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$_{1-x}$Co$_x$AsO and GdFe$_{1-y}$Co$_y$AsO, 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$_{1-x}$Co$_x$AsO and $0.05 < y < 0.25$ for GdFe$_{1-y}$Co$_y$AsO, showing a maximum transition temperature $T_\textup{sc}$ 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 $T_\textup{C}^\textup{Co} \approx$ 75 K at $x = 1$ and $y = 1$. In the Ce-compounds, the N\'{e}el temperature $T_\textup{N}^\textup{Ce}$ increases upon suppressing the SDW transition of Fe and then remains nearly unchanged with further increasing Co concentration up to $x \simeq 0.8$ ($T_\textup{N}^\textup{Ce}\approx$ 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 $0.7 \leq y < 1.0$, the system undergoes a possible magnetic reorientation below the N\'{e}el temperature of Gd ($T_\textup{N}^\textup{Gd}$). 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.