Researchers at KAIST, in collaboration with multiple institutions, have experimentally confirmed the three-dimensional vortex-shaped polarization distribution inside 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 made 20 years ago and holds potential for developing ultra-high-density memory devices.
A KAIST-led research team has successfully demonstrated the internal three-dimensional polarization distribution in ferroelectric nanoparticles, paving the way for advanced memory devices capable of storing over 10,000 times more data than current technologies.
Materials that remain magnetized independently, without needing an external magnetic field, are known as ferromagnets. Similarly, ferroelectrics can maintain a polarized state on their own, without any external electric field, serving as the electrical equivalent to ferromagnets.
It is well-known that ferromagnets lose their magnetic properties when reduced to nano sizes below a certain threshold. What happens when ferroelectrics are similarly made extremely small in all directions (i.e., into a zero-dimensional structure such as nanoparticles) has been a topic of controversy for a long time.
The research team led by Dr. Yongsoo Yang from the Department of Physics at KAIST has, for the first time, experimentally clarified the three-dimensional, vortex-shaped polarization distribution inside ferroelectric nanoparticles through international collaborative research with POSTECH, SNU, KBSI, LBNL and University of Arkansas.
About 20 years ago, Prof. Laurent Bellaiche (currently at University of Arkansas) and his colleagues theoretically predicted that a unique form of polarization distribution, arranged in a toroidal vortex shape, could occur inside 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 over 10,000 times greater than existing ones. However, experimental clarification had not been achieved due to the difficulty of measuring the three-dimensional polarization distribution within ferroelectric nanostructures.
Advanced Techniques in Electron Tomography
The research team at KAIST successfully solved this 20-year-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 with the CT scans used in hospitals to view internal organs in three dimensions; the KAIST team adapted it uniquely for nanomaterials, utilizing an electron microscope at the single-atom level.
Three-dimensional polarization distribution of BaTiO3 nanoparticles revealed by atomic electron tomography. (Left) Schematic of the electron tomography technique, which involves acquiring transmission electron microscope images at multiple tilt angles and reconstructing them into 3D atomic structures. (Center) Experimentally determined three-dimensional polarization distribution inside a BaTiO3 nanoparticle via atomic electron tomography. A vortex-like structure is clearly visible near 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 distinct vortex structure can be observed.
Using atomic electron tomography, the team completely measured the positions of cation atoms inside barium titanate (BaTiO3) nanoparticles, a well-known ferroelectric material, in three dimensions. From 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 revealed, for the first time experimentally, that topological polarization orderings including vortices, anti-vortices, skyrmions, and a Bloch point occur inside the 0-dimensional ferroelectrics, as theoretically predicted 20 years ago. Furthermore, it was also found that the number of internal vortices can be controlled depending on their sizes.
Prof. Sergey Prosandeev and Prof. Bellaiche (who proposed with other co-workers the polar vortex ordering theoretically 20 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 utilized into next-generation high-density memory devices that can store more than 10,000 times the amount of information in the same-sized device compared to existing ones.
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 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, 8 May 2024, Nature Communications.
DOI: 10.1038/s41467-024-48082-x
The study was mainly supported by the National Research Foundation of Korea (NRF) Grants funded by the Korean Government (MSIT).