Transport, magnetic, and 57Fe and 155Gd Moessbauer spectroscopic properties of GdFeAsO and the slightly overdoped superconductor Gd0.84Th0.16FeAsO

The results of x-ray diffraction, electrical resistivity, magnetic susceptibility, and 57Fe and 155Gd Moessbauer spectroscopy studies of polycrystalline samples of nonsuperconducting GdFeAsO and superconducting Gd0.84Th0.16FeAsO are reported. The superconductor Gd0.84Th0.16FeAsO has the onset transition temperature of 54.4(1) K. We find that Fe and Gd magnetic moments in GdFeAsO order antiferromagnetically at 132.7(1) K and 4.1(1) K, respectively. In Gd0.84Th0.16FeAsO, Fe atoms carry no magnetic moment down to 1.9 K whereas Gd magnetic moments order below 2.9(1) K. This is taken as evidence for the coexistence of Gd magnetic order and superconductivity. The Debye temperatures of GdFeAsO and Gd0.84Th0.16FeAsO are, respectively, 409(4) and 389(3) K.