Low-field anomaly of vortex dynamics in iron-pnictide superconductors

Magnetic relaxations in Ba(Fe1−xCox)2As2 before and after swift particle irradiations are measured. The normalized relaxation rate S shows a clear suppression in a pristine sample at low fields below the self-field. Heavy-ion irradiations suppress these anomalies considerably, although there still remain weak features depending on ion species and/or energies. In a proton irradiated sample, the low-field anomaly of S survives although it is smeared compared with that in a pristine sample. All these results indicate that the low-field anomaly of S is suppressed by defects, and correlated defects have stronger effects. Possible origins of the low-field anomaly of S are discussed.

► Magnetic relaxations in optimally-doped Ba(Fe1–xCox)2As2 shows an anomalous suppression of relaxation rate at low fields.
► Swift particle irradiations weaken the anomaly.
► In the case of uncorrelated defects, the low-field anomaly of relaxation rate is only smeared.
► Effective suppression of the anomaly by correlated defects may suggest the important role of curved vortices related “Meissner hole”.