  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 Hund's rule coupling.
We found that the difference in the strength of the Hund's 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.
