Spin Waves and Magnetic Exchange Interactions in CaFe2As2

TL;DR

This study uses inelastic neutron scattering to analyze spin-wave excitations in CaFe2As2, revealing that its magnetism is a complex interplay of local and itinerant behaviors, challenging simple models.

Contribution

It provides the first comprehensive mapping of spin waves in CaFe2As2, demonstrating that its magnetic interactions cannot be fully described by a simple local or itinerant model.

Findings

Spin waves are well described by a 3D Heisenberg Hamiltonian.

Large in-plane anisotropy cannot be explained by local moment models.

Magnetism in CaFe2As2 is a hybrid of local and itinerant characteristics.

Abstract

Antiferromagnetism is relevant to high temperature (high-Tc) superconductivity because copper oxide and iron arsenide high-Tc superconductors arise from electron- or hole-doping of their antiferromagnetic (AF) ordered parent compounds. There are two broad classes of explanation for the phenomenon of antiferromagnetism: in the local moment picture, appropriate for the insulating copper oxides, AF interactions are well described by a Heisenberg Hamiltonian; while in the itinerant model, suitable for metallic chromium, AF order arises from quasiparticle excitations of a nested Fermi surface. There has been contradictory evidence regarding the microscopic origin of the AF order in iron arsenide materials, with some favoring a localized picture while others supporting an itinerant point of view. More importantly, there has not even been agreement about the simplest effective ground state Hamiltonian necessary to describe the AF order. Here we report inelastic neutron scattering mapping of spin-wave excitations in CaFe2As2, a parent compound of the iron arsenide family of superconductors. We find that the spin waves in the entire Brillouin zone can be described by an effective three-dimensional local moment Heisenberg Hamiltonian, but the large in-plane anisotropy cannot. Therefore, magnetism in the parent compounds of iron arsenide superconductors is neither purely local nor purely itinerant; rather it is a complicated mix of the two.