎ multilayer. a HAADF-STEM image of the film, recorded with the incident electron beam parallel to the [010]_pc direction. Scale bar, 10 nm. b EDS-mapping of Pb, Sr, and Ti shows that the interface of PTO/STO is atomically sharp. This region is extracted from the white box in a. Scale bar, 1 nm. c Ti atom displacement vector maps based on experiment, showing the domain evolution with changing PTO thickness. Red and blue regions indicate the clockwise and counterclockwise vortex pairs^–. d Superposition of the iDPC-STEM image and a structure model of PTO, showing the atom displacements around the vortex structure. The red, green, and yellow dots denote the positions of Pb²⁺, Ti⁴⁺, and O^2- columns, respectively. Arrows denote the polarization direction. The dashed lines indicate the interface between PTO and STO. The zoom-in images attached to this iDPC-STEM image exhibit the unit cell of PTO along [010]_pc in which positions of Ti, O, and Pb are clearly shown. Scale bar, 1 nm

Fig. 2

Manipulation of the ferroelectric domain in a PTO/STO multilayer. a Schematic diagram of the in-situ electric field experiment. The input voltage is applied along the c-axis. To capture the structural changes, a real-time analysis is performed in STEM mode. b–d Mapping of the real-time dynamic evolution of the polarization based on HAADF images under an external electric field and the corresponding simulation results. An evolution process of the polar structure from vortex to wave and finally polar down state was recorded. The three columnar sub-panels are schematic diagrams, experimental mappings of polar vector and phase-field simulations, respectively. e The out-of-plane strain (ε_yy) map of the PTO layer under different external bias, which remain almost unchanged. Scale bar, 2 nm

Fig. 3

EELS-mapping results in PTO₍₁₁₎/STO₍₁₀₎ where vortex domains exist. a HAADF image of a PTO layer of 11 uc with vortex domains, together with its polar map. Dashed circles represent the concentrated areas of negative charge. Scale bar, 2 nm. b The upper colored surface plot shows the Ti–L₃ energy splitting in the PTO layer where a vortex exists, the lower HAADF image shows the location of the corresponding vortex structure in the PTO layer. c Superposition of the Ti⁴⁺ and Ti³⁺ signal based on EELS analysis; the investigated area is extracted from the dashed box area in a. Scale bar, 1 nm. d Ti–L_2,3 spectra corresponding to three areas: the blue-A curve is from the vortex core, the green-B curve is from the PTO layer but away from the vortex cores, and the gray curve is the reference Ti-L_2,3 spectrum acquired in the bulk-PTO. These three areas show a distinct variation of the EELS spectrum, which is a clear indication of the valence changes of Ti

Fig. 4

HAADF-STEM images and EELS characterization across the multilayer. a HAADF-STEM image of PTO₍₆₎/STO₍₁₀₎. The yellow arrows indicate the polarization direction. Line scan EELS were performed across different regions of the wave domain. Scale bar, 1 nm. b Energy splitting value in the PTO and STO layers across the corresponding regions in a, showing that the Ti³⁺ component is constrained in the wave opening. c–e HAADF-STEM image of the polar down region and corresponding EELS results of Ti–L_2,3 and O–K edges across the PTO layer. The blue, green and red curve are acquired at the regions indicated by arrows in c. The reference O–K spectrum acquired in PTO bulk could be seen in Supplementary Fig. 11. Subtle changes are observed as a result of an electronic-structure reconstruction. These results provide evidence for the appearance of oxygen vacancies on the negative polar interface of PTO and Ti³⁺ component on the positive polar interface. Scale bar, 1 nm