FeSe1−xSx superconductors are commonly considered layered van der Waals materials with negligible inter-layer 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 not only significant, as it affects excitations up to \textcolor{black}{15} meV, but also ferromagnetic in nature, making the system different from most unconventional superconductors including iron pnictides. Our observation provides a new standpoint to understand the absence of magnetic order in FeSe1−xSx. Since intercalating between the Fe layers is known to enhance superconductivity and suppress the inter-layer coupling, superconductivity appears to be a more robust phenomenon in the two-dimensional limit than antiferromagnetic order.
@article{arxiv.2407.05548,
title = {Ferromagnetic inter-layer coupling in FeSe$_{1-x}$S$_{x}$ superconductors revealed by inelastic neutron scattering},
author = {Mingwei Ma and Philippe Bourges and Yvan Sidis and Jinzhao Sun and Guoqing Wang and Kazuki Iida and Kazuya Kamazawa and Jitae T. Park and Frederic Bourdarot and Zhian Ren and Yuan Li},
journal= {arXiv preprint arXiv:2407.05548},
year = {2025}
}