Magnetic Excitations in the Iron-based Superconductors*

We calculate the expected inelastic neutron scattering response based on the spin-orderings found in the iron-based superconductors, using spin-wave theory. For the two-sublattice collinear antiferromagnet, we consider two types of superexchange couplings between Fe atoms: nearest-neighbor coupling J$_1$ and next-nearest-neighbor coupling J$_2$. We show how to distinguish experimentally between ferromagnetic and antiferromagnetic J$_1$. We show the existence of saddlepoints near ($\pi$, $\pi/2$) and ($0$, $\pi/2$), which are expected to give rise to extra scattering intensity. We find that the sublattice magnetization can be reduced by the zero-point motion of spin waves, although not enough to account for the small moments observed in experiment. By comparison with experimental results on SrFe$_2$As$_2$, we estimate that the effective magnetic interlayer coupling is rather large, about $1/8$ the value of the in-plane couplings. References: 1) Phys. Rev. Lett. 101, 167203 (2008); 2) Phys. Rev. B 78, 052507 (2008)