Strong ferromagnetic exchange interaction in the parent state of the superconductivity in BaFe₂S₃

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Inelastic neutron scattering measurements have been performed to investigate the spin waves of the quasi-one-dimensional antiferromagnetic ladder compound BaFe₂S₃, where a superconducting transition was observed under pressure [H. Takahashi et al., Nat. Mater. 14, 1008-1012 (2015); T. Yamauchi et al., Phys. Rev. Lett. 115, 246402 (2015)]. By fitting the spherically averaged experimental data collected on a powder sample to a Heisenberg Hamiltonian, we find that the one-dimensional antiferromagnetic ladder exhibits a strong nearest neighbor ferromagnetic exchange interaction (SJᵣ=-71±4 meV) along the rung direction, an antiferromagnetic SJₗ=49±3 meV along the leg direction and a ferromagnetic SJ₂=-15±2 meV along the diagonal direction. Our data demonstrate that the antiferromagnetic spin excitations are a common characteristic for the iron-based superconductors, while specific relative values for the exchange interactions do not appear to be unique for the parent states of the superconducting materials.