Physicists reveal the ‘3D vortex’ of zero-dimensional ferroelectricity

Researchers at KAIST, in collaboration with multiple institutions, experimentally confirmed the vortex-shaped 3D polarization distribution inside photovoltaic nanoparticles. Using atomic electron tomography, they mapped the atomic positions in barium titanate nanoparticles and calculated the internal polarization distribution. This discovery confirms theoretical predictions made 20 years ago, and holds the potential for developing ultra-dense memory devices.

a kaistThe research team he leads has successfully demonstrated three-dimensional internal polarization distribution in ferroelectric nanoparticles, paving the way for advanced memory devices capable of storing 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. Likewise, ferroelectricity can maintain a state of polarization on its own, without any external electric field, acting as the electrical equivalent of ferromagnets.

It is known that ferromagnets lose their magnetic properties when reduced to nanoscale sizes below a certain threshold. What happens when ferroelectric materials are made identically in a very small volume in all directions (i.e. in a dimensionless structure like nanoparticles) has been a subject of debate for a long time.

The research team led by Dr. Youngsu Yang from the Department of Physics at KAUST has, for the first time, elucidated the 3D vortex-shaped polarization distribution inside ferroelectric nanoparticles through international collaborative research with POSTECH, SNU, KBSI, and LBNL. And the University of Arkansas.

About 20 years ago, Professor Laurent Belich (now 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 occur inside ferroelectric nanodots. They also suggested that if this vortex distribution could be properly controlled, it could be applied to high-density memory devices with capacities 10,000 times larger than existing devices. However, experimental clarification has not been achieved due to the difficulty of measuring the 3D polarization distribution within ferroelectric nanostructures.

Advanced techniques in electron tomography

The research team at KAIST has solved this 20-year-old challenge by implementing a technique called atomic electron tomography. This technology works by acquiring atomic-resolution transmission electron microscope images of nanomaterials from multiple tilt angles, then reconstructing them back into 3D structures using advanced reconstruction algorithms. Electron tomography can be understood as the same method used in CT scans used in hospitals to view internal organs in three dimensions; The KAIST team uniquely adapted it to nanomaterials, using electron microscopy on a single sample.corn 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. (Middle) The 3D polarization distribution was experimentally determined inside a BaTiO3 nanoparticle via atomic electron tomography. A vortex-like structure is clearly visible near the bottom (blue dot). (Right) 2D cross section of the polarization distribution, thinly sliced ​​at the center of the vortex, and together the color and arrows indicate the polarization direction. A distinct vortex structure can be observed.

Using atomic electron tomography, the team measured the positions of the entire cation atoms inside barium titanate (BaTiO3) nanoparticles, a ferroelectric material, in three dimensions. With precisely defined 3D atomic arrangements, they were able to further calculate the 3D internal polarization distribution at the single-atom level. Analysis of the polarization distribution has revealed, for the first time experimentally, that topological polarization arrangements, including vortices, antivortexes, skyrmions, and the Bloch point, occur inside zero-dimensional ferroelectrics, as predicted theoretically 20 years ago. Moreover, it has also been found that the number of internal vortices can be controlled by their sizes.

Professor Sergei Brusandev and Professor Belich (who together with other colleagues proposed the polar vortex arrangement theoretically 20 years ago) joined this collaboration and also demonstrated that the vortex distribution results obtained from experiments agree with theoretical calculations.
By controlling the number and direction of these polarization distributions, it is expected that this could be leveraged in next-generation high-density memory devices that can store more than 10,000 times the amount of information in the device itself compared to existing devices.

Dr Yang, who led the research, explained the significance of the findings: “This result suggests that controlling the size and shape of ferroelectric materials alone, without the need to tune the substrate or surrounding environmental influences such as epitaxial stress, can manipulate ferroelectric vortices or other topological arrangements at the nanoscale.” Further research can then be applied to developing the next generation of ultra-dense memory.

Reference: “Revealing the Three-Dimensional Order of Polar Topology in Nanoparticles” by Chihwa Jeong, Joo Hyuk Lee, Hyesung Jo, Jayohan Oh, Hyunsuk Baek, Kyung Joon Jo, Junwoo Son, Se Young Choi, Sergey Brusandev, Laurent Belich and Youngsoo Yang, May 8, 2024, Nature Communications.
doi: 10.1038/s41467-024-48082-x

This study was mainly supported by National Research Foundation of Korea (NRF) grants funded by the Korean government (MSIT).