Weyl fermions point the way to advanced computer memory

A team of Japanese researchers has successfully demonstrated a method for switching a novel material between two different non-volatile states at very high speeds and with great accuracy. The physical constituents of the material in question are also highly robust against external influences such as magnetic fields. As the researchers report in a paper in Nature, this should allow the material to be used to create a high-speed and high-capacity memory device, which would also be extremely energy efficient.

In 1929, theoretical physicist Hermann Weyl was exploring the newly derived Dirac equation, which describes many things in particle physics and led to the discovery of antimatter. He noticed that the equation implied the existence of a massless particle that became known as the Weyl fermion, which was once believed to be the elementary particle called the neutrino.

Almost a century later, in 2015, the Weyl fermion was finally discovered in reality, and in the years since physicists have not only begun to understand more about it but also to find potential uses for it. A team including researchers from the laboratory of Satoru Nakatsuji at the Institute for Solid State Physics and the Department of Physics at the University of Tokyo has now found a way to use Weyl fermions to make advanced memory devices.

"Spintronics is a word likely to excite those interested in the future of technology. Broadly, it is something that could supersede and replace many electronic functions in present-day devices," explained Tomoya Higo, a research associate at the University of Tokyo. "For a while now, ferromagnetic materials – magnets that behave in a familiar way – have been used to explore spintronic phenomena. But there is a better class of magnetic materials for this purpose called antiferromagnetic materials, which seem harder to work with but have many advantages."

Antiferromagnets are interesting materials because they offer researchers many of the same useful properties as ferromagnetic materials, but they are less subject to external magnetic fields, due to a unique arrangement of their constituent parts. This is a benefit when working towards memory devices, as accuracy and robustness are important, but this special arrangement also makes it harder to manipulate the material as needed.

"It was not at all obvious whether you can control an antiferromagnetic state with a simple electrical pulse as you can a ferromagnetic one," said Nakatsuji. This is where the aforementioned Weyl fermions come in.

"In our sample (antiferromagnetic manganese-tin alloy Mn₃Sn), Weyl fermions exist at Weyl points in momentum space (not a physical space but a mathematical way of representing momentums of particles in a system). These Weyl points have two possible states which could represent binary digits," explained Hanshen Tsai, a postdoctoral research fellow at the University of Tokyo. "Our breakthrough finding is that we can switch a Weyl point between these states with an external electrical current applied to neighboring thin layers of Mn₃Sn and either platinum or tungsten. This method is called spin-orbit torque switching."

"Our discovery indicates the massless Weyl fermion pursued by physicists has been found in our magnet, and moreover can be electrically manipulated," added Nakatsuji.

The very large signal produced by Weyl fermions in Mn₃Sn makes it possible to detect spin-orbit torque switching. The switching rate, which corresponds to how fast memory based on such technology could be written to or read from, is in the region of trillions of times a second, or terahertz. Current high-end computer memory switches a few billion times a second, or gigahertz. So, when realized, this new form of memory could lead to quite a jump in performance. But there is still a way to go.

"There were two big challenges in our study. One was optimizing the synthesis of Mn₃Sn thin films. The other was figuring out the switching mechanism," said Higo. "We are excited not only because we found some interesting phenomena, but because we can expect our findings may have important applications in the future. By creating new materials, we discover new phenomena which can lead to new devices. Our research is full of dreams."

This story is adapted from material from the University of Tokyo, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.