Researchers at Johannes Gutenberg University Mainz (JGU) are the first to directly utilize orbital currents without the need for conversion of the orbital current into a spin current. "We have thus realized the first purely orbitronic device approach," said Dr. Christin Schmitt, a scientist in the research group of Professor Mathias Kläui at the JGU Institute of Physics. Orbitronics is a promising technology for future memory devices, as it could enable the realization of large-scale storage media with extremely low energy consumption. It is based on orbital moments, which can be described in simplified terms as the quantum-mechanical "vortices" of electrons around atomic nuclei, as well as orbital currents, i.e., the movement of these circulations through an electrical conductor. "For the first time, we have been able to directly couple mobile orbital moments with localized orbital moments in a magnet. In doing so, we have achieved a milestone in orbitronics and laid the foundation for significantly more energy-efficient data storage. In this way, we obtain signals that are two orders of magnitude stronger than those in conventional spintronic devices," said Schmitt. Her work, performed in collaboration with an international team of more than 20 researchers, including some of Forschungszentrum Jülich, was recently published in the prestigious scientific journal Science.
Cobalt oxide with a copper layer as a model system
Orbital currents offer a significant advantage over spin currents: the measurable signals are orders of magnitude stronger. If orbital currents are used to write or read information in magnetic memories, substantially more efficient switching processes become possible. In the long term, orbitronics has the potential to produce extremely energy-efficient devices. Until now, the drawback has been that orbital currents had to be converted into spin currents to be used, for example, to write to or read from a memory device. "For the first time, we have succeeded in using orbital currents directly – thereby finding a way to fully exploit the potential of orbitronics in the future," said Schmitt. As a model system, the team used cobalt oxide as an insulating antiferromagnet with a layer of copper, which reacted at the surface to form copper oxide. The samples were provided by the research group of Professor Eiji Saitoh at the University of Tokyo. It was already known that orbital currents form at the copper-copper oxide layer and propagate toward the cobalt oxide. The team has succeeded in coupling the mobile orbital moments in the copper – which form the orbital current – to localized orbital moments in the cobalt. This coupling is required to read out magnetic information: depending on how the orbital moments in copper and cobalt are aligned with respect to one another, a "0" or a "1" is represented. "The coupling became possible because we used a magnet dominated by orbital angular momentum, whereas previous studies had always relied on spin-dominated magnets," said Schmitt.
Signals two orders of magnitude stronger than in spintronics
The team compared this system with a cobalt oxide/platinum system, in which information can be stored and read out using pure spin currents. "With the orbitronic system, we were able to increase the resulting signal by two orders of magnitude compared with the signal generated by pure spin currents," Schmitt summarized the results of the study. Dr. Sachin Krishnia, a senior member of the research team, emphasized that the effect is not only stronger but also fundamentally different in physical terms: "Beyond the magnitude of the signal, what is crucial is that the orbital current interacts with the cobalt oxide in a completely different way. It does not simply mimic a spin current; rather, it appears to activate hidden properties of the antiferromagnet. This makes orbital magnetism an active degree of freedom for future devices." Schmitt sees considerable potential for future applications: "Antiferromagnetic materials with strong orbital properties therefore constitute a good platform for future orbital devices. By enabling more energy-efficient memory and computing technologies, they could help address challenges related to resources, energy consumption, and climate change." The head of the research team, Mathias Kläui, emphasized that this breakthrough was only possible through long-term collaborations: "For more than ten years, we have been working with colleagues in Japan as part of projects funded by the German Academic Exchange Service and the Japan Society for the Promotion of Science. This enables JGU students to produce there the required high-quality materials together with colleagues on site. And the theoretical work was carried out within the framework of German and EU-funded projects. Such international collaborations make possible exciting new research that we could never have realized on our own."
