Comparing magnetic ground-state properties of the V- and Cr-doped topological insulator (Bi, Sb)2Te3

An insulating ferromagnetic ground state is a fundamental prerequisite for the quantum anomalous Hall (QAH) effect observed in magnetically doped topological insulators such as (Bi, Sb)(2)Te-3. So far, the QAH effect could only be induced by V and Cr doping, with V resulting in ferromagnetism with a higher T-C and a more robust QAH state. To better understand the difference between the two dopants, we have investigated epitaxial V0.1Sb1.9Te3 and Cr-0.1(Bi0.1Sb0.9)(1.9)Te-3 films using x-ray absorption spectroscopy and x-ray magnetic circular dichroism. Our analysis of the V and Cr L-2,L-3 spectra by multiplet ligand-field theory goes beyond existing studies by allowing several charge-transfer states. We find a strongly covalent ground state, dominated by the superposition of one and two Te-ligand-hole configurations, with a negligible contribution from ionic V3+ or Cr3+. Crucial for a comparison with theoretical models are the resulting d-shell fillings (n(d)(V) = 3.47 and n(d)(Cr) = 4.33), and spin (m(spin)(V) = 2.39 mu(B) and m(spin)(Cr) = 3.22 mu(B)) and orbital (m(orb)(V) = -0.55 mu(B) and m(orb)(Cr) = -0.03 mu(B)) magnetic moments, with the total magnetic moments being in good agreement with published magnetometry results. Our findings indicate the importance of the Te 5p states for the ferromagnetism in (Bi, Sb)(2)Te-3 and favor theories involving pd-exchange.