新型铁磁性纳米掺杂对钆钡铜氧薄膜磁通钉扎性能的影响及其钉扎机理研究

Carrying high electrical currents without losses in very high magnetic field is a difficult task for (Re) Ba2Cu3O7-x (Re BCO) coated conductors. Introducing magnetic pinning could be an effective method for the problems of pinning in (Re) BCO materials. Two factors will inhabit the applying of magnetic pinning technology. First, inter-diffusion between magnetic nanoparticles and (Re) BCO matrix would cause degradation of (Re) BCO intrinsic properties. Second, whether magnetic pinning is a truly pinning effect for magnetic nanoparticles is unclear. In order to solve inter-diffusion question, ferromagnetic La0.67Sr0.33MnO3 (LSMO) nanoparticles with better thermodynamic/chemical stability will be prepared for (Gd) Ba2Cu3O7-x (GdBCO) films. In this project, we will design a new experiment to elucidate magnetic pinning and core pinning effects of LSMO nanoparticles. We will focus on three important aspects: (a) By RF-Sputtering method, we will find the main parameters that control the shape/density and size of LSMO nanoparticles, and investigate the influence of LSMO nanoparticles with different shape/density and size on the pinning characteristics of GdBCO films; (b) We will investigate the pinning characteristics of GdBCO films deposited on or beneath LSMO nanoparticles. In particular, we plan to distinguish magnetic pinning from core pinning, which arises from microstructural features, defects and strain of the sample. Pinning types of magnetic pinning and core pinning will be elucidated. The anisotropic of magnetic pinning and core pinning contributions from LSMO nanoparticles with different magnetic field and temperature will be studied by measuring the angular dependent critical current density (Jc). By analysis of the anisotropic pinning and isotropic contributions in a large range of temperature, possible pinning sources will be identified, and they will be classified into the strong and weak flux pinning model; (c) A series of GdBCO/LSMO quasi-multilayer films will be deposited by RF-Sputtering method. We will optimize the flux pinning properties of GdBCO/LSMO system, to find the most enhanced Jc and increased flux pinning force density in high magnetic fields. Thus, researches on new magnetic artificial flux pinning in (Re) BCO films are significant not only for practical applications of superconductivity, but also understanding of fundamental physics , for example , potential peak effect, or vortex liquid issue in low fields.

提高（Re）BCO薄膜磁场下的载流能力是涂层导体领域的重大难题，人工引入“磁性钉扎中心”是解决该问题的有效方法。但是，铁磁材料与（Re）BCO的互扩散、“磁性钉扎中心”的钉扎特性尚不清楚，成为制约该方法推广的主要瓶颈。本项目拟采用LSMO铁磁材料掺杂的手段，对LSMO引入（Gd）BCO薄膜的“磁性钉扎”和“芯钉扎”的特性进行深入研究。具体内容有：（1）研究（Gd）BCO薄膜临界电流密度Jc与LSMO纳米颗粒的形状和密度等因素之间的关系；（2）用基底修饰和表面修饰两种手段，定量研究LSMO纳米颗粒引入GdBCO薄膜内部的“磁性钉扎”和“芯钉扎”的特性，确定各自的钉扎类型，研究两种钉扎的各向异性特征；（3）制备（GdBCO）m /（LSMO）n准多层膜,优化该体系的钉扎效果。本项目将为向（Re）BCO薄膜中引入有效的“磁性钉扎中心 ”奠定基础，具有重要的理论意义和工程应用价值。

本项目采用射频磁控溅射技术在STO(SrTiO3)单晶基底上获得了LSMO(La0.7Sr0.3MnO7)纳米颗粒可控制备的生长参数，成功制备了尺寸和密度可控的LSMO(La0.7Sr0.3MnO7)纳米颗粒。然后在LSMO颗粒之上沉积（Gd）BCO超导薄膜。我们测量分析了样品在不同磁场、不同温度下的磁性传输性能，研究了LSMO颗粒的钉扎特性。GBCO/LSMO系统的电流密度的磁性传输测量表明：LSMO纳米颗粒修饰法可有效的提高GBCO薄膜的磁场下的超导载流能力和最大“钉扎力密度”。同时可将最大“钉扎力密度”的磁场推向更高的磁场强度。在（Gd）BCO /LSMO系统中，我们发现了两种钉扎机制，它们分别是“体钉扎”和“磁性钉扎”机制。LSMO纳米颗粒物本身是有效的“磁性钉扎中心”。由LSMO纳米颗粒物造成的GBCO薄膜内部沿c轴的“线性相关缺陷”是有效的“体钉扎中心”。这为研究超导薄膜与磁性颗粒之间的耦合效应提供了确切的实验证据。同时充分证明了磁性纳米颗粒对提高超导薄膜的磁场钉扎性能非常有效，寻找新型的磁性纳米材料掺杂将对高温超导薄膜的实际应用打开广阔的前景。. 该项目还研究发现对于（Gd）BCO超导薄膜来说，基底生长温度和薄膜厚度，同样可以调控薄膜的磁场下的载流能力，影响到薄膜磁场下“钉扎能力”的各向异性。研究发现，较低基底温度生长的超导薄膜，磁场下载流能力各向异性较弱。随着（Gd）BCO薄膜厚度的增加，JC迅速减小。当样品厚度为0.5微米时， JC随磁场增加以较为慢的速率下降。当磁场强度H接近和高于某一个临界值Hd（700mT）时，JC的厚度依赖性变得不明显。这些结果进一步证实了超导薄膜层间磁耦合对降低高温超导厚膜的JC随厚度衰减效果起到起主导作用。. 项目研究期间培养了4名硕士研究生；发表了4篇论文，另外1篇已经被接收；其中4篇被SCI收录，1篇中文核心期刊收录。