Itinerant Spin Excitations in SrFe$_2$As$_2$ Measured by Inelastic Neutron Scattering

We have performed inelastic neutron scattering (INS) measurements of the magnetic excitations in SrFe$_2$As$_2$, an iron pnictide parent compound [1]. The data extend up to energies of $\sim 260$\,meV. We find that the spectrum calculated from a local-moment J$_1$-J$_2$ model fails to describe our data in several key respects. In particular, the data can only be fitted by using different exchange parameters at high and low energies, for which there is no obvious physical justification. In both cases the nearest-neighbor exchange parameters, J$_{1\rm{a}}$ and J$_{1\rm{b}}$, are very different. Also, on warming above the magnetic/structural ordering transition, one would expect J$_{1\rm{a}}$ $\rightarrow$ J$_{1\rm{b}}$ and hence a soft mode at $\mathbf{Q}=(0.5,0.5)$, due to frustration in the local-moment model. However we find that the spectrum is largely unaltered. We find that the qualitative features of the INS spectra that cannot be described by the J$_1$-J$_2$ model are readily explained by calculations from a 5-band itinerant mean-field model. This also implies that it is not necessary to invoke additional broken symmetry, such as electronic nematic or orbital order, to explain the lack of a soft mode. [1] R. A. Ewings et al, Phys. Rev. B \textbf{83}, 214519 (2011)