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.