Mapping gradient-driven morphological phase transition at the conductive domain walls of strained multiferroic films

Abstract

The coupling between antiferrodistortion (AFD) and ferroelectric (FE) polarization, universal for all tilted perovskite multiferroics, is known to strongly correlate with domain wall functionalities in the materials. The intrinsic mechanisms of domain wall phenomena, especially AFD-FE coupling-induced phenomena at the domain walls, have continued to intrigue the scientific and technological communities because of the need to develop the future nano-scale electronic devices. Over the past years, theoretical studies often show controversial results, owing to the fact that they are neither sufficiently nor directly corroborated with experimental evidences. In this work, the AFD-FE coupling at uncharged 180° and 71° domain walls in BiFeO 3 films are investigated by means of aberration-corrected scanning transmission electron microscopy with high resolution (HR-STEM) and rationalized by phenomenological Landau-Ginsburg-Devonshire (LGD) theory. We reveal a peculiar morphology at the AFD-FE walls, including kinks, meandering, triangle-like regions with opposite oxygen displacements and curvature near the interface. The LGD theory confirms that the tilt gradient energy induces these unusual morphology and the features would change delicately with different kinds of domain walls. Moreover, the 180° AFD-FE walls are proved to be conductive with an unexpected reduction of Fe-O-Fe bond angle, which is distinct from theoretical predictions. By exploring AFD-FE coupling at domain walls and its induced functionalities, we provide exciting evidences into the links between structural distortions and its electronic properties, which benefit a lot for fundamental understanding for domain wall functionalities as well as functional manipulations for novel nano-devices.

Controlling the domain structure of ferroelectric nanoparticles using tunable shells

“…Gregg [40] reviewed experimental and theoretical works to find out whether or not complex arrangements of dipoles (such as flux-closure, vortex, or skyrmion patterns) exist in multiaxial ferroelectrics. Metastable meandering domain walls can exist in thin BiFeO 3 films [41,42]. Rodriguez et al [43] experimentally studied two-dimensional arrays of ferroelectric lead zirconate titanate nanodots with the help of piezoresponse force microscopy.…”

Elastic distortion determining conduction in BiFeO3 phase boundaries

“…buckling rather than straightening of the bond angle). 37 To our knowledge, localised evidence of Fe-O-Fe bond angle straightening (or buckling) in mixed-phase BiFeO 3 has not yet been observed experimentally. Direct measurement of bond angles is challenging in these systems since their large epitaxial strain necessitates a relatively large TEM lamella thickness to avoid thin foil relaxation effects.…”

Mesoscopic theory of defect ordering–disordering transitions in thin oxide films

“…remain a subject of active research since the discovery of ferroelectricity in late 1920s 4 . Topological defects in ferroelectric materials and coupling between the ferroelectric and semiconductor subsystems are actively explored in the context of surface and domain wall conductance 5 – 8 .…”

“…Furthermore, in multiferroic materials fundamentally new properties can emerge at the topological defects 17 . Examples of the former include the emergence of suppressed order parameter and associated topological defects at the domain walls, surfaces and interfaces, domain wall and vortex core conduction, incipient ferroelectricity and magnetism, and many others 8 , 18 , 19 . Remarkedly that conductive domain walls in bulk samples and thin films can act as mobile charged channels (being also mobile topological defects) opening the way for “domain wall nanoelectronics” 20 .…”

“…The vast majority of the research in the field explored the coupling between the primary physical order parameters including magnetization, polarization, and ferroelasticity 8 , 18 . However, it is well known that chemical degrees of freedom can strongly affect the ferroic behavior 23 .…”