Spin waves and magnetic exchange interactions in CaFe2As2

Antiferromagnetism is relevant to high-temperature (high-Tc) superconductivity because copper oxide and iron arsenide superconductors arise from electron- or hole-doping of their antiferromagnetic parent compounds. There are two broad classes of explanation for antiferromagnetism: in the `local moment' picture, appropriate for the insulating copper oxides, antiferromagnetic interactions are well described by a Heisenberg Hamiltonian; whereas in the `itinerant model', suitable for metallic chromium, antiferromagnetic order arises from quasiparticle excitations of a nested Fermi surface. There has been contradictory evidence regarding the microscopic origin of the antiferromagnetic order in iron arsenide materials, with some favouring a localized picture and 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 antiferromagnetic order. Here, we use inelastic neutron scattering to map spin-wave excitations in CaFe₂As₂ (refs 26, 27), 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.