Spintronics uses electron spins to perform logic operations or store information. Ideally, spintronic devices could operate faster and more energy-efficiently than conventional semiconductor devices. However, creating and manipulating spin textures in materials remains challenging.
Graphene, a two-dimensional honeycomb structure made of carbon atoms, is considered an interesting candidate for spintronic applications. Graphene is typically deposited on a thin film of heavy metal.
At the interface between graphene and the heavy metal, a strong spin-orbit coupling develops, which gives rise to various quantum effects, including spin-orbit splitting of energy levels (Rashba effect) and a tilt in the spin alignment (Dzyaloshinskii-Moriya interaction). The spin tilt effect is particularly necessary to stabilize vortex-like spin textures, called skyrmions, which are particularly suitable for spintronics.
A Spanish-German team has shown that these effects are significantly enhanced when a few monolayers of cobalt, a ferromagnetic element, are inserted between graphene and the heavy metal (here iridium). The samples were grown on insulating substrates, a necessary condition for the implementation of multifunctional spintronic devices exploiting these effects.
The research is published in the journal ACS Nano.
“Within the framework of BESSY II, we analyzed the electronic structures at the interfaces between graphene, cobalt and iridium,” explains Dr. Jaime Sánchez-Barriga, physicist at the HZB. The most important discovery: contrary to expectations, graphene interacts not only with cobalt, but also, via cobalt, with iridium.
“The interaction between graphene and iridium, a heavy metal, is ensured by the ferromagnetic layer of cobalt,” explains Sánchez-Barriga. The ferromagnetic layer improves the separation of energy levels.
“We can influence the spin-canting effect by the number of cobalt monolayers; three monolayers are optimal,” Sanchez-Barriga explains.
This result is confirmed not only by experimental data, but also by new calculations using density functional theory. The fact that the two quantum effects influence and reinforce each other is new and unexpected.
“We were only able to obtain this new information because BESSY II offers extremely sensitive instruments for measuring spin-resolved photoemission (Spin-ARPES). This allows us to determine the presumed origin of spin canting, i.e. Rashba-type spin-orbit splitting, very precisely – probably even more precisely than spin canting itself,” emphasizes Professor Oliver Rader, who heads the “Spin and Topology in Quantum Materials” department at the HZB.
Only a few institutions worldwide have instruments with these capabilities. The results show that graphene-based heterostructures have great potential for the next generation of spintronic devices.
More information:
Beatriz Muñiz Cano et al., Rashba-type spin textures in graphene promoted by ferromagnet-mediated electronic hybridization with a heavy metal, ACS Nano (2024). DOI: 10.1021/acsnano.4c02154
Provided by the Helmholtz Association of German Research Centers
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