Observation of a C -type short-range antiferromagnetic order in layer spacing expanded FeS

Here, we report neutron diffraction studies of FeS single crystals obtained from RbxFe2-yS2 single crystals via a hydrothermal method. While no $\sqrt{5}$×$\sqrt{5}$ iron vacancy order or block antiferromagnetic order typical of RbxFe2-yS2 is found in our samples, we observe C-type short-range antiferromagnetic order with moments pointed along the c axis hosted by a different phase of FeS with an expanded interlayer spacing. The Néel temperature for this magnetic order is determined to be 170 ± 4 K. Our finding of a variant FeS structure hosting this C-type antiferromagnetic order demonstrates that the known FeS phase synthesized in this method is in the vicinity of a magnetically ordered ground state, providing insights into understanding a variety of phenomena observed in FeS and the related FeSe1-xSx iron chalcogenide system.