Superconductivity in NdFe1-xCoxAsO (0.05 NdCoAsO

The phase diagram of NdFe1-xCoxAsO 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) mu(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) angstrom. A comparison of the maximum reported critical temperatures of well-characterized cobalt-doped 122- and 1111-type superconductors is presented.