The phase transition of ThFe_1-xCo_xAsN from superconductor to metallic paramagnet

Differently from most of the other 1111-type iron-based superconductors, ThFeAsN itself shows superconductivity at 30 K without antiferromagnetism, even in the absence of chemical doping and other treating. In order to understand its peculiar behavior better, it is necessary to investigate the evolution of the superconducting phase through electron doping. Chemically, Co doping is a more effective way to introduce electrons, as carriers are doped directly into the FeAs planes. It also could provide information on how well the ThFeAsN tolerates in-plane disorder. Here we have substituted Co for Fe to synthesize ThFe1-xCoxAsN. It is found that the superconductivity of ThFeAsN parent compound is quickly suppressed upon Co doping. With a doping amount of), the superconductivity of ThFe1-xCoxAsN vanishes. ThCoAsN has been synthesized and characterized. It shows no superconductivity at 1.8 K. As both its crystal structure and transport behaviour are similar to those of itinerant ferromagnets LaCoAsO and LaCoPO, it is expected that ThCoAsN would be a kind of itinerant ferromagnet. However, until 1.8 K, the expected itinerant ferromagnetism could not be confirmed. The experimental data support that ThCoAsN is a kind of metallic paramagnet above 1.8 K.