Collective magnon excitations observed in a dilute ferromagnetic topological insulator

Inelastic neutron scattering measurements are presented on ferromagnetic Mn-doped Bi₂Te₃ with a concentration of 0.1 Mn per formula unit. Susceptibility and neutron diffraction measurements find a Curie temperature (T_C) of 12 K and ordered moments oriented perpendicular to the quintuple-layer structure. Sharp magnetic diffraction peaks suggest long-ranged and homogeneous ferromagnetism. Despite the dilute nature of the magnetic constituents, collective ferromagnetic magnon excitations are observed below T_C with a bandwidth of ~3 meV, although the dispersive features are broadened by site disorder. The magnons are two-dimensional in character, indicating that the strongest magnetic interactions occur within the quintuple-layer with weaker interlayer interactions. In addition, sharp, low-energy (< 1 meV) localized excitations are observed which are consistent with free Mn pairs or small clusters separated from the main percolated ferromagnetic Mn network.