Spintronic devices utilize spin textures generated by quantum-physical interactions. A collaborative effort between Spanish and German researchers has recently investigated graphene-cobalt-iridium heterostructures using the BESSY II facility. Their findings reveal that two beneficial quantum-physical effects can enhance one another within these heterostructures, paving the way for the development of novel spintronic devices using these materials.
Spintronic devices rely on spin textures created by quantum-physical interactions. A collaboration between researchers from Spain and Germany has examined graphene-cobalt-iridium heterostructures at BESSY II. The study’s results indicate that two favorable quantum-physical effects can support and enhance each other within these heterostructures, which could lead to innovative spintronic devices derived from these materials.
Spintronics harnesses the electron spins to carry out logic operations or store data. Ideally, spintronic devices could function faster and use less energy compared to traditional semiconductor devices. However, generating and managing spin textures in materials remains a challenge.
Graphene’s Role in Spintronics
Graphene, a two-dimensional structure made of carbon atoms arranged in a honeycomb pattern, is viewed as a promising option for spintronic applications. Typically, graphene is placed on a thin layer of heavy metal. At the interface between graphene and the heavy metal, strong spin-orbit coupling occurs, leading to various quantum effects, such as energy level splitting (Rashba effect) and a change in spin alignment (Dzyaloshinskii-Moriya interaction). The spin canting effect is particularly vital for stabilizing vortex-like spin textures called skyrmions, which are ideal for spintronic applications.
Adding Cobalt Monolayers
Recent research by a Spanish-German team has demonstrated that the previously mentioned effects can be significantly enhanced by inserting several monolayers of ferromagnetic cobalt between the graphene and the heavy metal (in this case, iridium). The samples used in this study were grown on insulating substrates, which is essential for creating multifunctional spintronic devices that leverage these effects.
Observing Interactions
“At BESSY II, we analyzed the electronic structures at the interfaces composed of graphene, cobalt, and iridium,” explains Dr. Jaime Sánchez-Barriga, a physicist at HZB. The key discovery revealed that, unexpectedly, graphene interacts with not only cobalt but also with iridium through the cobalt layer. “The interaction between graphene and the heavy metal iridium is facilitated by the ferromagnetic cobalt layer,” Sánchez-Barriga clarifies. The ferromagnetic layer amplifies the splitting of energy levels. “We can manipulate the spin-canting effect by adjusting the number of cobalt monolayers, with three layers being optimal,” he adds.
This conclusion is bolstered by experimental findings as well as new calculations based on density functional theory. It is both novel and surprising that the two quantum effects can interact and amplify each other.
SPIN-ARPES at BESSY II
“Our ability to gain these new insights was made possible by the highly sensitive instruments at BESSY II, which measure photoemission with spin resolution (Spin-ARPES). This advantageous situation allows us to very accurately determine the source of spin canting, specifically the Rashba-type spin-orbit splitting, possibly even more accurately than the spin canting itself,” emphasizes Prof. Oliver Rader, who leads the “Spin and Topology in Quantum Materials” department at HZB. Very few institutions around the world possess instruments with such capabilities. The findings illustrate that graphene-based heterostructures hold significant promise for future spintronic devices.
