Magnetic Order and Magnetic Excitations in FeTe: How Good is a Short-Range Heisenberg Model?

We revisit the fitting of the Fang-Bernevig-Hu model [Europhys. Lett. 86, 67005 (2009)] to inelastic neutron scattering data carried out by Lipscombe et al. [Phys. Rev. Lett. 106, 057004 (2011)]. We demonstrate that there are many quite different parameter choices within the experimentally observed phase (AFM3) that provide approximately equally good fits to the neutron data. We note that (a) all of these parameter sets lie very close to a point of transition between the AFM3 phase and one of its neighbors in the classical phase diagram, and (b) many of them involve rather large values of the third-neighbor coupling $J_3$. In light of these observations, we discuss whether the modeling of FeTe may need revision to allow for a slightly non-AFM3 ground state, orbital as well as spin physics on the Fe sites, the effects of electron itinerancy, or a combination of these.