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

We report neutron diffraction studies of FeS single crystals obtained from Rb${}_x$Fe${}_{2-y}$S${}_2$ single crystals via a hydrothermal method. While no $\sqrt{5}\times \sqrt{5}$ iron vacancy order or block antiferromagnetic order typical of Rb${}_x$Fe${}_{2-y}$S${}_2$ is found in our samples, we observe $C$-type short range antiferromagnetic order with moments pointed along the $c$-axis hosted by a new phase of FeS with an expanded inter-layer spacing. The N'{e}el temperature for this magnetic order is determined to be 165 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 FeSe${}_{1-x}$S${}_x$ iron chalcogenide system.