Computers and smartphones have different types of memory, varying in speed and power efficiency depending on where in the system they are used. Typically, larger computers, especially those in data centers, use a lot of magnetic hard drives, which are less common in consumer systems today. The magnetic technology on which they are based offers very high capacity, but lacks the speed of solid-state system memory. Upcoming devices based on spintronic technology may be able to bridge this gap and radically improve even the theoretical performance of conventional electronic devices.
Professor Satoru Nakatsuji and Project Associate Professor Tomoya Higo from the University of Tokyo’s Department of Physics, together with their team, are exploring the world of spintronics and other related areas of solid-state physics – basically, physics things that work without moving. Over the years they have studied special types of magnetic materials, some of which have very unusual properties. You’re familiar with ferromagnets, because they’re the types that exist in many everyday applications like computer hard drives and electric motors – you’ve probably even stuck some to your fridge. However, the team is more interested in more obscure magnetic materials called antiferromagnets.
“Like ferromagnets, the magnetic properties of antiferromagnets arise from the collective behavior of their constituent particles, in particular their electron spins, something analogous to angular momentum,” Nakatsuji said. “Both materials can be used to encode information by modifying localized groups of constituent particles. However, antiferromagnets have a distinct advantage in the high speed at which these changes to information-storage spin states can be performed, at the cost of increased complexity.
“Some spintronic memory devices already exist. MRAM (magnetoresistive random access memory) has been commercialized and can replace electronic memory in some situations, but it is based on ferromagnetic switching,” Higo said. “After much trial and error, I believe we are the first to report the successful switching of spin states in the Mn3Sn antiferromagnetic material using the same method used for ferromagnets in MRAM, which means we have persuaded the antiferromagnetic substance to act as a simple memory device.
This method of switching is called spin-orbit torque switching (SOT) and is generating excitement in the technology industry. It uses a fraction of the power to change the state of a bit (1 or 0) in memory, and although the researchers’ experiments involved changing their sample of Mn3Sn in as little as milliseconds (thousandths of a second), they are confident that SOT switching could occur on the picosecond (trillionth of a second) scale, which would be orders of magnitude faster than the switching speed of today’s advanced electronic computer chips.
“We achieved this through the unique Mn3Sn material,” Nakatsuji said. “It turned out to be much easier to work in this way than other antiferromagnetic materials could have been.”
“There are no rules on how to make this material. Our goal is to create a pure, flat crystal lattice of Mn3Sn from manganese and tin using a process called molecular beam epitaxy,” said said Higo “There are many parameters of this process that need to be refined, and we are still refining the process to see how it could be scaled up if it is to become an industrial method one day.”