Signatures of Dirac fermion-mediated magnetic order

The spin–momentum locking of topological states offers an ideal platform to explore novel magnetoelectric effects. These intimately depend on the ability to manipulate the spin texture in a controlled way. Here we combine scanning tunnelling microscopy with single-atom deposition to map the evolution of topological states under the influence of different magnetic perturbations. We obtain signatures of Dirac fermion-mediated magnetic order for extremely dilute adatom concentrations. This striking observation is found to critically depend on the single adatoms’ magnetic anisotropy and the position of the Fermi level. Our findings open new perspectives in spin engineering topological states at the atomic scale and pave the way to explore novel spin-related topological phenomena with promising potential for applications. The spin texture of a topological insulator is defined by spin-momentum locked Dirac fermions in its non-trivial surface states. Here, Sessi et al. show how the spin texture of Bi 2 Te 3 may be modified by extremely dilute magnetic adatoms, with magnetic order mediated via the RKKY interaction.