Superconductivity in NdFe_1-xCo_xAsO (0.05<x<0.20 ) and rare-earth magnetic ordering in NdCoAsO

The phase diagram of NdFe_1-xCo_xAsO for low cobalt substitution consists of a superconducting dome (0.05<x<0.20) with a maximum critical temperature of 16.5(2) K for x=0.12. The x=1 end member, NdCoAsO, is an itinerant ferromagnet (T_C =85K) with an ordered moment of 0.30 (1) µB at 15 K. Below TN =9K, 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 (2) and x=0.065 (2) show distortions to the orthorhombic Cmma structure at 72(2) and 64(2) K, respectively. The temperature dependences of the critical fields H_c2 (T) near Tc are linear with almost identical slopes of 2.3 (1) T K^-1 for x=0.065 (2), x=0.118 (2), and x=0.172 (2). The estimated critical field H_c2 (0) and correlation length for optimally doped samples are 26(1) T and 36 (1) Å. A comparison of the maximum reported critical temperatures of well-characterized cobalt-doped 122- and 1111-type superconductors is presented. © 2010 The American Physical Society.