Neutron-irradiation effects on critical current densities in single-crystalline ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

Neutron-irradiation experiments were made on two high-quality ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ single crystals in the fluence range from 2×${10}^{21}$ to 2×${10}^{22}$ ${\mathrm{m}}^{\mathrm{-}2}$ (E>0.1 MeV). Critical current densities ${\mathit{J}}_{\mathit{c}}$ and volume-pinning forces ${\mathit{P}}_{\mathit{V}}$ 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 ${\mathit{J}}_{\mathit{c}}$ anisotropy, and significant changes of the temperature dependence of ${\mathit{J}}_{\mathit{c}}$. 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×${10}^{22}$ ${\mathrm{m}}^{\mathrm{-}2}$ (E>0.1 MeV).