FLUX JUMPS IN MELT-TEXTURED Y-BA-CU-O

The magnetization of a melt-textured Y-Ba-Cu-O superconducting sample has been measured in magnetic fields up to 5.5 T using a SQUID magnetometer. Partial flux jumps were observed below the temperature of 7.6 K. With the applied field parallel to the sample's c axis, the upper bound of the instability field was found to be the maximum applied field, while with the applied field perpendicular to the c axis, the sample's dimensions determined the upper bound. Magnetization loops with flux jumps were calculated employing a critical-state model which incorporates the flux-jump instability criterion and considers the incompleteness of jumps. The different shapes of the magnetization loops are determined by four model parameters of which the instability field of the first virgin jump has the strongest effect on the loop shape. To predict the observed loop asymmetry and the appearance of solitary jumps, the simple Bean critical-state model was found to be insufficient. Instead, a critical-state model where the critical current density decreases as a function of the magnetic field is needed. It is shown that the use of a simple Kim-type critical current density is adequate to explain in detail all our experimental flux-jump data.