Various exotic phenomena have emerged due to topological defects (TDs) in diverse fields of materials and biological sciences. Intriguing functionality in materials includes the domain wall’s ability to host dynamic electrons, making the domain walls electrically conductive and superconductive.
Ferroelastic domains and their boundaries are fascinating objects for researchers in fundamental and applied sciences. A recent study published in MRS Advances presented ferroelastic twins captured via Bragg coherent diffractive imaging (BCDI) in barium hexaferrite BaFe12O19 (BHF) individual nanocrystal.
Reconstruction of the displacement field and morphology of the BHF (200) plane by BCDI helped recognize ferroelastic domains that were separated by domain boundaries and had homogenous displacement fields. BCDI therefore serves as an efficient imaging platform for studying TDs in three-dimensional (3D) structures.
Ferroelectricity and Ferroelectric Materials
Ferroelectricity defines the magnitude and order of electric dipoles. The phase transitions of ferroelectric materials to dielectric occur above the Curie temperature. Under an external electric field and below the Curie temperature, the crystal domains rearrange, decreasing the internal electric field. The realignment of the crystal domains induces structural anisotropy with various ion movements in the crystal lattice, generating ferroelectricity.
Among the two types of ferroelectric materials: in displacive ferroelectric materials generating ferroelectricity entails mobile atoms in unit cells, resulting in the formation of dipoles in the material.
On the other hand, ferroelectric materials of order-disorder type show both order-disorder and displacive ferroelectricity. Here, the atom’s displacement leads to the breaking and making of bonds along with changes in bond lengths, thereby forming dipoles. As a result, this ferroelectricity type is termed incorrect ferroelectricity.
Non-volatile random-access memories (NRAM), high-performance capacitors, energy materials, piezoelectricity-based quantum computation, and functional engineering materials are a few applications of transition-metal-based perovskite oxides in basic and applied sciences. Here, the researchers focused on TDs in transition-metal-based perovskite oxides.
BHF and barium titanate (BaTiO3, BTO) are two exemplary systems that have been extensively studied owing to their novel electronic and structural features that emerge under the influence of related conjugate fields. While BHF exhibited improper ferroelectricity, BTO exhibited local polarization and the consequent piezo- and ferroelectricity owing to the displacement of barium and titanium ions.
Electron Beam Lithography and BCDI
Although electron beam lithography is a widely utilized technique for the fabrication of high-resolution lenses, the electron backscattering effect, also termed the proximity effect, is its major drawback.
The process of electron beam lithography patterning should ideally affect only the sample regions subjected to the electron beam. Electron backscattering, on the other hand, causes undesirable structural or morphological changes in the sample substrate regions adjoining the selected patterned areas.
In this regard, BCDI is a lens-less X-ray microscopy technique that eliminates the challenge of lenses by using advanced reconstruction algorithms to iteratively invert diffraction patterns with interference patterns from reciprocal to real space.
BCDI provides a phase-contrast, non-invasive, and high-resolution imaging of transition-metal-based perovskite oxides in 3D space to investigate their ferroelectric domain, topological textures, structural heterogeneities, and domain wall.
Probing Ferroelectric Nanocrystals Using BCDI
In the present study, a transition metal-based perovskite oxide, BHF, bearing both types of ferroelectrics was examined in its single-crystal form, utilizing BCDI to probe lattice strain, local structural heterogeneities, and domain morphologies.
Here, synchrotron sources emitted by particle accelerators produced X-ray photons, whose beams were narrowly focused and controlled for spectroscopy and imaging, thereby enabling the observation of small excitations in solid-state materials at the nanoscale level for the first time.
A 3D Bragg diffraction pattern was recorded for the BHF nanocrystals, and the reconstruction results revealed the presence of multiple domains with superficial string-like textures. The generation of ferroelectricity in the BHF crystals was attributed to their room-temperature multiferroicity.
Moreover, twinned structures were observed in single-crystal BHF nanoparticles, as indicated by the reconstruction algorithm’s phase component. In ferroelastic systems, twinning impacts ferroelectric and magnetic ordering and is a signature of geometrical defects.
Employing BCDI in the present study helped identify TDs at the boundaries of elastic domains with surprising phenomena such as conductivity and superconductivity. Within this framework, one of the authors of the present work, Fohtung, highlighted the importance of TDs by stating, “at the nanoscale, features such as dislocations and global TDs are almost like building blocks in the large-scale applications of these materials”.
This research presented X-ray BCDI as a high-resolution probe to examine twinned magnetic ferroelectric nanocrystal and captured the atomic displacement field, morphology, and ferroelastic domains in them.
The interaction between ferroelastic twins and consequent ferroelectric polarization has impacted the functionality and properties of these material systems. Thus, BCDI can be used to probe operando processes in a wide range of hard and soft condensed matter systems.
"In regenerative medicine and biology, TDs can be seen as the building blocks that control collective cell dynamics. The ability to visualize such defects in their native environments is, therefore, a high priority" said Fohtung.
Shi, X., Nazirkar, N. P., Barringer, Z., Williams, S., Harder, R., Fohtung, E. (2022). Topological defects and ferroelastic twins in ferroelectric nanocrystals: What coherent X-rays can reveal about them. MRS Advances. https://doi.org/10.1557/s43580-022-00352-w
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