Magneto-optical study of flux penetration in heavy-ion irradiated high-T_c single crystals

Single crystals of Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) and of DyBa₂Cu₃O_7-δ have been exposed to high-energy heavy-ion irradiation (230 MeV ⁵⁸Ni-ions, 0.5 GeV ¹²⁷I-ions and 1.0 GeV ²⁰⁸Pb-ions) with various fluences perpendicular to the sample surface and hence parallel to the c-axis of the crystals. The irradiation with Ni-ions produces spherical regions of amorphized material, whereas the iodine and lead ions create columnar defects along the paths of the projectiles in the superconductors. Using the high-resolution Faraday effect (HRF) technique, domain patterns of the Shubnikov phase are investigated. From the measured flux-density profiles, the acting local pinning forces and critical current densities are determined. To allow direct comparison of the irradiation effects under the same experimental conditions, one half of each crystal was covered by an absorber during the irradiation. Fluence-dependent measurements of the pinning forces are performed and the influence of irradiation on flux-creep effects is also investigated. All heavy-ion irradiated samples show an enhancement of the measured critical current densities up to a factor of 33 as compared to the unirradiated region of the same crystal. The effective activation energies for flux creep remain unchanged after each kind of irradiation in both types of single crystals.