Analysis of pinning effects in YBa₂Cu₃O_7-δ single crystals after fast neutron irradiation

Flux-pinning effects were studied by sequentially irradiating a YBa₂Cu₃O_7-δ single crystal to fast neutron fluences of 8×10²¹ m^-2. From superconducting quantum interference device magnetometry, the transition temperature T_c and the magnetic moments were obtained for fields parallel and perpendicular to the crystallographic c axis. Irreversibility lines and critical current densities were evaluated as a function of neutron fluence. In order to explain the data, a qualitative model is developed that is based on the layered structure of the system and the nature of the various defects introduced by irradiation. Because of the dominating influence of large defect cascades on flux pinning, different summation models have to be employed, depending on temperature, field orientation, field strength, and pin density.