We report on magnetic measurements carried out in a textured YBa$_2$Cu$_3$O$_{ 7-\delta}$ and YBa$_2$(Cu$_{ 1-x}$Ag$_x$)$_3$O$_{ 7-\delta}$ (at $x \approx$ 0.02) crystals. The so-called fishtail-effect (FE) or second magnetization peak has been observed in a wide temperature range 0.4~$<T/T_c<$~0.8 for $\textbf{ H}\parallel c$. The origin of the FE arises for the competition between surface barrier and bulk pinning. This is confirmed in a non-monotonically behavior of the relaxation rate $R$. The value $H_{ max}$ for Ag-doped crystals is larger than for the pure one due to the presence of additional pinning centers, above all on silver atoms.
Categories: cond-mat.supr-con