Why magnetism in CeO$_{1-x}$F$_x$FeAs and LaO$_{1-x}$F$_x$FeAs is different

TL;DR

This study uses advanced first-principles calculations to compare the magnetic behaviors of CeOFeAs and LaOFeAs, revealing differences in magnetic coupling, structural distortion origins, and doping effects.

Contribution

It provides a detailed first-principles analysis of magnetic structures and structural distortions in CeOFeAs, highlighting key differences from LaOFeAs and the effects of electron doping.

Findings

Ce layer moments are perpendicular to Fe layers.

Weak Ce-Ce magnetic coupling with strong Fe-Ce coupling.

Structural distortion in CeOFeAs is driven by Fermi level spectral weight changes.

Abstract

Using state-of-the-art first-principles calculations we study the magnetic behaviour 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-Ce coupling. Comparison of the origin of the tetragonal to orthorhombic structural distortion in CeOFeAs and LaOFeAs show marked differences; in CeOFeAs the distortion is stabilized by a lowering of spectral weight at the Fermi level, while in LaOFeAs by a reduction in magnetic frustration. Finally, we investigate the impact of electron doping upon CeOFeAs and show that while 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 N\'eel temperature.