Polar Vortexes and Charged Domain Walls in a Room Temperature Magnetoelectric Thin Film

Abstract

Multiferroic domain walls are an emerging solution for future low-power nanoelectronics due to their combined tuneable functionality and mobility. Here we show that the magnetoelectric multiferroic Aurivillius phase Bi6TixFeyMnzO18 (B6TFMO) crystal is an ideal platform for domain wall-based nanoelectronic devices. The unit cell of B6TFMO is distinctive as it consists of a multiferroic layer between dielectric layers. We utilise atomic resolution scanning transmission electron microscopy and spectroscopy to map the sub-unit-cell polarisation in B6TFMO thin films. 180˚ charged head-to-head and tail-to-tail domain walls are found to pass through > 8 ferroelectric-dielectric layers of the film. They are structurally similar to BiFeO3 DWs but contain a large surface charge density (σ_s) = 1.09 |e|per perovskite cell, where |e| is elementary charge. Although polarisation is primarily in-plane, c-axis polarisation is identified at head-to-tail domain walls with an associated electromechanical coupling of strain and polarisation. Finally, we reveal that with controlled strain engineering during thin film growth, room-temperature vortexes are formed in the ferroelectric layer. These results confirm that sub-unit-cell topological features can play an important role in controlling the conduction properties and magnetisation state of Aurivillius phase films and other multiferroic heterostructures.

Nanoscale Design of High-Quality Epitaxial Aurivillius Thin Films

“…We confirm the uniaxial in-plane polarization along the NGO [0 1 0]-direction, which corroborates the absence of crystal twinning in the films. All three films exhibit stripe in-plane-polarized domains, which result in an array of alternating nominally charged head-to-head and tail-to-tail domain walls (sketched in Figure b), as previously characterized at the nanoscale using STEM . The precise control that we have over the thickness of our films allows us to probe interlayer coupling for BFTO homologues of different n .…”

“…OPBs are translation boundary defects routinely imaged with STEM , as an out-of-phase unit cell shift in the vertical direction by a fraction of unit cell (uc). Such structural defects have been linked to inhomogeneous strain fields and, in some instances, even to suppressed polarization. , Recent experiments further highlighted their impact on the topology of polar states in ferroelectric thin films . Hence, their occurrence should be controlled to perfect the ferroelectric performance.…”