Evolution of Fishtail-effect with aging in pure and Ag-doped MG-YBCO

$M(B)$-curves were experimentally investigated. Fishtail-effect (FE) was observed in MG YBa$_2$Cu$_3$O$_{7-\delta}$ and YBa$_2$Cu$_{3-x}$Ag$_x$O$_{7-\delta}$ (at $x \approx$ 0.02) crystals in a wide temperature range 40 K $<T<$ 75 K at the orientation of magnetic field $\textbf{H}\parallel c$. It was obtained that the influence of bulk pinning on FE is more effective at low temperatures while the influence of surface barriers is more effective at high temperatures. The value $H_{max}$ for Ag-doped crystals is larger than for a pure one that due to the presence of additional pinning centers, above all on silver atoms.