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New Material for Generating Energy-Efficient Spin Currents
(Professor Byong-Guk Park (left) and Professor Kab-Jin Kim) Magnetic random access memory (MRAM) is emerging as next-generation memory. It allows information to be kept even without an external power supply and its unique blend of high density and high speed operation is driving global semiconductor manufacturers to develop new versions continuously. A KAIST team, led by Professor Byong-Guk Park in the Department of Materials Science and Engineering and Professor Kab-Jin Kim in the Department of Physics, recently has developed a new material which enables the efficient generation of a spin current, the core part of operating MRAM. This new material consisting of ferromagnet-transition metal bilayers can randomly control the direction of the generated spin current unlike the existing ones. They also described a mechanism for spin-current generation at the interface between the bottom ferromagnetic layer and the non-magnetic spacer layer, which gives torques on the top magnetic layer that are consistent with the measured magnetization dependence. When applying this to spin-orbit torque magnetic memory, it shows the increased efficiency of spin torque and generation of the spin current without an external magnetic field. High-speed operation, the distinct feature of spin-orbit torque-based MRAM that carries its non-volatility, can significantly reduce the standby power better than SRAM. This new material will expect to speed up the commercialization of MRAM. The research team said that this magnetic memory will further be applied to mobile, wearable, and IoT devices. This study, conducted in collaboration with Professor Kyung-Jin Lee from Korea University and Dr. Mark Stiles from the National Institute of Standards and Technology in the US, was featured in Nature Materials in March. The research was funded by the Creative Materials Discovery Program of the Ministry of Science and ICT. (Figure: Ferromagnet-transition metal bilayers which can randomly control the direction of the generated spin current)
A New Spin Current Generating Material Developed
(Professor Park(left) and Ph.D. candidate Kim) Magnetic random-access memory (MRAM) is a non-volatile device made of thin magnetic film that can maintain information without an external power supply, in contrast to conventional silicon-based semiconductor memory. It also has the potential for high-density integration and high-speed operation. The operation of MRAM involves the control of the magnetization direction by exerting spin current-induced torque on a magnetic material. Spin current is generated using electricity in conventional MRAM, but this study developed materials technology that generates spin current using heat. A KAIST research team led by Professor Byong-Guk Park of the Department of Materials Science and Engineering developed a material that generates spin current from heat, which can be utilized for a new operation principle for MRAM. There have been theoretical reports on the spin Nernst effect, the phenomenon of the thermal generation of spin current, but is yet to have been experimentally proven due to technological limitations. However, the research team introduced a spin Nernst magnetoresistance measurement method using tungsten (W) and platinum (Pt) with high spin orbit coupling which allows for the experimental identification of the spin Nernst effect. They also demonstrated that the efficiency of spin current generation from heat is similar to that of spin current generated from electricity. Professor Park said, “This research has great significance in experimentally proving spin current generation from heat, a new physical phenomenon. We aim to develop the technology as a new operational method for MRAM through further research. This can lower power consumption, and is expected to contribute to the advancement of electronics requiring low power requirement such as wearable, mobile, and IOT devices”. This research was conducted as a joint research project with Professor Kyung-Jin Lee at Korea University and Professor Jong-Ryul Jeong at Chungnam National University. It was published in Nature Communications online on November 9 titled “Observation of transverse spin Nernst magnetoresistance induced by thermal spin current in ferromagnet/non-magnet bilayers.” Ph.D. candidate Dong-Jun Kim at KAIST is the first author. This research was funded by the Ministry of Science and ICT. (Schematic diagram of spin Nernst magnetoresistance) (Research result of new spin current generating materials)
High-Speed Motion Core Technology for Magnetic Memory
(Professor Kab-Jin Kim of the Department of Physics) A joint research team led by Professor Kab-Jin Kim of the Department of Physics, KAIST and Professor Kyung-Jin Lee at Korea University developed technology to dramatically enhance the speed of next generation domain wall-based magnetic memory. This research was published online in Nature Materials on September 25. Currently-used memory materials, D-RAM and S-RAM, are fast but volatile, leading to memory loss when the power is switched off. Flash memory is non-volatile but slow, while hard disk drives (HDD) have greater storage but are high in energy usage and weak in physical shock tolerance. To overcome the limitations of existing memory materials, ‘domain wall-based, magnetic memory’ is being researched. The core mechanism of domain wall magnetic memory is the movement of a domain wall by the current. Non-volatility is secured by using magnetic nanowires and the lack of mechanical rotation reduced power usage. This is a new form of high density, low power next-generation memory. However, previous studies showed the speed limit of domain wall memory to be hundreds m/s at maximum due to the ‘Walker breakdown phenomenon’, which refers to velocity breakdown from the angular precession of a domain wall. Therefore, there was a need to develop core technology to remove the Walker breakdown phenomenon and increase the speed for the commercialization of domain wall memory. Most domain wall memory studies used ferromagnetic bodies, which cannot overcome the Walker breakdown phenomenon. The team discovered that the use of ‘ferrimagnetic‘ GdFeCo at certain conditions could overcome the Walker breakdown phenomenon and using this mechanism they could increase domain wall speed to over 2Km/s at room temperature. Domain wall memory is high-density, low-power, and non-volatile memory. The memory could be the leading next-generation memory with the addition of the high speed property discovered in this research. Professor Kim said, “This research is significant in discovering a new physical phenomenon at the point at which the angular momentum of a ferrimagnetic body is 0 and it is expected to advance the implementation of next-generation memory in the future.” This research was funded by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MSIP) (No. 2017R1C1B2009686, NRF-2016R1A5A1008184) and by the DGIST R&D Program of the Ministry of Science, ICT and Future Planning (17-BT-02). (Figure 1. Concept Map of Domain Wall Memory Material using Ferrimagnetic Body) (Figure 2. Scheme and Experimental Results of Domain Wall Speed Measurements)
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