Delicate Ferromagnetism in MnBi₆Te₁₀

Abstract

Tailoring magnetic orders in topological insulators is critical to the realization of topological quantum phenomena. An outstanding challenge is to find a material where atomic defects lead to tunable magnetic orders while maintaining a nontrivial topology. Here, by combining magnetization measurements, angleresolved photoemission spectroscopy, and transmission electron microscopy, we reveal disorder-enabled, tunable magnetic ground states in MnBi 6 Te 10 . In the ferromagnetic phase, an energy gap of 15 meV is resolved at the Dirac point on the MnBi 2 Te 4 termination. In contrast, antiferromagnetic MnBi 6 Te 10 exhibits gapless topological surface states on all terminations. Transmission electron microscopy and magnetization measurements reveal substantial Mn vacancies and Mn migration in ferromagnetic MnBi 6 Te 10 . We provide a conceptual framework where a cooperative interplay of these defects drives a delicate change of overall magnetic ground state energies and leads to tunable magnetic topological orders. Our work provides a clear pathway for nanoscale defect-engineering toward the realization of topological quantum phases.

Recent progress in MnBi2nTe3n+1 intrinsic magnetic topological insulators: crystal growth, magnetism and chemical disorder

“…It is worth noting that, for MnBi 4 Te 7 and MnBi 6 Te 10 with different growth methods or growth conditions, FM ground states were also observed [ 70 , 72 ]. The disparities are likely attributed to varying degrees of crystal defects within samples grown by different groups, which we discuss extensively in the next section.…”

“…They have played an important role in the magnetism and band topology in the MnBi 2 n Te 3 n +1 family. For example, in the case of MnBi 6 Te 10 , an FM state can be achieved by having additional Mn vacancies in the sample [ 72 ]. In addition to these defects in the bulk samples, when the crystals are exfoliated and fabricated into devices, the fabrication process can introduce defects too.…”

“…Scanning transmission electron microscopy (STEM), STM, X-ray and neutron diffractions, and magnetization analysis have identified and quantified the chemical defects in MnBi 2 n Te 3 n +1 [ 15 , 16 , 23 , 25 , 34 , 51 , 58 , 72 , 78–84 ]. Each technique has its pros and cons.…”

Antisite-Defects Control of Magnetic Properties in MnSb2Te4

“…As the Mn Bi concentration is small, MBT still exhibits an overall A-type AFM ground state (i.e., the interlayer AFM interactions are dominant). In MST, due to similar ionic radii of Sb 3+ and Mn 2+ (smaller than that of Bi 3+ ), a higher Mn–Sb antisite concentration (around 15% Mn Sb and 30% Sb Mn ) is reported when compared to the Mn–Bi antisite concentration (around 5% Mn Bi and 20% Bi Mn ) in MBT. ,,,− Therefore, as introduced above, these antisite defects exert significant influences on the overall magnetic structures (AFM or FIM) and transition temperatures of MST. ,,− In addition to magnetism, the Mn–Bi/Sb antisite defects would also induce electron/hole doping and alter the Fermi surface, band inversion, and topology of Mn­(Sb/Bi) 2 n +2 Te 3 n +4 . − Despite the importance of the defects, their formation mechanism, the methods to tune their concentration and distribution, and how they can control the magnetism remain unclear.…”

Comparative Study of Magnetic Properties of (Mn$_{1-x}$A$_x^{IV}$)Bi2Te4 A$_x^{IV}$ = Ge, Pb, Sn

“…However, as the number of [Bi 2 Te 3 ] QLs increases, significant changes occur in the magnetic properties. For the 147 phase, T N is already 13 K and H SF does not exceed 0.3 T. The phase 1610 possesses an uncertain type of magnetic ordering which can be either AFM or FM, depending on growth conditions and the presence of defects [23,24]. For phases with higher m, the system resembles non-interacting 2D ferromagnets formed by the [MnBi 2 Te 4 ] SL [21].…”