Theoretically, some 20 years ago, Prof. Laurent Bellaiche (now at the University of Arkansas) and his associates anticipated that inside ferroelectric nanodots, a special type of polarization distribution could be structured in the shape of a toroidal vortex.
They also proposed using this vortex distribution to create ultra-high-density memory devices with capacities more than 10,000 times larger than current ones, provided that it could be managed appropriately. However, the challenge of quantifying the three-dimensional polarization distribution inside ferroelectric nanostructures prevented experimental clarification.
For the first time, the three-dimensional, vortex-shaped polarization distribution inside ferroelectric nanoparticles has been experimentally clarified by the research team headed by Dr. Yongsoo Yang from the Department of Physics at KAIST.
This 20-year problem was successfully resolved by the KAIST research team by using an approach known as atomic electron tomography. This method involves taking pictures of the nanomaterials from various tilt angles using an atomic-resolution transmission electron microscope and then utilizing sophisticated reconstruction techniques to put the images back into three-dimensional structures.
Electron tomography is basically the same process as CT scans, which are used in hospitals to inspect internal organs in three dimensions. The KAIST researchers modified this process specifically for nanomaterials by using an electron microscope to examine individual atoms.
The researchers used atomic electron tomography to precisely determine the cation atom locations inside ferroelectric barium titanate (BaTiO3) nanoparticles in three dimensions. Based on the accurately measured 3D atomic configurations, they were also able to compute the internal three-dimensional polarization distribution at the single-atom level.
As theoretically anticipated twenty years ago, the polarization distribution analysis demonstrated for the first time experimentally that topological polarization orderings, including skyrmions, anti-skyrmions, vortices, and a Bloch point, occur inside the 0-dimensional ferroelectrics. In addition, the quantity of internal vortices was discovered to be controllable based on their respective diameters.
In this collaboration, Professors Sergey Prosandeev and Bellaiche (who, together with other colleagues, established the polar vortex ordering theoretically 20 years ago) participated and further demonstrated that the experimental results of the vortex distribution match theoretical calculations.
It is anticipated that by manipulating the quantity and direction of these polarization distributions, next-generation high-density memory devices will be able to store almost 10,000 times as much data in a unit of the same size as those currently on the market.
Dr. Yang, who led the research, explained the significance of the results: “This result suggests that controlling the size and shape of ferroelectrics alone, without needing to tune the substrate or surrounding environmental effects such as epitaxial strain, can manipulate ferroelectric vortices or other topological orderings at the nano-scale. Further research could then be applied to developing next-generation ultra-high-density memory.”
Journal Reference:
- Jeong, C., Lee, J., Jo, H. et al. Revealing the three-dimensional arrangement of polar topology in nanoparticles. Nat Commun 15, 3887 (2024). DOI: 10.1038/s41467-024-48082-x