Strong ferromagnetic exchange interaction under ambient pressure in BaFe2 S3

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, Nat. Mater. 14, 1008 (2015)1476-112210.1038/nmat4351; T. Yamauchi, Phys. Rev. Lett. 115, 246402 (2015)PRLTAO0031-900710.1103/PhysRevLett.115.246402]. 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.