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.Comment: 4 pages, 3 figure
