The low-energy spin excitations of the Fe pnictide parent compounds have been determined by inelastic neutron scattering and interpreted within the local moment $J_{1a,b}\text{−}{J}_2$ Heisenberg model with orthorhombic symmetry. This has led to alternative exchange models that strongly differ in the size of anisotropy. Although the compounds are itinerant the localized spin model can explain basic features of the excitations. The inherent frustration of this model leads to quantum fluctuations and possible moment reduction. We investigate this question in detail using spin-wave approximation and partly exact diagonalization Lanczos calculations for finite clusters. We find that the orthorhombic anisotropy stabilizes the columnar antiferromagnetic phase and its moment. For the exchange models proposed from inelastic neutron scattering we can exclude a strong influence of frustration on the moment size. We also investigate dependence of magnetization and susceptibility on field and temperature.

• Received 19 November 2009

DOI:https://doi.org/10.1103/PhysRevB.81.165101