Reviewed by Danielle Ellis, B.Sc.Sep 23 2024
According to a study published in ACS Nano, a Spanish-German team discovered that inserting a few monolayers of the ferromagnetic element cobalt between graphene and the heavy metal (in this case, iridium) significantly enhances these effects.
Symbolic illustration of a graphene layer on a microchip. In combination with a heavy-metal thin film and ferromagnetic monolayers, graphene could enable spintronic devices. Image Credit: Dall-E/arö
Spintronic devices utilize spin textures created by quantum-physical interactions. A Spanish-German collaboration has now investigated graphene-cobalt-iridium heterostructures at BESSY II. The findings demonstrate how two desired quantum-physical effects reinforce one another in these heterostructures. This could result in new spintronic devices based on these materials.
Spintronics uses electron spins to perform logic operations or store information. Ideally, spintronic devices would be faster and more energy efficient than traditional semiconductor devices. However, it remains difficult to create and manipulate spin textures in materials.
Graphene for Spintronics
Graphene, a two-dimensional honeycomb structure made of carbon atoms, is regarded as a promising candidate for spintronic applications. Graphene is typically deposited on a thin layer of heavy metal. A strong spin-orbit coupling develops at the interface between graphene and heavy metal, resulting in a variety of quantum effects such as spin-orbit splitting of energy levels (Rashba effect) and canting in spin alignment (Dzyaloshinskii-Moriya interaction).
The spin canting effect is especially important for stabilizing vortex-like spin textures known as skyrmions, which are ideal for spintronics.
Plus Cobalt Monolayers
The samples were grown on insulating substrates, which is a requirement for the development of multifunctional spintronic devices that exploit these effects.
Interactions Observed
At BESSY II, we have analyzed the electronic structures at the interfaces between graphene, cobalt and iridium.
Dr. Jaime Sánchez-Barriga, Physicist, Helmholtz-Zentrum Berlin
The most significant discovery is that, in contrast to predictions, graphene interacts with cobalt and, via cobalt, with iridium.
“The interaction between the graphene and the heavy metal iridium is mediated by the ferromagnetic cobalt layer,” explains Sánchez-Barriga. The splitting of the energy levels is improved by the ferromagnetic layer.
Sánchez-Barriga further added, “We can influence the spin-canting effect by the number of cobalt monolayers; three monolayers are best.”
New calculations using density functional theory and experimental data both corroborate this result. It is novel and unexpected that the two quantum effects reinforce and influence one another.
SPIN-ARPES at BESSY II
We were only able to obtain these new insights because BESSY II offers extremely sensitive instruments for measuring photoemission with spin resolution (Spin-ARPES). This leads to the fortunate situation that we can determine the assumed origin of the spin canting, i. e., the Rashba-type spin-orbit splitting, very precisely, probably even more precisely than the spin canting itself.
Oliver Rader, Assistant Professor, Helmholtz-Zentrum Berlin für Materialien und Energie
He is the head of the HZB department “Spin and Topology in Quantum Materials.”
Only a few institutions worldwide have instruments with these capabilities. The findings indicate that graphene-based heterostructures hold great promise for the next generation of spintronic devices.
Journal Reference:
Cano, B, M., et al. (2024) Rashba-like Spin Textures in Graphene Promoted by Ferromagnet-Mediated Electronic Hybridization with a Heavy Metal. ACS Nano. doi.org/10.1021/acsnano.4c02154