Commensurate Spin Density Wave in LaFeAsO: A Local Probe Study

We present a detailed study on the magnetic order in the undoped mother compound LaFeAsO of the recently discovered Fe-based superconductor ${\mathrm{LaFeAsO}}_{1\ensuremath{-}x}{\mathrm{F}}_{x}$. In particular, we present local probe measurements of the magnetic properties of LaFeAsO by means of $^{57}\mathrm{Fe}$ M\"ossbauer spectroscopy and muon-spin relaxation in zero external field along with magnetization and resistivity studies. These experiments prove a commensurate static magnetic order with a strongly reduced ordered moment of $0.25(5){\ensuremath{\mu}}_{B}$ at the iron site below ${T}_{N}=138\text{ }\text{ }\mathrm{K}$, well separated from a structural phase transition at ${T}_{S}=156\text{ }\text{ }\mathrm{K}$. The temperature dependence of the sublattice magnetization is determined and compared to theory. Using a four-band spin density wave model both, the size of the order parameter and the quick saturation below ${T}_{N}$ are reproduced.