Grain geometry effect on the magnetic properties of a granular iron-based superconductor LaFeAsO1−xFx

We have studied and compared the magnetic response of LaFeAsO0.92F0.08, in the form of granular bulk and powder, to the application of ac magnetic fields with different frequencies. The study of the third harmonics of the ac susceptibility has shown that the ac frequency plays an important role in the electromagnetic coupling of the grains in the granular sample. In particular, the characteristic harmonic response of the bulk sample at the highest frequency resembles the response of the unconnected grains of the powders, even at low frequencies. Moreover, the possibility of the occurrence of magnetic field focusing in the region between the grains, ascribed to both superconducting shielding and geometric effect of the grains, has been considered to explain the observed peak effect in the dc magnetization loops on the bulk sample. Although our investigation has be confined to the LaFeAsO0.92F0.08 granular, these results may be applied to any granular sample consisting of flat grains connected by weak links. Moreover, they also show that a peak effect in a granular superconductor can exist without necessarily invoke the existence of vortex dynamic processes.