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Flux pinning in Y-Ba-Cu-O crystals is studied as a function of fluence of 3-MeV protons, which create random local defects. Order-of-magnitude increases in the critical current density are deduced from magnetic hysteresis loops, with values up to 2x 10 5 A/cm 2 observed at 77 K and 1 T. However, the irreversibility line in the field-temperature plane and the pinning potentials deduced from flux-creep studies are hardly changed. These results are compared to melting and pinning models.

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Deliberately introducing defects by particle irradiation is an effective way to increase the flux pinning in high-critical-temperature superconductors, which is a requirement for technological applications of these materials. Proton irradiation generates a random distribution of point defects, which largely enhances the critical current in YBa 2 Cup7_x single crystals; but it is not effective in shifting the irreversibility line to higher magnetic fields. The aligned columnar defects created by high-energy heavy-ion irradiation generate even stronger vortex pinning, resulting in higher critical currents at high temperatures and fields and a large displacement of the irreversibility line to higher fields. -1 a " ~ 0 o 1 ><10 15 cm-2

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