Breach of the logarithmic magnetic relaxation behavior of HTSC with strong pinning centers

The dynamics of magnetic flux trapped in low constant fields are investigated experimentally in YBCO single crystals with strong pinning centers that are formed by the unidirectional twin boundary system. It is found that within the domain of strong thermal fluctuations in the vicinity of the superconducting phase transition the logarithmic isothermal relaxation of magnetization differs essentially from the quasilogarithmic one previously observed only at high magnetic fields. Using the of Anderson-Kim linear model of thermal activated creep the effective pinning potential was estimated. It is shown that the Zeldov inverse logarithmic model offers a much correct description of magnetization drop much at temperatures close to the critical ones, especially for long measurement times.