The Investigation Of Superconducting And Magnetic Properties For YBCO And Its Composites

In recent years, room temperature ferromagnetism has been reported in small-scale superconducting materials. Within the BCS theory region, it is difficult to understand these. Furthermore, numerous experiments have revealed ferromagnetism can coexist with superconducting state. A great number of works have also shown that ferromagnetism is an intrinsic nature of nonorganic nanoparticles materials, and a defect-induced-ferromagnetism mechanism is established. But for low-dimensional superconducting systems, the origination of ferromagnetism is open question. A key problem is whether ferromagnetism can coexist with superconductivity at low temperature. It remains controversy that superconductivity is based on magnetism. Inspired by these, we try to explore the mechanism of ferromagnetism in low-dimensional superconductors. The main content and results is as follows:1. Room temperature ferromagnetism in YBa2Cu3O7-δ particlesIn this section, we employed a citrate pyrolysis technique to prepare superconducting YBa2Cu3O7-δ system. Magnetic measurement revealed room temperature ferromagnetism in the samples. Magnetic data at low temperature indicated the existence of superconductivity. Electron spin resonance spectra at low temperatures further confirmed that there is a transition from the normal to the superconducting state. The ZFC-FC curves at various applied magnetic fields showed that the superconducting transition temperature is92K, and it decreases with increasing of the magnetic field. Post-heating treatment at argon gas displayed that the ferromagnetism is enhanced but the superconductivity suppressed, suggesting that the ferromagnetism maybe originate from the oxygen defects. In addition, the upturn of FC curve at low temperature indicated the coexistence of ferromagnetism and superconductivity.2. Room temperature ferromagnetism in Y2BaCuO5particlesGreen phaseY2BaCuO5particles were prepared via a citrate pyrolysis technique. Scanning electron microscopy and transmission electron microscopy observation showed the formation of a particle chain Y2BaCuO5phase with the size of hundreds of nanometers. Magnetic measurements showed that all annealed samples exhibit room temperature ferromagnetism and the saturation magnetization decreases with increasing sintering temperature. The magnetic data of the precursor and the EDS result showed that there is not any magnetic contamination. So the ferromagnetism is intrinsic nature. Two magnetic phase transitions located at about11and110K were revealed at the ZFC-FC curves. The peak at about11K was associated with antiferromagnetic phase transition and a spin-glass transition at about110K was confirmed. Similarly, post-heating treatment under argon atmosphere was also performed on the samples. After heating treatment, the ferromagnetism increased greatly. And it was further enhanced as the flow rate increased. These results suggested that the magnetism of Y2BaCuO5particles is attributed to the surface oxygen defects.3. Room temperature ferromagnetism in Y2O3nanoparticlesWe adopted a glycine-nitrate method to synthesize nanoparticles of Y2O3. Magnetic measurement showed the existence of room temperature ferromagnetism. X-ray photoelectron spectroscopy results indicated that there is not any magnetic contamination and then the observed ferromagnetism is intrinsic. After post-heated at vacuum and pressed, an obvious increase of the ferromagnetism was observed. But for the sample heated in the air, the ferromagnetism decreased. These results suggested that the ferromagnetism is responsible for oxygen defects. By analyzing the multicomponent fitting to the O1s peaks for these samples, the correlation between the ferromagnetism and oxygen defects in Y2O3nanoparticles is established.4. Room temperature ferromagnetism in YBa2Cu3O7-δ-Y2BaCuO5compositesIn this section, we prepared YBa2Cu3O7-δ-Y2BaCuO5composites by a citric acid complexing-hydrothermal synthesis coupled method and a citrate pyrolysis technique. Firstly, the composite system was obtained via the citric acid complexing-hydrothermal synthesis coupled method. Scanning electron microscopy observation showed the formation of microspheres. Magnetic measurement showed room temperature ferromagnetism in these composites. Electron spin resonance spectra and magnetic data at low temperature confirmed the superconducting behavior. Also the superconducting transition located at93K was revealed by ZFC-FC curves. X-ray photoelectron spectroscopy and EDS results indicated that there is not magnetic contamination, suggesting the intrinsic nature of the ferromagnetism. Post-heating treatment at argon gas showed that the observed ferromagnetism should not be attributed to oxygen defects and be associated to the interface at the composites, which was observed in HRTEM. In addition, we employed the citrate pyrolysis technique to prepare the YBa2Cu3O7-δ-Y2BaCuO5composites. Magnetic measurement revealed that the ferromagnetism of the composites is greater than that of their pure phases. Also post-heating treatment at argon gas confirmed that the ferromagnetism results from oxygen defects and is attributed to the interface at the composites. A clear interface in the composite is observed in the HRTEM picture. These work revealed that the ferromagnetism in the YBa2Cu3O7-δ-Y2BaCuO5composites originates from the interface between the two phases.