Contrasting effects of dilute magentic impurities, Mn and Cr, in topological insulators: From competing interations to quantum anomalous Hall effect

Introducing magnetism into topological insulators (TIs) unlocks emergent quantum phenomena, such as quantum anomalous Hall effect (QAHE), axion states, and Weyl semimetallic phases. In dilute magnetic TIs, ferromagnetism arising from substitutional transition-metal impurities breaks time reversal symmetry and enables QAHE. However, the microscopic nature of magnetic interactions critically determines the stability of these quantized states. Using inelastic neutron scattering and density-functional theory (DFT), we previously revealed that Mn substitution in Te-based TIs (Sb₂Te₃, SnTe) leads to antiferromagnetic (AFM) dimers that compete with long-range ferromagnetic order, thereby suppressing QAHE. In contrast, our recent studies of Cr-doped analogs show the absence of such competing AFM dimers, but with prevailing FM interactions, consistent with robust ferromagnetism and experimentally observed QAHE. This comparative analysis between Mn and Cr substitution elucidates how the emergence or absence of AFM dimers governs the topologically non-trivial quantum transport in dilute magnetic TIs.