Defect-Driven Competing Magnetic Interactions in MnSb₂Te₄

The antiferromagnetic topological insulator MnBi₂Te₄ displays interesting properties related to the interaction between topological electronic bands and magnetic order. In isostructural MnSb₂Te₄, anti-site mixing between Mn and Sb atoms introduces magnetic vacancies and defects and determines whether the ground state is ferromagnetic or antiferromagnetic. Here, we present inelastic neutron scattering on single crystals of MnSb₂Te₄, where clear evidence of defect-induced magnetic excitations are observed. Qualitative analysis using linear spin wave theory and classical atomistic spin dynamic simulations are consistent with strong antiferromagnetic coupling to magnetic defects. We discuss how these defect-driven interactions control the global magnetic order in MnSb₂Te_4.