CRITICAL CURRENT AND RELAXATION OF ORIENTED GRAINED YBA2CU3O7-X AFTER FAST-NEUTRON IRRADIATION

We investigated the effects of fast neutron irradiation on the superconducting properties of oriented grained YBa2Cu3O7-x after a fluence of 10(17) cm-2. The critical current density j(c) increases up to a factor of 4 reaching 2.7.10(4) A/cm2 when B perpendicular-to a,b and 5.3.10(3) A/cm2 when B parallel-to a,b, respectively, at 77 k, 2 T. This enhancement is accompanied by a decrease of the activation energy U(o) in the field and temperature region at which U(o) passes through a maximum. The unexpected effect between U(o) and j(c) points to weak pining centers introduced by atomic collision or to a change of the electronic structure via oxygen rearrangement. Upon irradiation the irreversibility field increases slightly in both geometries. We further investigated whether intragrain decoupling was introduced and/or increased by irradiation. A simple check from magnetization measurements allows one to determine the degree of nonuniform current flow as a precursor of granularity. This investigation indicates no deviation from the nongranular case after irradiation if the current flows within the a,b plane and a tendency towards a more uniform current flow in the other geometry.