Moderate interaction between 3d- and 4f-electrons and ferrimagnetism in Co-doped GdFeAsO

We synthesized a series of GdFe${}_{1-x}$Co${}_x$AsO polycrystalline samples ($0 \leq x \leq 1$) by using a solid state reaction method and present a systematic study on the physical properties by means of electrical resistivity $\rho (T)$, magnetic susceptibility $\chi (T)$ and specific heat $C(T)$. The parent compound GdFeAsO undergoes a spin-density-wave (SDW) transition associated with Fe 3d-electrons around 130 K, followed by an antiferromagnetic (AFM) transition of Gd at $T_{\text{N}}^{\text{Gd}}\approx$ 4 K. The SDW transition is quickly suppressed by Fe/Co substitution and superconductivity appears in a narrow doping range of $0.05<x<0.25$, showing a maximum $T_{\text{sc}}$ $\approx$ 20 K around $x=0.1$. On the other hand, the 4f-electrons of Gd are antiferromagnetically ordered over the entire doping concentration ($0 \leq x \leq 1$), while the Co 3d-electrons exhibit a ferromagnetic (FM) transition above $x\approx 0.8$, with the Curie temperature ($T_{\text{C}}^{\text{Co}}$) reaching 75 K in GdCoAsO. These two magnetic species (Gd and Co) are coupled antiferromagnetically to give rise to ferrimagnetic behavior in magnetic susceptibility on the Co-rich side. For $0.7\le x<1.0$, the system undergoes a possible magnetic reorientation below $T_{\text{N}}^{\text{Gd}}$.