Ferroelectric materials - those that carry a spontaneous electric polarisation that can be switched by an electric field - are central to many modern technologies like sensors, memory devices, and energy harvesters. Until now, imaging their intricate domain structures in three dimensions has been extremely challenging. Traditional methods reveal only surface or two dimensional slices, leaving the inner architectures elusive. Researchers from the University of Southampton and collaborators used a technique called multi-peak Bragg coherent X-ray diffraction imaging (BCDI) at the I16 Beamline to tackle this challenge. This groundbreaking approach lets scientists capture 3D maps of tiny ferroelectric domains in nanocrystals. The study, published in Nature Communications, pushes the frontiers of nanoscale imaging and deepens our understanding of how these materials work.
In electromagnetism, a dielectric material is one that will react to an electric field by becoming polarised and have the capacity to store electric charges. These materials are widely used in electronics in capacitors and oscillators. Ferroelectric materials exhibit similar properties, as they always possess an electric polarisation, but this one can be reversed by applying an electric field. Ferroelectric materials allow the production of tuneable capacitors that possess many applications in electronics.
Ferroelectric materials like hexagonal YMnO₃ (yttrium manganite) form fascinating vortex-like patterns within their internal structures. These patterns are not just scientific curiosities - they affect how the materials switch polarisation and could play roles in miniaturised memory or sensor technologies. However, up till now, most imaging has been surface-based or two-dimensional, leaving the rich 3D structure of domains largely unknown. The research focuses on visualising these elusive internal landscapes in a single nanocrystal using multi-peak BCDI.
The study material - YMnO₃ nanocrystals - harbours a Mexican-hat-like energy landscape, leading to six possible domain configurations with opposing polarisations and trimerisation orders. One lingering challenge in the field is accurately mapping how these domains are arranged and how they interact in three dimensions. Capturing this would help scientists understand domain wall behaviour and topological arrangements, which are vital for designing future devices.
The team conducted a multi-peak Bragg coherent X-ray diffraction imaging (BCDI) experiment at the I16 beamline, illuminating a single YMnO₃ nanocrystal with coherent X-rays at 9 keV and capturing diffraction across multiple reflections. They then applied advanced phase-retrieval algorithms (HIO Mask and error-reduction cycles) to reconstruct the 3D displacement field and full strain tensor within the crystal.
Multi peak X-ray Coherent Diffraction Imaging offers unique access to the inner world of micrometric objects with nanometric resolutions. The pioneering study presented here are essential to improve the experimental and the data analysis methodologies leading to application of X-ray BCDI to a variety of quantum and functional materials.
Alessandro Bombardi, Principal Beamline Scientist of the I16 beamline
Crucially, using multi-peak Bragg CDI enabled them to resolve displacements along all three spatial directions. Their measurements revealed two adjacent ferroelectric domains with opposite polarisations (β⁺ and γ⁻), separated by a domain wall, and arranged in an anti-vortex (clockwise) configuration, consistent with a simulation based on the Mexican-hat symmetry model. Without access to the unique capabilities of Diamond’s beamline, particularly multi-peak BCDI driven by I16's unique combination of high coherent flux, the kappa-diffractometer and an in-vacuum Merlin detector, such fine, volumetric domain‐mapping would not have been possible. The results confirm theoretical models and open the door to deeper understanding of ferroelectric switching and domain walls in multiferroics.
To find out more about beamline I16 please contact the Principal Beamline Scientist: Alessandro.bombardi@diamond.ac.uk
Mokhtar, A. H. et al. Three-dimensional domain identification in a single hexagonal manganite nanocrystal. Nat. Commun. 15, 3587 (2024).
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