  Mackinawite is a naturally-occurring layer type FeS mineral well-known to be
important in biogeochemical cycles and, more recently, to the development of
microbial fuel cells. Conflicting results have been published as to the
magnetic properties of this mineral, with M\"ossbauer spectroscopy indicating
no magnetic ordering and density functional theory predicting an
antiferromagnetic ground state, similar to the Fe-based high-temperature
superconductors with which it is isostructural and for which it is known that
magnetism is suppressed by strong itinerant spin fluctuations. We investigated
this latter possibility for mackinawite using photoemission spectroscopy (PES),
near-edge X-ray absorption fine structure spectroscopy (XAS), and density
functional theory (DFT) computations. Our Fe 3s core-level PES spectrum of
mackinawite showed a clear exchange-energy splitting (2.9 eV) consistent with a
1 {\mu}B magnetic moment on the Fe ions, while the Fe L-edge XAS spectrum
indicated rather delocalized Fe 3d electrons in mackinawite similar to those in
Fe metal. DFT computations demonstrated that the ground state of mackinawite is
single-stripe antiferromagnetic, with an Fe magnetic moment (2.7 {\mu}B) that
is significantly larger than the experimental estimate and has a strong
dependence on the S height and lattice parameters. All of these trends signal
the existence of strong itinerant spin fluctuations.If strong spin fluctuations
prove to be the mediators of electron pairing in these Fe-based
superconductors, we conjecture that mackinawite may be one of the simplest
Fe-based superconductors.
