Global Ferromagnetism and Localized Dimers in (Sn_0.9Mn_0.1)Te

Neutron scattering and magnetic susceptibility experiments of the dilute magnetic topological insulator (Sn_0.9Mn_0.1)Te reveal long-range ferromagnetic (FM) order below T_C = 12 ± 1 K. Neutron scattering data provide additional evidence for short-range magnetic correlations within the FM state. Analysis of the short-range correlations reveals competing pairwise magnetic interactions from nearest-neighbor FM Mn-Mn direct interactions, next-nearest neighbor antiferromagnetic (AFM) Mn-Te-Mn super-exchange interactions, and longer-range AFM Mn-Te-Sn-Te-Mn interactions. The AFM interaction via the linear Mn-Te-Mn bond (J ~ 0.7 meV), quantified in previous measurements of dilute Sn_0.97Mn_0.03Te, is nearly an order of magnitude greater than other interactions and likely supports the formation of strong Mn-Mn dimer singlet states. Such dimers would not participate in the global FM order. However, the dimer singlet spectrum cannot be resolved in our data due to the higher Mn concentration and interactions of the dimer quantum state with other Mn neighbors and the internal molecular field.