Frequency Dependent Flux Dynamics and Activation Energies in Pnictide Bulk (Ba0.56K0.44)Fe2As2 Superconductor

Abstract

Thermally activated flux de-pinning and flux activation de-pinning energies are studied in a (Ba0.56K0.44)Fe2As2 (Tc=38.5K) bulk superconductor in DC magnetic fields up to 18 T. Ac susceptibility was measured as a function of temperature, DC and AC magnetic fields, and frequency. Ac susceptibility curves shift to higher temperatures as the frequency is increased from 75 to 1997Hz in all fields. We model this data by Arrhenius law to determine flux activation energies as a function of AC and DC magnetic fields. The activation energy ranges from 8822K at μ0Hdc = 0 T to 1100K at 18 T for Hac=80 A/m. The energies drop quickly in a non-linear manner as DC field rises above 0 T and around 1 T, which we describe as pinning transition field, the drop levels and continues more slowly in a linear like manner as DC field approaches to 18 T. Furthermore, the activation energy drops quickly as AC field increases from 80 A/m to 800 A/m at 0 DC field. As the DC field rises above 0, the activation energy has significantly weaker dependence on the AC field amplitude. Extensive map of the de-pinning, or irreversibility, lines shows broad dependence on the magnitude of the small AC field, frequency, in addition to the DC field.