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

We report neutron diffraction studies of FeS single crystals obtained from ${\mathrm{Rb}}_{x}{\mathrm{Fe}}_{2\ensuremath{-}y}{\mathrm{S}}_{2}$ single crystals via a hydrothermal method. While no $\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5}$ iron vacancy order or block antiferromagnetic order typical of ${\mathrm{Rb}}_{x}{\mathrm{Fe}}_{2\ensuremath{-}y}{\mathrm{S}}_{2}$ is found in our samples, we observe $C$-type short-range antiferromagnetic order with moments pointed along the $c$ axis hosted by a different phase of FeS with an expanded interlayer spacing. The N\'eel temperature for this magnetic order is determined to be $170\ifmmode\pm\else\textpm\fi{}4$ 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 ${\mathrm{FeSe}}_{1\ensuremath{-}x}{\mathrm{S}}_{x}$ iron chalcogenide system.