Researchers from Osaka University, in collaboration with the National Institute of Advanced Industrial Science and Technology (AIST) and Grenoble Alpes University, succeeded in efficiently controlling the direction of a nano-sized magnet by heating at high speed. The researchers also discovered that nano-magnets amplify microwave signals. This group's achievements will contribute to reducing power consumption of magnetoresistive random access memory (MRAM) and artificial intelligence (AI) devices. This will make AI devices read and write to their memory more efficiently, thereby suppressing the power consumption of AI functions such as machine learning and decision making. This is another step towards achieving a super-smart society.
Reducing the power consumption of information and communication devices means they could continue running for a long time, even in times of disaster. Spintronics is a widely-researched field in which MRAM technology has been developed using magnetic tunnel junctions (MTJs). MRAM uses the direction of a magnetic pole to store information, so it can retain memory without standby power. Using these technologies, researchers have tried to reduce the energy use of AI devices.
By controlling the magnetic alignment of an MTJ by using a small current and voltage, it is possible to reduce device power consumption. Spin-transfer-torque MRAM (STT-MRAM) has been developed; however, the problem is that its voltage increases rapidly when its write speed is high, causing a great deal of power to be used.
The research group has discovered that it is possible to write information using less energy than STT-MRAM by changing the magnetic anisotropy in an MTJ via controlling the applied voltage. To make this method practical, it is necessary to increase the magnitude of voltage-controlled magnetic anisotropy. In addition to finding proper materials, other methods for changing magnetic anisotropy have been sought.
The researchers succeeded in inducing a giant magnetic anisotropy change in an MTJ with double-insulator layers by Joule heating. As temperature in the metal (free) layer of an MTJ increases, magnetic anisotropy changes, so it becomes possible to change the direction of a magnetic pole. They found that the magnetic anisotropy depended on the bias voltage due to Joule heating. This shows that the temperature increase induced by Joule heating changed magnetic anisotropy. When we evaluated the maximum value of the magnetic anisotropy change for a given electric field, the size of the heat effect was 300 fJ/Vm, which was almost the same as the reported maximum value of the fast voltage control of the magnetic anisotropy (VCMA) using pure electronic effect. Although the current is much larger in heat effect, compared with VCMA, it is more efficient than STT for high speed application. In addition this value will increase by improving the heat system in an MTJ.
The research group also found that a microwave was amplified by an MTJ using the giant magnetic anisotropy change. Microwave amplification had been previously attempted using a microwave-frequency magnetic field; however, the microwave power obtained by conventional methods was 0.005, and there was no amplification. The group achieved a microwave power reflectivity of 1.6 with a magnetic field of 50 mT and a microwave frequency of 0.4 GHz; that is, the microwave was amplified by approximately 60% compared with the input microwave.
First author Minori Goto says, "Our study is the first report of microwave amplification using spintronics devices. This research will open the way to developing high-performance microwave devices. Moving forward, we anticipate that our technology will be applied to new microwave devices with high sensitivity and high output. This will also contribute to low-power-consumption technology for MRAM and AI hardware."
[Full bibliographic information]
Article: Microwave amplification in a magnetic tunnel junction induced by heat-to-spin conversion at the nanoscale
Journal: Nature Nanotechnology
Authors: Minori Goto1, Yosuke Wakatake1, Ugwumsinachi Kalu Oji1, Shinji Miwa1, Nikita Strelkov3, Bernard Dieny3, Hitoshi Kubota2, Kay Yakushiji2, Akio Fukushima2, Shinji Yuasa2, and Yoshishige Suzuki1
Affiliations: 1 Osaka Univerisity, Japan, 2 National Institute for Advanced Science and Technolgy, Japan, 3 Grenoble Alpes University, France
[About Osaka University]
Osaka University was founded in 1931 as one of the seven imperial universities of Japan and now has expanded to one of Japan's leading comprehensive universities. The University has now embarked on open research revolution from a position as Japan's most innovative university and among the most innovative institutions in the world according to Reuters 2015 Top 100 Innovative Universities and the Nature Index Innovation 2017. The university's ability to innovate from the stage of fundamental research through the creation of useful technology with economic impact stems from its broad disciplinary spectrum.