A research team led by Ryo Shimano of the University of Tokyo has successfully visualized two distinct mechanisms through which up and down spins, inherent properties of electrons, switch in an antiferromagnet, a material in which spin alignments cancel each other out. One of the visualized mechanisms provides a working principle for developing ultrafast, non-volatile magnetic memory and logic devices, which could be much faster than today's technologies. The findings are published in the journal Nature Materials.
Paper slips with holes, small metal rods, vacuum tubes, and transistors: these are technologies that have been used to encode 0s and 1s, the basis of classical computation. However, the world's ever-growing computational needs demand yet more powerful tools. Antiferromagnets are a class of materials whose magnetic properties, or lack thereof, could be leveraged to encode 0s and 1s in a novel way.
"For many years," says Shimano, "scientists believed that antiferromagnets like Mn₃Sn (manganese three tin) could switch their magnetization extremely quickly. However, it was unclear whether this non-volatile switching could complete within a few to several tens of picoseconds or how the magnetization really changed during the switching process."
The biggest question about the mechanism was whether it was driven by the heat generated by the electric current or by the current itself. The researchers thus set out to find the answer to this question by visualizing the mechanism. They prepared a thin layer of Mn3Sn and sent short electric pulses through it. Then, using precisely controlled ultrafast flashes of light with varying delays compared to the electric pulse, they tried to create a "time-lapse image" of the change in magnetization.
"The most challenging part of the project," Shimano remembers, "was measuring the infinitesimal changes in the magneto-optical signal. However, we were surprised how clearly we could finally observe the switching process once we established the right method."
Their result was something that had never been seen before: a frame-by-frame visualization of the change in the magnetic pattern. The frames revealed that switching occurs in two distinct processes depending on the current amplitude: one driven by a thermal process under a large current and another driven without substantial heating under a weak current. The latter process could provide the base for applications in developing reliable next-generation spintronic devices for computing, communications, and advanced electronics. To Shimano, this means one thing: boundaries of knowledge waiting to be expanded.
"Our present fastest time-resolved observation of electrical switching in Mn₃Sn is 140 picoseconds, mainly limited by how short the current pulses can be generated in our device setup. However, our findings suggest that the material itself could switch even faster under appropriate conditions. In the future, we aim to explore these ultimate limits by creating even shorter current pulses and by optimizing the device structure."
