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

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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.