We examine the relevance of several major material-dependent parameters to the magnetic softness in iron-base superconductors by first-principles electronic structure analysis of their parent compounds. The results are explained in the spin-fermion model where localized spins and orbitally degenerate itinerant electrons coexist and are coupled by Hunds rule coupling. We found that the difference in the strength of the Hunds rule coupling term is the major material-dependent microscopic parameter for determining the ground-state spin pattern. The magnetic softness in iron-based superconductors is essentially driven by the competition between the double-exchange ferromagnetism and the superexchange antiferromagnetism.
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