Effects of 18 MeV Fe⁸⁺ ion irradiation on superconductivity and microstructure of Bi₂Sr₂CaCu₂O_x superconductor

Bi₂Sr₂CaCu₂O_x single crystals were irradiated with 18 MeV Fe⁸⁺ ion at room temperature. Superconductive property and microstructure of the samples after the irradiation were investigated by magnetization hysteresis (ΔM) and transmission electron microscopy (TEM). ΔM was enhanced by increasing ion dose up to 1×10¹² ions/cm², but decreased at the density of 1×10¹³ ions/cm². The ion irradiation produced ellipse-like amorphous defects with the size of several nm, which may behave as flux pinning centers inside the crystals. The defects were formed in the band region of 0.5 μm at the depth of about 4.5 μm from the surface. The number of defects were equal to several times of applied ion dose. The optimum dose for improving superconductivity should be dominated by the defect size and spacing of defects to avoid overlapping among the defects.