Evaluation method for flux pinning properties in the mixed state of superconductors

1. 采用电子束蒸发法在高J_c的Si衬底上制备了MgB_2薄膜。在O_2气氛中沉积MgB_2薄膜提高了J_c值。在O_2气氛中沉积的MgB_2薄膜在磁场中表现出较高的J_c。研究发现，o_2掺杂薄膜的高J_c是由于沿晶界引入MgO加强晶界的磁通钉住作用所致。在强磁场中，观察到E-J曲线分两段变化的奇特行为。这种行为也源于强化晶界的通量钉住。采用PLD法制备了BaZrO_3纳米棒，对ErBa_2Cu_3O_y和YBa_2Cu_3O_y薄膜的磁钉钉性能进行了测试，探讨了磁钉钉钉机理。通过测量J_c对磁场的依赖性，证实了J_c的增强作用。J_c的角依赖性在B‖c轴处有一个宽峰。结果表明，BaZrO_3纳米棒作为c轴相关钉钉中心。根据电场-电流密度特性对钉钉参数m进行了评估，m的峰值出现在磁场依赖性中。m的特征行为是由BaZrO_3纳米棒的密度与流体的密度匹配引起的。结果表明，含BaZrO_3纳米棒的ErBa2Cu_3O_y薄膜的局部临界电流密度相对分布宽度小于含BaZrO_3纳米棒的YBa_2Cu_3O_y薄膜。

1. As-grown MgB_2 thin films on Si substrates with high J_c under magnetic fields were prepared by electron-beam evaporation. The value of J_c has been enhanced by the deposition of MgB_2 thin film in an O_2 atmosphere. The MgB_2 thin film deposited in the O_2 atmosphere O_2-doped film has exhibited considerably higher J_c in magnetic fields among MgB_2 thin films reported before. It has been found that the high J_c of the O_2-doped film is attributable to the flux pinning with grain boundaries strengthened by an introduction of MgO along grain boundaries. In a high magnetic field, a peculiar behavior of E-J characteristics where E-J curves vary in two stages was observed. This behavior also originates from the flux pinning with strengthened grain boundaries.2. Flux pinning properties in ErBa_2Cu_3O_y and YBa_2Cu_3O_y thin films with BaZrO_3 nano-rods prepared by PLD were measured to investigate the flux pinning mechanism. The enhancement of J_c was confirmed by the measurement of the dependence of J_c on magnetic field. The angular dependence of J_c has a broad peak at B‖c-axis. This result indicates that the BaZrO_3 nano-rods work as the c-axis-correlated pinning centers. The pinning parameter m was evaluated from the electric fields versus current density characteristics and the peak of m appears in the magnetic field dependence. The characteristic behavior of m is caused by the matching of the density of BaZrO_3 nano-rods with that of fluxoids. It is found that the relative distribution width of the local critical current density for the ErBa2Cu_3O_y film with BaZrO_3 nano-rods is smaller than that of the YBa_2Cu_3O_y films with BaZrO_3 nano-rods.