ABSTRACT
For an applied field between HC1 and HC2, the magnetic flux penetrates the superconductor in the form of vortices. The magnetization measurements M(H) show that the curves in increasing fields never coincide with those obtained in decreasing fields (see Fig. 5.1). This is because the vortices have difficulties in moving as well as when they penetrate the material as when they leave. The origin of this phenomenon is the existence of defects which trap the vortices and thus allow the superconductor to support very high densities of current without a loss of energy. The critical current density is the extrinsic current quantity that liberates the vortices. Among the defects which can exist in the material let us quote the chemical defects such as the lacuna and the substitutions, and the structural defects such as the dislocations and the grains boundaries. The pinning effects are the result of three types of interaction, the vortex-defect interaction, the vortex-vortex interaction and the vortex-spin interaction.
