Tunable interplay between3dand4felectrons in Co-doped iron pnictides

Abstract

We study the interplay of $3d$ and $4f$ electrons in the iron pnictides CeFe${}_{1\ensuremath{-}x}$Co${}_{x}$AsO and GdFe${}_{1\ensuremath{-}y}$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${}_{1\ensuremath{-}x}$Co${}_{x}$AsO and $0.05<y<0.25$ for GdFe${}_{1\ensuremath{-}y}$Co${}_{y}$AsO, showing a maximum transition temperature ${T}_{\mathrm{sc}}$ 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 ${T}_{\mathrm{C}}^{\mathrm{Co}}\ensuremath{\approx}$ 75 K at $x=1$ and $y=1$. In the Ce compounds, the N\'eel temperature ${T}_{\mathrm{N}}^{\mathrm{Ce}}$ increases upon suppressing the SDW transition of Fe and then remains nearly unchanged with further increasing Co concentration up to $x\ensuremath{\simeq}0.8$ (${T}_{\mathrm{N}}^{\mathrm{Ce}}\ensuremath{\approx}$ 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 $0.7\ensuremath{\le}y<1.0$, the system undergoes a possible magnetic reorientation below the N\'eel temperature of Gd (${T}_{\mathrm{N}}^{\mathrm{Gd}}$). 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.