MAGNETIC AND THERMAL INSTABILITIES IN A TEXTURED YBA2CU3OX SUPERCONDUCTOR WITH HIGH TRANSPORT JC

Magnetization hysteresis measurements on a melt-textured YBa2Cu3Ox superconductor have been carried out. Saw-toothed flux jumps occurred more frequently when the relaxation time (the stable-field-time interval during measurement) was reduced or the field sweep rate was increased. The jump-starting field HFJ was found to be related with the relaxation time in a linear form. A large amount of trapped flux due to flux jumps was observed when the direction of the sweeping field was reversed and illustrated using an adiabatic critical state model. An evaluation of the temperature variation from flux jumping has been made by comparing the hysteresis loops in some particular field at various temperatures. It is estimated that the thermal pulses induced by flux jumps at 5 K can cause an instantaneous temperature rising of the sample as high as 32 K, a value about 30 times higher than that of the conventional type-II materials. An intergranular effect of the field-dependent magnetization was also discussed.