Topological Insulator*

Inelastic neutron scattering (INS) techniques reveal the predominant magnetic interactions in the dilute magnetic topological insulator (Sb_1-xMn_x)₂Te₃ (x = 0.03). The INS shows a series of local excitations that are associated with the formation of antiferromagnetically coupled Mn-Mn dimers, (confirming Mn²⁺ with s = 5/2).  Modeling the momentum-space dependence of these excitations confirms the formation of the dimer, and furthermore identifies them as interlayer next-nearest-neighbors (NNNs) Mn that substitute the Sb atoms in the adjacent A-B stacked bilayers.  We note that the dimers are connected through a Te with an Mn-Te-Mn bond angle of 180^o following the Goodenough-Kanamori-Anderson rule for p-d hybridized orbitals with dominant AFM. Low energy excitations (0.1 to 0.3 meV) in the (H,0,0) and (0,0,L) plane suggest in-plane FM nano-clusters coupled ferromagnetically across the van der Waals gap and antiferromagnetically in the A-B bilayer. The magnetic susceptibility of the dilute system is consistent with the INS results.  It is interesting that such low Mn concentrations (~3%) reveal interactions that are relevant to the intercalated MnSb_2nTe_3n+1 and with implications on the isostructural Bi-based compounds.