Observation of in-plane anisotropy of vortex pinning by inclined columnar defects

The anisotropy of the critical current density introduced in the (a,b) plane of ${\mathrm{DyBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ single crystals by oblique irradiation with heavy ions is observed by magneto-optics. The pinning forces on vortices oriented perpendicular to the (a,b) plane are largest when directed perpendicular to the columnar defects. Two modes of vortex motion are considered. Effects of flux creep and thermal depinning on the critical current density are discussed.