Researchers in Germany and Japan have been able to increase the spread of magnetic vortices, so-called skyrmions, by a factor of ten.
In today’s world, our lives are unimaginable without computers. Until now, these devices have processed information using primarily electrons as charge carriers, and the components themselves heat up significantly during the process. Active cooling is therefore necessary, which comes with high energy costs. Spintronics aims to solve this problem: Instead of using the flow of electrons for information processing, it relies on their spin, or their intrinsic angular momentum. This approach is expected to have a positive impact on the size, speed and durability of computers or specific components.
Magnetic Whirls stores and processes information
Science often considers not just the spin of a single electron, but rather magnetic vortices made up of many spins. These vortices, called skyrmions, appear in magnetic metallic thin layers and can be thought of as two-dimensional quasiparticles. On the one hand, the vortices can be intentionally moved by applying a small electric current to the thin layers; on the other hand, they move randomly and extremely efficiently due to diffusion. The possibility of creating a functional computer based on skyrmions was demonstrated by a team of researchers from the Johannes Gutenberg University Mainz (JGU), led by Professor Dr. Mathias Kläui, with the help of a first prototype. This prototype consisted of thin, stacked metallic layers, some only a few atomic layers thick.
Increase energy efficiency
In collaboration with the University of Konstanz and Tohoku University in Japan, researchers at Mainz University have now taken another step towards spin-based, unconventional computing: They were able to increase the scattering of skyrmions by a factor of ten using synthetic antiferromagnets, drastically reducing energy consumption and increases the speed of such a potential computer. “The reduction of energy use in electronic devices is one of the biggest challenges in basic research,” emphasized Professor Dr. Ulrich Nowak, who led the theoretical part of the project in Konstanz.
The power of antiferromagnets
But what is an antiferromagnet and what is it used for? Normal ferromagnets consist of many small spins, all linked together to point in the same direction, creating a large magnetic moment. In antiferromagnets, the spins are directed alternately antiparallel, i.e. a spin and its direct neighbors point in opposite directions. As a result, there is no net magnetic moment, although the spins remain antiferromagnetically ordered. Antiferromagnets have significant advantages, such as three orders of magnitude faster switching dynamics, better stability, and the potential for higher storage densities. These properties are studied intensively in several research projects.
To understand why these antiferromagnets are useful in this context, we need to delve a little deeper. When skyrmions move very fast, an additional force component arises in ferromagnetic layers perpendicular to the direction of motion. This force component drives the skyrmions off course. They end up hitting the wall, getting stuck and blocking the way for others. At higher speeds, they can even be destroyed. However, it is theoretically known that this effect either does not occur in antiferromagnets or occurs to a very limited extent.
Advances in synthetic antiferromagnets
To create such an antiferromagnet artificially, the researchers connected two of their ferromagnetic layers in such a way that the magnetization in the two layers is precisely aligned in opposite directions, eliminating their magnetic fields. This provides two advantages: They reduce the force that pushes the vortices out of their path, thereby increasing diffusion. “With this, we have created a synthetic antiferromagnet in which the diffusion of skyrmions is about ten times higher than in the individual layers,” says Klaus Raab, physicist at JGU. “This diffusion can be implemented to realize stochastic computation – a form of computation in which stochastic processes such as the random movement of particles are used.”
The team of researchers investigated the effects of the compensation of the magnetic layers in addition to the effect of temperature and size of the skyrmions on the diffusion and consequently on the movement of the skyrmions, both experimentally and through simulations. Intricate connections have been found: As the temperature rises, the skyrmions have more energy to diffuse faster. The heat also reduces the size of the skyrmions, which positively affects their mobility. The compensation of the vertical force component also has a positive effect on the diffusion. All these effects are difficult to disentangle from each other. “The increased diffusion appears to be due not only to the pure compensation of the magnetic fields but also to the associated reduction in the size of the skyrmions,” Raab concluded.
Professor Mathias Kläui, who led the study, is satisfied with the fruitful collaboration with Tohoku University: “We have been working with this leading Japanese university for about ten years and there are even joint study programs. With the support of the German Academic Exchange Service – DAAD – and other research funders, over a dozen students from Mainz University have already participated in exchanges with Tohoku University. I am delighted that this collaboration has been made possible through this collaboration.”
The research results have recently been published in the journal Nature communication.
Reference: “Enhanced thermally activated skyrmion diffusion with tunable effective gyrotropic force” by Takaaki Dohi, Markus Weißenhofer, Nico Kerber, Fabian Kammerbauer, Yuqing Ge, Klaus Raab, Jakub Zázvorka, Maria-Andromachi Syskaki, Aga Shahee, Morbias Böttcher, Philipp Pirro, Gerhard Jakob, Ulrich Nowak and Mathias Kläui, 11 September 2023, Nature communication.
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