Magnetic history dependence of the AC susceptibility of GdSr2RuCu2Oz

The AC susceptibility of GdSr2RuCu2Oz, samples has been measured as function of the temperature, at different AC field amplitudes (from 1 to 16 G) and frequencies (in the range [10-10(3) Hz]), applied parallel to the major axis of the sample, and without any additional DC magnetic field. In particular we have analysed the temperature region above the Superconducting critical temperature T,, and around the magnetic transition temperature T-m. Our attention has been focused on the differences between the curves measured after cooling the sample in zero AC field (Zero AC Field Cooling or ZAFC) and those measured after cooling in presence of a non zero AC field (AC Field Cooling or AFC). Clear differences between the ZAFC and AFC susceptibility curves appear in the temperature range between T, and T., and they depend on the measuring field amplitude and frequency. Finally, indications of a spin glass behaviour in Ru-1212 system observed from its AC magnetic response are reported.