Nanoscale Bubble Domains and Topological Transitions in Ultrathin Ferroelectric Films

Abstract

Observation of a new type of nanoscale ferroelectric domains, termed as "bubble domains"-laterally confined spheroids of sub-10 nm size with local dipoles self-aligned in a direction opposite to the macroscopic polarization of a surrounding ferroelectric matrix-is reported. The bubble domains appear in ultrathin epitaxial PbZr Ti O /SrTiO /PbZr Ti O ferroelectric sandwich structures due to the interplay between charge and lattice degrees of freedom. The existence of the bubble domains is revealed by high-resolution piezoresponse force microscopy (PFM), and is corroborated by aberration-corrected atomic-resolution scanning transmission electron microscopy mapping of the polarization displacements. An incommensurate phase and symmetry breaking is found within these domains resulting in local polarization rotation and hence impart a mixed Néel-Bloch-like character to the bubble domain walls. PFM hysteresis loops for the bubble domains reveal that they undergo an irreversible phase transition to cylindrical domains under the electric field, accompanied by a transient rise in the electromechanical response. The observations are in agreement with ab-initio-based calculations, which reveal a very narrow window of electrical and elastic parameters that allow the existence of bubble domains. The findings highlight the richness of polar topologies possible in ultrathin ferroelectric structures and bring forward the prospect of emergent functionalities due to topological transitions.

Deterministic Switching of Ferroelectric Bubble Nanodomains

“…Previously, we have shown that even under a very weak AC field the bubble domains can easily morph into classical cylindrical domains and eventually merge to form a mesoscale labyrinthine polydomain network . This transition from the bubble domain to the cylindrical domain state under electrical bias manifests a polarization rotation process, which results in a significant enhancement of the electromechanical response.…”

“…The crystallographic details of (001)‐oriented epitaxial PZT/STO/PZT sandwich films on La 0.67 Sr 0.33 MnO 3 (LSMO) buffered STO substrate used here can be found elsewhere . For this study, we chose PZT layer fixed at 7 unit cells (u.c.)…”

“…Topological structures (or defects) in ferroic materials, such as skyrmions, vortices, flux closures, incommensurate curl domains, and bubble domains, have recently attracted attention due to their emergent nature. They can display large current‐induced spin‐orbit torques, giant electromechanical response, enhanced or ballistic electron transport, chirality or colossal optical coefficients, to name just a few of their exciting properties. Often these are distinctly absent in their parent ferroic phases, which provides us with a new paradigm for functional nanodevices …”

“…However, controlled switching at the single defect level in ferroelectrics remains a substantive challenge2c,10,14 even though this has been achieved at the mesoscale . The unambiguous demonstration of deterministic switching between various nontrivial ferroelectric topologies, particularly at the single defect level would lead to a unprecedented handle over the emergent properties and hence entirely new cross‐coupled electro‐mechanical‐optical‐polarization functionalities, as outlined by Martin et al The primary impediment with ferroelectrics is that single topological configurations (i.e., vortex‐like or bubble domain states) have a size only several unit‐cells, which is several orders magnitude smaller than their classical magnetic counterparts. While this drastic size reduction imposes a considerable level of difficulty for manipulation and measurement, it also offers an ideal platform for employing advanced scanning probe microscopy (SPM) approaches.…”

Giant Uniaxial Strain Ferroelectric Domain Tuning in Freestanding PbTiO3 Films

“…The recent discovery of novel domain patterns or complex domain arrangements in nanoscale ferroelectrics has regenerated much attention to the search for phases that do not exist in the parent bulk materials . Structures such as vortex pairs, super‐tetragonal phase, and polar skyrmions have been experimentally reported . In essence, the formation of ferroelectric domain structures strongly relies on the balance between electrostatic and elastic energies which is sensitively affected by film thickness, substrate mismatch strain, depolarization field, cooling rate, etc .…”