Magneto-optical study of Ba(Fe_1−xM_x)₂As₂ (M=Co and Ni) single crystals irradiated with heavy ions

Optimally doped single crystals of Ba(Fe_1−xM_x)₂As₂ (M=Co, Ni) were irradiated with 1.4 GeV ²⁰⁸Pb⁵⁶⁺ ions at fluences corresponding to matching fields of B_ϕ=0.1, 0.5, 1, and 2 T. Magneto-optical imaging has been used to map the distribution of the magnetic induction in the irradiated samples. The imaging is complemented by the magnetization measurements. The results show a substantial enhancement of the apparent critical current densities as revealed by the much larger Bean penetration fields and an increase in the hysteretic magnetization. However, the effect depends on the compound, temperature, and applied magnetic field. In Ba(Fe_0.926Co_0.074)₂As₂ crystals, at 15 K and low fields, the enhancement appears to scale with the irradiation dose at a rate of about 0.27 MA⋅cm⁻² T⁻¹, whereas in Ba(Fe_0.954Ni_0.046)₂As₂ crystals, higher irradiation doses are less effective. Our results suggest that moderate irradiation with heavy ions is an effective way to homogeneously enhance the current-currying capabilities of pnictide superconductors.