NEUTRON-IRRADIATION EFFECTS ON CRITICAL CURRENT DENSITIES IN SINGLE-CRYSTALLINE YBA2CU3O7-DELTA

Neutron-irradiation experiments were made on two high-quality YBa2Cu3O7-delta single crystals in the fluence range from 2 X 10(21) to 2 X 10(22) m-2 (E > 0.1 MeV). Critical current densities J(c) and volume-pinning forces P(V) were obtained from magnetization measurements in the temperature range from 5 to 77 K, for magnetic fields up to 8 T, and for field orientations parallel and perpendicular to the crystallographic c direction. All evaluations were made on the basis of an extended anisotropic Bean model. The results show large enhancements of flux pinning, in particular at low neutron fluences, a strong reduction of J(c) anisotropy, and significant changes of the temperature dependence of J(c). The complicated (nonlinear) dependence of critical current densities and pinning forces on neutron fluence is discussed in terms of an interaction between the radiation-induced defects with the preirradiation defect structure, based on electron microscopy of individual defects and the defect distribution measured in one crystal after neutron irradiation to 2 X 10(22) m-2 (E > 0.1 MeV).