Tunable interplay between 3d and 4f electrons in Co-doped iron pnictides

We study the interplay of 3d and 4f electrons in the iron pnictides CeFe_1-xCo_xAsO and GdFe_1-yCo_yAsO, which correspond to two very different cases of 4f-magnetic moment. Both CeFeAsO and GdFeAsO undergo a spin-density-wave (SDW) transition associated with Fe 3d electrons at high temperatures, which is rapidly suppressed by Fe/Co substitution. Superconductivity appears in a narrow doping range: 0.05<x<0.2 for CeFe_1-xCo_xAsO and 0.05<y<0.25 for GdFe_1-yCo_yAsO, showing a maximum transition temperature T_sc of about 13.5 K for Ce and 19 K for Gd. In both compounds, the 4f electrons form an antiferromagnetic (AFM) order at low temperatures over the entire doping range and Co 3d electrons are ferromagnetically ordered on the Co-rich side; the Curie temperature reaches TCCo≈ 75 K at x=1 and y=1. In the Ce compounds, the Néel temperature TNCe increases upon suppressing the SDW transition of Fe and then remains nearly unchanged with further increasing Co concentration up to x≃0.8 (TNCe≈ 4 K). Furthermore, evidence of Co-induced polarization on Ce moments is observed on the Co-rich side. In the Gd compounds, the two magnetic species of Gd and Co are coupled antiferromagnetically to give rise to ferrimagnetic behavior in the magnetic susceptibility on the Co-rich side. For 0.7≤y<1.0, the system undergoes a possible magnetic reorientation below the Néel temperature of Gd (TNGd). Our results suggest that the effects of both electron hybridizations and magnetic exchange coupling between the 3d-4f electrons give rise to a rich phase diagram in the rare-earth iron pnictides.