Spin waves and magnetic exchange interactions in CaFe2As2

Antiferromagnetism is relevant to high-temperature (high-T-c) superconductivity because copper oxide and iron arsenide superconductors arise from electron- or hole-doping of their antiferromagnetic parent compounds(1-6). There are two broad classes of explanation for antiferromagnetism: in the 'local moment' picture, appropriate for the insulating copper oxides(1), antiferromagnetic interactions are well described by a Heisenberg Hamiltonian(7,8); whereas in the 'itinerant model', suitable for metallic chromium, antiferromagnetic order arises from quasiparticle excitations of a nested Fermi surface(9,10). There has been contradictory evidence regarding the microscopic origin of the antiferromagnetic order in iron arsenide materials(5,6), with some favouring a localized picture(11-15) and others supporting an itinerant point of view(16-20). More importantly, there has not even been agreement about the simplest effective ground-state Hamiltonian necessary to describe the antiferromagnetic order(21-25). 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.