Atomic imaging of mechanically induced topological transition of ferroelectric vortices

Abstract

Ferroelectric vortices formed through complex lattice–charge interactions have great potential in applications for future nanoelectronics such as memories. For practical applications, it is crucial to manipulate these topological states under external stimuli. Here, we apply mechanical loads to locally manipulate the vortices in a PbTiO3/SrTiO3 superlattice via atomically resolved in-situ scanning transmission electron microscopy. The vortices undergo a transition to the a-domain with in-plane polarization under external compressive stress and spontaneously recover after removal of the stress. We reveal the detailed transition process at the atomic scale and reproduce this numerically using phase-field simulations. These findings provide new pathways to control the exotic topological ferroelectric structures for future nanoelectronics and also valuable insights into understanding of lattice-charge interactions at nanoscale.

Engineering polar vortex from topologically trivial domain architecture

“…2c acquired from a region that only includes the (PTO) 10 /(STO) 10 superlattice layers. It reveals superlattice reflections both in-plane and out-of-plane, from which the in-plane lattice parameter a and out-of-plane lattice parameter c are calculated to be ~3.9 Å and ~4.0 Å, similar to those observed in reference 40 40 . In particular, the enlarged view of satellite diffraction spots around (001) reflection (inset in Fig.…”

“…We also calculated the corresponding tetragonality in terms of c / a ratio (Fig. 2e ), wherein the red sinusoidal wave-like pattern also suggests the vortex pair as previously reported 8 , 38 – 40 . Pairs of clockwise and anticlockwise vortices are schematically shown in Fig.…”

Atomic-scale observations of electrical and mechanical manipulation of topological polar flux closure

“…Du et al (30) reported that vortices can split, transform to polar waves, and finally become a stable c domain that does not recover spontaneously. According to Chen et al (31), under mechanical stress, a polar vortex can be switched to an a domain and reversibly recover after removal of stress. In our study, the switching behaviors of flux-closures are even more complicated than those of vortices, because the transformation of a flux-closure involves not only the topological core but also the ordinary ferroelectric 90°and 180°domain walls.…”

“…Phase-field simulations have demonstrated the possibility of switching polar vortices: under the application of an electrical field, vortex cores with opposite curls move closer until they reach the same lateral position and then produce new a domains, and the reverse process of back-switching takes place when the applied field is removed ( 37 ). The evolution of vortices driven by temperature change ( 29 ), electric field, and stress have been probed experimentally by X-ray diffraction, PFM ( 29 ), and TEM ( 30 , 31 ). Damodaran et al ( 29 ) found phase coexistence of vortex and ferroelectric phases and electric-field-driven interconversion between them.…”

“…On the other hand, surface probe-based PFM is usually limited in its spatial resolution and does not have the capability to obtain full information regarding nanosized polar topological structures, which have highly structured inhomogeneities and strong charge–lattice coupling, hidden within thin films. By contrast, recent advances in atomically resolved in situ scanning transmission electron microscopy (STEM) have presented the possibility of capturing structural evolutions of individual polar vortices ( 30 , 31 ). Therefore, in the present study we utilize this cutting-edge technique to investigate whether or not topological polar flux-closures can be controllably switched to ordinary ferroelectric domains, and we further reveal the detailed transition behavior and underlying mechanism by combining this experimental investigation with phase field simulations.…”