The surprisingly rapid relaxation of the sustainable current density in the critical state of single-crystalline Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$ 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 $\mu$. At low magnetic fields, below the second magnetization peak, $\mu$ 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.”

• Received 22 April 2012

DOI:https://doi.org/10.1103/PhysRevB.86.024515