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

The influence of the size of columnar defects on pinning properties of high -T-c superconductors has been investigated. Bi2Sr2CaCu2O8 single crystals ( Bi-2212) were irradiated with heavy ions using different experimental condi tions. The irradiation induced defect morphology was studied by High Resolu tion Electron Microscopy (HREM). Different values of columnar defect radius , R, were achieved by changing the electronic stopping power and the incide nt ion energy. Pinning properties of columnar defects were studied through magnetic measurements ((H) over right arrow//(c) over right arrow). It is s hown that pinning properties are improved by the enlargement of defect size . A new strategy was also used: by tilting the crystal during the irradiati on process, columnar defects of elliptical section in the ((a) over right a rrow,(b) over right arrow) planes were introduced. The section size of defe cts in the ((a) over right arrow,(b) over right arrow) planes, R-ab, is lar ger for these tilted samples than for defects parallel to the (c) over righ t arrow axis. It is shown that such a larger size of the track projections on the ((a) over right arrow,(b) over right arrow) planes can also enhance the pinning efficiency. The enhancement of pinning properties observed for larger R-ab could be directly related to the enhancement of pinning energy with R-ab. (C) 1998 Elsevier Science B.V. All rights reserved.