Comparison of superconducting flux pinning due to ion irradiation and fishtail effects

Heavy-ion irradiation experiments have been carried out for single crystals of YBa2Cu4O8 in order to compare the magnetic-field dependence of the flux pinning for damage channels with the intrinsic open-quote open-quote fishtail close-quote close-quote pinning that occurs in this material before irradiation. The two pinning effects seem to be additive in that irradiation increases the hysteresis at all fields and the fishtail hump persists in the irradiated samples with roughly the same magnitude as before the damage channels are produced. For irradiated samples, there are two regions where pinning rises with increasing field, one near zero field due to a matching effect of the defect density with the vortex lattice spacing and a second at the same field as the original fishtail. Both effects appear in the same sample in different magnetic-field ranges. copyright 1996 The American Physical Society