Towards an understanding of magnetic interactions and anisotropies in iron superconductors

The itinerant or strong coupling origin of magnetism in iron pnictides is still unsettled. The localized description generally assumes AF exchange constants satisfying $2J_2>J_1$, with$J_1$ and $J_2$ referring to first and second nearest neighbors respectively. The itinerant picture relies on the nesting of the Fermi surface. Both descriptions reproduce the columnar ordering found experimentally. The role played by the Hund's coupling $J_H$ and the orbital degree of freedom are also highly debated. Orbital ordering has been invoked to explain the anisotropic resistivity and optical conductivity. We make connection between these two pictures by studying the same five-orbital model within Heisenberg and mean field descriptions [1]. We have found that $J_2/J_1$ strongly depends on the charge and orbital filling what results in an unexpected sensitivity of the AF ordering to crystal field parameters. $J_1$ and $J_2$ can become ferromagnetic at large $J_H$. Consistent results are obtained in the mean field description. We also analyze the resistivity and optical conductivity anisotropies and show that they are a consequence of magnetism and not of orbital ordering [2].\\[4pt] [1] M.J. Calder\'on et al, arXiv:1107.2279. E. Bascones et al, PRL 104, 227201 (2010).\\[0pt] [2] B. Valenzuela et al, PRL 105, 207202 (2010).