Pinning effect of columnar defects as a probe of anisotropy of Bi2Sr2Can-1CunO2n+1 thin films

We have studied the influence of columnar defects produced by heavy-ion irr adiation on the superconducting properties of Bi2Sr2Can.1CunO2n+1, thin fil ms (2212 and 2201) and a 2212/2201 multilayer. We will show that: first, a minimum irradiation dose is needed to observe an enhancement of pinning by the columnar defects and to overwhelm the effects of other pinning centers such as natural defects. Second, in the case of the weakly anisotropic syst em 2201, a directional effect is observed in a given T and H range indicati ng an anisotropic pinning enhancement by the irradiation. In contrast, for highly anisotropic 2212 and the multilayer this increase of pinning efficie ncy is isotropic. It is demonstrated that columnar defects allow us to prob e the anisotropy of the films and the vortex dimensionality.