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=85  K) with an ordered moment of 0.30(1)  µ_B at 15 K. Below T_N=9  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(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 T_c 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