Physicists discover «three-dimensional vortex» of zero-dimensional ferroelectrics

Researchers at KAIST, in collaboration with several institutions, have experimentally confirmed the three-dimensional distribution of polarization in the form of vortices inside ferroelectric nanoparticles. Using atomic electron tomography, they mapped the positions of atoms in barium titanate nanoparticles and calculated the internal polarization distribution. This discovery confirms theoretical predictions from 20 years ago and has the potential for the development of ultra-high-density memory devices.

AND KAISTA research team under the leadership has successfully demonstrated an intrinsic 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 magnetized independently, without the need for an external magnetic field, are known as ferromagnets. Similarly, ferroelectrics can maintain a polarized state by themselves, without an external electric field, serving as the electrical equivalent of 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 (ie into a zero-dimensional structure such as nanoparticles) has been a topic of controversy for a long time.

A research team led by Dr. Yongsoo Yang from the Department of Physics at KAIST has, for the first time, experimentally elucidated the three-dimensional vortex-shaped polarization distribution inside ferroelectric nanoparticles through international collaborative research with POSTECH, SNU, KBSI, LBNL -om , 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 the form of a toroidal vortex, could appear inside ferroelectric nanodots. They also suggested that if this distribution of vortices 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 has not been achieved due to the difficulty in measuring the three-dimensional polarization distribution within ferroelectric nanostructures.

Advanced techniques in electron tomography

A research team at KAIST successfully solved this 20-year-old challenge by implementing 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 back into three-dimensional structures using advanced reconstruction algorithms. Electron tomography can be thought of 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 on oneatom level.

Three-dimensional polarization distribution of BaTiO3 nanoparticles revealed by atomic electron tomography

Three-dimensional polarization distribution of BaTiO3 nanoparticles revealed by atomic electron tomography. (Left) Schematic of the electron tomography technique, which involves the acquisition of transmission electron microscope images at multiple tilt angles and their reconstruction into 3D atomic structures. (Center) Experimentally determined three-dimensional polarization distribution inside a BaTiO3 nanoparticle by means of atomic electron tomography. At the bottom, a vortex-like structure is clearly visible (blue dot). (Right) Two-dimensional cross-section of the polarization distribution, thinly sliced ​​at the center of the vortex, with color and arrows collectively indicating the direction of polarization. A distinct vortex structure can be observed.

Using atomic electron tomography, the team fully measured the positions of cation atoms inside nanoparticles of barium titanate (BaTiO3), a well-known ferroelectric material, in three dimensions. From the precisely determined 3D arrangements of the atoms, they were able to further calculate the internal three-dimensional polarization distribution at the level of a single atom. Analysis of the polarization distribution revealed, for the first time experimentally, that a topological order of polarization including vortices, anti-vortices, skyrmions and Bloch points appears within 0-dimensional ferroelectrics, as predicted theoretically 20 years ago. Furthermore, it has also been found that the number of internal vortices can be controlled depending on their size.

Prof. Sergey Prosandeev and prof. Bellaiche (who with other collaborators proposed the theoretical order of the polar vortex 20 years ago), joined this collaboration and further proved that the results of the vortex distribution obtained by experiments were in accordance with theoretical calculations.
By controlling the number and orientation of these polarization distributions, it is expected that this can be exploited in next-generation high-density memory devices that can store more than 10,000 times more information in the same size device compared to existing ones.

dr. Yang, who led the research, explained the significance of the results: «This result suggests that by just controlling the size and shape of ferroelectrics, without the need to adjust the substrate or environmental effects such as epitaxial deformation, one can manipulate ferroelectric vortices or other topological arrangements at the nanoscale. Further the research could then be applied to the development of the next generation of ultra-high-density memory.”

Reference: “Discovering the Three-Dimensional Polar Topology Arrangement 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 grants from the National Research Foundation of Korea (NRF) funded by the Korean government (MSIT).

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