TEMPERATURE-DEPENDENCE OF THE SURFACE-BARRIER ON YBA2CU3O7-DELTA GRAINS DETECTED BY AC SUSCEPTIBILITY MEASUREMENTS

The surface barrier on YBa2CU3O7-delta grains has been studied in dyna mic regime by performing measurements of the two components of the ''W ide-Band'' Susceptibility, chi(a) and chi(r), as function of the appli ed AC field amplitude at different temperatures and frequencies. The b est fit of the measured curves was obtained with the analytical expres sions of chi(a) and chi(r) calculated from a model in which the magnet ic behaviour of the sample is supposed to be governed by the intergrai n shielding current and by a surface barrier on the grains of the samp le. From the fitting results, the height of the surface barrier on the grains has been valued independently for each measured curve, allowin g us to determine the temperature dependence of the surface barrier us ing an AC technique at different frequencies. The result is a barrier which decreases with increasing temperatures, but its temperature depe ndence at low (up to similar to 10 kHz) frequencies of the applied AC field is different respect to the dependencies obtained at higher freq uencies.