The phase diagram of ${\text{NdFe}}_{1-x}{\text{Co}}_x\text{AsO}$ for low cobalt substitution consists of a superconducting dome $\left(0.05<x<0.20\right)$ with a maximum critical temperature of 16.5(2) K for $x=0.12$. The $x=1$ end member, NdCoAsO, is an itinerant ferromagnet $\left(T_C=85\text{K}\right)$ with an ordered moment of $0.30\left(1\right){\mu }_{\text{B}}$ at 15 K. Below $T_N=9\text{K}$, Nd spin ordering results in the antiferromagnetic coupling of the existing ferromagnetic planes. Rietveld analysis reveals that the electronically important twofold tetrahedral angle increases from $111.4°$ to $115.7°$ in this series. Underdoped samples with $x=0.046\left(2\right)$ and $x=0.065\left(2\right)$ show distortions to the orthorhombic $Cmma$ structure at 72(2) and 64(2) K, respectively. The temperature dependences of the critical fields $H_{\text{c}2}\left(T\right)$ near $T_c$ are linear with almost identical slopes of $2.3\left(1\right)\text{T}{\text{K}}^{-1}$ for $x=0.065\left(2\right)$, $x=0.118\left(2\right)$, and $x=0.172\left(2\right)$. The estimated critical field $H_{\text{c}2}\left(0\right)$ and correlation length for optimally doped samples are 26(1) T and $36\left(1\right)\text{Å}$. A comparison of the maximum reported critical temperatures of well-characterized cobalt-doped 122- and 1111-type superconductors is presented.

• Received 11 September 2009

DOI:https://doi.org/10.1103/PhysRevB.81.064511