Flux pinning and critical current densities in proton irradiated single crystal yttrium barium copper oxide

Radiation damage has been traditionally used to introduce artificial pinning centers into superconducting material in order to enhance their critical current densities. The objective of this study was to determine the effect of defects induced by irradiation with 3.5 MeV protons on the critical current densities and to investigate the structure of the defects using SQUID magnetization and magnetoresistive measurements. There are three general types of defects produced by proton irradiation at room temperature. They are (1) a relatively high concentration of point defects on all the sublattices, (2) clusters of point defects, and (3) the defect cascades. SQUID magnetization measurements suggest that the defect clusters and cascades are effective in pinning the vortices at 70 K for $\vec{\rm H}\Vert\vec{\rm c}$ axis. Additional contribution to pinning at 10 K for $\vec{\rm H}\Vert\vec{\rm c}$ axis comes from the vacancies formed upon irradiation with protons. The defect clusters seem to be the only contributors to pinning for $\vec{\rm H}\Vert\vec{\rm ab}$ plane. The anisotropy of the critical current densities in detwinned crystals between field orientations parallel to the $\vec{\rm a}$ and $\vec{\rm b}$ axes determined from magnetization measurements at 10 K consistently show that $\rm J\sbsp{c}{c}(\vec H\Vert\vec b)\u3eJ\sbsp{c}{c}(\vec H\Vert\vec a)$ at all fields after 2 irradiations and annealing for 2 and 4 weeks. The effect of proton irradiation on the resistive transition and the anisotropy of the defect structure was studied using angular dependence of the resistivity in $\rm YBa\sb2Cu\sb3O\sb{7-\delta}$ single crytals. The expected drop in resistivity when the field is aligned parallel to the $\vec{\rm c}$ axis due to the alignment of the vortices with the defect cluster as observed in the case of the twin boundaries was absent