Low ordered magnetic moment by off-diagonal frustration in undoped parent compounds to iron-based high-Tc superconductors

TL;DR

This paper analyzes a Heisenberg model for Fe-As high-Tc superconductor parent compounds, revealing hidden magnetic order due to off-diagonal frustration, explaining low magnetic moments and predicting specific low-energy spin-wave excitations.

Contribution

It introduces a classical and small-cluster analysis of a Heisenberg model showing how off-diagonal frustration leads to hidden magnetic order in Fe-As compounds.

Findings

Hidden magnetic order explains low magnetic moments.

Low-energy spin-wave excitations predicted at specific wavevectors.

Frustration violates Hund's rule in the model.

Abstract

A Heisenberg model over the square lattice recently introduced by Si and Abrahams to describe local-moment magnetism in the new class of Fe-As high-Tc superconductors is analyzed in the classical limit and on a small cluster by exact diagonalization. In the case of spin-1 iron atoms, large enough Heisenberg exchange interactions between neighboring spin-1/2 moments on different iron 3d orbitals that frustrate true magnetic order lead to hidden magnetic order that violates Hund's rule. It accounts for the low ordered magnetic moment observed by elastic neutron diffraction in an undoped parent compound to Fe-As superconductors. We predict that low-energy spin-wave excitations exist at wavenumbers corresponding to either hidden Neel or hidden ferromagnetic order.