Researchers from KAIST, in collaboration with multiple institutions, have experimentally confirmed the three-dimensional vortex-shaped polarization distribution in ferroelectric nanoparticles. Using atomic electron tomography, they mapped atomic positions in barium titanate nanoparticles and calculated the internal polarization distribution. This finding confirms theoretical predictions from two decades ago and holds potential for the development of ultra-high-density memory devices.
a KAISTThe team-led research team successfully demonstrated the internal three-dimensional polarization distribution in ferroelectric nanoparticles, paving the way for advanced memory devices that can store more than 10,000 times more data than current technologies.
Materials that remain independently magnetized, without the need for an external magnetic field, are known as ferromagnets. Similarly, ferroelectric materials can maintain a polarized state on their own, without any external electric field, serving as the electrical equivalent of ferromagnets.
Ferromagnets are known to lose their magnetic properties when reduced to nanosizes below a certain threshold. What happens when ferroelectric materials are made extremely small in all directions in the same way (i.e. in a zero-dimensional structure such as nanoparticles) has long been a subject of controversy.
The research team led by Dr. Yongsoo Yang of KAIST’s Department of Physics has experimentally elucidated the three-dimensional, vortex-shaped polarization distribution in ferroelectric nanoparticles for the first time through international joint research with POSTECH, SNU, KBSI, LBNL and the University of Arkansas.
About twenty years ago, Prof. Laurent Bellaiche (currently at the University of Arkansas) and his colleagues theoretically predicted that a unique form of polarization distribution, arranged in a toroidal vortex shape, could occur in ferroelectric nanodots. They also suggested that if this vortex distribution could be properly controlled, it could be applied to ultra-high-density memory devices with capacities more than 10,000 times larger than existing ones. However, experimental elucidation had not been achieved due to the difficulty in measuring the three-dimensional polarization distribution within ferroelectric nanostructures.
Advanced techniques in electron tomography
The KAIST research team has successfully solved this two-decade-old challenge by implementing a technique called atomic electron tomography. This technique works by acquiring atomic resolution transmission electron microscope images of the nanomaterials from multiple tilt angles, and then reconstructing them back into three-dimensional structures using advanced reconstruction algorithms. Electron tomography can be understood as essentially the same method as the CT scans used in hospitals to view internal organs in three dimensions; the KAIST team uniquely adapted it for nanomaterials, using an electron microscope on the single-atom level.
Three-dimensional polarization distribution of BaTiO3 nanoparticles revealed by atomic electron tomography. (Left) Schematic representation of the electron tomography technique, which acquires transmission electron microscope images at multiple tilt angles and reconstructs them into 3D atomic structures. (Middle) Experimentally determined three-dimensional polarization distribution within a BaTiO3 nanoparticle via atomic electron tomography. A vortex-like structure is clearly visible at the bottom (blue dot). (Right) A two-dimensional cross-section of the polarization distribution, thinly sliced at the center of the vortex, with the color and arrows together indicating the direction of the polarization. A clear vortex structure can be observed.
Using atomic electron tomography, the team measured the positions of cation atoms in barium titanate (BaTiO3) nanoparticles, a well-known ferroelectric material, completely in three dimensions. Based on the precisely determined 3D atomic arrangements, they were able to further calculate the internal three-dimensional polarization distribution at the single-atom level. The analysis of the polarization distribution experimentally revealed for the first time that topological polarization orders, including vortices, anti-vortices, skyrmions and a Bloch point, exist within the 0-dimensional ferroelectric elements, as theoretically predicted twenty years ago. Moreover, it was also found that the number of internal vertebrae can be controlled depending on their size.
Prof. Sergey Prosandeev and Prof. Bellaiche (who theoretically proposed the polar vortex ordering together with other colleagues twenty years ago), joined this collaboration and further proved that the vortex distribution results obtained from experiments are consistent with theoretical calculations.
By controlling the number and orientation of these polarization distributions, it is expected that this can be used in next-generation high-density memory devices, which can store more than 10,000 times the amount of information in devices of the same size compared to existing devices.
Dr. Yang, who led the research, explained the significance of the results: “This result suggests that just controlling the size and shape of ferroelectric materials, without having to tune the substrate or surrounding environmental effects such as epitaxial stress, ferro- can manipulate electrical vortices or other topological orders at the nanoscale. Further research could then be applied to the development of next-generation ultra-high-density memory.”
Reference: “Revealing the Three-Dimensional Arrangement of Polar Topology in Nanoparticles” by Chaehwa Jeong, Juhyeok Lee, Hyesung Jo, Jaewhan Oh, Hionsuck Baik, Kyoung-June Go, Junwoo Son, Si-Young Choi, Sergey Prosandeev, Laurent Bellaiche and Yongsoo Yang, May 8, 2024, Nature communication.
DOI: 10.1038/s41467-024-48082-x
The research was mainly supported by grants from the National Research Foundation of Korea (NRF), funded by the Korean Government (MSIT).