Effect of doping and pressure on magnetism and lattice structure of iron-based superconductors

Using first-principles calculations, we analyze structural and magnetic trends as a function of charge doping and pressure in BaFe2 As2, and compare to experimentally established facts. We find that density-functional theory, while accurately reproducing the structural and magnetic ordering at ambient pressure, fails to reproduce some structural trends as pressure is increased. Most notably, the Fe-As bond length which is a gauge of the magnitude of the magnetic moment, μ, is rigid in experiment but soft in calculation, indicating residual local Coulomb interactions. By calculating the magnitude of the magnetic ordering energy, we show that the disruption of magnetic order as a function of pressure or doping can be qualitatively reproduced but that in calculation, it is achieved through diminishment of |μ|, and therefore likely does not reflect the same physics as detected in experiment. We also find that the strength of the stripe order as a function of doping is strongly site dependent: magnetism decreases monotonically with the number of electrons doped at the Fe site but increases monotonically with the number of electrons doped at the Ba site. Intraplanar magnetic ordering energy (the difference between checkerboard and stripe orderings) and interplanar coupling both follow a similar trend.