Superconductivity and magnetism in Ir-doped GdFeAsO

The 5d-transition metal, Ir has successfully been doped at Fe site and induced superconductivity in GdFeAsO at T_c = 18.9 K and ∼20 atom%. The Ir-doping shortened the c-axis length and stretched the a-axis one, which led to enhance the coupling between the FeAs- and SmO-layer, and to weaken the bonding between Fe and As atom. Paramagnetism was observed in all of the samples, which was resulted from the magnetic Gd ion as in the F-doped GdFeAsO. An upper critical field of GdFe_0.8Ir_0.2AsO was extrapolated to around 24 T, much smaller than that of F-doped GdFeAsO owing to a relatively low T_c and small value of dH_c2/dT.