Magnetic Phase Diagram of FeAs based superconductors

The recently discovered high-temperature superconductivity in doped quaternary iron oxypnictides correlates experimentally with a magnetic instability. We have used first-principles calculations to determine a magnetic phase diagram of ReO$_{1-\delta}$FeAs (Re=La--Dy) as a function of the doping $\delta$, of the FeAs in-plane lattice constant $a$, and of the distance between the Fe and As planes, that is qualitatively consistent with recent experimental findings on the doping, internal (chemical) and external pressure dependence. The existence of a tricritical point (TCP) in the phase diagram suggests new ways of enhancing $T_c$.