Weak ferromagnetism of and its evolution with Co doping

We have investigated electronic and magnetic properties of iron pnictide CuxFe1+yAs, which is isostructural to iron pnictide superconductors LiFeAs and NaFeAs, using polycrystalline and single-crystal samples. We found that CuxFe1+yAs is characterized by strong electron correlations and shows weak ferromagnetic behavior with the Curie temperature T-c similar to 42 K, in contrast with the superconductivity in LiFeAs and NaFeAs. Its electronic transport properties exhibit quasi-two-dimensional characteristics: While the in-plane resistivity follows quadratic temperature dependence for T < 10 K, the c-axis resistivity shows nonmetal-to-metal crossover near 20 K. In addition, we have studied the Co-doping effect on CuxFe1+yAs and found that Co doping enhances the ferromagnetism with T-c increasing up to 83 K for CuxFe0.5Co0.5As. From these results, we discussed the origin of the ferromagnetism of CuxFe1+yAs and its relevance with the magnetism of iron pnictide superconductor parent compounds.