Magnetic Skyrmions Hold Promise for Next-Gen Memory Devices

　　Researchers at Singapore's Agency for Science, Technology and Research (A*STAR) and Nanyang Technological University (NTU) have created a thin film material that allows them to control the size and density of magnetic skyrmions — a critical milestone in the creation of a skyrmion-based memory device.

　　The A*STAR / NTU discovery has been published in Nature Materials.

　　Skyrmions were theorized to exist in 1962 by British physicist Tony Skyrme, explained Anjan Soumyanarayanan, lead author of the paper and one of the A*STAR researchers behind the discovery. Magnetic skyrmions were first predicted in 1989 and discovered in 2009 at very low temperatures (-250 °C) as crystalline materials.

　　“It's really cold and pretty much unusable," Soumyanarayanan said.

　　Since then there has been considerable excitement over their use in memory devices, especially after the discovery of room temperature skyrmions in 2013 through sequential stacking of metals, he said. “That's what really changed the game," he added.

　　The stability of skyrmions are part of what makes them promising. As with many emerging memory technologies, the potential is a device that can hold more information while using less power. “They're very small, and very stable," Soumyanarayanan said.

　　He added that the potential is not only smaller non-volatile memory devices, but smaller by an order of magnitude. “Skyrmons could impact the road map for a few decades," he said.

　　The A*STAR / NTU paper outlines how the researchers achieved electrical detection of these skyrmions, which are small particle-like magnetic structures about 400 times smaller than a red blood cell. They can be created in magnetic materials, and their stability at small sizes makes them ideal candidates for memory devices.

　　In a conventional ferromagnet, the magnetic moments, or spins, are all aligned parallel to each other to form a uniformly magnetized state of arbitrary size. In contrast, explains Soumyanarayanan, a skyrmion is a finite sized spin structure. The spin at the skyrmion center points opposite to the background. The spins wind around the center in a chiral fashion and are smoothly restored to the background orientation. The spins in a skyrmion whirl around the center somewhat similarly to a vortex generated in a pool of water.

　　But it is more useful to think of skyrmions in terms of their emergent behavior, he said. They behave like nanometer-scale magnetic particles that can be seen with magnetic microscopes. They can self-organize into ordered arrays, or lattices. They can be singly created, moved, and deleted with electrical currents. “They can be thought of as analogous to game pieces on a checkerboard," he said.

　　The researchers have developed a new multilayer thin film platform for skyrmions. It consists of a sequential stacking of Ir, Fe, Co and Pt layers, said Soumyanarayanan. “This four-layer stack is repeated several times to generate the full stack structure." The multilayer film is deposited using sputtering techniques on a CMOS-compatible silicon substrate, a fabrication process that is currently used to commercially develop memory devices, he said.

　　"Our material platform allows us direct control of magnetic interactions that govern skyrmion properties simply by varying the thickness of the constituent layers. This allowed us to vary the size of skyrmions by a factor of two, and their stability and density by a factor of 10," said Soumyanarayanan. It also enabled researchers to achieve skyrmion configurations tailored to contrasting device requirements while at the same time using industry-compatible fabrication techniques. Researchers also demonstrated the electrical detection of ambient skyrmions, known as the Hall effect, which is also essential for realizing device applications, he said.

　　A*STAR and NTU initiated a joint research program on skyrmions more than two years ago, and it's taken more than year of experiments and simulations to identify this particular multilayer stack as ideally suited to address the problem. “To this day, it is a unique material host of tunable skyrmions," said Soumyanarayanan.

　　And although several groups around the world discovered room temperature skyrmions in 2015-16, they were unable to develop tunability of skyrmion properties or electrical detection, he said, which is a key requirement for moving from films to devices. The Lawrence Berkeley National Laboratory in California has been collaborating with A*STAR and NTU on their skyrmion research.

　　The next hurdles the researchers must clear is fabricating devices while ensuring consistency of magnetic properties. The details of the physical mechanisms governing the stabilization of skyrmions in nanostructures, and their reading and writing, remain to be fully established, said Soumyanarayanan, and he wouldn't speculate on timelines despite being rather optimistic. “Once we can demonstrate deterministic reading and writing of skyrmions in devices with reasonable electrical parameters, and demonstrate the scalability and reproducibility of these phenomena, the technology should be ripe for commercialization."