Understanding the roles of heavy ion and gamma-irradiations on the magnetic and transport properties of superconductors

The roles of point defects created by γ-irradiation and chemical doping along with the roles of extended defects such as grain boundary regions and columnar defects created by Pb-ion irradiation on the behavior of the critical current density, J_c have been investigated in several superconductors. The critical current density in YBa₂Cu₃O_7-Δ (YBCO) polycrystalline sample doped with Mn was found to be significantly enhanced. The enhancement of J_c was found to be more significant at the lower temperatures. Magnetization measurements were carried out on YBa₂(Cu_1-xB_x)3O_7-Δ (x = 0, 0.05, 0.1) polycrystalline samples. Considerable increase in the hysterisis width of the magnetization M versus the applied magnetic field H with increasing boron concentration was observed in these samples suggesting an enhancement of the vortex pinning forces in these samples. A limitation of the role of point and columnar defects on enhancing J_c was found in all our samples. We report an explanation of the effect of γ- irradiation and Pb-ion irradiation doses on J_c in these materials. The explanation is based on combining several competing mechanisms of irradiation which we believe to take place mainly in the regions of the grain boundaries. The influences of these mechanisms were found to vary with the irradiation dose level. The effect of γ-irradiation on the normal state resistivity of MgB₂ polycrystalline superconducting specimens was also investigated. An increase in the normal state resistivity and a broadening of the resistive transition to the superconducting state were observed with increasing γ-irradiation dose. Different temperature dependence of normal-state resistivity and different residual resistivity ratios, RRR were obtained for different doses.