Flux Pinning In Oxide Superconductor Quasi-Multilayers And Proximity Effect Of Superconductor/Ferromagnet Heterostructures

High-temperature superconducting REBa2CuO7-δ(RE denotes rare earth) thin film expitaxially grown on single crystal exhibit the highest critical current density due the deletion of weak linking and a large amount of dislocations arising from island-like growing mechanism. The secondary-generation superconductor coated conductors based on the RE123 thin films have been the hot topic of applied superconducting materials. However, it is recently found that the Jc at high fields is mainly limited by the properties of intragrains. To make the second generation high temperature superconducting cable broadly applied in electrical power project, improving the critical current density , irreversibility field and decreasing the AC loss must be first resolved. So improving flux pinning within the R123 grains is therefore one area of chief concern for the development of conductor technology. Thus, reaserches on arficial flux pinning RE123 thin film and its coated conductor, are significant not only for understanding of fundamental physics (To see potential peak effect, or re-entrant of vortex liquid in low fields), but also practical applicationscience.On the other hand, the fabrication of heterostructures for devices incorporating high-temperature superconductors (cuprates) and magnetic oxides in the lanthannum mangnite family (manganites) is a relatively new application. Ferromagnetic (F) and superconducting (S) orders are antagonistic in the sense that ferromagnetism produces parallel and superconductivity antiparallel aligment of the spins. When a superconductor is placed in contact with a ferromagnet both long-range phenomena compete at the interface, giving rise to a variety of exotic phenomena likeπjunctions, spatially modulated order parameter, etc. Thus, reaserches on the proximity effect of superconductor and ferromagnet are significant not only for understanding of fundamental physics, but also the development of new device. In this dissertation, we will focus on two important aspects: (a) a kind of growth-controlled quasi-multilayer consisting of YBCO and Yttria Stablized Zirconia (YSZ) is prepared with a given ratio of two compounds. The original doping phase of YSZ is a widely used substrate or buffer material for the growth of YBCO films, having a matchable lattice constant with YBCO as well. Magneto-transport properties at various temperarure and mangeic fields are investigated with respect to flux pinning properties. (b) The La1-xCaxMnO3 systems with different doping level exhibit variable magnetic orders. In order to further understand and investigate the proximity effect in oxide FM/SC heterostructures, we study some distinct FM/SC heterostructures including the ferromagneic layers with different magneic orders. In addition, Ferromagnet/superconductor/ferromagnet (F/S/F) spin valve systems in the dirty limit, described by Usadel equations, was theoretically investigated with respect to superconducting transition temperature.The major results are summarized as follows:(1) A series of quasi-multilayer films of YBCO/YSZ are prepared by means of pulsed laser deposition (PLD). X-ray diffraction measurements reveal that a chemical reaction takes place in YSZ-doped films, leading to a heterogeneous BaZrO3 perovskite phase. The presence of the second Jc peak observed from the angular dependence of Jc at H//c implies that the c-axis correlated defects increase due to lattice mismatch induced by YSZ doping. The quasi-multilayers with low doping content may show the increased flux pinning force density and then enhanced Jc in high magnetic fields, leading to a crossover behavior in Jc–H curves compared to the pure YBCO thin film. Furthermore, a strong temperature dependence of such a crossover behavior is observed. As the temperature increases, the artificial tailoring of flux pinning may appear clearer since the mixed vortex state moves towards to the region of weak vortex glass and vortex liquid.(2) The temperature and magnetic field dependences of anisotropic pinning contributions of YBCO/YSZ quasi-multilayers have been investigated by the measurement of angular dependent critical current density (Jc) . With the isotropic and anisotropic pinning contributions in a wide range of temperature, we identify the possible sources of the pinning centers, and classify them into the weak and the strong flux pinning model. Angular-dependent Jc(H, T) measurements have demonstrated that the growth control strategy is very effective in preventing the vortex motion at high fields and high temperatures. It is suggested that at high applied fields, such as 7T, the pinning contribution of the nanostructured quasi-multilayers is dominated by the anisotropic disorders, while at intermediate-low fields (such as 1T) the pinning contribution is determined by both isotropic and anisotropic disorders, suggesting the co-existence of isotropic and anisotropic pinning.(3) The YBa2Cu3O7-ō/La0.88Ca0.12MnO3 (YBCO/LCMOi) and YBa2Cu3O7-ō/ La0.33Ca0.67MnO3 (YBCO/LCMOj) bilayers are fabricated with the method of pulsed laser deposition, and the magnetic dependent proximity effects are investigated. The magnetic fields were applied parallel (in-plane) and perpendicular (out-of-plane) to the film plane. Magnetic properties curves in different temperature range and magnetic field along two crystal orientation show a complex behavior due to the interplay between Meissner currents in YBCO layer and the magnetic fields present in LCMOi (or LCMOj) layer. The ac susceptibility measurements show that the suppression of superconductivity in LCMOi and LCMOj bilayers. The obtained results are discussed in terms of the effect of hole charge transfer from YBCO to LCMOi (or LCMOj) on the proximity between superconductivity and ferromagnetism.(4) The oxide heterostructure consisting of La0.7Sr0.3MnO3/YBa2Cu3O7-δ/ La0.7Sr0.3MnO3/La0.33Ca0.67MnO3 is fabricated by using the technique of pulsed laser deposition (PLD). The magnetoresistance (MR) for the system is investigated in the region of mixed vortex state near the onset temperature of superconducting transition. The present experimental results indicate a reversal variation of MR is observed with increasing temperature in the system, implying the competition between a positive MR of YBa2Cu3O7-δand a negative MR of La0.7Sr0.3MnO3. Furthermore, our analysis further suggests that the large MR peaks are mainly caused by spin-dependent interface scattering, which depends on the relative orientation of magnetization in two ferromagnetic layers. And it is proved that the superconductivity is an essential factor for the large positive MR in the artificial heterostructure system.(5) Ferromagnet/superconductor/ferromagnet (F/S/F) spin valve systems in the dirty limit, described by Usadel equations, was theoretically investigated with respect to superconducting transition temperature. Their superconducting characteristics strongly depend not only on the mutual orientation and thickness of the ferromagnetic layers, but also on the the interface transparency as well as magnetic scattering. Especially, the introduction of magnetic scattering drastically reduces the spin-valve effect in our work. The obtained results could be used to understand experimental values ofΔT c =TcAP?TcP and to provide guidelines for optimizing the experimental systems.