by Meera Aravinth
Quantum computing and ‘spin-tronics’ have been increasingly viewed as the next step in the world of computing, as the need for memory storage and computational power is limited by space constraints. A magnetic phenomenon known as a skyrmion is a potential path forward to devices that store information bits on electron spins or other quantum mechanical features on the atomic level, dramatically reducing the physical space needed to store information. Electron spin is the intrinsic angular momentum of an electron and plays a large part in defining the magnetic characteristics of a material. Ferromagnets are materials where all the electron spins in a material align parallel to an applied magnetic field, and antiferromagnets are where the spins align anti-parallel.
Magnetic skyrmions refer to a type of chiral spin structure, where the electrons spins in a material form a spiraling pattern. Skyrmions have become an interesting potential candidate as a quantum information bit: in particular locations within a material, the presence or absence of skyrmions can correlate to a “0” or “1” bit. Skyrmions have been observed in both ferromagnetic materials and antiferromagnetic materials.
Ferromagnetic skyrmions have several disadvantages. Since the ferromagnetic material aligns with applied magnetic fields, these fields could influence skyrmion spins and break the chiral spin pattern, destroying the skyrmion. The ferromagnetic skyrmion is thus very vulnerable to external magnetic fields. Ferromagnetic materials also generate their own internal dipole fields, which can distort the skyrmions. Usually, unique magnetic structures like skyrmions are more stable at very cold, near-absolute zero temperatures, as they are less likely to be disrupted by random fluctuations that accompany higher temperatures. These vulnerabilities make ferromagnetic skyrmions unreliable as vehicle for information storage as they are relatively unstable.
A group of scientists led by William Legrand at the French National Center for Scientific Research observed stable skyrmions at room temperature in a material known as a synthetic antiferromagnet (SAF). The material was made by stacking coupled ferromagnetic layers of cobalt, ruthenium, and plutonium. The ferromagnetic layers are aligned anti-parallel to each other, and thus macroscopically look like an antiferromagnet, avoiding the vulnerabilities to destabilizing external fields. However, the individual ferromagnetic layers still have very localized dipole fields which make it easier to detect the skyrmions and study their behavior. The scientists observed that these stable skyrmions were around 10 nanometers across, about 100 times the diameter of an atom. The SAF skyrmions are easy to manipulate with electric fields, a quality that would be vital if skyrmions are ever to be used in technological applications. Lastly, the SAF skyrmion’s stability at room temperature is a quality that makes it attractive, as other materials that need to be cooled to absolute zero to work properly are not very practical choices for a real-world technology.
The researchers propose that skyrmions in synthetic antiferromagnets could have future applications as information bits in quantum computing. Their existence at room temperature, small size, and stability in response to external disturbances make them excellent candidates for quantum computing technology. Further study of skyrmions may lead us to new opportunities to better understand these magnetic phenomena and how we could utilize them in our future innovations.
Legrand, W., Maccariello, D., Ajejas, F. et al. Room-temperature stabilization of antiferromagnetic skyrmions in synthetic antiferromagnets. Nature Materials 19, 34–42 (2020) doi:10.1038/s41563-019-0468-3