Computer memory without memory loss
The operational core of mobile phones, computers, wearable devices, Internet of Things, and smart cities lies in the "computer memory" responsible for data computation and storage. However, the commonly used DRAM (Dynamic Random Access Memory) at present is not only power consuming, but also its volume is difficult to reduce. Prof. Chih-Huang Lai of the Department of Materials Science and Engineering, National Tsing Hua University, Taiwan, and Prof. Hsiu-Hau Lin of the Department of Physics, National Tsing Hua University, Taiwan, successfully applied electron spin current to achieve revolutionary breakthroughs in the new generation of MRAM (Magnetoresistive Random Access Memory), leading to the creation of "Memory without Memory Loss" featuring faster reading and writing, power saving, and without loss of information when the power is off. MRAM is a promising star in the realm of computer memory.
This R&D outcome reputed as the "dream memory" has been published in the top international journal Natural Materials recently. It is expected to greatly enhance the competitiveness of the computer memory industry.
The commonly used DRAM at present uses electrical charge to store data. A charged memory unit represents the state "1"; an uncharged memory unit represents the state "0". The computer memory needs to be constantly charged so as not to lose the original 1 and 0 information. The size of DRAM in the past was reduced in accordance with Moore's Law, reduced by half every 18 months. However, as the device became smaller and smaller, Moore's Law gradually became invalid. It also became harder to reduce the volume further. Prof. Chih-Huang Lai believes that as Moore's Law comes to an end, it is the advent of the age of spintronics.
Prof. Hsiu-Hau Lin explained that, in addition to charge, electrons also have another magical feature: spin. The extremely small magnetic moments come from self-rotation of electrons. Just as millions of tiny magnets are formed on a chip, the 0 or 1 states are determined by the magnets' north pole directed upwards or downwards. Therefore, power supply is not required when not computing. Even if the power is cut off while computing, data loss will not occur. Future mobile phones, tablet PCs, etc., that use magnetic memory will at least double the standby time.
Magnetic memory is structured like a sandwich. The upper layer is a magnet that can be flipped freely to quickly process data; the bottom layer is like a magnet fixated by a clamp, which is responsible for storing data; the middle layer is an oxide layer. When the magnetic moments of the two magnets have the same direction, spin electrons can easily pass, forming low resistance expressed as "1"; when the magnetic moments are in opposite directions, high resistance is formed, which is expressed as "0".
However, these small magnets are not stable at room temperature due to thermal energy. Scientists have come up with a way to solve this. "Stick" a layer of an anti-ferromagnetic layer to lock the direction of the magnets' magnetic moments, just as tightly fastening a clamp to fixate it. The troublesome part is the clamp can only be released and fastened by repeatedly increasing and decreasing the temperature. "These small magnets are restless and indeterminate little gizmos. Once clamped, they will not turn randomly, but they cannot be flexibly flipped to achieve computing", described by Prof. Hisu-Hau Lin.
Prof. Chih-Huang Lai studied materials and Prof. Hsiu-Hau Lin, who specialized in theoretical physics formed a cross-domain team. Through repeated experimentation, major breakthroughs were finally achieved. A method for flexibly controlling magnetic clamps was found. It involves adding a thin layer of platinum only a few nanometers thick to the magnetic structure. Through spin-orbit interaction, the spin current is generated to quickly and accurately flip fixated magnets.
"Just as a bowling lane, by applying a little friction, the direction of a spinning ball can be easily changed, which in turn knocks pins down.", said by Prof. Chih-Huang Lai. This is not only a breakthrough in magnetic memory, but also creates a new vision for spintronics development.
Samsung, Intel, Taiwan Semiconductor Manufacturing Company (TSMC), and other international manufacturers have engaged in MRAM development and research in recent years. According to the industry's forecast, high-density MRAM will enter mass production this year. The National Tsing Hua University team used electron spin to operate the magnetic flip of magnetic-antiferromagnetic nanofilms. It is expected to become the new generation of MRAM core technology.
The research team is currently applying this breakthrough finding to other nanofilms and nanostructures, and more influential results have been subsequently found. In addition to the academic contributions, the results exert a determining influence on the development of the computer memory industry. With the A.I. and IoT devices and more data collection and sensing needs, the magnetic memory market will achieve rapid growth. Prof. Chih-Huang Lai pointed out that whether the technological development of spin and magnetic memory can be put to good uses is bound to affect the competitiveness of the semiconductor industry in the post-Moore era.
More information:
Po-Hung Lin et al. Manipulating exchange bias by spin–orbit torque, Nature Materials (2019). DOI: 10.1038/s41563-019-0289-4
Provided by National Tsing Hua University
