Enhancement of critical current densities by heavy-ion irradiation in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ observed using the high-resolution Faraday effect

Using the high-resolution Faraday technique, domain patterns of flux penetration in heavy-ion-irradiated ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ thin films are obtained. To compare the influence of different projectiles, the samples were irradiated with 25-MeV oxygen ions, 230-MeV nickel ions, and 1-GeV lead ions with various fluences perpendicular to the sample surface and hence parallel to the c axis of the materials. The irradiation with ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ produces point-defect cascades mainly perpendicular to the path of the projectile whereas the irradiation using ${}_{}{}^{58}{}_{}{}^{}\mathrm{Ni}$ ions creates spherical regions of amorphized material along the path. The ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ ions create columnar tracks along the c axis of the material. For a simultaneous observation of irradiated and unirradiated regions of a single epitaxial thin film, only one-half of each sample was exposed to the irradiation. To obtain data for the acting-volume pinning forces and the local critical current densities, a numerical calculation also regarding the curvature of the flux lines is carried out. Oxygen and nickel irradiation lead to an enhancement of the critical current density by a factor of 2. By lead irradiation, an enhancement of the critical current density up to a factor of 5 is obtained. This critical current density at the largest fluence used reaches the theoretical predictions for an enhancement of the critical current density by means of irradiation-induced columnar tracks. The effects of the irradiation-induced defects are compared to those of the pinning sites present in the background pinning. The density of the irradiation-induced columnar defects and their influence on the pinning are compared to the density and effect of screw dislocations. Our results clearly indicate that screw dislocations do not act as effective pinning centers because of their small density.