Neutron-irradiation effects on critical current densities in single-crystalline YBa sub 2 Cu sub 3 O sub 7 minus. delta

Neutron-irradiation experiments were made on two high-quality YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} single crystals in the fluence range from 2{times}10{sup 21} to 2{times}10{sup 22} m{sup {minus}2} ({ital E}{gt}0.1 MeV). Critical current densities {ital J}{sub {ital c}} and volume-pinning forces {ital P}{sub {ital 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 {ital 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 {ital J}{sub {ital c}} anisotropy, and significant changes of the temperature dependence of {ital J}{sub {ital 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{times}10{sup 22} m{sup {minus}2} ({ital E}{gt}0.1 MeV).