Spintronic devices work with spin textures brought on by quantum-physical interactions. A Spanish-German collaboration has now studied graphene-cobalt-iridium heterostructures at BESSY II. The outcomes show how two desired quantum-physical effects reinforce one another in these heterostructures. This may lead to recent spintronic devices based on these materials.
Spintronics uses the spins of electrons to perform logic operations or store information. Ideally, spintronic devices could operate faster and more energy-efficiently than conventional semiconductor devices. Nonetheless, it remains to be difficult to create and manipulate spin textures in materials.
Graphene for Spintronics
Graphene, a two-dimensional honeycomb structure construct by carbon atoms, is taken into account an interesting candidate for spintronic applications. Graphene is often deposited on a skinny film of heavy metal. On the interface between graphene and heavy metal, a powerful spin-orbit coupling develops, which provides rise to different quantum effects, including a spin-orbit splitting of energy levels (Rashba effect) and a canting within the alignment of spins (Dzyaloshinskii-Moriya interaction. Especially the spin canting effectis needed to stabilise vortex-like spin textures, often known as skyrmions, that are particularly suitable for spintronics.
Plus Cobalt Monolayers
Now, nevertheless, a Spanish-German team has shown that these effects are significantly enhanced when a couple of monolayers of the ferromagnetic element cobalt are inserted between the graphene and the heavy metal (here: iridium). The samples were grown on insulating substrates which is a mandatory prerequisite for the implementation of multifunctional spintronic devices exploiting these effects.
Interactions observed
‘At BESSY II, we’ve analysed the electronic structures on the interfaces between graphene, cobalt and iridium,’ says Dr. Jaime Sánchez-Barriga, a physicist at HZB. Crucial finding: contrary to expectations, the graphene interacts not only with the cobalt, but in addition through the cobalt with the iridium. ‘The interaction between the graphene and the heavy metal iridium is mediated by the ferromagnetic cobalt layer,’ Sánchez-Barriga explains. The ferromagnetic layer enhances the splitting of the energy levels. ‘We are able to influence the spin-canting effect by the variety of cobalt monolayers; three monolayers are best,’ says Sanchez-Barriga.
This result’s supported not only by experimental data, but in addition by recent calculations using density functional theory. The undeniable fact that each quantum effects influence and reinforce one another is recent and unexpected.
SPIN-ARPES at BESSY II
‘We were only capable of obtain these recent insights because BESSY II offers extremely sensitive instruments for measuring photoemission with spin resolution (Spin-ARPES). This results in the fortunate situation that we are able to determine the assumed origin of the spin canting, i. e., the Rashba-type spin-orbit splitting, very precisely, probably much more precisely than the spin canting itself.,’ emphasises Prof. Oliver Rader, who heads the “Spin and Topology in Quantum Materials” department at HZB. There are only a only a few institutions worldwide which have instruments with these capabilities. The outcomes show that graphene-based heterostructures have great potential for the subsequent generation of spintronic devices.
