Magnetism dependent phonon anomaly in LaFeAsO observed via inelastic x-ray scattering

The phonon dispersion was measured at room temperature (above T$_{\mathrm{N}}$ ) along (0,0,L) in the tetragonal phase of LaFeAsO using inelastic x-ray scattering. Magnetostructural effects are well documented in the AFe$_{2}$As$_{2}$-based (A$=$Ca,Sr,Ba,Eu) systems. Only recently have single crystals of RFeAsO (R$=$La,Ce,Pr,Nd,Sm,Gd)-based compounds become available. The experimentally observed splitting between two A$_{\mathrm{1g}}$ phonon modes at 22 and 26 meV is only produced in spin-polarized first-principles calculations imposing various types of antiferromagnetic order. Magnetostructural effects similar to those observed in the AFe$_{2}$As$_{2}$ materials are confirmed present in LaFeAsO. This is discussed in terms of the strong antiferromagnetic correlations that are known to persist above T$_{\mathrm{N}}$ and into the tetragonal phase.