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

1.采用电子束蒸发法在磁场作用下在Si衬底上制备了高Jc的MgB_2薄膜。在O_2气氛中沉积MgB_2薄膜，提高了J_c值。在已报道的MgB_2薄膜中，在O_2气氛中沉积的掺氧MgB_2薄膜在磁场中具有较高的J_c。结果表明，沿晶界引入沿着MgO，增强了晶界的磁通钉扎作用，从而提高了薄膜的J_c。在强磁场下，观察到E-J特性曲线呈两个阶段变化的特殊行为。这种行为也源于强化晶界的磁通钉扎.采用PLD法制备了BaZrO_3纳米棒对ErBa_2Cu_3O_y和YBa_2Cu_3O_y薄膜的磁通钉扎性能进行了研究，探讨了磁通钉扎机理。通过测量J_c随磁场的变化关系，证实了J_c的增强。J_c的角度依赖性在B轴上有一个宽峰。这一结果表明BaZrO_3纳米棒作为c轴相关的钉扎中心。钉扎参数m由电场-电流密度特性估算，m的峰值出现在磁场依赖性中。m的特征行为是由于BaZrO_3纳米棒的密度与助熔剂的密度匹配引起的。结果表明，掺杂BaZrO_3纳米棒的ErBa_2Cu_3O_y薄膜的局域临界电流密度的相对分布宽度小于掺杂BaZrO_3纳米棒的YBa_2Cu_3O_y薄膜的局域临界电流密度的相对分布宽度。

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.