The dynamics of magnetic flux trapped in low dc magnetic fields (of the order of the Earth's field) was for the first time studied experimentally in single-crystal YBCO samples with unidirectional twin boundaries in the temperature range near T_c (0.8 < T/T_c < 0.99). Strong pinning in the system of unidirectional planar defects was demonstrated, and a significant deviation from monotonous behavior was established for the averaged effective pinning potential U_p(T) for the trapped flux of low density. In order to compare different methods of J_c determination, the field dependences of the magnetization loop width M(H), which are related to the effective pinning and J_c, were obtained, and resistive measurements on microbridges made from the same single crystals were carried out.
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