Magnon contribution to the thermodynamic properties of the iron based pnictides SrFe2As2

We investigate some of the thermodynamic properties of iron based pnictides, using an effective J1−J2−J⊥−K spin-model containing only the local Fe moments. Both the multi-layered structure of the material, as well as the single ion anisotropy, are taken into account through the J⊥ and K couplings, respectively. We derive a fully analytic expression for the magnetic contribution to the specific heat when J⊥=0; the case when J⊥≠0 is evaluated numerically. Both solutions (with no adjustable parameters) are found to compare favorably with the reported experimental data for SrFe2As2. An improvement is, however, noticed for the J⊥≠0 case, consistent with the layered character of these weakly coupled magnetic structures. We discuss the applicability of our formalism to other oxy/pnictides, or related magnetic materials.