Vortex pinning by Ni point defects inBi2Sr2Ca(Cu1−xNix)2O8+δsingle crystals

Abstract

We study the vortex pinning of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ (Bi2212) single crystals doped with up to 2 at. % of Ni atoms on the Cu position using measurements of the dynamic magnetic relaxation. We find that below an optimum Ni-doping level of about 1 at. % the thermal activation energy for vortex creep and the critical current density definitely increase with the Ni concentration. This proves that Ni point defects contribute to the pinning forces, as expected within the theory of weak collective pinning. At higher Ni concentrations we observe a strong decrease of the activation energy, probably caused by an overlap of the vortex potentials from neighboring Ni sites. The dynamic behavior of the vortex system at low temperatures and magnetic fields is dominated by elastic creep of single vortices whereas at higher temperatures and fields plastic creep of vortex dislocations becomes dominant. The phase with plastic creep grows in the $(B,T)$ plane with increasing strength of the collective pinning forces.