Superconductivity and Unusual Magnetism in the Solid Solution NdFe$_{1-x}$Co$_{x}$AsO

NdFeAsO is a low carrier concentration semimetal which undergoes a spin density wave transition, and is a parent phase of iron pnictide superconductors. Doping with fluorine is known to produce superconductivity with Tc up to 52 K. Through a study of the solid solution NdFe$_{1-x}$Co$_{x}$AsO, we demonstrate that replacement of iron with cobalt also induces superconductivity, as has been reported for several other iron pnictide systems. Of particular interest in this system is the end-member NdCoAsO, which shows unusual magnetic behavior. This material undergoes three magnetic phase transitions at low temperatures. We report a detailed study of NdCoAsO including results from transport, magnetization, and heat capacity measurements, as well as neutron diffraction.