GRAIN-SIZE DEPENDENCE OF HYSTERESIS LOOPS BY AC LOW MAGNETIC-FIELD MEASUREMENTS IN YBA2CU3O7-X CERAMICS

We have studied the hysteresis loops of three YBa2Cu3O7-x ceramics characterized by different grain sizes (2, 7 and 10 μm). The measurements are based on 16 bits analog to digital conversion, the applied magnetic field being alternating at a frequency of 20 Hz and with a maximum amplitude of 300 Oe. We observed that the irreversible phenomena (area of the loops) largely increase when reducing the grain size. Considering that the measured magnetization can be separated into two terms characteristics of the inter- and intragranular regions, we suggest that at 4.2 K and for H<300 Oe, the hysteretic phenomena are essentially localized at the surface of the grains. Indeed, this explains why the hysteresis loops are larger for smaller grains. Critical current density is also analyzed as a function of the field amplitude and the grain size. © 1990.