Commensurate Spin Density Wave in LaFeAsO: A Local Probe Study

Abstract

We present a detailed study on the magnetic order in the undoped mother compound LaFeAsO of the recently discovered Fe-based superconductor ${\mathrm{LaFeAsO}}_{1-x}{\mathrm{F}}_x$. In particular, we present local probe measurements of the magnetic properties of LaFeAsO by means of ${}^{57}\mathrm{Fe}$ Mössbauer 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 $0.25(5){\mu }_B$ at the iron site below $T_N=138\mathrm{K}$, well separated from a structural phase transition at $T_S=156\mathrm{K}$. 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.