Nonmonatonic effect of γ-rays on the critical current density in polycrystalline MgB₂ superconductors

MgB₂ polycrystalline superconducting specimens were irradiated with several doses of γ-rays up to 100 MR. An increase in the normal state resistance and a broadening of the resistive transition to the superconducting state were observed with increasing γ-irradiation dose. However, the change in the critical temperature was insignificant. After γ-irradiation dose of 10 MR, the critical current density, J_c was found to be significantly enhanced almost at all temperatures. Higher γ-irradiation doses did not produce significant further enhancement in J_c. The enhancement of J_c was also found to have a nonmonatonic behavior with temperature at all doses. These results are explained by the effects of γ-rays and temperature on the migration of MgO impurities into the grains and the oxygen segregation in the grain boundaries. PACS: 74.60.Ge; 74.60.Jg; 74.70.Ad