Study of the critical current and irreversibility line of Bi2Sr2CaCu2O8 thin films irradiated with low-energy Co+ and Fe+ ions

The effects of low-energy irradiation of the Bi2Sr2CaCu2O8 thin films on their properties, such as the superconducting transition temperature, critical current density and location of the irreversibility line on the phase plane, have been investigated. The films were irradiated with the Fe+ and Co+ ions with the energy of 40 keV and with the dose varying from 5 × 1011 to 3 × 1013 ions/cm2. The efficiency of vortex pinning by the irradiation-induced defects has been estimated by monitoring the magnetic flux profile and the hysteresis properties of samples. It has been established that the critical current density increases twice and the area of nondissipative current flow expands as a result of irradiation with Fe+ ions with the dose of 5 × 1011 ions/cm2. The further dose increase leads to the weakening of the pinning strength and, as a consequence, to the degradation of superconducting properties of thin films. The enhanced pinning in the samples irradiated with a low-dose may be explained by the formation of local amorphous regions, which effectively fix the magnetic flux lines. Under the irradiation with a high dose the amorphous regions overlap and form the continuous layer which does not prevent the vortex movement.