  We report on the quenching of single Ni adatom moments on Te-terminated
Bi2Te2Se and Bi2Te3 topological insulator surfaces. The effect becomes
manifested as a missing X-ray magnetic circular dichroism for resonant L3,2
transitions into partially filled Ni 3d states of occupancy n_d = 9.2. On the
basis of a comparative study of Ni and Fe using scanning tunneling microscopy
and ab initio calculations we are able to relate the element specific moment
formation to a local Stoner criterion. While Fe adatoms form large spin moments
of m_s = 2.54 mu_B with out-of-plane anisotropy due to a sufficiently large
density of states at the Fermi energy, Ni remains well below an effective
Stoner threshold for local moment formation. With the Fermi level remaining in
the bulk band gap after adatom deposition, non-magnetic Ni and preferentially
out-of-plane oriented magnetic Fe with similar structural properties on
Bi2Te2Se surfaces constitute a perfect platform to study off-on effects of
time-reversal symmetry breaking on topological surface states.
