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KAIST to Develop Technology to Control Topological Defects
(Professor Chan-Ho Yang and PhD candidate Kwang-Eun Kim) Professor Chan-Ho Yang and his team from the Department of Physics developed technology to create and remove topological defects in ferroelectric nanostructures. This technology will contribute to developing topological defect-based storage that will allow the saving of massive amounts of information in a stable manner. Topology refers to the property of matter upon deformation, in which a circle and a triangle are considered to be the same topologically. During the announcement of the 2016 Nobel Prize in Physics, the concept of topology was explained with a bagel with a hole, cinnamon bread without a hole, and a glass cup. Although the cinnamon bread and the glass cup have different appearances, they are topologically the same since neither has a hole. In the same sense, the bagel and the cinnamon bread are topologically different. In other words, topology of matter is conserved and its properties cannot be altered by continuous deformation. Using this topological texture can produce information storage devices that can protect the stored information from external stimuli, but the data can still be written and erased, resulting in ideal non-volatile memory. Unlike ferroelectrics, magnetic topological defect structures such as the ferromagnetic vortex and skyrmion have already been implemented. Ferroelectrics, which have aligned electric dipoles without external electric fields, can stabilize topological defect structures to a smaller size using less energy; however, further research on ferroelectrics has not been carried out sufficiently. This is due to a lack of research on stabilizing topological defect structures and how to control them in an experimental setting. To overcome this problem, the team applied inhomogeneous deformations to ferroelectric nanostructures to successfully stabilize the topological defect structures. The team manufactured a ferroelectric nanoplate structure on a special board, which can exert strong compression from the bottom surface while the sides and the upper surfaces of the structure is free from deformation. This structure led to radial compressive strain relaxation, in which deformations of the lattice stabilize the vortex structure of ferroelectrics. This could lead to the establishment of the core principle of topological ferroelectric memory of high density, high efficiency, and high stability. Professor Yang said, “Ferroelectrics are nonconductor but topological ferroelectric quasiparticles could carry electrical conductivity locally. This finding could be expanded to new quantum device research.” This research, led by the PhD candidate Kwang-Eun Kim, was published in Nature Communications on January 26. The study was co-conducted by Professor Si-Young Choi and Dr. Tae Yeong Koo from POSTECH, Professor Long-Qing Chen from The Pennsylvania State University, and Professor Ramamoorthy Ramesh from the University of California at Berkeley. Figure 1. Five different topological structures produced by controlling the number of topological defects
Technology to Allow Non-Magnetic Materials to Have Magnetic Properties by Professor Chan-Ho Yang
Professor Chan-Ho Yang and his research team from the Department of Physics at KAIST have developed a technology that allows non-magnetic materials to have magnetic properties or, in reverse, to remove magnetic properties from a magnet using an electric field. Based on this research, it is expected that if magnetic-material-based data storage is developed, applications for high-speed massive data transfer will be possible. The results of this research, with Ph.D. candidate Byung-Kwon Jang as the first author, were published online in Nature Physics on October 3. Very small magnets exist inside of any materials. If the direction of the minuscule magnets is dis-aligned, pointing multiple directions, it is non-magnetic. If the direction is aligned in a certain direction, the material holds magnetic property just like any magnet we normally see. Data storage capacity technology has rapidly advanced to the point where we can easily get a portable hard disk drive (HDD) with terabyte-level storage; however, the increase in storage is inevitably followed by slower data access speed for a storage device. Although HDDs are currently the most widely used data storage devices, their technical applications are limited due to their slow data access speed. Other methods such as solid-state drives (SSDs), floating gates, and resistive switching have been developed as alternatives. Yet, they leave tracks every time data is written, and this can cause fatigue cumulative damage. There have been many attempts to compose cells—the smallest data storage space on a storage device—with magnetic materials as that would enable faster data access speeds and remove fatigue cumulative damage. Generally, the techniques tried by researchers were to use induced magnetic fields through current flow. However, magnetic fields are very difficult to shield and can affect a large area. As a result, they alternate the magnetic property of adjacent cells. Because each cell cannot be adjusted one by one, it cannot also be arranged in a certain direction, and therefore, it is hard to change the magnetic state. Professor Yang and his team adjusted the magnetic state by using magnetoelectric interaction to deal with this issue. Instead of using magnetic fields, magnetoelectric interaction is a method that uses an electric field to adjust the magnetic state. It has the advantage of smaller energy consumption as well. Professor Yang's team demonstrated that cells facing random directions can be arranged in a certain direction by only inducing an electric field. In addition, the reverse was also proved to be feasible. Until this research, most cases of previous findings were only feasible at extremely low temperatures or high temperatures, but the technology developed by the research team is practicable at room temperature by manipulating chemical pressure. It allows for a reversible magnetic state, and moreover, is non-volatile. Therefore, the results of this research are expected to provide the basis for developing next-generation information storage device. Professor Yang said, “The changes in the electric magnetic state will be accompanied by entropy changes” and added, “Our research is expected to open new potential for future applications not only for magnetoelectric devices, but also for thermoelectric effect.” This research has been worked on jointly with Dr. Si-Yong Choi from the Korea Institute of Materials Science, Prof. Yoon-Hee Jeong from the Pohang University of Science and Technology, Dr. Tae-Yeong Koo from the Pohang Accelerator Laboratory, Dr. Kyung-Tae Ko from the Max Planck Institute for Chemical Physics of Solids, Dr. Jun-Sik Lee and Dr. Hendrik Ohldag from the SLAC National Accelerator Laboratory of the United States, and Prof. Jan Seidel from the University of New South Wales of Australia. The research was supported by the Mid-Career Researcher Program of the National Research Foundation of Korea, Global Research Network Support Project, Leading Research Center Support Project (Condensed Quantum Coherence Research Center), Global Frontier Project (Hybrid Interface Materials Research Group), and others. Picture: The concept graphic for the electric-field-induced magnetic phase switching the magnetic direction
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