Ferromagnetic interlayer coupling inFeSe1−𝑥⁢S𝑥superconductors revealed by inelastic neutron scattering

FeSe1−𝑥⁢S𝑥 superconductors are commonly considered layered van der Waals materials with negligible interlayer coupling. Here, using inelastic neutron scattering to study spin excitations in single-crystal samples, we reveal that the magnetic coupling between adjacent Fe layers is ferromagnetic in nature, making the system different from most unconventional superconductors including iron pnictides. The weak interlayer coupling is estimated to be 𝐽𝑐∼ 0.2 meV, in agreement with the short spin-spin correlation length 𝜉𝑐∼0.2⁢𝑐 along the 𝑐 axis. The results provide an experimental basis for establishing a microscopic theoretical model to describe the absence of magnetic order in FeSe1−𝑥⁢S𝑥.