Defect-Induced Hedgehog Polarization States in Multiferroics

Abstract

Continuous developments in nanotechnology require new approaches to materials synthesis that can produce novel functional structures. Here, we show that nanoscale defects, such as nonstoichiometric nanoregions (NSNRs), can act as nano-building blocks for creating complex electrical polarization structures in the prototypical multiferroic BiFeO_{3}. An array of charged NSNRs are produced in BiFeO_{3} thin films by tuning the substrate temperature during film growth. Atomic-scale scanning transmission electron microscopy imaging reveals exotic polarization rotation patterns around these NSNRs. These polarization patterns resemble hedgehog or vortex topologies and can cause local changes in lattice symmetries leading to mixed-phase structures resembling the morphotropic phase boundary with high piezoelectricity. Phase-field simulations indicate that the observed polarization configurations are mainly induced by charged states at the NSNRs. Engineering defects thus may provide a new route for developing ferroelectric- or multiferroic-based nanodevices.

Recent Progress on Topological Structures in Ferroic Thin Films and Heterostructures

“…[ 58 ] Direct evidence for the formation of rotating polarization at atomic scale was made in 2011 when experimentalists were able to directly observe continuous polarization rotation near the interfaces of ferroelectric thin films using aberration‐corrected (scanning) transmission electron microscopy (TEM). [ 59,60 ] Since then, several exotic polar topologies such as flux‐closure domains, [ 61–65 ] vortices, [ 66–71 ] non‐Ising‐like domain walls, [ 72–74 ] center‐type domains, [ 68,70,75,76 ] labyrinthine domains, [ 77 ] bubbles, [ 78 ] incommensurate curl domains, [ 79 ] spiral states, [ 80 ] hedgehog states, [ 81 ] and polar skyrmions, [ 82,83 ] have been revealed in various ferroelectric materials. Which kind of polar topology develops would depend on the relative magnitudes of various energies, such as elastic (i.e., strain), electrostatic (i.e., depolarization), polarization/chemical gradient, and interfacial coupling energies.…”

Oxygen vacancy-induced topological nanodomains in ultrathin ferroelectric films

“…Another rarely investigated pathway lies in the exploitation of ionic point defects, which can break the lattice symmetry locally via electric and strain fields. For example, hedgehog-and vortex-like polarization states were recently discovered in the vicinity of charged point defects in BiFeO 3 thin films 13 . The charged defects were immobile cation impurities intentionally introduced during film growth.…”

Control of Domain Structures in Multiferroic Thin Films through Defect Engineering

“…It is well known that some common types of defects, such as dislocations and vacancies, can interact with ferroelectric domains and DWs, pinning metastable polarization configurations. Recently, it was also discovered that impurity defects, another major type of defect with a structure different from the host material, can induce dramatic changes in domain structures of ferroelectric thin films . This finding suggests the possibility of using engineered impurity defects in combination with suitable interface boundary conditions to control domain formation and create complex domain structures.…”

“…This finding suggests the possibility of using engineered impurity defects in combination with suitable interface boundary conditions to control domain formation and create complex domain structures. Nevertheless, the observed impurity defects in literature are either accidentally formed in the films or coupled to polarization‐structure changes within a range of only a few nanometers of the defect . The approach of precisely creating ordered patterns of defects that can control the domain structure in the bulk of the film remains unexplored.…”