Magnetic properties of YBa₂Cu₃O_y crystal with various oxygen annealing conditions

It is known that the superconducting properties of YBa₂Cu₃O_y are strongly dependent on the oxygen content, which affects the anisotropy and the doping level. We measured the magnetic properties of the same YBa₂Cu₃O_y single crystal in order to systematically study the effect of oxygen annealing condition. The doping level could be controlled simply by changing the annealing temperature. The overdoped samples exhibited a very high irreversibility field of >7 T at 77 K and for the H∥c, although the critical temperature was slightly lower than that of the optimum doped sample. The secondary peak effects are observed for all the samples, however, the source of the peak effect was rather complicated and depended on the annealing conditions.