We consider dilute magnetic doping in the surface of a three dimensional topological insulator where a two dimensional Dirac electron gas resides. We find that exchange coupling between magnetic atoms and the Dirac electrons has a strong and peculiar effect on both. First, the exchange-induced single ion magnetic anisotropy is very large and favors off-plane orientation. In the case of ferromagnetically ordered phase we find a colossal magnetic anisotropy energy, of the order of the critical temperature. Second, a persistent electronic current circulates around the magnetic atom and, in the case of a ferromagnetic phase, around the edges of the surface.
@article{arxiv.1003.5931,
title = {Colossal anisotropy in diluted magnetic topological insulators},
author = {A. S. Núñez and J. Fernández-Rossier},
journal= {arXiv preprint arXiv:1003.5931},
year = {2015}
}