![]() If the static long-range antiferromagnetic order shown in Fig. 1a for the parent compounds of iron-based superconductors originates from a collective spin-density-wave order instability of itinerant electrons like in chromium, the velocity of spin-wave excitations c should be c=( v e v h/3) 1/2, where v e and v h are the electron and hole Fermi velocity, respectively 9. Using inelastic neutron scattering, we have measured the dispersion of spin-wave excitations in CaFe 2As 2 (refs 26, 27), one of the parent compounds of the FeAs-based superconductors, and determined the effective magnetic exchange interactions. A determination of the effective magnetic exchange coupling and ground-state Hamiltonian in the parent compounds of these materials is important because such an understanding will provide the basis against which superconductivity-induced changes can be identified. Since the discovery of static antiferromagnetic order (with a spin structure as in Fig. 1a) in the parent compounds of iron pnictide superconductors 5, 6, much effort has been focused on understanding the role of spin dynamics in the superconductivity of these materials 11, 12, 13, 14, 15, 16, 17, 18, 19, 20. 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. 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. Here, we use inelastic neutron scattering to map spin-wave excitations in CaFe 2As 2 (refs 26, 27), a parent compound of the iron arsenide family of superconductors. ![]() More importantly, there has not even been agreement about the simplest effective ground-state Hamiltonian necessary to describe the antiferromagnetic order 21, 22, 23, 24, 25. ![]() 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, 12, 13, 14, 15 and others supporting an itinerant point of view 16, 17, 18, 19, 20. 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. 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, 2, 3, 4, 5, 6. ![]()
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