Suppression of surface barriers for flux penetration in Bi₂Sr₂CaCu₂O_8+δ whiskers by electron and heavy ion irradiation

We have used micron-sized linear Hall probe arrays to investigate the effects of irradiation on surface barriers for flux penetration in individual superconducting Bi₂Sr₂CaCu₂O_8+δ whiskers. Samples were irradiated with 2.5-MeV electrons or 9-GeV heavy (Pb) ions. The magnetization was investigated in the temperature range between 5 K to above the superconducting transition temperature, in magnetic fields up to 1 T. At all temperatures, irradiation by high-energy electrons or swift heavy ions reduces the penetration field substantially. At low temperatures (T<50 K) we attribute this to the suppression of a Bean-Livingston (BL) surface barrier for two-dimensional "pancake" vortices, and our results are in reasonable agreement with recent theoretical predictions. At high temperatures (T>50 K) we tentatively propose that the reduction in H_p(T) is due to suppression of a BL surface barrier for flux lines. While electron irradiation strongly reduces magnetic irreversibility at high temperatures, the moderate hysteresis measured after heavy ion radiation suggests that this creates additional bulk pinning for flux lines on columnar defects.