Magnetic properties of Mn-doped Bi2Se3 topological insulators: Ab initio calculations

Abstract

Doping Bi2Se3 by magnetic ions represents an interesting problem since it may break the time reversal symmetry needed to maintain the topological insulator character. Mn dopants in Bi2Se3 represent one of the most studied examples here. However, there is a lot of open questions regarding their magnetic ordering. In the experimental literature different Curie temperatures or no ferromagnetic order at all are reported for comparable Mn concentrations. This suggests that magnetic ordering phenomena are complex and highly susceptible to different growth parameters, which are known to affect material defect concentrations. So far theory focused on Mn dopants in one possible position, and neglected relaxation effects as well as native defects. We have used ab initio methods to calculate the Bi2Se3 electronic structure influenced by magnetic Mn dopants, and exchange interactions between them. We have considered two possible Mn positions, the substitutional and interstitial one, and also native defects. We have found a sizable relaxation of atoms around Mn, which affects significantly magnetic interactions. Surprisingly, very strong interactions correspond to a specific position of Mn atoms separated by van der Waals gap. Based on the calculated data we performed spin dynamics simulations to examine systematically the resulting magnetic order for various defect contents. We have found under which conditions the experimentally measured Curie temperatures TC can be reproduced, noticing that interstitial Mn atoms appear to be important here. Our theory predicts the change of TC with a shift of Fermi level, which opens the way to tune the system magnetic properties by selective doping.

Twin Domain Structure in Magnetically Doped Bi2Se3 Topological Insulator

“…This assumption is supported by synchrotron experiments which show that Mn is not metallic in Bi Se and thus does not segregate there [ 54 ]. The influence of the magnetic defects on the crystal structure is reflected by local lattice relaxation similar to the previous bulk calculation of Bi Te and Bi Se [ 17 , 30 ]. According to supercell calculations [ 30 ], we have modified and used the Wigner–Seitz radii locally while the total volume of Bi-sublattice is retained.…”

“…The mentioned quite large difference between the magnitudes of Fe and Mn magnetic moment stems from a distinct character of the magnetic exchange interactions ( Figure 7 ), where they are evaluated by employing the Liechtenstein formula [ 30 , 55 , 56 ]. A comparison shows that unlike Mn related interactions, which are nearly positive except the nearest ones, the exchange interactions between Fe dopants are predominantly antiferromagnetic [ 57 ], regardless of the considered magnetic sublattices.…”

“…[ 17 ]. Small changes which stem from the presence of the intrinsic Se defects are neglected in correspondence to our previous work [ 30 ]. Besides, the relaxation corresponding to the presence of surfaces is not included there.…”

“…Besides, there are studies which suggest an occupation of the interstitial positions within vdW gap concerning the related Bi Te compound [ 29 ]. In our calculation, we especially employ Mn [ 23 , 27 , 30 , 31 ] as well as Fe [ 27 , 31 , 32 ] magnetic dopants. In addition in our calculations we also assume native defects like Bi or Se antisites (Bi resp.…”

High-throughput search for magnetic and topological order in transition metal oxides

“…At the intersection of these two quantum orders, the focus has mainly been on taking prototypical topological materials like (Bi, Sb) 2 Te 3 and introducing magnetic dopants ( 11 , 12 ). This doping approach has a number of drawbacks, namely, that dopants are hard to control ( 13 ) and the critical temperature for observing exotic physics is low (below 2 K) ( 14 ). Important challenges remain in assessing the stability (synthetic accessibility) of proposed materials ( 15 ) and coupling topological property prediction to magnetism.…”