  We have investigated magnetic interactions between transition metal ions in
bismuthene topological insulator with protected edge states. We find that these
topological states have a crucial role on the magnetic interactions in 2D
topological insulators. Using first-principles and model Hamiltonian we make a
comparative study of transition metal doped bulk and nanoribbon bismuthene.
While direct overlap between the transition metal prevails in gapped bulk
bismuthene, at the borders of nanoribbons a long-range magnetism is present.
The exchange interactions are well described by a RKKY-like Hamiltonian
mediated by topological states. Our results show a dominance of
antiferromagnetism promoted by the topological states, preserving the
spin-locked Dirac crossing states due to a global time-reversal symmetry
preservation. This extended magnetic interactions mediated by massless
electrons can increase the spin diffusion length being promising for fast
dissipationless spintronic devices.
