The magnetic interactions in iron pnictides

Using static linear response theory we studied the pair wise magnetic interaction parameters in many typical families of iron pnictides. Parameters have been obtained for a wide range of volumes and distance between Fe and As atoms. We demonstrate that two nearest neighbor couplings in plane dominate, with a third and fourth nearest neighbor coupling responsible for the appearance of non-collinear ordering when the magnetic moment is small. We found that the ratio between first and a second neighbor coupling is not universal and greatly varies as a function of pressure or Fe-As distance. A small interplane coupling is found, and it varies by a factor of 10-20 among pnictides. We analyze the Neel temperatures, adiabatic spin wave spectrum and a nature of magneto-structural transitions in different classes of pnictides.