Improvement of the vortex pinning properties by increasing the size of columnar defects

Abstract

The influence of the size of columnar defects on pinning properties of high-Tc superconductors has been investigated. Bi2Sr2CaCu2O8 single crystals (Bi-2212) were irradiated with heavy ions using different experimental conditions. The irradiation induced defect morphology was studied by High Resolution Electron Microscopy (HREM). Different values of columnar defect radius, R, were achieved by changing the electronic stopping power and the incident ion energy. Pinning properties of columnar defects were studied through magnetic measurements (H→//c→). It is shown that pinning properties are improved by the enlargement of defect size. A new strategy was also used: by tilting the crystal during the irradiation process, columnar defects of elliptical section in the (a→,b→) planes were introduced. The section size of defects in the (a→,b→) planes, Rab, is larger for these tilted samples than for defects parallel to the c→ axis. It is shown that such a larger size of the track projections on the (a→,b→) planes can also enhance the pinning efficiency. The enhancement of pinning properties observed for larger Rab could be directly related to the enhancement of pinning energy with Rab.