The Genealogical Relationship Between Trivial and Nontrivial Domains in Ferroelectric PbTiO3 Film System

Le résumé fourni par la source

In recent years, a variety of nontrivial topological structures discovered in ferroelectric PbTiO3/SrTiO3 (PTO/STO) system have attracted significant research interest. These structures, including skyrmions [1-3], vortices, merons, hopfions, dipole waves [4, 5], and flux-closures [6], hold great promise for integration into ferroelectric random-access memories (FeRAM) or ferroelectric field-effect transistors (FeFETs) to enable high-density information storage. However, the intrinsic correlations among these topological structures and their underlying formation mechanisms remain poorly understood, limiting their device-oriented implementation. Based on an extensive literature review and our current (scanning) transmission electron microscopy ((S)TEM) experiment results, we conduct a systematic comparative analysis of representative trivial and nontrivial domains and propose a unified mechanism in which nontrivial structures evolve from trivial domain configurations (Fig. 1). For trivial domain structures in single PbTiO3 thin films, 180° ferroelectric domains form when the lattice strain between the substrate and the PbTiO3 layer is negligible. With increasing strain, a/c 90° domains first emerge, followed by a mixed configuration of a/c 90° and a1/a2 domains, and eventually evolve into a fully developed a1/a2 domain configuration. Through comparative analysis, we find that the formation of 180° domains is weakly influenced by strain but strongly governed by electrical boundary conditions [7]. Therefore, 180° domains predominantly exhibit the characteristics of ferroelectric domains. In contrast, a/c 90° and a1/a2 domains are highly sensitive to strain while their dependence on electrical boundary conditions is negligible, indicating that a/c and a1/a2 domains primarily possess ferroelastic domain characteristics (Fig. 1). Under the same strain conditions, when a single PTO thin film is replaced by a PTO/STO superlattices, the resulting domain structures depend strongly on the domain configuration of the individual PTO layer. If 180° domains are formed in a single PTO film, skyrmion-bubble structures emerge in the PTO/STO superlattices (Figs. 1a,c). TEM diffraction contrast images reveal the morphological features of skyrmion-bubbles. Notably, in the absence of ion-displacement analysis and plan-view morphological characterization, the observed contrast may be readily interpreted as arising from 180° domains (Fig. 1a). In fact, this occurs because the depolarization field fragments the stripe-like 180° domains into discrete, block-like or cylindrical skyrmion-bubble configurations, suggesting that these skyrmion-bubble structures inherit certain characteristics of 180° ferroelectric domains [7]. Moreover, a skyrmion structure can be regarded as a combination of two merons. Similarly, when a/c or a1/a2 domains form in a single PTO layer, flux-closure or vortex domain structures develop in the PTO/STO superlattices. The formation of flux-closure and vortex domains is primarily governed by the thickness of the PTO layers within the superlattices [7]. As the PTO thickness decreases in PTO/STO system, the domain evolution follows a sequence from flux-closure domain to vortex domain and eventually to dipole wave states (Figs. 1b,d). The polarization distribution maps corresponding to the vortex and dipole wave states were derived from high-angle annular dark-field image (HAADF) STEM images acquired from PTO/STO superlattices with varying thicknesses. We further observe that under external stimuli, such as electric fields, temperature, or mechanical stress, both flux-closure and vortex structures tend to transform into ferroelastic domains, such as a/c or a1/a2 domain. Here, our claim can be further strengthened by analyzing the effect of the depolarization field on the topological structure. When the insulating STO layers in the PTO/STO superlattices are substituted with conductive SrRuO3 layers, which completely screen the depolarization field, flux-closure (vortex) structures remain stable in the PTO/STO superlattices. This behavior indicates that these domain configurations (flux-closures, vortices, dipole waves) possess ferroelastic characteristics, rather than ferroelectric characteristics, and are highly sensitive to strain. In conclusion, over the past decade, through integrating our own (S)TEM research results with a comprehensive review of studies on ferroelectric topological structures, we clarify that non-trivial structures originate from trivial ones. The newly proposed formation mechanism provides a guiding principle for the exploration of emerging topological structures [8]. Genetic relationship between the trivial and nontrivial domain. (a) TEM diffraction contrast image showing ferroelectric skyrmion-bubbles in a (PTO23/STO23)10 superlattices grown on the STO substrate. (b) Ferroelectric flux-closures, vortices, and dipole waves. The polarization distribution mapping of the vortex and dipole waves are derived from the (PTOm/STOm)10 (m = 12, 7 u.c.) superlattices grown on the DyScO3 substrate. (c) Ferroelectric 180° domain. (d) Ferroelastic a/c 90° and a1/a2 domain. The bottom illustration is the TEM morphology image of the a1/a2 domain.

Ce résumé expose les affirmations des auteurs. BNTIC ne l’interprète pas comme une validation indépendante des résultats.