Frustrated local moment models for Fe-pnictide magnetism

TL;DR

This paper examines the magnetic excitations in Fe-pnictide compounds using local moment models, finding that orthorhombic anisotropy stabilizes magnetic order and frustration has limited impact on magnetic moment size.

Contribution

It provides a detailed analysis of spin wave and quantum fluctuations in Fe-pnictides, clarifying the role of anisotropy and frustration in magnetic properties.

Findings

Orthorhombic anisotropy stabilizes the AF phase and magnetic moment.

Frustration has limited influence on the size of the magnetic moment.

Magnetization and susceptibility depend on field and temperature.

Abstract

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-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 localised 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 diagonalisation Lanczos calculations for finite clusters. We find that the orthorhombic anisotropy stabilizes the columnar AF phase and its moment. For the exchange models proposed from inelastic neutron scattering we can exclude a strong influcence of frustration on the moment size. We also investigate dependence of magnetisation and susceptibility on field and temperature.