Magnetic susceptibility and short-range order in iron pnictides: anisotropic [J.sub.1]-[J.sub.2] heisenberg model

A spin-rotation-invariant Green-function theory for the dynamic spin susceptibility in the anisotropic non-frustrated [J.sup.1]-[J.sub.2] model for the iron pnictides A[Fe.sub.2][As.sub.2] (A = Ca, Sr, Ba) is presented. The two-spin correlation functions, the spin-excitation spectrum, and the staggered magnetization are calculated self-consistently as functions of temperature and spin. The temperature dependence of the magnetic (uniform static) susceptibility, in particular the linear increase with T in a wide temperature region that is in good quantitative agreement with experiments, is ascribed to a strong in-plane antiferromagnetic short-range order. The values of the Neel temperature resulting from the localized spin model with realistic out-of-plane couplings are found to be much higher than the measured values. From this an extension of the model by including itinerant degrees of freedom is suggested.