Anomalous Spin-flip Excitations in the Uniaxial Ising Ferromagnet TbV₆Sn₆

The topological metal TbV₆Sn₆ reportedly features non-magnetic V kagome layers and Tb triangle layers that exhibit strong uniaxial anisotropic ferromagnetic order below T_c = 4 K. Inelastic neutron scattering (INS) confirms a high-spin Ising doublet ground state for the Tb, which for the ferromagnetic phase should result in the observation of a single spin-flip excitation. However, INS results reveal two equally intense, sharp excitations that appear inconsistent with a uniaxial Ising ferromagnet. Ruling out the excitations being due to a structural distortion or the hyperfine splitting, we suggest that the Tb ions are coupled to another time-reversal breaking field presumably originating from the V kagome layer. A model where Tb moments are parallel or antiparallel to V magnetization can explain the two equal-intensity Ising spin-flip excitations and their temperature dependence.