Researchers at KAIST, in collaboration with multiple institutions, have experimentally confirmed the three-dimensional distribution of vortex-shaped polarization inside ferroelectric nanoparticles. Using electron atomic tomography, they mapped the atomic positions in the barium titanate nanoparticles and calculated the internal polarization distribution. This discovery confirms theoretical predictions made 20 years ago and has potential for the development of ultra-high-density memory devices.
or KAISTThe research-led team has successfully demonstrated intrinsic 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 independently magnetized, without the need for an external magnetic field, are known as ferromagnets. Similarly, ferroelectrics can maintain a self-polarized state without any external electric field, serving as the electrical equivalent of ferromagnets.
It is known that ferromagnets lose their magnetic properties when reduced to nano size below a certain threshold. What happens when ferroelectrics are made similarly extremely small in all directions (ie in a zero-dimensional structure like nanoparticles) has long been a controversial topic.
The research team led by Dr. Yongsoo Yang from the Department of Physics at KAIST has experimentally elucidated, for the first time, the three-dimensional vortex-shaped polarization distribution inside ferroelectric nanoparticles through international collaborative research with POSTECH, SNU, KBSI, LBNL. , and the University of Arkansas.
About 20 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 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 larger 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 challenge by applying a technique called atomic electron tomography. This technique works by taking atomic-resolution transmission electron microscope images of nanomaterials from multiple tilt angles, and then reconstructing them into three-dimensional structures using advanced reconstruction algorithms. Electron tomography can be understood as essentially the same method as CT scans used in hospitals to view internal organs in three dimensions; the KAIST team adapted it uniquely for nanomaterials, using an electron microscope in oneatom level.
Three-dimensional polarization distribution of BaTiO3 nanoparticles revealed by electron atomic tomography. (Left) Schematic of the electron tomography technique, which involves taking 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 electron atomic 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 through the center of the vortex, with color and arrows together indicating the direction of polarization. A special structure of vortices can be observed.
Using atomic electron tomography, the team fully measured the positions of cation atoms inside nanoparticles of barium titanate (BaTiO3), a known ferroelectric material, in three dimensions. From precisely defined 3D atomic arrangements, they were able to further calculate the internal three-dimensional polarization distribution at the single-atom level. Analysis of the polarization distribution revealed, for the first time experimentally, that topological polarization orders including vortices, anti-vortices, skyrmions and a Bloch point occur within 0-dimensional ferroelectrics, as predicted theoretically 20 years ago. Moreover, it was also found that the number of internal vortices can be controlled depending on their sizes.
Sergey Prosandeev and Prof.
By controlling the number and orientation of these polarization distributions, it is expected to be used in next-generation high-density memory devices that can store more than 10,000 times the amount of information on the same size 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 having to adjust the substrate or surrounding environmental effects, such as epitaxial strain, can manipulate ferroelectric vortices or other ordering topological nanoscale. Further research can then be applied to the development of next-generation ultra-high-density memory.”
Reference: “Discovering 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 Bellai, and Yongsoo Yang , May 8, 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).