Chiral Magnetic Interactions in Zigzag Graphene Nanoribbons Deposited on Topological Insulators

It is known that the interplay between strong spin-orbit coupling and broken inversion symmetry can lead to chiral configurations in low-dimensional magnetic systems deposited on heavy-element surfaces. This phenomenon is due to the Dzyaloshinskii-Moriya interaction, which favours non-collinear magnetic structures. In this work we investigate zigzag graphene nanoribbons on the (111) surface of the topological insulator Sb₂Te₃ by first-principles simulations based on Density Functional Theory. We consider unpassivated nanoribbon edges, which bind strongly to the substrate. We show that the edge magnetism is preserved, in spite of the strong chemical interaction between the edge carbon atoms and the substrate. Furthermore, we show that the Dzyaloshinskii-Moriya interaction leads to a twisting of the two antiferromagnetically-coupled edge states of the graphene nanoribbon. We also study the effects of this magnetic configuration on the surface states of the topological insulator. We find that the resulting net magnetization of the nanoribbon shifts the Dirac point of the surface state and induces a small gap.