In a ferroelectric material, the electric polarisation (a preferred orientation of charges within the crystal) can normally only be reversed by applying an electric field parallel to this polarisation and directed in the opposite direction. Researchers from the Theoretical Materials Physics Department at the University of Liège, in collaboration with teams from universities in the United States and Spain, have shown that in a layered ferroelectric material, bismuth titanate (Bi₄Ti₃O₁₂), a field applied perpendicular to the main polarisation is sufficient to reverse it.
Ferroelectric materials are used in the manufacture of numerous devices, including non-volatile memory (such as smart cards or access badges), sensors (for shock or pressure) and actuators (fuel injectors). They are also being studied with a view to future low-power computing technologies. In the bismuth titanate examined here, two polarisation components coexist: one, which is significant, lies within the plane of the atomic layers (approximately 50 µC cm⁻²), whilst the other, which is weaker, is oriented out of this plane (in the order of 2 to 5 µC cm⁻²). The researchers have shown that an electric field applied out of plane, acting on the small out-of-plane component, deterministically causes the reversal of the in-plane component, even though the latter is significantly larger.
This behaviour can be explained by a set of couplings between different deformations of the crystal structure. “The theoretical work, which we carried out at ULiège in collaboration with Professor Javier Junquera (University of Cantabria), has enabled us to understand its origin,” explains Fernando Gómez-Ortiz, a physicist and research fellow in the research team of Professor Philippe Ghosez at ULiège. Using first-principles calculations and an analysis based on symmetries, we have identified a network of so-called ‘trilinear’ couplings, which link the two components of polarisation to the rotations and tilts of the oxygen octahedra forming the crystal. These couplings act like a system of gears: turning one wheel forces the others to move. Similarly, reversing one polarisation triggers a cascade of atomic rearrangements that leads to the inversion of the perpendicular polarisation.” On this basis, researchers from Liège and Cantabria proposed a switching pathway compatible with the material’s symmetries.
This proposal was subsequently confirmed by the group led by Ramamoorthy Ramesh (University of California, Berkeley) and his colleagues. Thin films of bismuth titanate were prepared by pulsed laser deposition and then examined using piezoelectric force microscopy, atomically resolved electron ptychography, photoelectron emission microscopy and electron diffraction imaging. These direct, al observations of the switching process validated the theoretical model and indicated that the reversal could be achieved reproducibly.
The study builds on a line of research to which ULiège had previously contributed. In 2008, Eric Bousquet and Philippe Ghosez had helped to demonstrate a so-called ‘improper’ new form of ferroelectricity in artificial stacks of materials, showing that polarisation can arise from the coupling between non-polar deformations rather than from the classical ferroelectric mechanism. “This work exploits these same couplings in a naturally layered material to derive a novel functionality,” the researcher continues. “And this approach also has practical applications. In many miniaturised electronic architectures, the useful polarisation lies in the plane of the material, but its conventional reversal requires a high lateral voltage. An out-of-plane field, on the other hand, can be applied through a film just a few nanometres thick: the field required for switching is then achieved with a much lower voltage.”
Based on these results, the authors propose a deterministic mechanism for switching in-plane polarisation using a perpendicular field, and suggest that comparable structural couplings may exist in other layered or low-symmetry materials. Beyond its fundamental interest, this approach could contribute to the development of ferroelectric memories and logic components that are more energy-efficient and compatible with existing thin-film architectures.
Gupta, S. Puebla, F. Gómez-Ortiz, X. Li, S. Husain, T.-R. Liu, P. Meisenheimer, V. Srikrishna, D. Nikonov, M. Chen, Y. Kumar, A. Omar, K. Das, B. Achinuq, S. Roy, Y.-T. Shao, Y. Han, S. Salahuddin, A. Mathuriya, S. Manipatruni, Ph. Ghosez, J. Junquera and R. Ramesh, ‘Perpendicular switching of polarisation in layered ferroelectrics’, Nature, https://doi.org/10.1038/s41586-026-10839-3
Cyanobacteria found in the waters of the Meuse in Liège were analysed by Professor Annick Wilmotte’s laboratory at ULiège.
Elise Lebreton a obtenu le prestigieux Mason Hale Award remis par l'International Association of Lichenology (IAL) pour l'excellence de sa recherche et les travaux exceptionnels résultant de sa thèse de doctorat.
An international team has identified an evolutionary constraint in mammals that allows them to use the same tooth for two distinct functions.
