  The electronic and magnetic properties of individual Fe atoms adsorbed on the
surface of the topological insulator Bi$_2$Te$_3$(111) are investigated.
Scanning tunneling microscopy and spectroscopy prove the existence of two
distinct types of Fe species, while our first-principles calculations assign
them to Fe adatoms in the hcp and fcc hollow sites. The combination of x-ray
magnetic circular dichroism measurements and angular dependent magnetization
curves reveals out-of-plane anisotropies for both species with anisotropy
constants of $K_{\text{fcc}} = (10 \pm 4)$ meV/atom and $K_{\text{hcp}} = (8
\pm 4)$ meV/atom. These values are well in line with the results of
calculations.
