FLUX MOTION DURING PULSED-FIELD MAGNETIZATION IN Y-BA-CU-O SUPERCONDUCTING BULK MAGNET

We have studied the dynamical motion of magnetic-flux lines penetratin g into a melt-processed Y-Ba-Cu-O bulk superconductor during pulsed fi eld magnetization. Subsequent Aux creep was investigated by measuring signals from three Hall sensors mounted at the center, periphery, and the midpoint on the top surface of the sample at 77 K. The penetration of the magnetic flux into the sample is quite sluggish as long as the applied field is low. However, the magnetic Aux penetrates suddenly a nd moves rapidly in response to the changing external field during the ascending stage of the pulsed field, when the intensity exceeds 1.5-2 T. Furthermore, when the rise time of the pulsed field is very short, the local magnetization measured at the midpoint is found to oscillat e with time, as soon as the sudden penetration of the magnetic flux ta kes place. The fast motion of the magnetic flux generates heat and inc reases the temperature of the sample. This effect appears as a reducti on of the trapped flux, already in the descending stage of the pulsed field and it contributes to the suppression of the flux creep after th e pulse. The creep is suppressed more when the applied field is large, and it diminishes most in the center of the sample. (C) 1997 American Institute of Physics.