Spintronic devices operate using spin textures caused by quantum-physical interactions, and a recent study from a Spanish-German collaboration focused on graphene-cobalt-iridium heterostructures at BESSY II. The research revealed that desired quantum-physical effects can strengthen each other in these heterostructures, opening up possibilities for new spintronic devices based on these materials. Spintronics involves using electron spins for logic operations and information storage, with the aim of achieving faster and more energy-efficient operation compared to traditional semiconductor devices. However, creating and manipulating spin textures in materials has proven to be challenging.
Graphene, a two-dimensional carbon atom structure, is a promising candidate for spintronic applications. When graphene is deposited on a thin film of heavy metal, a strong spin-orbit coupling develops at the interface, leading to quantum effects such as spin-orbit splitting of energy levels (Rashba effect) and a canting in the alignment of spins (Dzyaloshinskii-Moriya interaction). The spin canting effect is crucial for stabilizing vortex-like spin textures called skyrmions, which are highly suitable for spintronics applications. The addition of a few monolayers of cobalt between the graphene and heavy metal, in this case iridium, was found to enhance these effects significantly.
The research team at BESSY II analyzed electronic structures at the interfaces between graphene, cobalt, and iridium. Surprisingly, they discovered that graphene interacts not only with cobalt but also through cobalt with iridium. The ferromagnetic cobalt layer serves as a mediator for the interaction between graphene and iridium, enhancing the splitting of energy levels. By controlling the number of cobalt monolayers, the researchers were able to influence the spin-canting effect, with three monolayers being the most effective. These findings were supported by experimental data and new calculations using density functional theory, revealing an unexpected reinforcement of both quantum effects.
The use of Spin-ARPES at BESSY II enabled the researchers to gain new insights into the graphene-based heterostructures. Spin-ARPES allowed for extremely sensitive measurements of photoemission with spin resolution, providing a precise understanding of the origin of the spin canting, such as the Rashba-type spin-orbit splitting. Professor Oliver Rader, who heads the “Spin and Topology in Quantum Materials” department at HZB, emphasized the unique capabilities of BESSY II’s instruments in achieving these precise measurements. The results of the study highlighted the potential of graphene-based heterostructures for the development of the next generation of spintronic devices, showcasing the importance of these materials in advancing spintronics technology.
In conclusion, the study on graphene-cobalt-iridium heterostructures has demonstrated the significant enhancement of quantum effects when cobalt monolayers are inserted between graphene and heavy metal. The unexpected interactions observed between graphene, cobalt, and iridium, mediated by the ferromagnetic cobalt layer, have opened up new possibilities for manipulating spin textures in materials for spintronic applications. The use of advanced techniques such as Spin-ARPES at BESSY II has allowed for precise measurements and a deeper understanding of the underlying quantum-physical processes in these heterostructures. Overall, the research paves the way for the development of innovative spintronic devices that could revolutionize the field with faster and more energy-efficient operation.
