CHANGES IN THE PHASE DIAGRAM OF Zn-DOPED YBCO SINGLE CRYSTALS

The Zn -doped YBCO single crystals were prepared with Zn contents of 0.5%, 1.5%, 2.5%, 3.0% and 4.3%. The magnetic measurements were made under magnetic field up to 5 T using SQUID. The peak effect was observed in these samples in all the measured temperature range from 0.2 ≤ T/Tc ≤ 0.95. The region between peak field Hp and irreversibility field H irr increased consistently with increasing Zn concentration showing broadening in the phase diagram and indicating increased disorder for higher Zn content samples. The anisotropy in the peak field Hp for these samples measured for applied field H along c-axis and along ab-plane (Hp‖c/Hp‖ab) was found to increase from 0.15 to 0.18 for samples with Zn concentration up to 3% and decreased to 0.142 for sample with Zn content of 4.3% at 0.9 Tc. In the temperature range of 0.8–0.95 Tc, the anisotropy in the critical current density (Jc|c/Jc‖ab) was also found to increase in the order of 1.9–2.166 for the samples with Zn content up to 3%, whereas for 4.3% Zn sample it was 0.926. The anisotropic changes in the irreversibility field H irr were also found to be in the range of 0.254–0.279 for the Zn concentration up to 3% and decreased for the 4.3% Zn sample in temp range of 0.8–0.95 Tc. In addition to the effect of local magnetic moments induced by Zn impurity, the changes in the anisotropic factor with the increase of Zn content seems to contribute effectively for increase in Hp, Jc and H irr for Zn contents up to 3% samples even at elevated temperature of 0.8–0.95 Tc. The decrease in Jc and H irr for 4.3% Zn sample may be due to increase in the impurity level beyond threshold leading to decrease in superconducting volume fraction.