Enhancement of the critical current density by increasing the collective pinning energy in heavy ion irradiated Co-doped BaFe₂As₂ single crystals

We investigate the effect of heavy ion irradiation (1.4 GeV Pb) on the vortex matter in Ba(Fe_0.92Co_0.08)₂As₂ single crystals by superconducting quantum interference device (SQUID) magnetometry. The defects created by the irradiation are discontinuous amorphous tracks, resulting in an effective track density smaller than 25% of the nominal doses. We observe large increases in the critical current density (J_c), ranging from a factor of ∼3 at low magnetic fields to a factor of ∼10 at fields close to 1 T after irradiation with a nominal fluence of B_Φ = 3.5 T. From the normalized flux creep rates (S) and the Maley analysis, we determine that the J_c increase can be mainly attributed to a large increment in the pinning energy, from <50 K to ≈500 K, while the glassy exponent μ changes from ∼1.5 to <1. Although the enhancement of J_c is substantial in the entire temperature range and S is strongly suppressed, the artificial pinning landscape induced by the irradiation does not modify significantly the crossover to fast creep in the field-temperature vortex phase diagram.