| We introduce a minimal model addressing the puzzles of the magnetism in iron pnictides. The key ingredient of the model is a mixing of different spin states of Fe3+, which makes the magnetic moment flexible and accounts for the intermediate experimental values of the ordered moment. In an effective spin model, this mixing leads to a strong biquadratic exchange, suggested recently to explain the observed dispersion of magnetic excitations in iron pnictides. In the paramagnetic phase, we explain the origin of the linear temperature dependence of the static spin susceptibility. Finally, we show that, apart from the commonly used spin-fluctuation mediated interpocket pairing, our model leads to a large intrapocket pairing. |
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