The properties of a locally frozen ͑in a region of diameter 0.5 mm͒ magnetic field in a YBa 2 Cu 3 O 7−x slab 0.5 m thick are investigated as a function of the value of the excitation field, the regime of freezing, and the transport current through the sample. The first regime is cooling of the ceramic to 77 K in the excitation field with a subsequent turning off of the excitation field, and the second regime is cooling in the Earth's magnetic field with a subsequent turning on and off of the excitation field. At an excitation field up to 2000 A / m in these regimes two different types of macroscopic current vortex structures, which generate the frozen field, are formed. The local critical field of excitation when the vortex structure is formed in the second regime exceeds the uniform perpendicular critical field of the slab by a factor of 10 and equals 1700 A / m. On the other hand, the vortex structure of the first type can be formed by practically any weak excitation field, including fields smaller than the critical field of the vortex structure of the second type. In a representation of the ceramic as a Josephson medium, physical models of the two types of vortex structures are proposed which correspond most fully to the results of experiments. The displacement of the vortex structure of the first type upon the passage of transport current through the slab, as a result of the action of the Lorentz force on that structure, is registered. This makes it possible to calculate the pinning force F p and to estimate the value of the viscosity for the motion of such a vortex structure in the ceramic: F p =6ϫ 10 −8 N, =6 ϫ 10 −5 kg/ s.

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The dynamics of low-density trapped magnetic fluxes, including local fluxes, are investigated in single-crystal YBCO in the region of strong thermal fluctuations near the superconducting phase transition. A substantial difference of the isothermal magnetization-relaxation behavior from the quasi-logarithmic behavior observed earlier in strong magnetic fields is established for the first time. The effective pinning potential in the temperature interval close to the critical temperature is estimated in terms of the simplest model of thermally activated creep.