We report muon spin relaxation and rotation ($\mu \mathrm{SR}$) measurements on hydrothermally grown single crystals of superconducting tetragonal FeS, which help to clarify the controversial magnetic state and superconducting gap symmetry of this compound. $\mu \mathrm{SR}$ time spectra were obtained from 280 K down to 0.025 K in zero field (ZF) and applied fields up to 75 mT. In ZF, the observed loss of initial asymmetry (signal amplitude) and increase of depolarization rate ${\mathrm{\Lambda }}_{\mathrm{ZF}}$ below 13 K indicate the onset of static magnetism, which coexists with superconductivity below $T_c$. TF $\mu \mathrm{SR}$ results indicate a linear temperature dependence of the superfluid density at low temperature, consistent with nodal superconductivity. The $s+d$-wave model gives the best fit to the observed temperature and field dependencies, and yields an in-plane penetration depth value ${\lambda }_{ab}(T=0$) = 241(3) nm.

• Received 29 January 2018
• Revised 15 May 2018

DOI:https://doi.org/10.1103/PhysRevB.97.174524