  The indirect RKKY interaction in iron pnictide and chalcogenide metals is
calculated for a simplified four bands Fermi surface (FS) model. We investigate
the specific multi-band features and show that distinct length scales of the
RKKY oscillations appear. For the regular lattice of the local moments, the
generalized RKKY interaction is defined in momentum space. We consider its
momentum dependence in paramagnetic and spin density wave (SDW) phases, discuss
its implications for the possible type of magnetic order and compare it to the
results obtained from more realistic tight-binding type Fermi surface model.
Our finding can give important clues on the magnetic ordering of the 4f- iron
based superconductors.
