  The effects of replacing small amounts of Co with Fe in NdCoAsO are reported.
Polycrystalline materials with compositions NdCo1-xFexAsO (x = 0.05, 0.10,
0.15, and 0.20) are studied and the results compared to previous reports for
NdCoAsO. Rietveld analysis of powder x-ray diffraction data shows that as Fe
replaces Co on the transition metal (T) site, the T-As distance increases, and
the As tetrahedra surrounding the T-site become more regular. Electrical
resistivity and magnetization measurements indicate that the three magnetic
phase transitions in NdCoAsO are suppressed as Co is replaced by Fe, and these
transitions are not observed above 1.8 K for x = 0.20. Based on these results,
the magnetic phase diagram for the Co-rich side of the NdCoAsO-NdFeAsO system
is constructed.
