Inter- and Intra-Granular Flux Pinning Properties in Ba(Fe $$_{0.91}$$ 0.91 Co $$_{0.09})_{2}$$ 0.09 ) 2 As $$_{2}$$ 2 Superconductor in AC and DC Magnetic Fields

Abstract

Flux pinning dynamics are studied in a Ba(Fe\(_{0.91}\)Co\(_{0.09})_{2}\)As\(_{2}\) (\(T_\mathrm{{c}}=25.3\) K) bulk samples via ac susceptibility measurements. Ac susceptibility curves shift to higher temperatures as the frequency of small ac fields is increased from 75 to 1997 Hz in all magnetic fields ranging from 0 to 18 T. The temperature profile of the ac susceptibility curves shows narrower ac loss distribution in temperature for higher frequencies and gradually narrowing frequency shift as the temperature sweeps the full range from 2 K to the upper critical field temperature. The frequency (\(f)\) shift of the susceptibility curves is modeled by the Anderson–Kim Arrhenius law \(f = f_{0} \mathrm {exp}(- {E}_\mathrm{{a}} /kT)\) to determine flux activation energy \(E_\mathrm{{a}}/k\) as a function of magnetic field. Extensive mapping of the irreversibility lines shows broad dependence on the magnitude and the frequency of the ac field, in addition to the dc magnetic field. The irreversibility lines were just below the upper critical field \(H_\mathrm{{c2}}\) lines at 0 T in the \(H-T\) plane, but they moved significantly below the \(H_\mathrm{{c2}}\) line at higher magnetic fields, placing constraints on the use of these materials at higher magnetic fields such as 10 T and above.