Non-local effect of impurity states on the exchange coupling mechanism in magnetic topological insulators

Abstract

Since the discovery of the quantum anomalous Hall (QAH) effect in the magnetically doped topological insulators (MTI) Cr:(Bi,Sb)2Te3 and V:(Bi,Sb)2Te3, the search for the magnetic coupling mechanisms underlying the onset of ferromagnetism has been a central issue, and a variety of different scenarios have been put forward. By combining resonant photoemission, X-ray magnetic circular dichroism and density functional theory, we determine the local electronic and magnetic configurations of V and Cr impurities in (Bi,Sb)2Te3. State-of-the-art first-principles calculations find pronounced differences in their 3d densities of states, and show how these impurity states mediate characteristic short-range pd exchange interactions, whose strength sensitively varies with the position of the 3d states relative to the Fermi level. Measurements on films with varying host stoichiometry support this trend. Our results explain, in an unified picture, the origins of the observed magnetic properties, and establish the essential role of impurity-state-mediated exchange interactions in the magnetism of MTI.

Superexchange dominates in magnetic topological insulators

“…We find that the superexchange dominates not only in (Cd,Mn)Te [23,24], but also in topological (Hg,Mn)Te. We also show that the conclusion about the predominant role of the superexchange substantiates experimental results on (Cd,Hg,Mn)Te [25,26] and explains hitherto challenging chemical trends in magnetic properties of V, Cr, Mn, and Fe-doped tetradymite topological insulators observed experimentally [9,10,27] and found in ab initio studies [28][29][30].…”

The AiiDA-KKR plugin and its application to high-throughput impurity embedding into a topological insulator

“…Relativistic corrections arising within the scalar-relativistic approximations as well as spin-orbit coupling have been taken into account fully selfconsistently for both host and impurity calculations. This approach has been applied successfully in the past to study magnetic and non-magnetic defects in topological insulators where a good agreement between our theoretical predictions and different experimental observations was verified 23,[33][34][35] .…”

“…A shift in the host's Fermi level can experimentally be achieved by appropriate doping with Bi Sb impurities 38 . This change in the impurity moment has been seen previously both theoretical 34 as well as experimental 35 and can be attributed to the competition between the impurity seeking charge neutrality and the strong change in the hybridization with the host's electronic structure with varying position of the Fermi level due to the presence of the bulk band gap in TI materials. This leads, for instance, to decreasing (increasing) spin moments for V Sb (V Te ) with increasing position of the Fermi level.…”

“…This results from the half-filling of the Mn d orbital that make the spin moment relatively insensitive to small changes in the hybridization with the host's electronic structure. Nevertheless, the magnetic interactions among multiple magnetic impurity atoms can, even at small changes in the impurity hybridization, be strongly affected 34,35 .…”

“…A future study focusing on this effect might give more insights into the effect of structural relaxations. Some experiments 35,46,47 additionally report on the measured impurity DOS for V and Cr defects from resonant photoemission spectroscopy or scanning tunneling spectroscopy. A comparison to our calculated impurity DOS spectra shows good agreement for defects at the substitutional Sb site which is in line with the reasonable agreement of the spin moment reported in Table 1.…”