Vortex creep and critical current densities in superconducting (Ba,K)Fe ₂As ₂ single crystals

The surprisingly rapid relaxation of the sustainable current density in the critical state of single-crystalline Ba _1-xK _xFe ₂As ₂ is investigated for magnetic fields oriented parallel to the c axis and to the ab plane, respectively. Due to the inadequacy of standard analysis procedures developed for flux creep in the high-temperature superconducting cuprates, we develop a simple, straightforward data treatment technique that reveals the creep mechanism and the creep exponent μ. At low magnetic fields, below the second magnetization peak, μ varies only slightly as a function of temperature and magnetic flux density B. From the data, we determine the temperature and field dependencies of the effective activation barrier for creep. At low temperatures, the measured current density approaches the zero-temperature critical current density (in the absence of creep) to within a factor 2, thus lending credence to earlier conclusions drawn with respect to the pinning mechanism. The comparable values of the experimental screening current density and the zero-temperature critical current density reveal the limited usefulness of the widely used "interpolation formula."