Three-dimensional topological insulators have conducting surface states in the bulk band gap, protected by time-reversal symmetry, which locks spin with momentum. It was shown, however, that surface magnetic doping can break time-reversal symmetry and induces backscattering of Dirac states . We present the results of our investigations of the magnetic properties of individual atoms of 3d transition metals (Mn, Fe, Co) deposited on the surface of the topological insulator Bi2Te3. All studied adatoms present an out-of-plane magnetic anisotropy associated with sizeable orbital moments and we find evidence of surface-mediated magnetic exchange interactions which are of opposite sign for Mn and Co, as a result of the competition of several exchange mechanisms.
 P. Sessi et al., Nat. Commun. 5, 5349 (2014).