Flux pinning and critical current density in $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+y}$ and $\mathrm{Eu}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+y}$ superconductors

We have studied the magnetic characteristics of the critical states of Y-Ba-Cu-O and Eu-Ba-Cu-O high-$T_c$ superconductors. Magnetization data at various temperatures have revealed a very large magnetic hysteresis. The highly irreversible behavior was found to exist both in bulk samples and in well-isolated particles. These effects are consistent with the existence of large pinning forces caused by the structural defects in these materials. By analyzing the hysteretic magnetization data, the critical current density $J_c$ as a function of the external magnetic field has been obtained at several temperatures. The variation of the trapped flux with temperature (up to $T_c$) has also been measured.