Coexistence of low moment magnetism and superconductivity in tetragonal   FeS and suppression of $T_\mathrm{c}$ under pressure
  We report local probe ($\mu$SR) measurements on the recently discovered tetragonal FeS superconductor which has been predicted to be electronically very similar to superconducting FeSe. Most remarkably, we find that low moment ($10^{-2}-10^{-3}\mu_\mathrm{B})$ disordered magnetism with a transition temperature of $T_\mathrm{N}\approx 20$ K microscopically coexists with bulk superconductivity below $T_\mathrm{c}=4.3(1)$ K. From transverse field $\mu$SR we obtain an in-plane penetration depth $\lambda_\mathrm{ab}(0)=223(2)$ nm for FeS. The temperature dependence of the corresponding superfluid density $\lambda_\mathrm{ab}^{-2}(T)$ indicates a fully gapped superconducting state and is consistent with a two gap s-wave model. Additionally, we find that the superconducting $T_\mathrm{c}$ of FeS continuously decreases for hydrostatic pressures up to 2.2 GPa. 
S. Holenstein, U. Pachmayr, Z. Guguchia, S. Kamusella, R. Khasanov, A.   Amato, C. Baines, H.-H. Klauss, E. Morenzoni, D. Jo...
cond-mat.supr-con cond-mat.str-el