  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 (TC = 85 K) with an ordered moment of 0.30(1) BM at 15 K. Below TN
= 9 K, Nd spin-ordering results in the antiferromagnetic coupling of the
existing ferromagnetic planes. Rietveld analysis reveals that the
electronically important two-fold tetrahedral angle increases from 111.4 to
115.9 deg. 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 Hc2(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 Hc2(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.
