Magnetism in CeFeAsO1-x Fx and LaFeAsO1-x Fx from first principles

Using state-of-the-art first-principles calculations we study the magnetic behavior of CeOFeAs. We find the Ce layer moments oriented perpendicular to those of the Fe layers. An analysis of incommensurate magnetic structures reveals that the Ce-Ce magnetic coupling is rather weak with, however, a strong Fe-Fe and Fe-Ce coupling. Comparison of the origin of the tetragonal to orthorhombic structural distortion in CeOFeAs and LaOFeAs shows marked differences; in CeOFeAs the distortion is stabilized by a lowering of spectral weight at the Fermi level, while in LaOFeAs by increase in Fe spin moment. Finally, we investigate the impact of electron doping upon CeOFeAs and LaOFeAs and show that (a) while in CeOFeAs the ground-state Fe moment remains largely unchanged by doping, the stability of magnetic order goes to zero at a doping that corresponds well to the vanishing of the Néel temperature and, (b) in contrast the LaOFeAs system remains magnetic with a slowly vanishing moment as a function of doping.