Magnetic properties and pinning mechanism of high Jc melt textured YBaCuO bulk sample

The magnetic relaxation and hysteresis loops of melt-textured YBa 2Cu 3O y bulk sample were measured at different temperatures and in different fields. The absolute value of magnetization M was found to increase with the applied field B when 2T��B��8T in the temperature range 20K-70K. Also found was that the relaxation curve deviates from logarithmic law apparently, and the relative relaxation rate decreases rapidly as temperature increases or field increases. The pinning mechanism was investigated in order to explain these properties. The function between the effective activation energy U and current density J over a wide range of J was gained through two methods. Based on this, the collective pinning theory was used to explain the experimental results. The field dependence of ��, J c0 and U 0 was obtained through direct fit of the measured U(J) curves, which is in good agreement with the collective pinning theory. Also discussed was the difference between the sample and the YBa 2Cu 3O y single crystal.