Magnetic relaxation of superconducting YBCO samples in weak magnetic fields

For the first time, magnetization of high-Tc samples with different crystalline structure and its isothermal relaxation is studied at very weak constant fields (H <= 0.1 Oe) for temperatures close to the critical ones. Essential influence of twin boundaries in YBCO single crystals on magnetic relaxation rate is shown. An estimation of effective pinning potential is made in the framework of the collective pinning model.