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Page 12 of 15 Keeney et al. Microstructures 2023;3:2023041 https://dx.doi.org/10.20517/microstructures.2023.41

[27]
under minimal strain ; however, the angular anisotropy of the pseudo-cubic template would likely give rise
to preferential twinning. The experimental observations of 45 orientated stripe domains, rather than
expected in-plane uniaxial anisotropy, may be the result of crystal twinning in B6TFMO created by
mimicking the pseudo-cubic symmetry of the underlying NGO substrate.

CONCLUSIONS
In summary, we demonstrate the use of AFM-based nano-machining using a commercially available
diamond probe to remove surface contaminants from ultrathin B6TFMO films with nanometer-level
precision. Complete Aurivillius layers are uncovered beneath the as-grown surface, with subsequent lateral
PFM imaging revealing distinct stripe domain configurations along the a-b plane (at 45° to the NGO
substrate b-axis), of stark contrast to the randomly distributed piezoresponse observed for the pristine film
surface. We attribute the differences in piezo-configuration between the pristine surface and exposed sub-
surfaces to the effectiveness of AFM-based nano-machining in removing growth surface artifacts (fractional
Aurivillius phase 2D islands/pits and secondary phase contaminants) that, otherwise, mask the domain
configurations of the underlying planar B6TFMO film. The experimental observations of 45° orientated
stripe domains rather than in-plane uniaxial anisotropy are likely ensuing from the presence of crystal
twinning in B6TFMO to conform to the pseudo-cubic symmetry of the underlying NGO substrate. The
width of the 45° stripe domains is narrower for the 5.6 nm B6TFMO films (0.08 µm) compared to the 7.9
nm B6TFMO films (0.14 µm), consistent with the Landau-Lifshitz-Kittel scaling law [48-50] . Moreover, while
previous PFM investigations of multiferroic m = 5 B6TFMO films demonstrated the persistence of stable
ferroelectricity close to the unit cell level (5 nm to 8 nm) [27,28] , the AFM-based nano-machining
investigations in this work indicate that ferroelectricity persists at thicknesses lower than this. Stable sub-
surface ferroelectric domain structures and piezoresponses persist along the in-plane directions throughout
the film depth, down to less than half of an Aurivillius phase unit cell (< 2.5 nm). These findings, along with
[29]
[26]
evidence for sub-unit cell ferroelectricity in exfoliated flakes and planar thin films of m = 4 Aurivillius
phases, demonstrate the technological potential of Aurivillius phase B6TFMO for future miniaturized
memory storage devices. For example, devices based on in-plane tunnel junctions are an appealing prospect,
as they would not be expected to be obstructed by opposing depolarization fields upon thickness-scaling to
ultrathin dimensions. This means that higher tunnelling current ratios could be achieved with non-
destructive read processes. To date, no such in-plane devices have been commercialized; only theoretical
device models based on monochalcogenides have been reported. However, it is difficult to synthesize high-
quality ultra-thin monochalcogenides by scalable growth methods [55-58] . As such, optimized multiferroic
B6TFMO Aurivillius phases, which can be synthesized by scalable DLI-CVD growth methods, are perfect
candidates for utilization in next-generation devices based on ultrathin multiferroic tunnel junctions.

DECLARATIONS
Acknowledgments
The authors gratefully acknowledge the support of the Royal Society-Science Foundation Ireland (SFI)
University Research Fellowship URF\R\201008 and the SFI Frontiers for the Future Project 19/FFP/6475.

Authors’ contributions
Conceiving and coordinating the project, developing the DLI-CVD processes, synthesizing the ultra-thin
B6TFMO thin films, and performing AFM and PFM characterization and AFM-based nanomachining
experiments; interpreting the results in discussion with the other authors; initially writing of the first draft of
the manuscript, with the final version being crafted through contributions from all authors: Keeney L
Making substantial contributions to the conception and design of the AFM-based nanomachining studies:
Colfer L