Defeating depolarizing fields with artificial flux closure in ultrathin ferroelectrics

Abstract

Material surfaces encompass structural and chemical discontinuities that often lead to the loss of the property of interest in so-called dead layers. It is particularly problematic in nanoscale oxide electronics, where the integration of strongly correlated materials into devices is obstructed by the thickness threshold required for the emergence of their functionality. Here we report the stabilization of ultrathin out-of-plane ferroelectricity in oxide heterostructures through the design of an artificial flux-closure architecture. Inserting an in-plane-polarized ferroelectric epitaxial buffer provides the continuity of polarization at the interface; despite its insulating nature, we observe the emergence of polarization in our out-of-plane-polarized model of ferroelectric BaTiO3 from the very first unit cell. In BiFeO3, the flux-closure approach stabilizes a 251° domain wall. Its unusual chirality is probably associated with the ferroelectric analogue to the Dzyaloshinskii–Moriya interaction. We, thus, see that in an adaptively engineered geometry, the depolarizing-field-screening properties of an insulator can even surpass those of a metal and be a source of functionality. This could be a useful insight on the road towards the next generation of oxide electronics.

What lies beneath? Investigations of atomic force microscopy-based nano-machining to reveal sub-surface ferroelectric domain configurations in ultrathin films

“…The 45° stripe domain configurations along the lateral a-b plane were observed to persist through the film, disappearing at depths consistent with the substrate. This is an interesting observation, as it suggests that the ferroelectric critical size limit may be absent for in-plane polarization directions in B6TFMO, similar to observations in optimized HfO 2 and BiFeO 3 films and thickness-dependent tomographic PFM studies of m = 4 Aurivillius phase films [7,51,52] . However, more detailed experiments correlating precise thickness measurements (e.g., using aberration-corrected scanning TEM) with PFM experiments are required in order to confirm whether or not a critical thickness exists for in-plane ferroelectricity in B6TFMO.…”

Defining ferroelectric characteristics with reversible piezoresponse: PUND switching spectroscopy PFM characterization

“…Piezoresponse force microscopy (PFM) is a powerful technique utilized for visualizing polarization orientation and locally reversing polarization in ferroelectrics [1]. PFM holds the potential to serve as a valuable tool for examining the dynamics of ferroelectric domains [2], inspection of polarization orientation at the nanoscale in the flux closure structures [3], vortexes [4], and other complicated domain patterns [5], investigation the shape and roughness of the domain walls [6], and measurements local piezoresponse specifically near interfaces [7], within individual material grains [5], and in the proximity of defects [8][9][10]. Moreover, PFM offers extensive capabilities to characterize quantum materials [11], and organize charge defects via local electrical stimuli [9,12].…”

Tilting and Distortion in the Multiferroic Aurivillius Phase Bi6Ti3Fe1.5Mn0.5O18

“…The (Bi 2 O 2 ) 2+ layers play a crucial role in charge compensation, contributing to reduced leakage currents and enhanced fatigue resistance characteristics compared to conventional perovskite materials. Consequently, Aurivillius phases, like SrBi 2 Ta 2 O 9 , SrBi 2 NbTaO 9 , and SrBi 4 Ta 4 O 15 , have found applications in high-speed, low-power, nonvolatile ferroelectric random access memory devices. , The use of in-plane polarized Bi 5 Ti 3 FeO 15 Aurivillius phase as a buffer layer in geometrically engineered BaTiO 3 and BiFeO 3 films has been observed to provide continuity of polarization at the interface and circumvent depolarization field issues typical in out-of-plane ferroelectrics, particularly at reduced dimensions . The underlying Aurivillius phase facilitates the growth of subsequent thin films, ensuring out-of-plane polarization from the very first unit cell.…”

“… 18 , 19 The use of in-plane polarized Bi 5 Ti 3 FeO 15 Aurivillius phase as a buffer layer in geometrically engineered BaTiO 3 and BiFeO 3 films has been observed to provide continuity of polarization at the interface and circumvent depolarization field issues typical in out-of-plane ferroelectrics, particularly at reduced dimensions. 18 The underlying Aurivillius phase facilitates the growth of subsequent thin films, ensuring out-of-plane polarization from the very first unit cell. Consequently, it offers depolarizing–field-screening properties and aids in stabilizing ultrathin out-of-plane ferroelectricity in oxide heterostructures.…”