Researches On The Superconductivity In R_efeaso And Magnetism Of R_ecoaso

In this dissertation, the superconductivity and magnetism of the quaternary -ZrCuSiAs type compounds RETMAsO (RE=Sm, Gd, Ce; TM=Fe, Co) have been investigated, in order to understand the correlations between the electronic behaviors and the superconductivity and magnetism more comprehensively. On the basis of these researches, exploration of the new superconductors has also been on progress. The main contents involved are presented as follows:effects of chemical doping at RE, Fe and O sites on the crystal structure, microstructure and the superconductivity of REFeAsO; the magnetism of RECoAsO (RE= Sm, Gd, Ce) compounds and the effect of Fe doping at Co site on the magnetic properties of SmCoAsO.In Chapter 1, the research background, importance, development of the iron-based superconductors, as well as the main purpose and content in this dissertation has been introduced.In Chapter 2, related preparation and measurement method of the samples has been clarified in detail.In Chapter 3,5d-transition metal Ir doping-induced superconductivity in the SmFeAsO and GdFeAsO systems has been investigated. The changes of the electrical properties for these two systems proved to be consistent with each other. However, the superconductivity depending on the Ir doping content shows different behavior in SmFeAsO and GdFeAsO system. SmFe1-xIrxAsO system is more sensitive to the Ir doping level whereas the GdFe1-xIrxAsO system shows superconductivity in a wider range of doping concentration. The highest superconducting transition in GdFe1-xIrxAsO system reaches 18.9 K which is higher than 17.3 K for SmFe1-xIrxAsO system. This highest Tc behavior is opposite to that in the F-doped REFeAsO systems, indicating a different superconducting mechanism between the doping at Fe site and O site. More investigation is needed to clarify the differences.A new low-temperature preparation process, in which the nano-scaled REF3 has been used as the starting materials, has been developed for REFeAsO1-xFx superconductors in Chapter 4.A series of SmFeAsO1-xFx and GdFeAsO1-xFx samples have been synthesized using both of the traditional solid state method and the present low temperature method. With increasing F content, the lattice constants shrunk and the superconducting transition temperature increased in both systems. A gradually saturation of the lattice constant and transition temperature are observed, leading to the appearance of the phase booundary. It is evident that the samples, prepared using low-temperature method, possess relatively larger shrinkage of the lattice constants and increase of Tc, higher top Tc and phase purities than the ones with the same doping level prepared by the traditional solid state method, suggesting that the low-temperature preparing process is more effective and practical in synthesizing the F-doped REFeAsO superconductors.In Chapter 5, Different sheathed (Cu/Nb and Ta) SmFeAsO0.8F0.2 wires have been fabricated by an in situ powder-in-tube method. The main investigations have been focused on the properties of the wires with different sheaths. Cu was observed diffusing into the cores under high temperature, leading to a fuzzy boundary between the sheath and the core. This diffusion destroyed the superconducting phase of SmFeAsO0.8F0.2-However, superconductivity at Tc=52.5 K. has been detected in the Ta-sheathed SmFeAsO0.8F0.2core. High intragrain Jc up to 2×106 A cm-2, a severe weak-link effect and a peak effect with a strongly temperature-dependent peak field Hpear has been observed in the Jc-H curves over the range 10-40 K, indicating that the iron-based superconductors might become another kind of competitive material for application.Effects of chemical doping on the superconductivity of SmFeAsO0.8F0.2 and SmFeAsO have been discussed in Chapter 6. Y doping at Sm site in SmFeAsO induced the shrinkage of the lattice parameters and the decrease of the TSDW without superconducting transition. The superconductivity of Sm1-xYxFeAsO0.8F0.2 is suppressed by Y doping for the negative pressure effect. It is concluded that the shrinkage of the lattice parameters and chemical pressure effect would just affect superconductivity but not induce superconductivity. The lattice parameters of SmFeAsO0.8F0.2 have been stretched by Zn doping at Fe site whereas the Tc has been sharply suppressed. With increasing Zn doping contents, the SmFe1-xZnxAsO0.8F0.2 compounds changes from superconductor to semiconductor.In Chapter 7, the magnetic and transport properties of RECoAsO (RE=Sm, Gd,Ce) have been investigated. All of the compounds present to be metallic behavior with low resistivity at room temperature. Under various temperatures and magnetic fields, multiple magnetic properties including ferromagnetism, antiferromagnetism, paramagnetism and ferrimagnetism etc., have been observed. Furthermore, metamagnetic transition caused by the spin canting has also been detected below the antiferromagnetic transition temperature in SmCoAsO and GdCoAsO. However, CeCoAsO shows different magnetism. It presents to be hard ferromagnetism with successive ferromagnetic transition under low temperature.In Chapter 8, effects of Fe doping on the magnetic and transport properties of SmCoAsO have been investigated. The ferromagnetism and antiferromagnetism coexists in the SmCo1-xFexAsO system. With Co partially replaced by Fe, the antiferromagnetism was suppressed whereas the ferromagnetism was enhanced. The F doping in SmCoAsO enhanced the interactions between two Co sublattices, stabilized ferromagnetic state under high temperature, inducing the increase of Tc. The investigations above revealed that the magnetism in SmCoAsO is originated from the interactions between the Co-As layers or Co the sublattices. The transport properties indicated that both of the SmCo1-xFexAsO and SmCoAsO1-xFx systems possess metalic characteristics.