Effects of a magnetic Fe monolayer on the structural and surface electronic properties of Sb2Te3

In this paper, we investigate the properties of Fe monolayers deposited on the (111) surface of the topological insulator Sb2Te3 by density functional theory simulations. We consider high-coverage monolayers and assume ferromagnetic configurations. We show that upon relaxation, the Fe atoms partly penetrate into the surface and that their magnetic moments are reduced due to the chemical interaction with the Te and Sb atoms of the two topmost layers. We compute the magnetic anisotropy energies and show that the easy axis is in-plane. We investigate hexagonal warping effects and find that the clean Sb2Te3(111) displays a large warping term. In spite of this, the surface-state gap for in-plane magnetization is below 1 meV. For very high coverages corresponding to three Fe atoms in the unit cell of Sb2Te3(111), no surface Dirac cones are observed except in the case of a metastable model consisting of a hexagonal Fe monolayer lying on top of the substrate, which also exhibits an out-of-plane easy axis. Finally, we discuss the relevance of our paper to recent experiments about magnetic impurities deposited on topological insulators.