Domain wall nanoelectronics

TLDR

Recent atomic‑resolution imaging has revealed that ferroelectric domain walls are far more complex than the previously assumed rectilinear Ising structures, exhibiting Bloch‑ and Néel‑like features and functional properties in magnetoelectric materials. This review surveys recent studies of ferroelectric and multiferroic domain walls, compares them with magnetic walls, and highlights device concepts that use the walls themselves as active elements. The authors discuss nanoelectronic devices that exploit ferroelectric domain wall conductivity, size, and emergent ferromagnetism, contrasting them with magnetic wall devices that rely on high mobilities and supersonic velocities. While magnetic wall microelectronics is already mature, ferroelectric wall devices are emerging, offering slower but potentially more versatile functionality due to their unique electrical and magnetic characteristics.

Abstract

Domains in ferroelectrics were considered to be well understood by the middle of the last century: They were generally rectilinear, and their walls were Ising-like. Their simplicity stood in stark contrast to the more complex Bloch walls or N\'eel walls in magnets. Only within the past decade and with the introduction of atomic-resolution studies via transmission electron microscopy, electron holography, and atomic force microscopy with polarization sensitivity has their real complexity been revealed. Additional phenomena appear in recent studies, especially of magnetoelectric materials, where functional properties inside domain walls are being directly measured. In this paper these studies are reviewed, focusing attention on ferroelectrics and multiferroics but making comparisons where possible with magnetic domains and domain walls. An important part of this review will concern device applications, with the spotlight on a new paradigm of ferroic devices where the domain walls, rather than the domains, are the active element. Here magnetic wall microelectronics is already in full swing, owing largely to the work of Cowburn and of Parkin and their colleagues. These devices exploit the high domain wall mobilities in magnets and their resulting high velocities, which can be supersonic, as shown by Kreines' and co-workers 30 years ago. By comparison, nanoelectronic devices employing ferroelectric domain walls often have slower domain wall speeds, but may exploit their smaller size as well as their different functional properties. These include domain wall conductivity (metallic or even superconducting in bulk insulating or semiconducting oxides) and the fact that domain walls can be ferromagnetic while the surrounding domains are not.