Do columnar defects produce bulk pinning?

From magneto-optical imaging performed on heavy-ion-irradiated YBa2 Cu3 O7 − δ single crystals, it is found that at fields and temperatures where strong single vortex pinning by individual irradiation-induced amorphous columnar defects is to be expected, vortex motion is limited by the nucleation of vortex kinks at the specimen surface. In the material bulk, vortex motion occurs through (easy) kink sliding. Depinning in the bulk determines the screening current only at fields comparable to or larger than the matching field, at which the majority of vortices is not trapped by an ion track. © 2000 American Physical Society.