Critical current and relaxation of oriented grained YBa2Cu3O7-x after fast neutron irradiation

The authors investigated the effects of fast neutron irradiation on the superconducting properties of oriented grained YBa₂Cu ₃O_7-x after a fluence of 10¹⁷ cm^-2 . The critical current density J_c increases up to a factor of 4, reaching 2.7×10⁴ A/cm² when B ⊥ a,b and 5.3×10³ A/cm² when B || a,b, respectively, at 77 K, 2 T. This enhancement is accompanied by a decrease in the activation energy U₀ in the field and temperature region at which U₀ passes through a maximum. The unexpected relationship between U₀ and J_c points to weak pinning 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. The authors also investigated whether intragrain decoupling was introduced and/or increased by irradiation. A simple check from magnetization measurements makes it possible to determine the degree of nonuniform current flow as a precursor of granularity. This investigation indicates no deviation from the nongranular case after irradiation of the current flows within the a,b plane and a tendency towards a more uniform current flow in the other geometry