【作者】 彭麟；

【导师】 蔡传兵；

【作者基本信息】 上海大学 ， 凝聚态物理， 2010， 博士

【摘要】 在单晶上外延生长的REBa2CuO7-δ(RE=Y或Nd等稀土元素)超导薄膜由于高度的面内外晶粒织构消除了电的弱连接,以及岛状生长机制而产生的大量位错缺陷提供了有效的磁通钉扎中心,其超导临界电流密度在各类超导材料中是最高的。基于薄膜外延技术发展而来的第二代高温超导带材是当前超导材料研究的热点问题。然而高运行温度下的巨磁通跳跃、本征的结构各向异性和较短的相干长度使得高温超导材料在液氮温区的临界电流密度和不可逆场都需要进一步的提高,以满足各类强电应用的需求。另外一些高温超导块体中出现的物理现象(如磁化曲线的鱼尾效应和混合态的涡旋液相的再入等)在相应的薄膜材料中从未观察到。因此研究高温超导薄膜的磁通钉扎机制和探索各种增加人工磁通钉扎中心的有效途径不仅具有理解基础物性的意义,也是实现第二代高温超导带材强电应用的要求。另一方面,在基于薄膜外延的超导和铁磁异质结构中存在独特的近邻效应和自旋关联的传输行为。其中对于金属铁磁/超导异质结而言,两种长程序在界面发生竞争,可产生了一系列诸如π结、空间调制的序参数等奇异现象。近年来特别是在氧化物巨磁阻材料发现之后,氧化物超导和氧化物磁性物质构成的层状系统中的电磁特性研究引起了人们广泛的兴趣,超导与铁磁相互作用和共存等现象正有待深入的研究。本论文针对上述两个热点问题,开展如下方面的研究:(1)利用脉冲激光沉积技术和薄膜生长控制方法制备YBCO/YSZ准多层膜(YSZ,Yttria Stablized Zirconia的简称,它作为基底和缓冲层被广泛使用在YBCO薄膜的制备中,具有与YBCO匹配较好的晶格常数)。测量分析不同温度和外场下的磁传输特性,从而研究其磁通钉扎特性。(2)具有不同掺杂水平的La1-xCaxMnO3系统表现为不同的磁序行为。为了进一步理解高温超导/锰氧化物异质结的近邻效应,我们通过系统的磁化和传输测量研究了具有不同磁序的锰氧化物和高温超导所组成的异质结。此外,我们还利用Usadel方程,在考虑脏极限的情况下,从理论上分析了FM/S/FM自旋阀系统超导临界转变温度受铁磁层厚度的影响。主要研究成果如下:(1)在具有不同YSZ掺杂量的YBCO/YSZ准多层膜中观察到YSZ掺杂薄膜发生了化学反应,导致了具有纳米数量级钙钛矿异质相BaZrO3粒子的形成。BaZrO3粒子的形成促使了晶格的不匹配以致沉积中形成了c-轴关联的缺陷。这些c-轴关联的缺陷可能充当了强磁通钉扎中心,提高了高场下的临界电流密度,直接导致了Jc-H曲线上交叉行为。进一步对不可逆场、交叉场分析发现其对温度具有较强的依赖性。(2) YBCO/YSZ准多层膜系统临界电流密度J c的磁传输测量表明:生长控制方法对于高温高场下阻止磁通涡旋运动是非常有效的。角度相关的Jc ( H,T)在H//c区域附近出现的二次峰揭示了较多c-轴关联缺陷的存在。基于各向异性磁通钉扎理论,我们分离了各向同性钉扎和各向异性钉扎对临界电流密度的贡献。分析表明所研究的准多层膜中产生了各向异性的缺陷,由此形成强钉扎中心,使得准多层膜在高场下仍具有较高的临界电流密度。(3)在双层膜YBCO/LCMOi和YBCO/LCMOj系统中分析了磁相关的近邻效应特征。其交流磁化率测量表明,由于LCMOi或LCMOj层的存在,近邻超导层的临近转变温度受到了抑制。双层膜YBCO/LCMOi和YBCO/LCMOj磁化特征揭示了YBCO层中的迈斯纳电流和YBCO/LCMOi或YBCO/LCMOj中的磁场存在较强的相互作用。另一方面,含有反铁磁LCMO的YBCO/LCMOj系统中可能由于YBCO层中空穴向LCMOj层中的传输,使得靠近界面的LCMOj薄层中的电荷重新分布,从而发生从反铁磁向铁磁的磁序转变,表现出铁磁特征行为。(4)在LSMO/YBCO/LSMO/LCMO异质结构中观察到其特有的磁阻效应。该异质结构在靠近超导转变温度起始端的混合态区域具有正的磁阻效应,这可能有自旋相关的界面散射而引起;在温度提高是YBCO进行正常态后,由于YBCO层正的磁阻与LSMO层负的磁阻之间的竞争,系统产生了反转的磁阻效应。(5)利用Usadel方程对含有一个超导薄层的铁磁/超导/铁磁自旋阀系统的磁散射效应进行了理论分析。结果表明磁散射机制直接影响了自旋阀效应。一方面,对于具有小界面透明度的自旋阀系统,自旋阀效应很弱,磁散射对系统的超导电性和自旋阀效应影响较小,ΔT c /Tc随磁散射系数α呈指数下降;另一方面,对于大的界面透明度情况,自旋阀效应强,磁散射对系统的超导电性影响较大并能降低自旋阀效应。更多还原

【Abstract】 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.更多还原