Anisotropy, Itineracy, and Magnetic Frustration in High-TC Iron Pnictides

Using first-principles density functional theory calculations combined with insight from a tight-binding representation, dynamical mean field theory, and linear response theory, we have extensively investigated the electronic structures and magnetic interactions of nine ferropnictides representing three different structural classes. The calculated magnetic interactions are found to be short range, and the nearest (J(1a)) and next-nearest (J(2)) exchange constants follow the universal trend of J(1a)/2J(2)similar to 1, despite their itinerant origin and extreme sensitivity to the z position of As. These results bear on the discussion of itineracy versus magnetic frustration as the key factor in stabilizing the superconducting ground state. The calculated spin-wave dispersions show strong magnetic anisotropy in the Fe plane, in contrast with cuprates.