Critical current density of the single crystal and polycrystalline Bi-2212 in the weak magnetic field regime: signatures of electromagnetic granularity

Abstract

We have studied the magnetisation of the single crystal and polycrystalline Bi-2212 as a function of the magnetic field ( H) at three different temperatures. The ratio ( κ) of the magnetic moments per unit mass of the polycrystalline to that of the single crystal of Bi-2212 is extracted for different magnetic field. The ratio is less than unity but increases with the magnetic field. Also as the temperature decreases the ratio increases towards unity. We have discussed the variations of the magnetisation as a result of the random orientation of the grains with respect to the applied magnetic field. The critical current density ( J c) has been extracted from the field dependent magnetisation. The peak positions in the variation of J c( H) shifts towards higher magnetic field in polycrystalline sample which is caused by the granularity. The dependence of the critical current density on the magnetic field shows that the polycrystalline nature has an impact on the vortex matter phase and its transformation for all temperatures. The different exponents are extracted for both samples and correlation has been explained with the possible formation of the vortex matter phases. Changes in the pinning strength of vortices (such as bending) in the CuO planes by the grain boundaries and the possible thermal hopping of vortices through the same are dominant factors for the decrease in J c. In addition, the rate of decrease of J c with the applied magnetic field is enhanced in polycrystalline sample which is manifested from the exponent values.