Study of the second magnetization peak and the pinning behaviour in Ba(Fe0.935Co0.065)2As2 pnictide superconductor

Abstract

Isothermal magnetic field dependence of magnetization and magnetic relaxation measurements were performed for the axis on a single crystal of Ba(Fe0.935 Co0.065)2As2 pnictide superconductor having Tc = 21.7 K. The second magnetization peak (SMP) for each isothermal M(H) was observed in a wide temperature range from Tc to the lowest temperature of measurement (2 K). The magnetic field dependence of relaxation rate R(H), showed a peak (Hspt) between Hon (onset of SMP in M(H)) and Hp (peak field of SMP in M(H)), which is likely to be related to a vortex-lattice structural phase transition, as suggested in the literature for a similar sample. In addition, the magnetic relaxation measured for magnetic fields near Hspt showed some noise, which might be the signature of the structural phase transition of the vortex lattice. Analysis of the magnetic relaxation data using Maley’s criterion and the collective pinning theory suggested that the SMP in the sample was due to the collective (elastic) to plastic creep crossover, which was also accompanied by a rhombic to square vortex lattice phase transition. Analysis of the pinning force density suggested a single dominating pinning mechanism in the sample, which did not showing the usual and nature of pinning. The critical current density (Jc), estimated using the Bean critical state model, was found to be 5 × 105 A cm−2 at 2 K in the zero magnetic field limit. Surprisingly, the maximum of the pinning force density was not responsible for the maximum value of the critical current density in the sample.