R&D: Husker Scientists Fine-Tuning Skyrmions in Bid to Improve Data Storage, Processing

The quest to make components ever smaller is a key driver behind the development of the next generation of computer electronics, and University of Nebraska–Lincoln materials scientists’ success at fine-tuning a magnetic material to exhibit extremely small spiral structures at room temperature could play a central role.

Illustration of magnetic vortex, known as a skyrmion,
that could star in next generation of digital memory.
Each arrow indicates direction of the magnetic axis in an individual atom.

The particle-like magnetic configurations are called skyrmions (SKUR’-mee-ahns), a set of atoms whose poles, or magnetic moments, tilt further and further away from the magnetic axis as they approach the skyrmion’s center, with the atom at its core pointing in the opposite direction of that axis. These structures have the potential to improve data storage and processing in multiple computer applications.

David Sellmyer

Nebraska physicists have been studying skyrmions for several years to determine ways to make them smaller and more stable so that they’re useful in practical applications, such as electronics. The Husker team is led by David Sellmyer, Professor of physics and astronomy, George Holmes University; Balamurugan Balasubramanian, research assistant professor; and Ralph Skomski, research professor of physics and astronomy, along with collaborators from the Nebraska Center for Materials and Nanoscience and otherinstitutions.

Balamurugan Balasubramanian

Skyrmions do not form above the so-called magnetic ordering temperature of a material, below which the magnetic moments lock in parallel orientations. Conventional wisdom says that high ordering temperatures correspond to large skyrmions. A previous discovery in the nanomaterials field led to the creation of skyrmions in a magnetic material at room temperature, but these skyrmions were relatively large, with a diameter of about 50 nanometers, or roughly 2,000 times thinner than a human hair. While interesting, these skyrmions are not useful for future applications because their size must be much smaller, ideally about ten nanometers.

Ralph Skomski

An earlier milestone for the Nebraska team was the ability to create much smaller skyrmions, about 13 nanometers in size. However, that was only achieved at a temperature of about minus-382 degrees Fahrenheit. Now, they’ve successfully increased the magnetic ordering temperature of a material whose structure supports skyrmions beyond room temperature and simultaneously shrunk the skyrmion size to about 17 nanometers.

“That’s important because ever-smaller and energy-efficient components are essential to the continual miniaturization in electronics,” Sellmyer said. “In the field of information processing and data storage, the science and engineering community is trying to figure out new physics systems that will make these elements smaller and smaller and use less power so they don’t heat up — and be fast.”

The Nebraska researchers’ findings counter the conventional wisdom that only larger skyrmions could be realized at room temperature because the quantum-mechanical atomic inter¬action ‘stiffens’ the spin structure, simultaneously increasing thermal stability and skyrmion size.

The researchers countered that tendency by adding cobalt to the non-magnetic compound cobalt silicon, gradually replacing the silicon atoms with excess cobalt, which is known to have a positive effect on magnetic ordering temperature. The enhanced cobalt content was realized through a non-equilibrium fabrication process that involved cooling the molten material to room temperature at about one million degrees per second.

That process circumvented equilibrium physics and gave magnetic ordering temperatures of about 129 degrees Fahrenheit, which is among the highest of all metal silicides and germanides hosting skyrmions. An analysis of the exchange interactions in the cobalt-rich material showed that the excess cobalt had a bigger effect on the ordering temperature than on the skyrmion size, which explains their small size.

“Normally, if you enhance temperature, it increases the size of skyrmions,” Skomski said. “In this case, there’s a very complicated interaction preventing that.”

The discovery is important for future developments in information science. Researchers hope to develop something called ‘memory racetracks,’ which are nanoscopic stripes that could transport the magnetic vortexes from one group of atoms to another when propelled by an electric current. By bringing those bits to a data reader or writer rather than vice versa, racetrack designs could increase processing speeds and extend the lifespans of data-storage devices.

The team reported its findings in the journal Physical Review Letters. Collaborators include Nebraska doctoral students Rabindra Pahari and Ahsan Ullah and senior research associate Wenyong Zhang, as well as researchers from Cornell University, the University of Delaware and Howard University. The research was supported by the U.S. Department of Energy, with additional support for fabrication and facilities by the National Science Foundation and Nebraska Research Initiative.

Article: Chiral Magnetism and High-Temperature Skyrmions in B20-Ordered Co-Si

Physical Review Letters has published an article written by Balamurugan Balasubramanian, Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, Nebraska 68588, USA , and Department of Physics and Astronomy, University of Nebraska, Lincoln, Nebraska 68588, USA, Priyanka Manchanda, Department of Physics and Astronomy, Howard University, Washington, DC 20059, USA, Rabindra Pahari, Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, Nebraska 68588, USA , and Department of Physics and Astronomy, University of Nebraska, Lincoln, Nebraska 68588, USA, Zhen Chen, School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, USA, Wenyong Zhang, Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, Nebraska 68588, USA , and Department of Physics and Astronomy, University of Nebraska, Lincoln, Nebraska 68588, USA, Shah R. Valloppilly, Xingzhong Li, Anandakumar Sarella, Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, Nebraska 68588, USA, Lanping Yue, Ahsan Ullah, Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, Nebraska 68588, USA , and Department of Physics and Astronomy, University of Nebraska, Lincoln, Nebraska 68588, USA, Pratibha Dev, Department of Physics and Astronomy, Howard University, Washington, DC 20059, USA, David A. Muller, School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, USA, Ralph Skomski, Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, Nebraska 68588, USA , and Department of Physics and Astronomy, University of Nebraska, Lincoln, Nebraska 68588, USA, George C. Hadjipanayis, Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716, USA, and David J. Sellmyer, Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, Nebraska 68588, USA , and Department of Physics and Astronomy, University of Nebraska, Lincoln, Nebraska 68588, USA.

Abstract: “Magnets with chiral crystal structures and helical spin structures have recently attracted much attention as potential spin-electronics materials, but their relatively low magnetic-ordering temperatures are a disadvantage. While cobalt has long been recognized as an element that promotes high-temperature magnetic ordering, most Co-rich alloys are achiral and exhibit collinear rather than helimagnetic order. Crystallographically, the B20-ordered compound CoSi is an exception due to its chiral structure, but it does not exhibit any kind of magnetic order. Here, we use nonequilibrium processing to produce B20-ordered Co1+xSi1−x with a maximum Co solubility of x=0.043. Above a critical excess-Co content (xc=0.028), the alloys are magnetically ordered, and for x=0.043, a critical temperature Tc=328  K is obtained, the highest among all B20-type magnets. The crystal structure of the alloy supports spin spirals caused by Dzyaloshinskii-Moriya interactions, and from magnetic measurements we estimate that the spirals have a periodicity of about 17nm. Our density-functional calculations explain the combination of high magnetic-ordering temperature and short periodicity in terms of a quantum phase transition where excess-cobalt spins are coupled through the host matrix.“