電腦、平板、手機在生活中處處可見，其背後運作的核心功能，不外乎資訊的處理與儲存。開發適當的電子元件，既可以同時快速處理資訊，又能夠穩定儲存資訊，吸引學界業界各個領域的專家投入，而其中MRAM是極被看好的後摩爾定律世代的記憶體。其結構有如三明治，上層是自由翻轉的鐵磁層，可快速處理資料，底層則是釘鎖住的鐵磁層，可用作儲存資料，兩層中則有氧化層隔開。當此二鐵磁層的磁化方向相同，是低電阻態，代表「1」 ；此二鐵磁層的磁化方向相反，是高電阻態，代表「0」。有別於目前的主流記憶體(SRAM 與 DRAM)，MRAM兼具處理與儲存資訊的功能，且斷電時資訊不會流失，電源開啟可即時運作，耗能低、讀寫速度快，是多方看好的明日之星。
目前研究團隊將此突破性的發現，應用到其它結構的奈米膜層，陸續發現更多具影響力的結果，除了學術的貢獻外，經由科技部半導體射月計畫的連結，將對於國內記憶體產業發展有決定性的影響力。這項技術在學理上的存取速度接近 SRAM，具快閃記憶體的非揮發性特性，平均能耗遠低於 DRAM，應用於嵌入式記憶體（Embedded Memory）極具潛力，隨著人工智慧、物聯網裝置與更多的資料收集與感測需求，MRAM的市場也將迅速成長。
A Memory that Remembers
The Ministry of Science and Technology and the National Tsing Hua University have long supported Professor Chih-Huang Lai and Professor Hsiu-Hau Lin’s research team to study characteristics, process and control of magnetic random access memory (MRAM). Setting the world’s first record, they succeeded in manipulating the magnetic switch of the ferromagnetic-antiferromagnetic nanolayers by spin current. This influential breakthrough was published in the prestigious journal “Nature Materials” recently.
Computers, tablets, and mobile phones are modern necessities in everyday life. The key functions behind these devices are nothing but information processing and storage. How to process information quickly, while maintaining the stability of stored information simultaneously, is the billion-dollar question attracting experts from all fields. In the post Moore’s law era, MRAM is the most promising candidate among the competing materials.
The structure of MRAM is like a sandwich: the upper layer is a free ferromagnetic layer (quick for information processing), the bottom layer is a pinned ferromagnetic layer (stable for information storage) and an oxide layer in-between to separate them. When the magnetization of the two ferromagnetic layers are parallel, it is in the low-resistance state, representing “1”. When magnetizations in the two layers are opposite, it is in the high-resistance state, representing “0”. MRAM are capable of information processing and storage at the same time. The stored information will not be lost even when the power is suddenly off. When the power is on, it can operate immediately with no booting process required. It consumes less energy with fast read and write speeds. Despite some technical bottlenecks to conquer, MRAM is viewed as the rising star in the future generation.
One of the key technical bottleneck is to manipulate the pinned ferromagnetic layer. How to pin a ferromagnetic layer in a specific direction? Simple yet magical. You only need to “glue” an antiferromagnetic layer on top. This phenomenon was discovered back in 1956 by scientists at General Electric in the United States and is referred as “exchange bias”. Although it has been discovered for over 60 years, the underlying mechanism remains unknown despite of its extremely wide applications. And, it is hard to manipulate the exchange bias. The device must be heated up first, followed by cooling process in the presence of magnetic field to set the direction of the exchange bias. This procedure is incompatible with existing manufacture processes in electronic industry.
Research teams around the world are seeking resolutions to conquer the technical bottleneck. Prof. Lai and Prof. Lin’s group achieve the goal by utilizing spin current. An electron has both charge and spin: when the change flows, it leads to the familiar (charge) current in everyday life. And, by driving the spin to move, the spin current is generated. They inject the spin current through the ferromagnetic-antiferromagnetic film layer and demonstrate how the magnitude and direction of the exchange bias can be manipulated. This breakthrough provides a brand new technical approach, seamlessly integrated with the existing electronic components. It is a milestone in magnetic memory and opens up a new horizon for the development of spintronics.
Using spin current to control the exchange bias has never been achieved before. When their manuscript was submitted to Nature Materials, many questions were raised during review processes. The common suspicion was that the observed results are due to thermal heating, not spin current injection. In the face of difficult questions and challenges, the strength of the interdisciplinary collaboration pays off. Combining the scopes of materials science and physics, the research team found ways to resolve the challenges with creative thinking and effective execution. For example, combining theoretical analysis and experimental investigation, the research team developed a new measurement technique to detect temperature variations within microsecond. This technique measures the temperature of the device at any instant, thereby explicitly ruling out the thermal effect and successfully eliminating all doubts from the reviewer.
And, this is not the end of the story. At present, the research team applies the breakthrough to devices with similar layer structures and discovers exciting phenomena. In addition to academic impacts to fundamental science, this breakthrough may have decisive influence in industrial developments. The Ministry of Science and Technology encourages interdisciplinary collaborations with long-term supports. With supports from the Engineering Division and the Natural Science Division, Professor Lai and Professor Lin form the interdisciplinary team and conquer the long-standing challenges with dazzling research results. The power of collaborations across different fields is self-evident.
Professor Chih-Huang Lai
Department of Materials Science and Engineering, National Tsing Hua University
Professor Hsiu-Hau Lin
Department of Physics, National Tsing Hua University
Dr. Chin-Wei Chen
Program Manager/Assistant Research Fellow,
Department of Natural Sciences and Sustainable Development,
Ministry of Science and Technology