Magnetism dependent phonon anomaly in LaFeAsO observed via inelastic   x-ray scattering
  The phonon dispersion was measured at room temperature along (0,0,L) in the tetragonal phase of LaFeAsO using inelastic x-ray scattering. Spin-polarized first-principles calculations imposing various types of antiferromagnetic order are in better agreement with the experimental results than nonmagnetic calculations, although the measurements were made well above the magnetic ordering temperature, T_N. Splitting observed between two A_{1g} phonon modes at 22 and 26 meV is only observed in spin-polarized calculations. Magneto-structural effects similar to those observed in the AFe_2As_2 materials are confirmed present in LaFeAsO. The presence of Fe-spin is necessary to find reasonable agreement of the calculations with the measured spectrum well above T_N. On-site Fe and As force constants show significant softening compared to nonmagnetic calculations, however an investigation of the real-space force constants associates the magnetoelastic coupling with a complex renormalization instead of softening of a specific pairwise force. 
S. E. Hahn, G. S. Tucker, J.-Q. Yan, A. H. Said, B. M. Leu, R. W.   McCallum, E. E. Alp, T. A. Lograsso, R. J. McQueeney, B....
cond-mat.supr-con